tnt^fCtlpoflmigork College of l^f)piitiani anb burgeons! Hibrarp Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons http://www.archive.org/details/americantextbookOOturn LIST OF CONTRIBUTORS. BURCHARD, HENRY H., M.D., D.D.S. ESSIG, CHARLES J., M.D., D.D.S. GRITMAN, A. DeWITT, D.D.S. HILLYER, ELLISON, D.D.S. HODGEN, JOSEPH D., D.D.S. LeCRON, D. O. M., M.D., D.D.S. OTTOLENGUI, RODRIGUES, M.D.S. PEESO, FRED A., D.D.S. THOMPSON, ALTON H., D.D.S. TRUEMAN, WILLIAM H., D.D.S. TURNER, CHARLES R., D.D.S., M.D. WILSON, GEORGE H., D.D.S. THE AMERICAN TEXT-BOOK OP PROSTHETIC DENTISTRY. IN CONTRIBUTIONS BY EMINENT AUTHOBITIES. EDITED BY CHARLES R. TURNER, D.D.S., M.D., PROFESSOR OF MECHANICAL DENTISTRY AND METALLURGY, DEPARTMENT OF DENTISTRY, UNIVERSITY OF PENNSYLVANIA, PHILADELPHIA. FOURTH EDITION, REVISED AND ENLARGED. ILLUSTRATED WITH 900 ENGRAVINGS LEA & FEBIGER PHILADELPHIA AND NEW YORK 1913 /^/ 'O Entered according to Act of Congress, in the year 1913, by LEA & FEBIGER in the Office of the Librarian of Congress. All rights reserved. WITH THE CORDIAL CONSENT OF THE CONTRIBUTORS THIS VOLUME IS DEDICATED TO EDWARD CAMERON KIRK, D.D.S., Sc.D., IN RECOGNITION OF HIS EMINENCE IN DENTAL LITERATURE SCIENCE, AND EDUCATION AND AS A MODEST TOKEN OF THE REGARD OF HIS FRIEND AND COLLEAGUE THE EDITOR. PREFACE TO FOURTH EDITION. The editor offers a new edition of the American Text-hook of Pros- thetic Dentistry in the hope that it may continue to fill the important })lace accorded it for so long a time by the teachers, students, and prac- titioners of dentistry in this country. Being intermediate in character between the hand-book, which is intended to furnish the main facts upon a subject briefly and succinctly, and the work of reference, which as a final authority must give an exhaustive treatise upon its topic, the book is primarily designed as a teaching volume. It stands for and ad- heres to the general pedagogical principle that a thorough knowledge of the fundamental principles of the subject must first be inculcated, so that the student may attack his problems by the analytical method instead of by rote or authority. After this foundational knowledge is obtained, the technical procedures necessary to construct the various appliances employed in prosthetic dentistry may be undertaken, instead of making the latter first in time and importance — a point of view not in vogue when prosthetic teaching was conducted empirically in the dental laboratory. That this method has been in accord with the present pedagogical practice is attested by the fact of the widespread adoption of the book as a text-book in the colleges of this country. To the dental practitioner it is hoped the technical teaching will appeal as being an exposition of the most recent approved methods of prosthetic practice. The plan of composite authorship permits the selection of men who have given special attention to the several subjects of which the}^ treat. Since the appearance of the last edition in no field of dentistry has more gratifying progress been made than in the increase of our knowledge upon the subject of the so-called "anatomical articulation" of artificial teeth. A broader knowledge of the natural masticating mechanism, greater accuracy in the articulators serving to represent a part of this mechanism, and vast improvements by the manufacturers in the de- signs of artificial teeth themselves have resulted from the activities of various ones concerned in treating this general problem. These facts have necessitated an enlargement of the text upon these several topics in order that it may embrace such details of these subjects as seem to be proved and established. While in none of them has the "ultima thule" been reached, there is a great general improvement in the scientific accuracy of our knowledge of the whole question. The chapter on Cast Metal Defitures, written by the late Dr. Clark L. Goddard, has not been replaced by a new one upon the subject. Since the extensive employment in dentistry within the past few years of the principle of casting metals under pressure, the use of the precious (vii) vm PREFACE TO FOURTH FD/TION. metals for base-plates by this plan has proved universally (lisai)point- ing, and none of the teehnic so far evolved can be recommended to the practitioner or student. The percentage of shrinkage and warpage of the metal when cast in large masses, its lower density and poorer texture than the swaged plate, and the general lack of uniformity of results have all pro\ed obstacles to those who demand accuracy in the finished product. While the methods of casting aluminum and the base metal alloys have all undergone improvement, it is not felt that anything authoritative can be given on this toj)ic at this stage of its de^■elol)nu•nt. It is believed that many of the defects of this method that held so much of promise to the prosthetic dentist may in time be o\crcome and its field of u^s('fulness extended. The technical details of metallurgical operations as regards obtaining the metals from their ores and all others not directly related to the practice of prosthesis in the laboratory have been omitted from the present chapter on the subject. As the workman must know his tools, so must he know the properties of the materials of which his products are wrought, but other details, interesting and important as they are, must be reserved for the separate work on metallurgy. The chapters originally contributed by Dr. H. H. Burchard on Tlic E.vcuni nation of the Mouth, The Taking of luijiirssions, and The Makincj of Piaster Casis, and revised by the late Dr. J. P. Gray, whose loss by death the staff of contributors has suffered since the last edition appeared, ha\e been replaced by chapters on these subjects by Dr. A. De Witt Gritman. The remaining chapters by Dr. Burchard and those by Dr. Essig have been thoroughly revised. In acknowletlging his indebtedness to the publishers for their unfail- ing kindness and valuable assistance during the preparation of this ^•olume, the editor especially wishes to express his appreciation of the sympathetic help of ]\Ir. Christian Febiger, with whom he has had the privilege of manv conferences in the progress of the work. C. R. T. 3930 Locust Street, Philadelphia. LIST OF CONTRIBUTORS. HENRY H. BURCHARD, M.D, D.D.S. Late Special Lecturer on Dental Pathology and Therapeutics, Philadelphia Dental College, Philadelphia. CHARLES J. ESSIG, M.D., D.D.S. Late Professor of Mechanical Dentistry and Metallurgy, Department of Dentistry, University of Pennsylvania, Philadelphia. A. DeWITT GRITMAN, D.D.S. Assistant Professor of Prosthetic Dentistry, Department of Dentistry, ITni- versity of Pennsylvania, Philadelphia. ELLISON HILLYER, D.D.S. Professor of Prosthetic Dentistry and Orthodontia in New York College of Dentistry, New York City. JOSEPH D. HODGEN, D.D.S. Professor of Chemistry and Metallurgy, College of Dentistry, University of California, San Francisco. D. O. M. LeCRON, M.D., D.D.S. Professor of Crown and Bridge-work and Porcelain Dental Art, Barnes Dental College, St. Louis, Mo. RODRIGUES OTTOLENGUI, m.d.s. Editor of the "Items of Interest." FRED A. PEESO, D.D.S. Assistant Professor of Crown and Bridge-work, Department of Dentistry, University of Pennsylvania, Philadelphia; Professor of Crown and Bridge- work, University of Pennsylvania Dental Post-Graduate School. ALTON HOWARD THOMPSON, D.D.S. Professor of Dental Anatomy, Kansas City Dental College, Kansas City, Mo. (ix) X LIST OF CONTRIliUTORS. WILLIAM H. TRUEMAX, D.D.S. Philadelphia, Pu. CHARLES R. TURNER, D.D.S., M.D. Professor of Mechanical Dentistry and Metallurgy, Department of Dentistry, Universitj^ of Pennsylvania, Philadelphia. GEORGE H. WILSON, D.D.S. Formerly Professor of Prosthetic Dentistry and Metallurgy, Dental Depart- ment, Western Reserve Universitj', Cleveland, Oliio. CONTENTS. CHAPTER I PAGE THE DENTAL LABORATORY: ITS EQUIPMENT AND ARRANGE- MENT 17 By Charles J. Essig, M.D., D.D.S., and Charles R. Turner, D.D.S., M.D. CHAPTER II. METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY 91 By Joseph Dupxjy Hodgen, D.D.S. CHAPTER HI. PORCELAIN TEETH 161 By Charles J. Essig, M.D., D.D.S. Revised by Ellison Hillyer, D.D.S. CHAPTER IV. THE HUMAN DENTAL MECHANISM: ITS STRUCTURE, FUNCTIONS, AND RELATIONS 192 By Charles R. Turner, D.D.S., M.D. CHAPTER V. THE HUMAN DENTAL MECHANISM AS MODIFIED BY TEMPERA- MENT, AGE, AND USE 2.55 By Alton H. Thompson, D.D.S., and Charles R. Turner, D.D.S., M.D. CHAPTER VI. THE EXl^MINATION, PREPARATION, AND STUDY OF THE MOUTH PRELIMINARY TO THE INSERTION OF ARTIFICIAL TEETH. . 271 By a. DeWitt Gritman, D.D.S. CHAPTER VII. IMPRESSIONS OF THE MOUTH 282 By a. DeWitt Gritman, D.D.S. (xi) Xll CONTENTS. CHAPTER Mil. PAGE THE MAKING OF PLASTER CASTS 302 By a. DEWirr Guitmax, D.D.S. CHAPTER IX. DIES, COUNTER-DIES, AND MOLDING 310 By William H. Trueman, D.D.S. CHAPTER X. SECURING THE VARIOUS DATA TO BE USED IN CONSTRT'CTING ARTIFICIAL DENTURES. TAKING THE BITE. ARTICULA- TORS 335 By Charles R. Turner, D.D.S., M.D. CHAPTER XI. THE PRINCIPLES UNDERLYING THE RETENTION OF PLATE DENTURES 386 By Charles R. Turner, D.D.S., M.D. CHAPTER XII. THE SELECTION, ARRANGEMENT, AND ARTICULATION OF ARTI- FICIAL TEETH 407 By Charles R. Turner. D.D.S., M.D. CHAPTER XIII. VULCANIZED RUBBER AS A BASE FOR ARTIFICIAL DENTURES.. 472 By George H. Wilson, D.D.S. CHAPTER XIV. SWAGED METALLIC PLATES 649 By William H. Trueman, D.D.S. CHAPTER XV. CONTINUOUS-GUM DENTT^RES 635 By D. O. M. LeCron, M.D., D.D.S. CHAPTER XVI. ARTIFICIAL CROWNS 661 By Henry H. Burchard, M.D., D.D.S., and Fred. A. Peeso, D.D.S. CONTENTS. xiii CHAPTER XVII. AN ASSEMBLAGE OF UNITED CROWNS (BRIDGE-WORK) 725 By IIenuy H. Burchaud, M.D., D.D.S., and Fued. A. Peeso, D.D.S. CHAPTER XVIII HYGIENIC RELATIONS AND CARE OF ARTIFICIAL DENTURES . 775 By Charles J. Essig, M.D., D.D.S. CHAPTER XIX. PALATAL MECHANISM 786 By Rodrigues Ottolengut, M.D.S. PROSTHETIC DENTISTRY, CHAPTER I. THE DENTAL LABORATORY, ITS EQUIPMENT AND ARRANGEMENT,, By Charles J. Essig, M.D.. D.D.S. AND Charles R. Turner, D.D.S., M.D. THE LABORATORY. It is higlily important that the mechanical laboratory should be suitably arranged and equipped for the purposes to which it is devoted. In planning the dental offices a room should be selected which is appro- priate in size and location. It frequently happens that the labora- tory is relegated to a room which cannot be used for other purposes, and which answers the requirements of a workshop in no degree. It should be easily accessible from the operating room to facilitate such opera- tions as band-fitting for crowns which are partly performed in both places. From a hundred to a hundred and fifty feet of iioor space are necessary for one or two workmen, and an advantage will be gained if the room is situated with its longest side toward the light. The uniformity of illumination afforded by a northern light recommends this exposure where it is obtainable. Ample light should be provided by sufficient window space so that none of the laboratory operations are required to be performed in shadow. Adequate ventilation and strict attention to the demands of sanitation are necessary hygienic requisites which must not be overlooked. The furniture and appliances should be especially adapted to the needs of the worlonan, and arranged to facilitate ease and celerity in his manipulations. Every article which is used in the laboratory should have a place provided for it, to which it may be returned when not in service. Systematic care of the tools and instruments and orderliness and cleanliness in the various mechanical procedures should characterize the laboratory work. The usual equipment of a dental laboratory consists in a suitable work-bench, carefully adapted to the purpose for which it is to be used; a molding box; plaster table and sink; a swaging block and anvil; at least two lathes, one designed especially for the grinding and fitting of 17 18 THE LABORATORY. teeth, the otlier for finishing and poHshing only; a mechanical blowpipe table, supplied with gas-burner on the Bunsen principle, of sufficient capacity to allow of the soldering of full dentures. In addition to these permanent articles of laboratory furniture it will be necessary to provide a suitable furnace for the melting of zinc, lead, and alloys commonly used in making dies and counter-dies, and also another and difi'erent one to be used for the occasional melting of gold and silver and in the formation of alloys to be used as solders. Besides these a vulcanizer to be used in vulcanizing dental rubber, porcelain furnaces for the baking of inlays, crowns, and continuous-gum dentures, and for staining teeth, and a gold casting a])])aratus for casting molten gold are necessary adjuncts to a fully ecjuij)])ed laboratory. The accessories of soldering, molding rings and flasks, ingot molds, rolling mills, draw-plates, pickling solutions, with the most suitable vessels for holding the same, grinding and polishing materials, fluxes, varnishes, adhesive wax, and bench tools, all necessarily form part of the equipment of the dental laboratory, and will each be described in this chapter or in the chapter devoted to that special subject. The Work-bench. — The work-bench should be constructed of cherry, ash, or well-seasoned oak; it should be provided with not less than two sets of drawers, one to contain the ordinary bench tools, being ar- ranged in a tier at the right hand side; the other directly in front of the workman and over his lap, and intended for the storing of the materials he is using and the filings and scraps from the same. Such a bench will presently be described. The height of the work-bench should be about 34 inches, which places the work the proper distance from the eyes when the workman is seated upon a stool of usual height (about 18 inches). Jewelers' benches which have been used for dental work are too high, as they have been designed for operations, which necessitate closer vision. It may be said that the stool upon which the operator sits, his own physique, and the height of the bench should be so correlated that the work is not so close to the workman's eyes as to put a continuous strain upon the accommoda- tion, and yet is clear enough for comfortable vision without his bending over. The bench should be located so that the workman faces the light, and for this reason it should be given the preference in position over any other laboratory furniture. The length, when designee! for the conveni- ence of two workmen, should be about .') feet () inches; the width may be 24 inches. The top should be at least 1 inch in thickness, and immedi- ately over the tool drawers should be arranged a rest for convenience in filing and finishing. This rest is usually made of the same hard wood as the top of the bench, 2 inches wide and about 3 inches long, tapering from IJ inches in thickness where it is mortised into die table to \ inch at its extremity. It is desirable that the bench l)e firm and solid to withstand any force exerted upon it in use, and it is advisable in some cases to have it fixed to the wall, especially if it is to be used as a support for the vise in drawing wire. Where it is possible, se})arate benches should be provided for the several classes of lal)oratory operations. The tools and materials for each kind of work are then kept together, and a mixing of the scraps and debris of the various operations is prevented. THE WORK-BENCH. 19 It is particularly important that a bench or portion of the bench be kept solely for work with the precious metals. A very convenient and elaborate work-bench of good design is shown in Figs. 1 and 2. It is especially adapted to meet the wants of those who do crowm and bridge-work. It is provided with a rolling top, which automatically locks the drawers when closed and yet does not obstruct the light when open. It is provided with a foot-bellows, and drawers for gold solder, phite, wire, files, scrapers, corundum wheels, pliers, cutters, benders, etc.; some of the most complete being shown by Figs. 3, 4, 5, and 6. This bench is a highly finished piece of cabinet work, and would be suitable for the operating room for such work as it is desired to do while the patient is in the chair. Three benches are shown in Figs. 7, 8, and 9 for vulcanite and gold and general plaster and sand work, respectively. The higher part of Fig. 7 is surmounted by a marble slab, 16^ X 14^ inches, for the accommoda- tion of the vulcanizer. Beneath the slab are two drawers 3 inches deep, for flaslvs, wrenches, and tools used about the vulcanizer. The third drawer contains a series of shallow compartments for the reception of scrapers, etc. The fourth drawer is partially divided by grooved com- partments similar to drawer No. 3, and is intended for files, etc. Drawers five and six are for hammers, horn mallets, and other large tools, and for rubber, w^ax, and other supplies. The cupboard contains a shelf, and \\dll be found convenient. Adjacent to the series of drawers described above, and under the right- hand end of the lower top, is a cupboard designed for the tooth stock. This cupboard contains a removable case diAaded horizontally through the centre and of a wddth to receive the boxes in which teeth are sold, the ends of the boxes projecting beyond the case to allow an easy with- drawal. For convenience the contents of each box may be marked upon its end, and the inner case and its contents may be removed and placed in a safe. At the left-hand end of the bench are two drawers, one containing a rack into the holes of which the mandrels of the lathe wdll drop, allowing the grinding wheels to rest upon the surface of the rack and keeping the wheels separated and in place. To the right of this drawer is another drawer of the same size for brush wheels, felt wheels, and cones. BetW'Cen these last two drawers runs the belt of the lathe. The lower top of the bench is of hard maple, finished smooth, and a 4-inch rail extends along the back and end. The fihng block is placed in such a location that the workman, when using it, is within easy reach of his tool draw^ers. The top of the bench represented by Fig. 8 is a marble slab 16 x 34 inches, and a rail extends around the two ends and back. Under the right-hand end is a slide, and beneath this slide is a drawer arranged w^th shallow compartments, concave in shape, for files, etc., each compartment being intended for a single instrument.^ Beneath this drawer are two drawers containing divisions of suitable size to hold the various plate-cutters, benders, punches, shears, pliers, etc. Drawers four and five are for bulky tools and supplies. The lower compartment contams the foot bellows fixed permanently Work bench, closed. 20 THE WORKBENCH. 21 in place, the treadle only of which projects in front of the bench. The bellows is connected with a metal pipe in the rear right-hand corner of the bendi, and this pipe extends upward behind the drawers and through Fig. 3 Fig. 4 Drawer for gold, solder, plate, and ware, with bora.K block, etc. Drawer for files and scrapers. Fig. 5 Fig. 6 Drawer for mounted grinding wheels. Drawer for pliers, cutters, benders, etc. the marble slab, and is surmounted by a neat polished brass casting with a horizontal nozzle to which the blowpipe tubing may be attached. This arrangement of the bellows overcomes the disadvantages of having the bellows continually under foot, and the tubing lying across the bench and hanging down in front. The bellows is made specially for this bench and is shipped connected and ready for use, but the ordinary bellows can be adapted to the compartment and easily connected to the metal pipe. If the compressed air tank be used instead of the bellows, the lower compartment may. be utilized as a dravrer by withdrawing and reversing it, the other end of the drawer being finished and furnished with .a drawer pull. The front end then becomes the rear end and the opening is closed by a slide furnished with all benches. Under a filing block, which is furnished with the bench, near the left- hand end of the bench, is a drawer arranged with concaved blocks cut out of the solid wood. One of these concavities is elliptical in shape for gold plate, wire, etc., and five are circular in shape for different grades of gold solder or for other purposes. The drawer also contains a slightly con- caved borax block or marble | inch thick, and a grooved block from which brushes and small instruments may be readily taken. This drawer can only be opened by one who knows the location of a secret lock. Below the gold drawer is a slide of zinc in a wooden frame for catching gold filings. This slide gradually slopes downward to a cen- tral recess, which has a screw cap removable from below. The top of the bench represented by Fig. 9 is a marble slab 19^^ X 24 oo THE LABORATORY. Fio. 7 Work bench for vulcanite work. Fig. 8 Work bench for gold work. inches, and overhangs the front, so that the dentist may stand close without touching the lower part. Around the sides and back is a rail, and at the back of the marble slab is an opening If X 6 inches, which ACCESSORIES OF THE WORK-BENCH. 23 enters a metal cliiite for carrying away waste plaster, etc., through the bench, back of the drawers, to a galvanized iron receptacle beneath. This receptacle rests upon a shelf, and can be drawn forward by its two handles and removed when its contents are to be disposed of. The left upper drawer is for rubber bowls and articulators. i3eneath the upper drawer is a shallower one arranged with compart- ments, concave in shape, for spatulas, plaster knives, etc., the arrange- ment of the compartments being such that the various tools remain where placed. At the right of the two drawers already mentioned is a tight drawer lined with zinc, for sand, molding rings, ladles, and all appliances used in casting. The long drawer above the two lower Fig. 9 Work-bench for plaster and sand work. compartments contains divisions for eighteen impression trays. The two lower compartments are V shaped and hinged at the lower point. One is for impression plaster and the other for cast plaster. The centre of gravity of these drawers is such that they remain in either an open or closed position as placed. These benches have not been im- proved upon for convenience. In laboratories, w'here space must be economized, one of the combination benches designed for the several classes of work may be desirable. Figs. 10, 11, and 12 show two recent approved patterns. Accessories of the Work-bencli. — A good vise is an important adjunct to the work-bench, and is indispensable w^hen the draw-plate is used for reducing the size of gold, platinum, or silver wire. 24 TJiE LABORATORY. Rubber slabs h incli in thickiu-ss l)y (i inclu's s(|uaro aiford excel- lent rests, not only lor the protection of the to]) ot" the beneh from injury by contact with dies and counter-dies in the j)reliniinary stage of plate-making, but also as pliant and elastic rests for the metallic or rubber denture durin), furni.shed with a loose bottom, made of 2-inch seasoned oak or ash, and provided with four pieces of solid rubber cylinder (Fig. 14, E) \\ inches in diameter by 2 inches long, let into it by holes of the same dimensions bored to a depth of IJ inches. Two thicknesses of rubber are thus interpo.sed between the block upon which the anvils rests and the floor of the labor- n ASTER TABLE ASD SISK. 27 atorv, and so much of the sound due to the percussive force of the hammer is thereby deadened that scarcely any noise or vibration will be observed by persons in other parts of the house. The anvil (Fig. 16), which should weigh not less than 40 pounds, may be securely fastened to the block upon which it rests by strong iron staples (G), and the box or outside covering of the block reinforced by iron bands, as shown at H. A swaging block so constructed may be looked upon as a permanent piece of laboratory furniture, and one that will not be likely to get out of order. An anvil resting upon a bed of sand contained in its base of sheet galvanized iron sold in the supply stores has proved very satisfactory in laboratories where space is a con- sideration. The sand takes up a great deal of the vibration imparted to the anvil in swaging. Fig. 17 shows the anvil and base. Two swaging hammers are required — one, weighing about 2 pounds, is of much use Fig. 17 Fig. 16 An%'il mounted upon swaging block. Swaging anvil. in starting the plate. The heavier one, which should weigh 5^ or 6 pounds, is used with greater force after the plate has been made to partially conform to the zinc die when there is no longer danger of its pleating or folding. Plaster Table and Sink. — The working of plaster, which forms so im- portant a part of the operations of the dental laboratory, is entitled to much more care and attention than it usually receives at the hands of the mechan- ical dentist. It may be employed with neatness and precision, when its results become truly artistic, or, as is too often the case, it may be handled in so slovenly and untidy a manner as to greatly lower the standard of results, and, unless kept carefully within the precincts assigned it, cause the laboratory to become a most unattractive place. It is of importance, therefore, that a suitable table be provided upon which the casting and subsequent trimming of plaster casts and other parts of the laboratory 28 THE LABORATORY. work ck'pciKling upon tlic ('iii])l()yiii(Mit of plaster may be performed. The plaster table should also be supplied with a receptacle for the cut- tings and refuse fragments. The table already described on page 22 has proved of very useful design. The accessories of the plaster table consist, first, of two short, broad- necked bottles, for sandarac and shellac varnish, two or more flexible rubber plaster bowls, the same number of bone, ivory, or steel spatulas for mixing, one or more plaster knives, such as are sold at the dental depots for the purpose of reducing the size of plaster casts, for flasking in rubber or celluloid work, and a number of different sizes of camel's- hair brushes, which are indispensable in carrying the plaster into the deeper parts when running or casting impressions for partial dentures, and, indeed, all impressions having deep and more or less inaccess- ible points, which might not be perfectly reached l)y the gravitation of the plaster unassisted by some such means as is suggested l)y the use of the camel's-hair pencils or brushes. Two kinds of varnish are usually employed in the ])reparation of the surfaces of impressions for running out the casts, so as to prevent too close adhesion of one to the other. One is transparent and dries upon the plaster without color. The other is of the color of burnt sienna, and imparts a dark-yellow stain to the plaster. The first is made by dissolv- ing 5 ounces of gum sandarac in a c)uart of alcohol. The latter is formed of gum shellac and alcohol in the same proportions. Gum sandarac dis- solves rather slow'ly, and recjuires a good c|uality of alcohol free from a very considerable percentage of water; otherwise it will have a milky appearance and will not afford a perfectly glazed surface when applied to the plaster impression. These two varnishes are employed for totally different purposes. In running out an impression the object should be to obtain a perfect surface to the cast, one that is free from air-bells or roughness of any kind, as such imperfections will be represented on rubber or celluloid dentures by multitudes of minute globules which are highly irritating to the nmcous membrane of the mouth. The shellac varnish should be applied first, as it penetrates the plaster and di.scolors it sufficiently to serve as a guide in removing imjiressions from casts, and thus prevents the workman from injuring the teeth t)r prominent parts of the cast. After the shellac varnish has been allowed to dry, the sandarac should be applied with a canuTs-hair brush until the surface is glazed. It should be laid on of a uniform thickness, but not in such quantity as to fill up deep places or to injure the correctness of the fac-simile of the mouth. After the last coat of varnish has been allowed to dry, if the glazing of the surface is satisfactory, the plaster impression merely requires to be dipped in water to ensure saturation and to further harden the varnish, when it is ready for running the cast. Careful attention to these details will produce a cast possessing hardness of surface, and with the glazed appearance which is noticed when plaster is poured and allowed to set upon glass. This result, however, cannot be obtained when oil or solu- tions of soap have been used; such substances should never be applied to plaster impressions, as they do not afford surfaces sufficiently smcDth or hard upon which to form rubber or celluloid dentures. To get the PLASTER TABLE AND SLXE. 29 best results in the handling of plaster, the latter in mixing should be slowly dropped into water until it becomes saturated and settles to the bottom of the bowl, so as to expel the air. The sur])lus of water is then pouretl ort' and the plaster well stirred, when it should be earned to the surface of the impression and into the deep parts with a camel's-hair brush, and the balance built up with the spatula. Plaster of Paris is prepared from a native calcium sulphate known as g}'])sum (CaSO, + 2H20). There are other native sulphates of cal- cium: alabaster, which is a whitish translucent mineral; selenite, which is transparent; but gypsum, which occurs in opaque white masses, is the common source of plaster of Paris. The latter is obtained from g>'psum by a partial dehydration, die process removing one of the molecules of the water of crystallization and leaving plaster of Paris (CaSO^ + H^O), a substance which, when mixed with water, has the property of taking up the other molecule of water of crystallization and of crystallizing with the formation of the original hydrate (CaSO^ + 2H20). In making plaster of Paris, the g}^sum is crushed into conveniently sized masses, is freed of its impurities, and roasted in an oven, a kettle, or a rotating cylinder at a temperature not exceeding 400° F. At 212° F. it begins to give off some of its combined water; at about 261° F. the best plaster of Paris is made, and if heated above .400° F. more than one molecule of water is driven off, and the property of the plaster to again take up this water is impaired or entirely destroyed. After calcining, the plaster is ground to various degrees of fineness, according to the use to which it is to be put. When mixed with the correct proportion of water, plaster of Paris hardens by crystallization, and this hardening or setting is attended with a slight evolution of heat and a slight expansion, usually about -j-g-Q- of its volume. Various factors affect the amount of the expansion, which for obvious purposes of accuracy when used in the dental laboratory should be reduced to the minimum. The time occupied in setting varies with dif- ferent plasters used in dental work, and various factors affect this time. The relationship of these various factors to both the degree of expansion and the time of setting will be discussed in Chapters VII. and VIII. Two separate kinds of plaster are employed in the dental laboratory, one for taking impressions, and the other for dental casts. Plaster w^hen not being used should be kept covered to shield it from occasional dampness of the atmosphere and to protect it from water and foreign substances which might accidentally fall into it. The tin cans in which plaster is furnished by the dental depots are admirably suited to this purpose. The Ohmer dental bin is a more convenient receptacle. A closed tin container is provided at the bottom with a sieve actuated by a crank, by means of which the plaster may be slowly sifted into the mixing bowl, any large particles of extraneous matter being restrained by the sieve. The Ohmer bin is of good design, but should be of stronger construction. The author has recently obtained flour bins with a capacity of about 25 quarts which are of this same design, and, be- ing admirably constructed, have been extremely satisfactory. A sink with rimning water is practically a necessity for the work in plaster, and the plaster table should adjoin it. An ordinary iron sink, 30 THE LABORATORY. such as is used in the kitchen or laundry will answer every purpose. Unless it is galvanized, it should be given a coat of pitch to prevent rust. Precaution must be taken to prevent clogging of the wa^ite pipe with pieces of hardened plaster by providing guards and traps, and a slatted wooden false bottom which may be removed at will, mav be fitted to the sink to both stop the larger pieces of plaster and oft'er a less hard and resistent surface to anything which might be broken by an accidental fall upon it. Heat in the Laboratory, This agent is utilized in so many laboratory processes that it is deemed wise to discuss its sources and the principles underlying its production before taking up the methods which employ it. In the dental laboratorv- heat is obtained either from the combustion of fuel or from electric energy, the former being its commoner source. Solid fuel, as coal or coke, is no longer in use, having been succeeded almost entirely by either liquid or gaseous fuel. Alcohol, gasoline, and kerosene are the liquids used for this purpose; the gas is commonly either ordinary illuminating gas or natural gas in those regions in which it is available. These all owe their inflammability to the fact that they are hydrocarbons, combustion of which takes place when they are heated in air. One of the phenomena of combustion is the production of flame, which is simply burning gas. The simplest flames^ Tvath which we are acquainted are those of hydrogen and carbon monoxide burning in air or oxygen. In such as these the burning gas undergoes no decomposition. The combustion consists of the simple union of an inflammable gas with oxygen: H, + O = HjO. 2C0 + Oj = 2C0,. The flame of either of these gases burning from the end of a tube appears as a burning cone, which upon investigation is found to be hollow, the combustion only taking place on the surface of the cone where the inflammable gas is mixed by diffusion with the air. Substances which undergo decomposition and yield more than one product of combustion present a more complex flame structure. The flames of hydrocarbons commonly employed for illuminating purposes, such as the candle, illuminating gas, and oil, are practically identical in points of construction and a description of one will suffice for all. The Candle Flame. — On approaching the wick with the flame of a match, the wax (or other hydrocarbon of which the candle may be made) melts, is drawn up in the fibres of the wick by capillary attraction and there converted by the heat into gaseous hydrocarbons, which ignite, and in their chemical union with the oxygen of the air produce the flame. In such flames, as in the simpler ones already referred to, there is first, about the wick or burner, the dark cone. A, of heated unburned gases. Above and about the apex of this cone is a second cone, B, which in comparison with the rest of the flame, seems nearly opaque, and which emits a bright-yellow light. At the base of the flame there is a small cal^TC-like region, C, which appears bright blue in color and 1 Manuscript on the flame furnished by J. D. Hodgen. THE REDUCING FLAME. 31 IS non-lummous. Fig. 18 Then enveloping the entire flame there is a faintly luminous, hardly perceptible, bluish-purple mantle, D (Fig. 18). llie dark cone, A, as has been explained, consists of unburned gases and in reality is not a part of the flame. However, chemical changes are taking place therein, owing to the heat from the sheath of combustion surrounding it. Cone B is ordinarily spoken of as the luminous cone. It has been concluded probable that the luminosity in flame is due to: (1) the presence of solid matter, (2) the density of the flame gases, and (3) the temperature of the flame. The blue region, C, may be regarded as being largely made up of the combustion of carbon monoxide. The faintly luminous mantle, D, is probably a zone of complete combustion, in which those substances which have been incompletely oxidized in the other portions of the flame, chiefly hydrogen and carbon monoxide, are finally converted into water and carbon dioxide. The Bmisen and Blowpipe Flame. AMien a certain amount of air is mixed with coal gas or any other hydrocarbon gas before combustion, the gas burns with a pale-blue, non-luminous, smokeless flame, which has a three-cone structure (Fig. 19). Cone A contains the mixture of combustible gases and air (oxj'gen). In the Bunsen burner the air is drawn in through the openings near the base of the metal tube. The mouth blowpipe conveys a blast of air into the centre of the flame. In the com- pound blowpipe flame the blast of air (ox}^gen) is injected into the combustible gases from the lungs of the operator or by some mechanical means, such as a bellows, through a concentered tube, D, while the gas is conveyed by the outer and larger tube, E (Fig. 20). The Reducing Flame. — The inner cone, B, presents the gas burning with a pale-blue flame, rendered so by the pre- sence of oxygen in the gas. If an oxidized piece of copper be placed in a Bunsen or blowpipe flame in the position of Candle flame Fig. 19 ML Fig. 20 Blowpipe flame. the line marked B' B' , it will be noticed that the metallic sheet brightens in the area covered by the flame. This is accounted for by the fact that this region of the flame contains highly heated but unburnt hydrogen or hydro- carbons, which have the power to abstract and then combine with the oxygen of the copper oxide, thus freeing or reducing the copper; hence, this region is known as the deoxidizing or reducing flame. Bunsen flame 32 THE LABORATORY. This is the flame used for soldering, ixs it reduces any oxides that ma) be on the solder, or parts to he soldered, and, also cutting off the oxygen of the air from contact with the heated metals, it prevents any reoxid- ation of them. The Oxidizing Flame. — The outer cone, C, presents a pale-hlue or purple color and is tlic zone of complete combustion. Gases which have escaped combustion in the inner cone are oxidized in the outer one by the ample supply of oxygen in the atmosphere surrounding it. A bright piece of copper held in the position of the line C C" will be quickly darkened by the formation of copper oxide upon its surface. This is accounted for by the fact that the copper becomes heated, and, being unprotected, is unable to resist the affinity of the oxygen in the air surrounding it, and is therefore oxidized. Hence, the term oxidizinc/famc. Any attempt at soldering with this flame results in oxidation of the base metals of the solder and parts to be soldered, so that adcHtional fluxing will be necessary before the solder can flow. Continued misuse of the flame may so greatly raise the carat of the solder, by oxidizing out the base metals, as to make its fusing point dangerously high, or the presence of the oxides mixed with the solder may make its flowing impossible. Because of the chemical nature of combustion, it is evident that the proportion between the air and the gas must be definite and fixed to obtain the highest heat, and this must be regulated when the blowpipe is in use. If too little air is supplied, imperfect combustion takes place and the full degree of heat is not developed. On the other hand, the luminosity of the flame is increased, the heat being inversely related to this. If too much air is forced in, the temperature of the flame must necessarily be reduced by the current of cool and uncombined air. The various heat-producing appliances which burn gas operate under the principles above outlined. They will be described in the portions of this chapter which treat of their use. Gasoline and alcohol apfjliances will also be discussed later. One of the commonest uses for heat in the laboratory is in the melting of metals. Fig. 21 shows a form of Bunsen burner that has found favor for general laboratory uses. Greater heat may be obtained by using a larger burner. Recentlv an improvement on the Bunsen principle has been utilized by Dr. Meker of Paris in the construction of the Meker l)urner. In this the air and gas are more thoroughly mixed in the tube, the larger open- ings at the base admitting more air, and a wire gauze at the mouth of the tube prevents back firing. This burner, shown in Fig. 22, has greater heat-producing capacity than a Bunsen burner of equal size, and the flame is almost uniform in its heat. Modes of Melting Metals. — The means employed for this purpose will depend upon the character of the metal or alloy to be fused.' The fusing of such alloys as are used for dies and counter-dies in crown and bridge-work, which melt at temperatures ranging from 158° F. to 2.36° F. — and there are a large number of these alloys now in use — may be accomplished by simply placing a sample of any one of them in MODES OF MELTING METALS. 33 a snuill iron ladle provided with a suitable handle, or in a copper ladle recently designed for the purpose (Fig. 23), and holding it over a Bunsen Hume or die flame of an alcohol or oil lamp. Fig. 21. Fig. 22 Bunsen burner for laboratory use. Meker burner. Metals and alloys used in the formation of dies and counter-dies melting in an iron ladle or at below red heat may be fused in an ordinary Fig. 23 Ladle for fusible metal. Fig. 24 Fletcher's furnace. Fig. 25 Burner for furnace. stove or furnace, or, where gas is available, in one of the furnaces devised by INIr. Fletcher, of Warrington, England. Fig. 24 ^hows a furnace of 3 34 THE LAIiORATORV his (k'siirii for lufltiiii,' /.iiic, lead, and otht-r nu-tals for (lies and counter- dies, which, it is l)clicvcd. is unexcelled by any other yet made. .Milling; liidU' and handle. It works c(|ually well with any gas suj)j)]y available; the s])eed of working is, however, proportionate to the sup])ly of gas. The burner can be removed from the casing and used for other purposes if desired. The cast-iron ladle and handle are .shown in Fig. 20. Fid. 27 Fig. 28 Fig 2fl Carbon Mipport. It should be remembered that zinc will, under favorable conditions, unite with iron, and it frecpiently attacks the cast-iron ladle in which it is melted, and may penetrate the side and escape into the fire. Accidents MODES OF MET/riNC, MKTAL!^. 35 of this kind are more likely to occur when the ladle is new, and inay he avoided by coating the inside with whiting previous to the first nielting. This coating should he renewed until a protecting covering of oxide of iron has been formed after repeated use of the pot. Small ([uantities of gold or silver may be melted by means of the ordinary blowpipe upon a su})p()rt formed of charcoal! A good solid cylindrical piece of thoroughly charred pine coal should be selected, and divided into two equal halves by a vertical cut with a saw, as shown by Figs. 27, 28, 29. Upon the end of one half a depression should be cut for the reception of the metal to be melted {A). On the flat side of the other half, extending to the end, the ingot mold should be carved, of a size and shape governed by the requirements of the case (B). The two halves should then be brought together and secured by a piece of iron or copper wire, when they will be found to practically combine the recjuire^ ments of a crucible and ingot mold. The depression in which the metal is to be melted and the mold or receptacle should be connected by means of a gutter or groove. The flame of the blowpipe is directed upon the metal, and when thoroughly fluid the charcoal is tilted, so that the fused metal will run into the mold prepared for it in the opposite half of the charcoal. This is probably the simplest form of apparatus by which small quantities of metal can be melted, and is often employed in the dental laboratory and by jewellers. Mr. Fletcher has devised an apparatus embodying the same general principles as the one just described for quickly obtaining ingots of gold and silver without the use of a furnace (Fig. 30): A representing a crucible of molded carbon, supported in position by an iron side-plate; B, the ingot mold ; C, clamp holding ingot mold and crucible in position ; Fig. 30 Ingot mold and blowpipe. D, cast-iron stand upon which the latter swivels. The metal to be mehed is placed in the crucible {A), and the flame of the blowpipe is directed upon it until it is perfectly fused. The waste heat serves to make the ingot mold hot. The whole is tilted over by means of the 36 THE LABORATORY. upright liandle at the hack of the mold. A sound ingot may be obtained by tlie use of tliis sinij)Ie little aj)|)aratus in a few minutes. Fig. 31 represents an improved form of the ])reeeding melting arrange- ment. It differs in that the two parts of the ingot mold slide on each other to enable ingots of any width to be cast, and the blowpipe is part of the rocking stand. The bellows is connected to the upper tube and the gas to the lower by the usual means of india-rubber tubing. Contrivances of this kind are, however, not applicable to melting operations involving quantities exceeding one ounce. In such cases it is better to employ a crucible and any stove or furnace in which the temperature can be raised sufficiently. This may be accomplished in an ordinary cooking stove, a blacksmith's forge, or a small fire-clay furnace by the use of anthracite coal, coke, or charcoal. By far the most convenient, compact, and effective furnace for melting from one to ten ounces of gold which has ever been used is the crucible furnace (Fig. 32) invented by ^Ir. Fletcher, which can be obtained at the dental depots. It is perfectly adapted to the wants of the mechanical dentist. It is composed of a substance resembling fire-clay, but much lighter in weight, and said to possess only one-tenth its conducting power for heat. The furnace con- ' "• '^^ sists of a simple pot for holding the crucible, with a lid and a blowpipe, all mounted on a suitable cast-iron base. The casing holds the heat so perfectly that the most refractory substances can be fused with ease by the use of a common foot-blower. The power which can be obtained is far l)eyond what is refjuired for most purposes, and is limited only l)y the fusibility of the crucible and casing. The graphite crucible made especially for the Fletcher furnace will hold about ten ounces of gold. An ordinary gas-supply pipe of yV-inch or f-inch diameter will work it efficiently. The blast is obtained l)y means of a foot-blower connected with the blowpipe by a flexil)le rubber tube. It requires a much smaller supply of gas than any other furnace known : about ten cubic feet per hour is sufficient for most purposes. A gasoline generator has })een devised by which these furnaces can l)e satisfactorily used when ordinary illuminating gas is not obtainable. Fig. 33 shows the generator attached to the furnace with foot-blower complete. In size the furnace is but 4 inches in diameter by 3 in height. From six to eight ounces of gold rec|uire from seven to twelve minutes for perfect fusion, the time depending on the gas supply and the pressure of air from the blower. Fletcher's blowpijiu aud adjustable ingot mold. MODES OF MELTING METALS. 37 In melting any large amount of gold, particularly if the melting operation is performed in an ordinary coal stove, there is always danger Fig. 32 Fig. 33 Crucible furnace for melting silver and gold. of loss by the escape of the precious metal through some defect in the bottom or sides of the crucible, when its recovery from among the fuel and ashes of the stove is almost impossible; but should such an accident occur when using the Fletcher furnace the complete recovery of the gold and silver would not be attended with the least difficulty. A modification of the appara- tus has been made, adapting it to the use of refined petroleum instead of gas as a fuel (Fig. 34), Thus improved, it is said to be in no way inferior in efficiency to the gas furnace. The burner of this furnace is constructed upon the principle of an atomizer, which, of course, dispenses with a wick; it is furnished with a device for regulating the supply of oil, which is operated by the milled nut A^ shown on the top of the reservoir in the cut, and for the supply of an annular jet of air, which is regulated by turning the sleeve (-B). This burner is so arranged that in case any obstruction should occur it can be taken apart and cleaned by separating the burner from the reservoir, which is ac- complished by loosening the small screws, drawing out the oil tube, taking off the sleeve B, and removing the inside tube. Crucible furnace operated with i,a&olint. .38 Tin-: LABORATOHY. Tlu'so furnaces are so constructed that tliev may be used for either gas or jH'troleuni, the lani|) l)eiMg fitted for adjustment in place of the gas burner, so that the same apparatus may be used for either. The blast is obtained by means of the foot-blower, which is connected with the furnace by the India-rubber tni)ing, as seen in the illustration (Fig. 34). An injector gas furnace has also been ])erfected by Mr. Fletcher, which seems to be well adapted to the wants of die dentist or metal- Vw. 34 Crucibk' furnace using petroleum. lurgist (Fig. 35), and it is claimed that its power and .speed of working are practically without limit, depending only upon the gas and air supply. With a -2-inch gas pipe and the small foot-blower this furnace will melt a crucible full of cast-iron .scraps in ten minutes. The supply of gas required is exceedingly small. Allowing five cubic feet of gas for heating up, it consumes about four feet of gas for every pound of metal melted. It is very sim])le in construction, and consists of two parts — an upper portion, which forms the cover, and a lower part, which holds the crucible while in operation. Fig. 35 luji-ctor furnace. For melting platinum very high temperatures are required, and none of the appliances heretofore dcscriljcd produce sufficient heat for the purpose. In the soldering of continuous-gum dentures, in melting platinum .scraps, and in some gold casting operations, greater heat is MODES OF MELTING METALS. 39 necessary. Dr. J. Rollo Knapp made use of the principle of the axy- hyth'oiijen blowpipe in a nitrous oxide blowpipe of his design, in which the ordinary ilhuninating gas furnished the hydrogen element and a cylinder of nitrous oxide gas the oxygen element. The gases are mixed in a mixing chamber and issue from the blowpipe ready to com- bine, producing an intense heat when the proportions are properly regulated. LeCron's blowpipe, illustrated in Chapter XV., is built on this principle. It has been found, however, that when a new cylinder is first connected with the apparatus the escape of the nitrous oxide under great pressure Lane-Seymour nitrous oxide blowpipe. causes such a chilling of the yoke and mixing chamber, because of the absorption of heat by the nitrous oxide in passing from the liquid to the gaseous state, as to interfere Avith the working of the apparatus. Dr. W. H. Taggart has devised a means of preventing this by placing a very small burner, connected to the illuminating gas supply, beneath the nitrous oxide tube ; the latter is kept warm, condensation is prevented, and the appliance works smoothly. After the pressure in the gas cylinder has 40 THE LABORATORY. been considerably reduced in the natural course of usiiitr the gas, it is not necessary to keep the burner lighted, although better o])eration of the blow'j^ipe is obtained by so doing. The blo\vj)ipe of Drs. Lane and Sevniour, shown in Fig. 36, is constructed upon this plan. Platinum scraps may be melted by a method devised by Dr. L. E. Custer, of Dayton, ( )hio, which consists in the use of the intense heat of the electric arc. The 110-volt current is used. A large quantity of current is necessary, the fuse plugs being as large as No. 16 or LS wire. A resistance coil of eight pounds of Xo. 18 copper wire should be in the circuit to prevent fusing the ])lug and to give a large arc. The platinum scraps siu)uld be placed upon a block of lime connected with one wire, and the other wire attached to a platinum-pointed piece of metal about f inch in diameter. This platinum-tipped piece of metal is brought in contact with the scraps, and upon raising it a short dis- tance an arc is formed directly upon the metal and it is melted. The arc can be carried about at will until the pieces are all Ijrought into one mass. Crucibles. — The term "crucible" was originally applied to a chemist's melting-pot, made of earthenware or other material, and so-called from the superstitious habit of the alchemists of marking such vessels with the sign of the cross. The term is now generally understood as designat- ing vessels in which metals are melted at high temperatures. A crucible should possess the power of resisting high temperatures without fusing or softening. It should also be capable of retaining sufficient strength when hot to prevent its cnmibling or breaking when grasped with the tongs. Lastly, it should not crack either in heating or cooling. For the purpose of melting metals, crucibles are made of clay with admixture of silica, burnt clay, graphite, or other infusible material. For use in the dental laboratory, graphite crucibles, which can be ob- tained at the dental depots, will be found to answer every purpose; they are thoroughly reliable in strength and durability. They range in size fron 2 to 4 inches high, and are especially adapted for use in the Fletcher gas furnaces. When the quantity of metal to be melted is very small — say, a half- ounce of gold — the smallest-sized Hessian crucible may be used in the small Fletcher apparatus. Before melting any considerable quantity of gold the crucible should be tested, particularly if the melting operation is to be performed in an ordinary coal stove, where a defective crucible might be the means of a considerable loss. A small amount of borax should be placed in the vessel, which should then be exposed to a high temperature. Should it not be perfect, the borax glass will run through and glaze the surface on the outside. If the crucible is found to be impervious, it should be so inverted while yet hot that the borax glass may cover the surface of the lip or groove out of which the melted metal is to be poured. This facilitates the pouring and prevents any portion of the metal from adhering to the side of the crucible. CRUCIBLES. 41 Ingot molds are constructed of various substances. For the reception of platinum melted by the oxyhydrogen blowpipe they are formed of hme or coke; for gold and silver they are commonly made of cast iron, about 2 inches square, and from ^ to -i\ of an inch thick (Fig. 37), with slightly concave inner surfaces, as the shrinkage of the ingot is greatest in the centre. Ingot molds formed of soapstone are also employed, but they are not superior to those made of cast iron. Before pouring the ingot the mold should be heated, and when made to cast iron it should be held over a gas jet or oil flame until its inner surface is thoroughly coated with carbon : this at once prevents the possible contamination of the gold by contact with the iron, and Fig. 37 Adjustable ingot mold. the carbon layer, being a good non-conductor, protects the melted metal at the moment of pouring from too rapid cooling, w^hich otherwise might be the cause of a defective ingot. The ingot of gold or silver should be as nearly rectangular as possible (Fig. 38), and the operation of pouring the melted metal from the crucible into the ingot mold cannot be considered as successful unless this result has been attained. The experienced workman holds the ingot mold, which should be provided with a suitable handle, with the left hand, while with the right he removes the crucible from the furnace and quickly carries it to the ingot mold, W'hich he slightly tilts so that the melted metal may first strike the side of the mold; but he quickly brings the mold to a level before the last of the fused metal leaves the crucible, and thus avoids the danger of confining air at the deepest part of the ingot mold, which would cause the ingot to assume an irregular shape (Fig. 39). The necessity of heating the ingot mold just before it is to receive the melted metal becomes apparent when we remember that gold fuses at 2012° F., while the iron ingot mold at the temperature of the atmos- 42 THE LABORATORY phere would be about 70° F., aud when tlie amount of gold and silver to be melted is but two or three ounces, the ingot mold, weighing in the neighborhood of twelve ounces, would abstract so much heat from Fig. 38 Fig. 39 ( 'urivclly iiiaik' iiigdt. Ingot iiic'oirectly inadr ; f!iu>(.-(l Wy fonlliiiiiK !>ir. Fig. 40 lii.i,'(it iii(latiiiuiii solder, to heat the furnaee in continuous-gum and porcelain crown work, or a furnace used in inching the more refrac-tory metals, A heater constructed upon the prin- ci])le of tlie Bunscn gas 1 turner, which will be found most useful for the smal- ler heating operations is shown in Fig. C4. Recently a blowpipe burning wood alcohol made upon the same principle as the blowpipe described above has been introduced Supports. In melting small quan- tities of gold or silver or in soldering with the blowpipe flame it is necessary to perform these operations u])on a support made of some suitable body, such as charcoal, coke, pumice-stone, or asbestos and plaster, charcoal and plaster, etc. Well-burned charcoal is especially suited for both purposes, as it helps to increase the heat, and in the putting together of small quantities of gold or silver solders prevents oxidation of the base metals which are added to reduce the fusing point of the alloy and cause it to flow freely. Charcoal made from the light woods, such as pine, is best, because it is not so likely to throw sparks when the flame is directed upon it as are the harder coals, such as that made from oak; and, being softer, it is much better adapted to soldering operations in which it is necessary to hold the pieces to be united together by means of small nails or tacks thrust into the support; as, for instance, where a rim is to be soldered to a plate, the former must be brought in contact with the latter upon the charcoal, and so held during the preliminary soldering, which consists of uniting the rim to the plate with a small piece of solder at some one point, after which the accurate adjust- ment of the rim to the plate for final soldering is rendered much easier. A good solid piece of charcoal, sufficiently large, should be selected, and bound with iron or copper wire to prevent its breaking into pieces. It should then receive a coating of plaster, from a quarter to a half inch in thickness, on all sides except the one upon which the object to be soldered is to rest. This adds to its strength and prevents soiling the fingers while it is being handled, (jood charcoal, suitable for use in the dental laboratory, cannot, however, always be found when wanted, and it is therefore often necessary to use some other substance which may be more easily obtained. Thus those living in large cities may be com- pelled to employ pieces of coke as supports in soldering. Next to char- coal, coke is most suitable for this purpose. It is more durable than charcoal, and when such a support, composed of one large piece or even SOLDERING APPARATUS AND ACCESSORIES 67 several smaller pieces, is bound together with wire and coated with plaster, it will last a long time. Large pieces of pumice-stone also answer well for the purpose of holding small objects while the flame of the blowpipe is directed upon them. Neither of these, however, is so well adaptetl as charcoal for holders, when small fjuantities of metals are to be melted, in consequence of their greater porosity and their hard- ness, which prevent the cutting of suitable pits for the reception of the metal to be fused. A very good support for soldering purposes alone may be formed by filling a cup made of sheet iron or copper, 5 inches in diameter by 5 inches in depth, with a mixture of asbestos and plaster or plaster and finely broken charcoal. The vessel should be supplied with a wooden handle, fastened in the bottom, for convenience in handling. Plattner's Manual of Qualitative and Quantitative Analysis with the Blowpipe gives a method of artificially preparing good solid supports of charcoal which might be found of value in the dental laboratory. It consists of mixing charcoal dust (which must not be too finely ground) with starch paste. The latter is prepared by combining 1 part of starch with 6 parts of boiling water. These are stirred in an earthen pot until all the meal is converted into paste. This paste is rubbed in a porcelain mortar with frequent additions of rharcoal dust until the mass becomes too tough for further admixture, w^hen enough of the coal-dust is kneaded in with the hands to render the whole mass stiff and plastic. From this the desired fornos of supports can be made, allowed to dry gradually and thoroughly, and then heated to redness in a covered vessel, so as to char the starch paste. The charring may be regarded as complete when the evolution of gases from the mass ceases or when it has been heated to dull redness. Coals thus formed are of the proper firmness, and ring like ordinary good charcoal when thrown on the table. Blocks formed of graphite and fire-clay are now often used as supports for holding objects to be soldered. These are by no means perfect non-conductors, and when used without some protection to the hand they soon become so hot in the operation of soldering that it is impossible Fig 65 Fig. 66 Soldering support. Soldering support and handle. to hold them for any length of time. To overcome this difficulty, how- ever, a very convenient device for holding the carbon, graphite, or other support has been introduced. (See Figs. 65 to 67.) Soldering blocks have recently been formed of asbestos, and have found favor with many in preference to the "carbon block" for solder- 58 77//-; LABORATORY. ini'- purposes, 'i'ht'y are eircular, (le})res.se(l on each face, and 4 inches in diameter. The carbon cylinder, made of the same composition as the carbon block, is a new form of support admirably adapted for soldering small Fig. 67 Fig. 68 Asbestos soldering surijifrt with handle detached. Solderiiit,' support for small articles. articles, such as gold crowns, or for blowpipe assays. In size it is 1| inches in diameter by 3 inches in length (Fig. 68). Among the more recently introduced forms of asbestos soldering and melting supports are those shown in the annexed illustrations. Fig. 69 represents a combined soldering, melting, and ingot block, 6 Fig. 69 Combined soldering, melting, and casting block. Fig. 70 Soldering block. inches long, 2| inches wide, by h inch in thickness. Fig. 70 shows an asbestos support intended exclusively for soldering, 4h inches in diameter by If inches high, with concave top, and provided with a con- venient holder, which also prevents the support from being laid flat upon the table while hot. SOLDERING APPARATUS AND ACCESSORIES 59 Fn;. 71 Laboratory burner. The best method of " heating up " a denture })reparatory to soldering is to phice it over a burner sueh as is eni])l()ye y. S. White electric lathe. It has a range of speed varying from 1000 to 4000 revolutions per minute, and the torque, or pull, is as great at the lowest speed as at the highest. The regulation of .speed and the starting and stopping of the lathe are effected by a slight turn of the milled stud .shown in the cut. The chucks are fixed on the tapered ends of the .spindle by a light tap, with any metal tool which may be conveniently at hand, and can be instantly removed while revolving or stationary by the tool shown below the chucks, used as a lever to force the chuck off the end of the .spindle. The polishing lathe should be provided with a drawer for the safe keeping of mandrels, brush wheels, felt and cotton wheels, cones, etc., together with the abrading and polishing powders which are usually employed in the final finishing of the ditterent kinds of prosthetic pieces. Corundum wheels, spatulas, cements, etc., used in fitting and attaching teeth to the plate, should be kept in a drawer attached to the grinding and fitting lathe. The corundum wheels so extensively used in the dental laboratory are made of the mineral corundum found in Ceylon and in Pennsylvania, Georgia, Massachusetts, and North Carolina. It occurs in crystals of the form of double six-sided pyramids of various sizes, and in some localities in large masses without crystalline form. Corundum is an aluminum oxide haA^ng the formula AljOg. The ruby and sapphire are transparent LA TIIES. 81 varieties of this mineral, their eolor being (hie to the presence of a small amount of coloring oxides. Emery, the use of which preceded Fk;. 107 Lathe-head suitable for polishing lathe. corundum as an abrasive agent in the dental laboratory, is a coarse variety of corundum. Corundum is, with the single exception of the diamond, the hardest mineral known. It is prepared by pulverizing the crystals in an iron mortar by successive blows of a heavy steel pestle. The three grits which are employed in making wheels for dental pur- poses are obtained by passing the powdered corundum through sieves of different degrees of fineness; they are known as fine, medium, and coarse. The latter will cut most rapidly; the finest will not cut so fast, but will leave a mucli finer surface. The powdered corundum is mixed with finely ground gum shellac in the proportions of 3 ounces of corun- dum to 1 of shellac; this is carefully heated and thoroughly mixed until it becomes of a doughy consistence, when it is put into an iron mold made in two parts, previously oiled. This mold is placed in a small press and force enough applied to consolidate and distribute the mixture into all parts of the mold. Too much force should be avoided, as it is liable to drive out so much of the shellac that the particles of corundum will not be sufficiently adherent— a condition which will greatly lessen the wearing qualities of the wheel. After the wheel has been removed from the mold, which is done by tapping the latter sharply with a wooden mallet, it is washed in alcohol for the purpose of removing the shellac from the surface and leaving the wheel in a sharp or gritty condition. Carborundum wheels, which are also in general use for grinding purposes, have as their abrasive constituent "carborundum," or the carbide of silicon. This substance was made experimentally in 1893 by Mr. G. E. Acheson, and is now manufactured on an exiensive scale at Niagara Falls, where large amounts of electricity are economically available, the intense heat of the electric furnace being necessary to 6 82 THE LABORATORY. % ■.J? produce the combination of the carbon and sihcon. Carbon in the form of finely divided coke, silica as very pure and clean sand, common salt (NaCl), and sawdust mixed together in definite proportions constitute the charge for the furnace. This is built of brick about IG feet long by () feet wide and 4 feet high. It is packed with the charge up to the level of the electrodes, which enter it about the centre of each end, and a core of crushed coke is laid from one elec- ^^"- ' '"^ trode to the other. The remainder of the charge is then put in and piled up to a height of about 8 f(>et. The current is then turned on. About 1000 horse-power of energy is utilized at an average voltage of 185, and a temperature approximating 7500° F. is reached. The burning takes thirty-six hours, the sawdust burning out and rendering the mass porous for the escape of carbon monoxide and other gaseous products. A mass of crystals is formed around the core, which consists of carbide of silicon (CSi), the carbon and silicon having combined in atomic proportions. These crystals are crushed, treated to a bath of dilute sulphuric acid, washed, and sieved. The wheels are made of the various sizes of crystals mixed with feldspar and kaolin, which constitute the "bond." They are molded into shape under tremendous pressure and baked in a kiln for seven days. Carborundum is harder than corundum and is more brittle. AMieels made of it may be run as well wet as dry and will not clog. They do, however, wear unevenly, probably due to lack of homogeneity of the bond or its uneven baking. Frecjuent tooling with the dresser shown in Fig. 108 will keep them true. While grinding porcelain teeth the corundum wheel must be kept constantly wet to prevent the shellac from becoming heated by friction — a condition which instantly impairs its cutting properties. Numerous appliances have been devised in the form of "drip cups," designed to automatically sup]>ly sufficient water to the w^heel while in use to prevent heating; but these are objectionable in more than one respect, and are liable to obstruct the light and prevent it from directly falling upon the point of contact of the tooth with the wheel. A simple dish, oblong in form, with the dimensions of 8 inches in length by 5 inches wide, by 2\ in depth, partially filled serves as a good hand-rest, while a piece of sponge of the size of a large walnut, which the operator will soon accjuire the habit of holding between the index and middle finger of the right hand Avhile he keeps it in contact with the corundum wheel, is an excellent means of conveying water to the wheel and preventing it from splash- ing his face or clothing. Dreiser for render- ing worn oarborundum wheels true. with clean water, LATHES. 83 There are at least seven sizes of coninduin wheels made for dental laboratory purposes, ranging from f of an ineh in diameter to 3|- inches, but the author has found, after much experience in fitting carved blocks, rubber sections, and single-gum teeth, that a maxinunn of 1 inch in diameter and I of an inch in thickness is (juite large enough for joint- ing piu"poses, while the smaller sizes, which are indispensable, are obtained by the wearing away of the 1-inch v^heels. In finishing dentures the first step is the proper levelling of the surface; this is usually done in metallic cases with the corundum wheel, after which the scratches left by the sharp particles of corundum should be removed by a keen-edged vulcanite scraper. The piece is then ready for the "Scotch stone," a soft mottled stone much used by silversmiths and workers in the precious metals, furnished by the dental-supply houses in pieces of 6 inches in length by j inch in thickness. This material has decided abrasive qualities, and is used chiefly to remove the scratches left by the corundum wheel and scraper; it produces a fine silk-like surface and brings the case to the point where the buff wheels armed with the coarser powders, such as pumice, are to be used; these produce a surface which may be highly polished by the brushes which should follow the buff wheels, and should carry the finer polishing powders or those used for the purpose of obtaining high lustre, such as calcined buckhorn when the case is of gold or silver, and prepared chalk when it is of vulcanite or celluloid. In vulcanite or celluloid work the corundum wheel need not be used, the scraper being sufficient for the levelling of the surface, after which the finer numbers of emery paper, Nos. and h, are employed, until all traces of the scraper are removed, when it is ready for the pumice powder, which is generally applied with a small stick of soft wood, such as poplar or pine, after v/hich the denture is ready for the felt or other kind of buff wheel and fine pumice. Buff wheels and cones are made of felt, cotton, duck, leather, soft wood, cork, disks of cloth or chamois leather, stitched together, etc. Felt is probably the best of the various materials used in forming buff wheels; these wheels can be obtained at the dental depots in sizes ranging from 1^ to 2h inches in diameter. Buff wheels are intended to cut and not to polish. They are usually armed with pumice, and must be kept constantly wet while in use. The best size of buff wheel for dental-laboratory use is If inches in diameter by f of an inch in thick- Felt wheei-chuck. ness. Smaller sizes are obtained by the wearing away of the larger wheels. They are easily mounted upon the "screw-cone" mandrel, to which they do not ordinarily require to be cemented or shellacked. An ingenious felt wheel-chuck has been suggested by Dr. F. E. Pomroy; it is provided with three steel pins to prevent the wheel from revolving on the screw (Fig. 109). The brush wheel is employed for the purpose of obtaining a still finer surface than is attainable with the Scotch stone or buff wheel and for 84 THE LABORATORY. the final polishing. There is ciuite a variety of forms made for dental laboratory use, beginning with the wood-centre brush wheel with straight Fig. 110 JS-J-S Brush wheel. bristles in from one to four rows (Fig. 110); the brush wheel with con- verging bristles (Fig. Ill); the cup-shaped wheel with from one to four Brush wheel ; converging bristles. rows of brisdes (Fig. 112); cup-shaped brisdes with long wooden shanks; hub-shaped with straight bristles and hub-shaped with converging Fin 112 .x-»e when finished, and after the vulcanizing); to reduce it to the proper thick- ness with steel burs sold for the j)urpos(% and which, on account of the danojer when they are used of cuttinj^ through the plate, should have no place in the dental laboratory. The preliminary waxing of dentures of this class should be done with such care and ])recision that the waxed piece will represent not only the exact thickness of the plate when finished, but all the irregularities of surface which are found on the plaster cast. The rugae and other prominences of the mouth assist in enunci- ation and mastication, and should l)e represented in the plate. It is probable that when so arranged artificial dentures feel less like foreign objects when worn in the mouth. In order not to obliterate these natural irregidarities of surface the waxing may be done with two or three layers of wax, not much thicker than is used in making artificial flowers, laid on separately and pressed with the thimib, after being slightly softened in the flame of a spirit lamp or Bunsen burner, until in complete contact with the palatal portion of the cast. Any desired thickness can be ol)tained by additional sheets of wax, but the main point to be gained by this method of waxing is uniformity of thickness: and if the waxing is artistically done, little or no scraping or finishing will be needed after vulcanizing except at the edges. Indeed, the most skilful workers in the plastic bases have demonstrated that the best results in vulcanite and celluloid work are obtained by precision in waxing and the use of tin to prevent contact with the plaster of the investment, and to afford a polished surface which shall need but little alteration by the scraper. Sheet wax should not be over -^^ inch in thickness; it may be prepared by dipping a square piece of plate glass or hard wood | inch thick, previously oiled, into melted wax, allowing it to cool upon the slab, and repeating the dipping until the desired thickness is attained, after which it is stripped off, trimmed to the dimensions of 3 inches s(|uare, laid in a box with tissue paper between the sheets, and it is then ready for use. Bench Tools, etc. — The special application of tools will be found in the respective chapters devoted to the particular kinds of work in which each is used. Our remarks here will, therefore, be confined to their care and proper use. There are two infallible indications of the amount of training and skill possessed by a mechanical dentist : (1) the condition of his tools; (2) the state of the cast after he has made a denture upon it. Skilful and accurate workmen will do so little damage to plaster casts while constructing plates or clasps that little or no evidence of their having been used will be apparent after the work is finished, showing that the tools have been well selected, kept in good working order, and correctly applied. Tools used in work at the bench may l^e ke})t in racks at the back of the bench top, the most convenient location for those in frequent senice, or they may be kept in drawers at the workman's right hand (Figs. 7, 8), especially if they are not often needed. Each should be provided with a definite place, so that it may be easily found when the occasion demanding its use arises. Cutting tools, as gravers, chisels, BENCH TOOLS, ETC. 89 scrapers, etc., ought to be kept sharp, and a good Arkansas stone, inches long and 2 niches wide, should be provided to keep them in con- dition. This should b.e used with oil, and the blade of the tool not con- stantly sharpened in the same direction in order to avoid wearing gnjoves in the stone. It must be kept clean of the black mixture of finely divided steel and oil to ensure its best sharpening qualities. The workman must know how to put an edge on his tools. The following are the principle bench tools in use for metal work: Plate shears, straight and curved. Pliers (flat-nose), in at least three sizes — one pair large and strong enough to be used in drawing wire. Pliers (round-nose), two sizes. Pliers, one pair with one beak rounded and the other flat — very useful in fitting clasps. Side-cutting nippers for removing that portion of the platinum pins which projects beyond the backing. Punching forceps, for punching holes in gold backings, for the platinum pins. Clasp-bending forceps. Plate nippers are employed for removing redundant portions of a plate, which they do more rapidly than could be accomplished with files. Plate burnishers, straight and curved. Horn mallet. Riveting hammer. Draw plate for reducing the size of wire. Screw plate and taps, useful in the construction of regulating fixtures. Plate gauge, standard American. Solder tweezers and tongs. Jewellers' saw frame and saw^s. Small steel cold chisels for cutting out chamber. Small hammer, weighing about 2 ounces. Round-edged brass chaser for use in forming vacuum chambers and for carrying the plate into deep places. Hand vise and pin vise. A small variety of sizes of gravers, chisel and gouge forms. Those made for wood engravers are well tempered and answer admirably for dental laboratory uses. The graver will reach places during the finish- ing of dentures which would be inaccessible to the corundum^ wheel. They are also useful in correcting slight imperfections in zinc dies. Files, half round, 5 or 6 inches long, moderately fine cut; round files, small variety, ranging from 6 to 12 inches in length, coarse and fine; flat files with safe edge, moderately coarse and fine. Files should be kept in a suitable rack," and not in a drawer with pliers, shears, etc., as contact with these and with each other will be sure to damage them. Triangular steel scraper for removing file-marks on edges of plate and backings. Small anvil set in lead. Scissors, straight and curved, for cutting patterns for plates, etc. Several points, made from broken excavators or worn-out pluggers, 90 THE lahoratoiiy. used for marking upon gold or silver plates, picking wax or cement from invested cases, and numerous other ])urposes. Blue ])encil for marking plan of plate and clasj)s u]K)n plaster casts. The additional tools, ap])liances, and materials used in crown and bridge-work will be described in Chapter XVI 1 1. For vulcanite work the following are needed and will be discussed more in detail in the chapter on that subject: Vulcan izer. Vulcanite flasks and wrenches. Flask press. Vulcanite scrapers. Files, half round and "rat tail." Chisels. Calipers for measuring thickness of plate. Articulators. Gas fitter's pliers for occasional use in tightening the bolts of vulcanite flasks and other rough work which would damage the ordinary bench pliers are convenient articles to have, and a small wood saw for reducing size of hard plaster casts is likewise useful. The use of bench tools should be strictly confined to the purpose for which they were designed. They should be carefully kept from contact with plaster of Paris, the fumes of acids, and particularly from chlorine a.s evolved from nitro-hydrochloric acid in the quartation process of refining gold, which readily acts upon the surface of steel and iron. CHAPTER ri. METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. By Joseph Dupuy Hodgejsi, D.D.S. Seventy-eight elements are at present known to us, of wliicli tiie following is a complete list, arranged alphabetically, with their symbols and their atomic weights: Table of Elements with International Atomic Weights, 1903. =16.* H = l.* = 16.* H = l.* Aluminum . Al 27.1 26.9 Neodymium . Nd 143.6 142.5 Antimony . Sb 120.2 119.3 Neon . Ne 20.0 19.9 Argon . A 39.9 39.6 Nickel . Ni 58.7 58.3 Arsenicum . As 75.0 74.4 Nitrogen . . N 14.04 13.93 Barium . Ba 137.4 136.4 O.^mium . Os 191.0 189.6 Bismuth . . Bi 208.5 206.9 Oxygen . 16.0 15.88 Boron . B 11.0 10.9 Palladium Pd 106.5 105.7 Bromine . . Br 79.96 79.36 Pliosphorus P 31.0 30.77 Cadmium . . Cd 112.4 111.6 Platinum . Pt 194.8 193.3 Caesium . . Cs 133.0 132.0 Potassium K 39.15 38.86 Calcium . . Ca 40.1 39.8 Praseodymium Pr 140.5 139.4 Carbon . C 12.0 11.91 Radium . Rd 225.0 223.3 Cerium . Ce 140.0 139.0 Rhodium . Rh 103.0 102.2 Chlorine . . CI 35.45 35.18 Rubidium Rb 85.4 84.8 Chromium . Cr 52.1 51.7 Ruthenium Ru 101.7 100.9 Cobalt . Co 59.0 58.56 Samarium Sm 150.0 148.9 Columbium . Cb 94.0 93.3 Scandium Sc 44.1 43.8 Copper . Cu 63.6 63.1 Selenium . Se 79.2 78.6 Erbium . Er 166.0 164.8 Silicon Si 28.4 28.2 Fluorine . . F 19.0 18.9 Silver Ag 107.93 107.12 Gadolinium . Gd 156.0 155.0 Sodium Na 23.05 22.88 Gallium . Ga 70.0 69.5 Strontium Sr 87.6 86.94 Germanium . Ge 72.5 71.9 Sulphur . S 32.06 31.83 Glucinum . . Gl 9.1 9.03 Tantalum Ta 183.0 181.6 Gold . Au 197.2 195.7 Tellurium Te 127.6 126.6 Helium . He 4.0 4.0 Terbium . Tb 160.0 158.8 Hydrogen . H 1.008 1.0 Thallium . Tl 204.1 202.6 Indium . In 114.0 113.1 Thorium . Th 232.5 230.8 Iodine . I 126.85 125.90 Thulium . Tm 171.0 169.7 Iridium . Ir 193.0 191.5 Tin . Sn 119.0 118.1 Iron . Fe 55.9 55.5 Titanium . Ti 48.1 47.7 Krypton . . Kr 81.8 81.2 Tungsten . W 184.0 182.6 Lanthanum La 138.9 137.9 Uranium . U 2.38.5 236.7 Lead . Pb 206.9 205.35 Vanadium V 51.2 50.8 Lithium . Li 7.03 6.98 Xenon Xe 128.0 127.0 Magnesium . Mg 24.36 24.18 Ytterbium Yb 173.0 171.7 Manganese . Mn 55.0 54.6 Yttrium . Yt 89.0 88.3 Mercury . • Hg 200.0 198.5 Zinc Zn 65.4 64.9 Molybdenum . Mo 96.0 95.3 Zirconium Zr 90.6 89.9 VIZ These seventy-eight elements are classed under two great divisions, metallic and non-metallic. * It will be understood that these atomic weights are relative; thus we have a list, in the first column of which oxygen is assigned the value of 16.— i.e.. without fraction for greater convenience in calculation. In this column hvdrogen has a relative value of l.OOS. /l he second column is a list of the atomic weights with hydrogen taken as the standard, when relatively o.xygen becomes equal to 15.88 91 Barium, Cobalt, Manganesium, Cadmium, Arsenicum, Titanium, Chromium, ITranium. 92 METALS AM) ALLOYS r.S'AV) /.V PROSTIIKTW DKNTLSTRY. THE METALS. Of tlu' sixty-two (•l('iiu'iit;irv.sul)staiic(>.s known jis metals onl y t'ourtfcn arc ordiiiarily i'nij)k)y('(l in tlu-ir tnic nictallic condition. I'lu'.sc arc: Iron, .Muniiiiuni, Gold, Copper, Nickel, Silver. Lead, Antimony, Mercury, Zinc, Magnesium, Platinum. Tin, Bismuth, About twelve are more or les.s useful in the preparation of medicines, in the arts for coloring pigments, and for alloying purposes. These are: Potassium, Sodium, Calcium, Lithium, The remaining thirty-six are more or less rare, and as yet of little or no practical value in the metallic state. The metallurgist groups the metals into two classes, which are knowD as noble and base: Noble Metals. — Noble metals are those whose compounds with oxygen are decomposable by heat alone, at a temperature not exceeding redness. These are: Mercury, Platinum, Ruthenium, Silver, Palladium, Osmium, Gold, Rhodium, Iridium. Base Metals. — Base metals are those whose compounds with oxygen are not decomposable by heat alone, retaining oxygen at high temperatures. The base metals are further subdivided with reference to their affinity for oxygen and other chemical properties. First division. This contains five metals. They are very readily oxidized, and their oxides are all soluble in water, giving it a strongly alkaline reaction; so also are their phosphates and carbonates, with the exception of lithium phosphate, which is quite insoluble, and the carbon- ate, which is only sparingly soluble. They all energetically decompose water at ordinary temperatures, liberating hydrogen and forming hydrates in solution. They are soft, of low specific gravity, and fusible at low temperatures. These are: Potassium, Lithium, Caesium Sodium, Rubidium, Second division. This contains four metals, all of which, with the exception of magnesium, decompose water at ordinary temperatures, combining with the oxygen. Their oxides are more or less soluble in water, rendering it alkaline ; but their neutral carbonates and phosphates are insoluble. These are: Barium, Strontium, Calcium, Magnesium. PROrKRTIP:S OF Tlir. METALS. 93 Third division. This contains tliirtccn metals, of wliich but three are of much importance. Those which have been isolated do not decompose water at ordinary temperatures without the addition of a weak acid or a slight rise of temperature. Their oxides and carbonates are insoluble in water. These are: Aluminum, Thorium, Cerium, Chromium, Yttrium, Lanthanum, Titanium, Zirconium, Didymium, Glucinum, Erbium, Tantalum, Columbium Fourth division. This contains nine metals, the chief of which decompose water at a red heat. These are: Iron, Manganesium, Vanadium, Nickel, Zinc, Thallium, Cobalt. Uranium, Indium. Fiph division. This contains four metals, which do not decompose water at any temperature. These are: Cadmium, Leao, Bismuth, Coppei . Sixth division. This contains six metals. All the higher oxides of these metals have acid properties. These are: Tin, Arsenicum, Tungsten, Antimony, Molybdenum, Tellurium. PROPERTIES OF THE METALS. A metal is an elementary substance, solid at ordinary temperatures, with the single exception of mercury (a liquid solidifying at — 39° C), having a peculiar lustre, called "metallic lustre," and the property of replacing hydrogen in chemical reactions, as, for example, Zn + H2SO4 = ZnSO, + Hj, insoluble in water, a good conductor of heat and electricity, and possess- ing the quality of uniting with oxygen to form a basic oxide. No line can be sharply drawn between metals and non-metals; just as none can be drawn between soluble and insoluble, poisonous and non-poisonous substances, yet, from a general point of view, this class of elements admits of the foregoing definition. The metals possess a number of characteristic properties, which may be noted as follows: Non-transparency. — Metals, as a rule, are non-transparent, or opaque, yet some have proven to possess the property of transparency in a low degree at least. In the case of gold, through the leaf, or thin films produced chemically on glass plate, a light-green color is transmitted. Also very thin films of mercury are said to transmit light with a violet- 94 METALS AM) ALLOYS USED L\ I'ROSTlIi: IIC DKyTISTRY. blue color, and copper, it is claimed, is somewhat translucent; while silver in infinitely thin films is absolutely opaque. Color. — The color of most metals is white, and ranges from the pure white of silver and tin to the bluish hue of lead. Bismuth is a light gray, with a delicate tinge of red. Copper is called tlie "red metal." Gold is a rich yellow, barium and strontium a straw color, while calcium exhibits a little deeper shade of that color. Luster. — Polished metallic surfaces, like those of other solids, divide any incident ray into two parts, of which one Ls refracted, while the other is reflected, with this difference, however, that the former is completely absorbed, while the latter is reflected, which latter accounts in all likelihood for the metallic luster. Odor and Taste. — ^Nlost metals are destitute of odor and taste. Peculiar odors are, however, evolved from some of them when Ideated; in fact, one of the means of discriminating arsenicum consists in the recognition of its characteristic smell of garlic when heated. Ircn, copper or zinc when heated also evolve peculiar odors. The taste whi.h is perceived in some is no doubt due to their peculiar character, although in some cases it may depend upon voltaic action set up by the chemical agency of the saliva, the metal not being perfectly pure. If a piece of zinc be placed upon the tongue, and a piece of silver luider it, and the edges joined, a metallic taste will be perceived dependent on slow solution of the zinc under electric action. The odor. Dr. Essig says,* "ni^y be noticed in a marked degree when holding in the hand a mass of an alloy composed of gold, platinum, tin, and silver prepared for use as amalgam. The moisture of the hand, aided by its heightened temper- ature, seems to promote the electric action." Crystalline Form. — Most metals are capable of crystallization, and their crystals belong to the following systems: Regular — silver, gold, palladium, mercury, copper, iron, lead; quadratic — tin, potassium; rhombic — antimony, bismuth, tellurium, zinc, magnesium. Perhaps all metals in soldifying assume a crystalline structure, which differs only in degree of visibility. Antimony, bismuth, and zinc exliibit a very distinct crystalline structure plainly visible in broken ingots. Tin is also crystalline, which fact is evinced by the "tin cry" when a bar of the metal is bent, the crystal faces sliding over one another; but the bar is not easily broken, and exhibits an apparently non-crystalline fracture. Gold, silver, copper, aluminum, cadmium, iron, lead, cobalt, and nickel are practically amorphous, the crystals being so closely packed as to virtually produce a homogeneous mass. Malleability, Ductility, and Tenacity. — ^These are properties possessed by some metals by reast)n of the cohesive power of their molecules, and are to that extent kinrlred. IVIalleability. — Malleability is that quality possessed by a metal which permits it to be hammered or rolled into thin sheets without breach of continuity. While many metals possess this property to some degree, it is most wonderfully exemplified in gold. Leaves of this 1 Dental Metallurgy, p. 20. PROrEIlTIKS OF Till-: Mrri'ALS. 9") metal have been produced 1^7-770 0^5- of an ineh in tliickncss, each grain of which will cover an area of 75 square inches. Ductility. — Ductility is that property possessed by some metals by virtue of which they may be drawn into wire. The operation consists in forcibly drawing the metal through a series of holes, in a hard-steel draw-plate, which gradually decrease in size. Gold is also the most ductile of all metals, a single grain of it having been drawn into a wire 550 feet in length. This was accomplished by covering the gold wire with silver, which is also remarkably ductile, thus making a composite wire of greater thickness. After drawing this down to the greatest possible degree of tenuity, the silver was dissolved off by nitric acid, leaving a gold wire -g-oVo" ^^ ^^^ ^'^^^ "^ diameter. Tenacity. — Tenacity is tliat property possessed by metals, in conse- quence of which they resist rupture when exposed to tension. Their relative tenacity may be ascertained by preparing wires of exactly equal diamet rs and comparing the number of pounds weight each will sustain before rupture. These properties are shown relatively for some of the more important metals in the following table: Malleability. Ductility. Tenacity. 1. Gold. 1. Gold. 1. Iron. 2. Silver. 2. Silver. 2. Copper. 3. Copper. 3. Platinum. 3. Platinum, 4. Tin. 4. Iron. 4. Silver. 5. Cadmium. 5. Nickel. 5. Gold. 6. Platinum. 6. Copper. 6. Zinc. 7. Lead. 7. Palladium. 7. Tin. 8. Zinc. 8. Aluminum. 8. Lead. 9. Iron. 9. Cadmium. 10. Nickel. 10. Zinc. 11. Palladium. 11. Tin. 12. Lead. The two properties of malleability and ductility are closely related to each other, yet, as may be seen from the above table, they do not always parallel each other, for the reason that ductility in a higher degree than malleability is determined by the tenacity of the metal; for example, tin, though quite malleable, is very slightly ductile, and iron, while ninth in point of malleability, is fourth in ductility. In the operation of hammering out a metal which the quality of malleability permits the granular particles are flattened and spread in all directions, while in those allowed by its ductility each granular particle is elongated into a fibre. There are several conditions which materially modify the properties of malleability, ductility, and tenacity, one of the most important of which is the state of purity of the metal. Gold is the most malleable of all metals, yet if the merest trace of lead, itself a soft and malleable metal, be contained in it, the gold becomes too brittle to be worked, and especially is this the case if the gold contains any silver, as is frequently the case. This destruction of malleability and tenacity is yet more 96 METALS AM) ALLOYS USED L\ rROSTLIETIC DENTISTRY. proiiouiKcd when aiitiiiiony or similar metals are mixed with fjold, even in minnte (iiiantities.^ Temperature also exereises a very great modifying influenee over these properties; for example, a bar of zinc obtained by easting is exceedingly brittle, but when heated to 100° or 150° C. it may be rolled into thin sheets or drawn into wire. Such sheet or wire then remains malleable and ductile after cooling. The explanation of this remarkable fact is, that the loosely cohering crystals have become intertwisted and forced into absolute contact with each other, and this is supported hy the fact tliat the rolled zinc has a somewhat higher specific gravity than the original ingot. If the temperature be carried to 205° C. the metal again becomes so brittle that it may be powdered in a mortar. Extreme care, therefore, must be exercised in the handling of hot zinc dies, for if by accident one be dropped upon a hard surface it is likely to be spoiled. Aluminum, magnesium, and some other metals, which at ordinary temperatures possess little or no ductility, may be drawn into wire when heated. These qualities are greatly diminished in alloys by heating. Some forms of brass, for example, which are soft, tenacious, and ductile at ordinary temperatures, are made quite brittle by heating to dull redness. Again, it is quite certain that 18-carat gold solder is brittle at red heat. The tenacity of metals in general is greatly diminished })y heating. The exceptions to this are in the cases of iron, steel, and gold. The following table shows the results obtained by Wertheim' in his experiments on a number of the metals at temperatures from 15° to 20° C: Permanent vdre one square mm. sec- tion, weight in (in kilos) causing Name. Iron, drawn . ' annealed Copper, drawn " annealed . Platinum, drawn . " annealed Silver, drawn " annealed Gold, drawn " annealed Zinc, drawn " annealed Tin, drawn . " annealed Lead, drawn " annealed Permanent elon- gation of Breakage 1201100 32.0 61.0 Under 5.0 47.0 12.0 40.0 Under S.O 30.0 34.0 23.0 11.3 29.0 2.6 16.0 13.5 27.0 3.0 10.0 0.75 13.0 1.0 0.45 2.45 0.2 0.25 2.1 02 1.8 Annealing. — Pure iron, copper, silver, and other metals are easily drawn into wire, rolled into sheets, or flattened under the hammer. But 1 See Gold. 2 Annales de Chimie et de Phy.'^ique (III.), vol. xii. PROPERTIES OF THE METALS. 97 all these operations iviulor the metals harder, and reduce from their pliability. Their original softness can be restored to them by annealing — L e., by heating them more or less and then plunging them into cool water, oil, etc. In the case of iron, however, this applies only if the metal is perfectly pure. If it contains a few parts carbon per thousand, the annealing process, instead of softening the metal, gives it a "temper," meaning a higher degree of hardness and elasticity.^ Welding. — The process of joining two clean surfaces of a metal together by pressure is called v^elding. This property is possessed by iron at white heat, but lead and gold will cohere at ordinary temperatures in proportion to their purity. Two pieces of iron may be welded by a current of electricity sent through their junction, when the metal is heated by the resistance offered to the passage of the current. Forging. — The process of hammering metals out into various shapes is called forging. With some it may be done when they are cold, while in others tljpy must be hot. It illustrates the solid flow of metals. Elasticity, — All metals are elastic to this extent, that a change in their form brought about by a stress not exceeding certain limit values, will disappear when the stress is removed. Strains exceeding the "limit of elasticity" result in permanent deformation, or, if sufficiently great, in rupture. This property may be increased in some metals by compound- ing and alloying. Thus, iron compounded with the proper amount of carbon, has its elasticity increased to the very highest degree, while the metal itself is almost devoid of the quality. The same is true of copper and zinc in some forms of brass, also of gold and platinum when alloyed with copper ; both the latter are soft and have little elasticity, yet when combined in proper proportions with copper an alloy is produced which is quite elastic, and may be used for clasps for artificial dentures. Sonorousness. — ^This is a property possessed by the harder metals, and is quite marked in certain alloys, such as those of copper and tin known as bell-metal. Lead, which is but feebly, if at all, sonorous, may become so, it is claimed, if cast in the shape of a mushroom. Aluminum emits a characteristic sound when struck. The first article known to have been made of aluminum was a baby rattle for the infant prince imperial of France, for which purpose it was well fitted on account of its sonorousness. Impurities sometimes increase the sonorousness of a metal, as in the case of antimony in lead. Fusibility and Volatility. — All metals may be fused, and most of them are capable of being volatilized, but the temperature at which they become fluid differs greatly in different metals, as the following table shows : Fusing point. Fusing point. Name of metal. Centigrade. Fahrenheit. Authority. Mercury —39.0 —38.2 Csesium + 26 to 27.0 +78.8 Setterberg. Gallium 30.0 S6.0 L. de Boisbau Rubidium . 38.5 101.3 Bunsen. Potassium ■ . 62.5 1 See Iron. 144.5 Bunsen. 98 METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. Names of metal. Sodium Indium Lithium Tin . Bismuth . Thaliuni . Cadmium . Lead Zinc . Antimony . Incipient red heat Aluminum Magnesium Cherry-red heat . Silver Gold Yellow heat Copper Iron, wrought Iron, chemically pure Cobalt Nickel Uranium . Dazzling white heat Palladium Oxyhydrogen flame Platinum . Iridium Rhodium . Ruthenium Maximum temperature of Fusing point. Fusing point. Centigrade. Fabreulieit. Autliority. 95.5 203.9 Bunsen. 176.0 348.8 Richter (?). 180.0 356.0 (?) 228.0 442.4 Rudberg. 264.0 507.2 Rudberg. 290.0 554.0 Lamy. 320.0 608.0 Rudberg. 325.0 617.0 415.0 779.0 Person. 425.0 797.0 525.0 977.0 Pouillet. 625.0 1157.0 750.0 1382.0 700.0 1292.0 Pouillet. 1040.0 1904.0 Becquerel. ^ 1100.0 201 2.0*. 1100 2012.0 Pouillet. 1200.0 2192.0 1300 to 1400.0 2372 to 2552.0 higher-1600 2912 1400.0 2552 1600 2912.0 (?) (?) 1.500 to KiOO.O 2732 to 2912.0 Pouillet. 1600.0 2912.0 2000.0 3632.0 2S70.0 5198.0 2870° — i.e., is as yet infusible. Bunsen. oxyhydrogen flame . Osmivim does not melt at Metals may be characterized as fixed and volatile. Of their volatility we have little precise knowledge. The boiling points of a few are given in the following table: Authority. Regnault. Deville and Troost Deville and Troost Dewar and Dittmar. metals may be clas.sed as Name of metal. Mercury Cadmium Zinc Potassium Sodium Boiling point. 357.3° C. 860.0° " 1040.0° " Below 1040.0° " Above 1040.0° " For practical purposes the volatility of follows : 1. Distillable below redress: Mercury. 2. Those distillable at red heats. Cadmium, Zinc, Magnesium, Potassium, Sodium. > Jahresb. f. Chem., 1867, p. 41 ; Phil. Mag., xxxiv. 489. PROPERTIES OF THE METALS. 90 3. Those which are vohitiHzed more or less readily when heated beyond their fusing points in open crucibles : Antiuujiiy (very readily), Bismuth, Ijead, , Tin, Silver. 4. Those which are with very great difficulty volatilized, if at all : Gold, Copper (?). 5. Those* which are practically "fixed," or non-volatile : Copper (?), Cobalt, Lithium, Iron, Calcium, Strontium, Nickel, Aluminum, Barium. "In the oxyhydrogen flame silver boils, forming a blue vapor, while platinum volatilizes slowly, and osmium, though infusible, very readily."^ "It is doubtful," says Makins, "if it (gold) is volatile per se. But if gold be alloyed with copper, it has been shown by Napier to be con- siderably volatilized, so that quantities, amounting to 4^ grains, could be collected during the pouring out of 30 pounds' weight from a crucible. . . . That mixtures of gold, silver, and lead, when cupelled together, volatilize considerably." Specific Heat. — Equal weights of different metals have been found to absorb different amounts of heat when subjected to the same temper- ature. They, indeed, possess different capacities for heat. Thus, the amount of heat necessary to raise a given weight of water has been found to be 31 times as great as that required to raise an equal weight of platinum through the same range of temperature; or, in other words, the amount of heat required to raise a given weight of water through 100° C. will raise 31 times the same weight of platinum through 100° C. of temperature. Thus, water being taken as the standard or unit, the specific heat of platinum is ^\-, or 0.032 that of water. Table op Specific Heats 1. Iron 2. Nickel . 3. Cobalt . 4. Zinc 5. Copper . 6. Palladium 7. Silver . S. Cadmium 9. Tin 10. Antimony 11. Mercury 12. Gold . 13. Platinum 14. Lead 15. Bismuth 0.1138 0.1086 0.1070 0.0956 0.0952 0.0593 0.0570 0.0567 0.0562 0.0508 0.0333 0.0324 0.0322 0.0314 0.0308 1 William Dittmar. 100 METALS AND ALLOYS USED L\ PROSTHETIC DEyVISTRY. Expansibility. — The expansion of metals by lieat varies greatly. The coefficient of expansion is constant only in metals that crystallize in the regular system. The others expand differently in the direction of the different axes of their crystals, and to eliminate this source of uncer- tainty in making estimates of their expansibility, they are employed as compressed powders. The following table gives the linear expansion from 0° to 100° C, according to Fizeau, the length at 0° being taken as unity.^ Kxpansion. Name of metal. 0° to 100° C. Platinum, cast Gold, cast Silver, cast Copper, native Copper, artificial Iron, soft Steel, cast Bismuth, mean expansion Tin, compressed powder . Lead, cast Zinc .... Cadmium, compressed powder Aluminum, cast Mercury 0.000907 0.001451. 0.001936 0.001708 0.001869 0.001228 0.001110 0.001374 0.002269 0.002948 0.002905 0.003102 0.002336 0.018153 "The high rate of expansibility of zinc renders it particularly vaUiable as a metal for dies upon which to form plates for the mouth in many cases. The metal is ca.st while fluid and at its extreme limit of ex|)ansion, which upon cooling returns to its minimum dimensions, and thus furnishes a cast a little smaller than the plaster model which it represents. It has been found that this contraction of the zinc die a trifle more than compensates for the expansion which takes place in the plaster model in setting, and in the majority of cases a plate made thereon adapts itself more accurately to the mouth than one made upon a die of less expansible metal. Even if the contraction undergone by the zinc is so great as to produce a die somewhat smaller than the mouth, so far from being a detriment, it is a positive advantage in most cases of full upper replacement, as under such conditions the pressure of the finished plate i.s greater upon the alveolar riflge than upon the central portions of the hard palate — a state of affairs the advantages of which are sufficiently obvious without explanation."^ Conductivity. — Metals are good conductors of heat and electricity, but these fjualities are very differently exhibited in different metals. An exact knowledge of these conductivities is of great scientific and practical importance to the dentist, and too much attention cannot be given their consideration. The following table gives the thermic and electric conductivities of some of the more important metals and alloys: ^ W^illiam Dittmar. a Dr. E. C. Kirk, Am. System of Dentistry, vol. iii. p. 793. PROPERTIES OE THE METALS. 101 Name of metal. Silver Copper Gold Tin . Iron Lead Platinum Bismuth Brass Steel German silver Rose fusible metal Pianoforte Wire Relative conductivity. Thermic. Electric, at 0° C. 100.0 100.00 73.G 99.95 . 53.2 77.90 14.5 12.30 11.9 16.81 8.5 8.32 8.4 18.80 1.8 1.24 . 23.6 . 11.6 7.3 7.67 2.8 14.40 Makins states that among the results of Dr. Matthiessen's experiments upon the electric conductivity of metals "are the facts that impurity of a metal or alloying it greatly diminishes its conducting power. Rise of temperature again has the same effect. Thus between 32° F. and 212° (or 0° C. and 100°) great diminution takes place, and that not uniformly, as some lose it much more in proportion than others, by thus raising the temperature. Many lose as much as 25 per cent, of their conducting power." An illustration of the comparative conductivity of the metals is observed in the electric furnaces with platinum coils. The electricity is readily transmitted from its source by the copper efferent wire, but when it meets the platinum that metal offers so much resistance to the passage of the current, on account of its low conducting power, that it becomes white-heated — incandescent. Specific Gravity. — ^This property varies in different metals from 0.594 (lithium) to 22.48 (osmium), as the following table shows: Name of metal Specific gravity. Authority. Lithium 0.594 Bunsen. Potassium 0.875 Baumhauer. Sodium 0.9735 Baumhauer. Rubidium 1.52 Bunsen. . Calcium 1.578 Bunsen and Matthiessen Magnesium 1.743 Bunsen. Caesium 1.88 Setterberg. Glucinum ' . . 2.1 Debray. Strontium 2.5 Aluminum 2.583 MaUet. Barium . Over 4.0 Clarke. Zirconium 4.15 Troost. Vanadium 5.5 Roscoe. Gallium 5.9 Lecoq de Boisbaudran. Lanthanum 6.163 Lecoq de Boisbaudran. Didymium 6.544 Hillebrandt and Norton. Antimony- 6.715 Marchand and Scheerer. Cerium 6.728 Hillebrandt and Norton. Chromium 6.81 Wohler. 102 METALS AM) ALLOYS USED L\ PROSTll ETK' DKSTISTRY Name of metal. Zinc Manganesium Till Indiiiin Iron Nickel . Cadmium Cobalt . Molybdenum Copper Bismuth Silver . Lead . Palladium Thallium Rhodium Ruthenium Mercury Tungsten Uranium Gold . Platinum Iridium Osmium Specific gravity Authority, 6.915 Karsten. 7.14 Brunner. 7.29 - 7.42 Richter. 7.S44 Berzelius. 8.279 Richter. 8.546 Schroder. 8.5 8.6 Uebray. S.94 9.823 Holzmann. 10.4 HolzmauM. 11.25 Deville. 11.4 Deville and Debray 11.86 Crookes. 12.1 Bunsen. 12.26 Deville and Deliray 13.595 H. Kopp. 16.54 Wohler. 18.33 Peligot. 19.265 Matthiessen. 21.46 22.4 22.477 Deville and Debray COMPOUNDS OF METALS AND NON-METALS. Metals mix with each other indefinitely to form alloys, preserving the metallic appearance and properties. • They combine with non-metals in definite chemical proportions to form compomids of a more precise natnre, in which case the metallic characters are almost invariably lost. These definite compounds include the Oxides Sulphides, Chlorides, They also combine with Nitrogen, Phosphorus, Bromides, Fluorides, Cyanides, Boron, Silicon, Selenides, Tellurides Caibon. ALLOYS. An alloy is the compound or mixture of two or more metals elTected by fusion. An amalgam is an alloy of two or more metals, one of which is mercury. Few metals are employed in the pure state, with the exception of iron, copper, lead, tin, zinc, platinum, and ahnninum; they are more frequently used for technical purposes in the form of alloys. Every ALLOYS. 103 industrial application requires special (jualities that may not exist in any single metal, but which may be produced by the proper mixture of two or more. For example, silver and gold are much too soft and pliable for plate, coin, or jewelry, but by the addition of certain amounts of copper they are rendered harder and more elastic, while their color and other valuable qualities are not impaired. Copper is also too soft and tough to be wrought in a lathe, but when alloyed with zinc it forms a hard, beautiful, yellow-colored alloy known as brass, of great usefulness and more easily worked than the pure metal. Alloys are extremely interesting, from a scientific standpoint, for they may be regarded not only as mere mixtures of metals, but in many instances as true chemical compounds. Matthiessen^ regarded it as probable that the condition of an alloy of two metals in a melted state may be either that of (1) a solution of one metal in another; (2) a chemical combination ; (3) a mechanical mixture or ; (4) a solution or mixture of two or all of the above ; and that similar differences may exist as to its condition in the solid state, defining a solid solution as "a perfectly homogeneous diffusion of one body in another." The Physical Properties of Alloys. — ^The physical properties of an alloy cannot be anticipated from those of its constituent metals, and are only determinable by actual experiment. Very minute proportions of some metals added to others will produce an alloy with proper- ties foreign to either of the constituents. Thus, a small quantity of lead fused with gold will produce a brittle alloy, though each metal is malleable. Specific Gravity. — If this property be calculated as the mean of that of the component metals of the alloy, the result may be greater than, equal to, or less than the actual specific gravity of the alloy determined by experiment. Thus, the alloys of silver and gold have a less specific gravity than the theoretical mean of the components; whereas copper and zinc vary in the opposite direction. The following table,^ by Thenard, shows examples of this variation: Alloys possessing a greater specific Alloys 1 la^^ng a lower specific grav gravity than the mean of their than the mean of their components. components. Gold and Zinc. Gold and Silver. " tt Tin. " " Iron. It tt Bismuth. " " Lead ti ti It tt Antimony. Cobalt. tt " Copper. " Iridium Silver " Zinc. tt " Nickel. ti " Lead. Silver " Copper tc tt Tin. Copper " Lead. it it Bismuth. Iron " Bismuth. It it Antimony. ti " Antimony. Copper it Zinc. it " Lead. li'^ tt Tin. Tin " Lead. tt it Palladium. " " Palladium. tt tt Bismuth. it " Antimony. ti It Antimony. Nickel " Arsenic. Lead tt Bismuth Antimony. Zinc " Antimony. Platinum ti Molybdenum. ' Palladium tt Bismuth. 1 British } Lssociati on Reports, 1863, p. 97. 3 Phillips' Metallurgy. 104 METALS AND ALLOYS IISKI) IX PROSTHETIC DENTISTRY. It is common among authorities who pubhsh determinations upon specific gravities of the alloys to give the calculated as well as the ob- served .sj)eciHc gravity. The Color. — The color of an alloy usually resembles that of the metal which predominates. Some few exceptions are (juite notable; for instance, gold 2 to 6, and silver 1 part produces an alloy of a greenish color, and it is said that -^j part of silver is sufficient to modify the color of gold. Nickel and copper form alloys varying from copper- red to the bluish-white of nickel. With a content of 30 per cent, of nickel the alloy is silver white; while with zinc, copper yields a variety* of shades, from the silver white of brass consisting of copper 43, and zinc 57 parts, to that of red brass, which contains 80 per cent, or more of cop])er. Malleability, Ductility, and Tenacity. — These properties are generally very much modified by alloying. As a rule the malleability and duc- tility are decreased, even when two malleable and ductile metals, such as gold and lead, are alloyed together — a very small content of lead destroying the malleability and ductility of the noble metal. Again, copper 94 and tin 6 parts form an exceedingly brittle alloy. Generally the ductility decreases, while the hardness as compared with that of the constituent metals increases to a considerable extent; for example, gold and platinum, two very ductile and soft metals, afford an alloy much harder and of greater elasticity than either. Gold and silver, being too soft for currency, are alloyed with 10 per cent, of copper, which gives them the required hardness. A few metals, antimony, for instance, possess the property of making metals harder. IVIr. INIakins states that y^WiJ P^^^ of this brittle metal will make gold quite unworkable. As a rule, a brittle and a ductile metal afford a brittle alloy; yet copper and zinc yield a malleable and ductile alloy in brass. The tenacity is generally very much increased, as is shown by the following results of Matthiessen's experiments. Wires of the same gauge were employed, and the weights causing their rupture before and after alloying noted as follows: Copper, unalloyed Tin, Lead, " Gold, " Silver, " Platinuni, " Iron, " Pounds at rupture. 25 to 30 under 7 " 7 20 to 25 45 " 50 . 45 " 50 80 " 90 Copper, alloyed with 12 per cent. Tin . Tin, " " " " Copper Lead, " " Tin . Gold, " " Copper Silver, " " Platinum Steel (iron compounded with carbon) . Pounds at rupture. 80 to 90 7 7 70 75 to 80 . above 200 ALLOYS. 105 Fusibility. — The fusing point of an alloy i.s always lower than that oi the least fusible metal entering into its eomposition, and is .sometimes lower than that of any of the components. Thus an alloy composed of 10 parts lead and 4 parts tin fuses at 470° F., melting lower than the less fusible lead (()17° F.), but at a greater tem])erature than tin (442° F.); and an alloy composed of 4 parts lead, 2 parts tin, 5 to 8 parts bismuth, and 1 to 2 parts cadmium (Wood's metal) melts at 140° to 161° F., lower than that of any of its constituents — tin being the most fusible (442° F.). Alloys of lead and silver, containing a small f[uantity of the latter, are more fusible than lead, and sodium and potassimn form an alloy fluid at ordinary temperatures. jNIatthiessen^ explains why the fusing point of alloys is uniformly lower than the mean of those of their constituents: "It is generally admitted that matter in the solid state exhibits excess of attraction over repulsion, while in the liquid state these forces are balanced, and in the gaseous state repulsion predominates over attraction. Let us assume that similar particles of matter attract each other more power- fully than dissimilar ones attract each other. It will then follow that the attraction subsisting between the particles of a mixture will be sooner overcome by repulsion than will the attraction in the case of a homogeneous body; hence, mixtures should fuse more readily than their constituents." Sonorousness. — This property is most wonderfully developed in some instances by alloying. Copper and tin, two metals wdiich possess the quality in but a small degree comparatively, unite to form an alloy known as "bell metal." Conductivity. — The property of conductivity, either for electricity or heat, in an alloy is much inferior to that of the pure metals. Advan- tage is taken of the high electric resistance in some of the alloys, such as German silver, for measuring the resistance of long lines of telegraph wire, the electromotive force or working power of batteries, for making rheostats and other apparatus for controlling the electric current, etc. Decomposition. — Heat decomposes alloys containing volatile metals like mercury or zinc. It requires a temperature much above the boil- ing point of the metal, however, to completely separate all traces of it from an alloy, and in most instances this cannot be accomplished even then without the assistance of chemical agency. When gold is con- taminated with tin, the latter cannot be removed entirely by roasting; but if heated with small quantities of potassium nitrate, wliich serves to oxidize the base metal, it may be entirely removed. Mercury may be completely separated by roasting; it volatilizes at about 675° F. When endeavoring to expel it from old amalgam fiUings, however, the mass should be heated bright red. Annealing and Tempering. — AnneaHng is a process employed in the the w^orking of various metals and alloys to reduce the observed brittle- ness and stiffness which result from the change of molecular struc- 1 Makins' Metallurgy, p. 65. 106 METALS AND ALLOYS USED L\ PROSTHETIC DENTISTRY. ture, produced by liammcriii<^, loii ALLOYS JISKD IN PROSTHETIC DENTISTRY. and is admirably illustrated in the operation of soldering platinum bases with pure gold for eontinuous-gum dentures. By means of the blow- pipe the gold is Howed over the platinum surfaces thus joining them. If the joint is not well made, and an intervening space is hllcd with gold, this is not as strong as a close joint. This, however, is all reme- (lied during the process of baking the body and enamel, as the high heat re(juire(l for this so diminishes the cohesive power of the platinum that it readily and completely alloys with the gold, producing a strongei joint of a platinum-gold alloy, which is observed to be the same color as the platinum. Autogenous Soldering is a process of soldering by direct fusion of the contiguous parts, without the intervention of a more fusible alloy. It is extensively wsvd 'n\ uniting ends of bands for collar crowns. Preparation of Alloys. — This would seem to be a simple 1task, but in order to produce an accurate result it is far from being as easy as it may seem. INIost alloys are prepared by directly melting the metals together, but much skill, judgment, and experience are refjuired to determine when it is best to add each constituent, and the amount of each to be used, to protect the molten mass, and to handle it generally. The metal having the highest fusing point is generally melted first, and the others are added in the reverse order of their fusibility. The varying densities of the metals to be combined frefjuently ren- der the formation of a homogeneous mass very difficult. In some instances the heavier metal tends to sink to the bottom, carrying with it a small cjuantity of the other, while the lighter, floating above, retains a small quantity of the heavier. For instance, only a small proportion of zinc will unite with lead, or aluminum with bismuth; but, as a rule, metals mix perfectly in the fluid state. AMien, however, the fluid mix- ture is poured into the ingot mold, it rarely happens that the solidified mass is perfectly homogeneous. The reason of this is that the addition of one metal to another produces an alloy, the solidifying point of which is usually lower than it should be according to calculations based upon the proportionate amounts and fusing points of the constituents. One particular mixture has a lower fusing point than any other possible mixture of the metals employed, and this is termed the eutectic alloy of that series. Aside from those possibly true chemical combinations of metals, a fluid mixture of two metals may be expected to begin depos- iting its less fusible constituent first, and, as the temperature falls, more and more of this element will be separated, the other constituent con- centrating in the fluid residue until this has ac(juired the eutectic com- position, when it will solidify as a whole in the spaces left between the already solidified particles. The more slowly the material solidifies, the more marked will be the separation that occurs. To obtain as homogeneous an alloy as possible, the metals, while in a state of fusion, must not be allowed to remain quiescent, but an intimate mixture effected by vigorous stirring, sticks of dry, soft wood being used for the purpose. By stirring the fused mass with one of these sticks the wood is more or less carbonized according to the temperature of the mass, gases are evolved from the carbonizing wood, which, by ascend- ALLOYS. 109 ing in the fused mass, contribute to its intimate mixture. The stirring should continue for some time and the alloy then cooled a,s rapidly as possible. For preparing alloys in a small way a crucible is used, and the alloy is covered with a suitable flux to protect it from the action of atmos- pheric air. Four sources of loss must be guarded against: (1) loss by oxidation; (2) loss by volatilization; (3) loss by chemical combination with the flux; (4) loss by fracture or solution of the crucible. The first may be prevented by the use of one of the various fluxes, or covering the surface with pulverized charcoal. The second loss usually occurs through an endeavor to alloy a metal of a high fusing point with one which fuses at a low temperature. Under such circum- stances the one requiring a high temperature should be fused first and well covered with flux melted to extreme fluidity; the more fusible metal should then be added in as large a piece as convenient and quickly thrust beneath the molten surface. The third source of loss is princi- pally caused by the use of borax as a flux for some base metals. It is well known that in much borax a portion of the boric acid is not per- fectly saturated, and this is especially true of the prepared article; and if melted with some base metals the free acid is absorbed, which, with the sodium borate, forms double salts of a glassy nature. Hence, by fusing some metals and alloys under borax, a certain portion will be lost in chemical combination. The fourth cause is guarded against by careful selection of crucibles. If alloys of low fusing metals are to be made, the ordinary clay or Hessian crucible is all that is necessary, and, indeed, with proper care, noble metals may be alloyed in it with- out danger of loss; but it is liable to perforation by corrosive fluxes, allowing the molten alloy to escape. Therefore, for the preparation of expensive alloys from noble metals, the employment of tried graphite or graphite and clay crucibles often saves much trouble and expense. In some instances, especially when metals are known to form chem- ical combinations, it may be best to melt the one of lowest fusing point first, and then dissolve the other components in it. Or, those of low fusing point may be melted in one crucible, while those more difficult of fusion are melted in another, then combined in the molten state. "]Many alloys," says INIr. Brannt,^ "possess the property of chang- ing their nature by repeated remelting, several alloys being formed in this case, which show considerable dift'erences, physically as well as chemically. The melting points of the new alloys are generally higher than those of the original alloy, and their hardness and ductility are also changed to a considerable extent. This phenomenon is frequently connected with many evils for the further application of the alloys, and in preparing alloys showing this property the fusion of the metals and subsequent cooling of the fused mass should be effected as rapidly as possible." Although most of the heavier metals are at present used in the prep- aration of alloys, copper, zinc, tin, lead, silver, and gold are more fre- 1 MetalHc Alloys, p. 87. no METALS AM) ALLOYS USED L\ PROSTHETIC DEyTISTRY. quently employed than all others. Alloys. eontaining nickel have become of great importance as well as those in which alumiiunu is a con- stituent. Mr. IJrannt reconuneiids for experimeiitution that metals be added to each other in certain quantities by weight, that is, according to their atomic weights, and claims that in this manner alloys of determined, charact(M"istic propertifvs are, as a rule, produced; or, if such does not answer the demands of the alloy, the object may be obtained by taking two, three, or more e(|uivalents of the metal, exception being made in the cases of arsenic and such elements. GOLD. Aurum. Symbol, Au. Atomic weight, 195.7. Malleability, first rank. Melting point, 1100° (2012° F.). Tenacity, fifth rank. Ductility, first rank. Specific gravity (cast), 19.265. Conductivity (heat), 53.20. Conductivity (electricity), 77.96. (Silver being 100.) Occurrence. — Gold is found in nature chiefly in the metallic state, or as native gold, and less frequently in combination with tellurium, lead, and silver. It is also found combined, or, perhaps, more strictly speaking, minutely mixed with pyrites and other sulphides, more com- monly called "sulphurettes." Native Gold occurs rather frecjuently in crystals belonging to the cubic system, the octahedron being the commonest form, but other and complex combinations have been observed. Large crystals are rarely well defined, owing to the softness of the metal, the points being commonly rounded. The most characteristic forms, however, are the nuggets or pepites. These, when of a weight less than one-quarter to one-half an ounce, are known as gold dust. Except the larger nuggets, which are usually more or less angular or irregular, gold is generally found in a bean-shaped or somewhat flattened form, the smallest particles being scales of scarcely appre- ciable thickness, and owing to their small bulk, as compared with their surface, they are frequently suspended in water and may be washed away by a rapid current; hence, they are known as float gold. In the museum of the Mining Bureau in San Francisco are several plaster-of-Paris models of famous gold nuggets found in the various gold regions of the world. The largest single piece of gold ever found was taken out at Ballarat, Victoria, Australia. It weighed 2166 troy ounces, and was valued at $41,882. The second largest was discovered in the Ural Mountains district, and weighed 1200 ounces. The third largest, which was also found in Victoria, Australia, weighed 1121 ounces, and was valued at .$22,000. The physical properties of native gold are c|uite similar to those of the melted metal and its alloys. The composition varies considerably in different localities, as shown in the following table: GOLD. Ill Analysis of Native Gold from \ .VHIUUS LOC.VLITIKS. Locality. Gold. Silver. Iron. Cijpper. Europe : British Isles — \'igra and Clogau . 90.16 9.26 Trace. Trace. Wicklow (River) . , 92.32 6.17 0.78 Transylvania . 60.49 38.74 0.77 Asia : Russian Empire — Brezovsk . 91.88 8.03 Trace. 0.09 Ekaterinburg . 98.96 0.16 0.05 0.35 Africa : Ashantee . 90.05 9.94 America : Brazil . . 94.00 5.85 Central America . . 88.05 11.96 Titiribi . 76.41 23.12 0.87 California . 90.12 9.01 iVIariposa . 81.00 18.70 Cariboo . 84.25 14.90 0.03 Australia : South Australia . 87.78 6.07 6.15 Ballarat . 99.25 0.65 The most important minerals containing gold are: Sylvanite, or graphic tellurium , (AgAu)Te2, containing 24 to 26 per cent. Calaverite, AuTcj, containing 42 per cent. Nagyagite, or foliate tellurium, of a complex and rather indefinite composition, and containing from 5 to 9 per cent, only of gold. The calaverite, a nearly pure telluride of gold, has been found to some considerable extent in Calaveras County, California. The minerals of the second class, called auriferous, are compara- tively numerous, and include many of the metallic sulphides. The most important of these are iron pyrites and galena; the first of these is of great practical importance, being found in many districts exceed- ingly rich, and next to the native metal, is the most prolific source of gold. A Native Amalgam of gold is found in California, but rarely in any considerable quantities. Gold is so widely distributed throughout the earth's crust that few regions may be said to be destitute of slight traces of it ; yet it has been found in comparatively few localities in quantities sufficient for econom- ical extraction. The principal supplies of the metal have been derived from Africa, California, Australia, Mexico, Brazil, Ural Mountains, Transylvania, Alaska, etc. The association and distribution of gold may be considered under two different heads, namely, as it occurs in mineral veins, and in alluvial or other superficial deposits which are derived from the waste or disin- tegration of the former. As regards the first, it is usually found in quartz veins or reefs transversing slaty or crystalline rocks, either alone or associated with such metals as iron, copper, tellurium, and rarely 112 MIITALS ASI) ALLOYS VSKD L\ PROSTHETIC DENTIS'IRY. bismuth, or sucli iniiuTuls ;i,s inagiu'tio and arsenical pyrites, galena, specular iron ore, and silver ore, and rarely with the sulj)liides of molyb- denum, tungstate of calcium, bismuth, and tellurium minerals. In the second or alluvial class (placers) of deposits it is associated chieHy with those minerals of great density and hardness, such as platinum, iridum, and other metals of the platinum group, tinstone, chromic, magnetic, and brown iron ores, diamond, sapphire, ruby, topaz, etc., which re{)resent the more durable original constituents of the rocks whose disintegration has furnished the detritus. Refining Gold. — In less it can be utilized tlie accumulation of gold in the form of scraps, filings, etc., in the dental laboratory and operating room frequently becomes a source of considerable loss to the dentist, because he is not familiar with the methods of refining or lacks the necessary apparatus. Some forms of scrap gold, such as old fillings, need only to be melted with the proportion of silver, copper, or both, to produce the desired alloy. Others, as scrap plate of know-n carat, may be utilized by simply remelting and rolling. Old crowns, plates, l)ridges, mixed filings containing more or less iron from the file, zinc, lead, antimony, and other base metals may be converted into malleable gold by simply roasting with such fluxes as will combine chemically with the base metals and remove them. Sweepings may be washed and then carried through the same process which is known as " roasting." The Roasting Process. — A method for rendering brittle gold malleable. This process may be most satisfactorily employed where the approxi- mate carat of the bulk of the scraps is known and the gold is suspected of being unworkable because of the admixture of l)ase metals. The larger pieces should be removed from the accumulation and the smaller ones with the filings freed from as much iron and steel as possible by a good magnet. AH should then be placed in a previously well-boraxed and tested graphite crucible, with the addition of sufficient potassium carbonate to well cover the charge; the ol)ject of this addi- tion being to form, when heated, a thin flux, permitting the small par- ticles and filings to sink and accumulte in one mass. The furnace should be placed beneath a fume chimney or by a win- dow with an outward draught, that the fumes escaping from it during the roasting may not fill the lal)orat()ry, th('rel)y endangering the health of the operator and damaging such instruments and t(»()ls as may be unprotected. The most convenient place to avoid such results is the fireplace. The furnace may l)e placed beneath its chimney in such a maimer that all fumes will be readily c-arried off. When the metal has become thoroughly fused, the refining ])rocess may be begun by first adding small quantities of the oxidizing agent, potassium nitrate (KNO^), accompanied with borax, as needed to properly protect the mass and further the process. The object of the potassium nitrate is to furnish sufficient oxygen to oxidize the contaminating base metals beneath the flux, thus se})arating them from the gold. As most base metals are easily oxidized under these circumstances, a continuation of this process from ten minutes to one hour and a half, according to the GOLD. 113 quantity of material, and the proportion of base metals eontained, add- ino; the nitrate and ])()rax as re(|uired, and maintaining a state of perfect fusion of the metal, tiie ingot, when made by pouring into a previously warmed and oiled mold, will be found to be quite malleable. If, however, upon examination it is found to be still brittle, it sliould be plaeed in a clean, boraxed, and tested crucible, heated, and brought to a perfect state of fusion. A mixture of ecjual parts of finely pul- verized vegetable charcoal and ammonium chloride should then be added; at first sufficient to properly cover and protect the molten mass and afterward a small quantity at a time as it is needed. "When the metal has been sufficiently treated, which may be determined by remov- ing small quantities and subjecting them to the physical tests for mal- leability, the crucible is to be removed from the furnace and the metal cast into an ingot or allowed to cool in the crucible as a button. The rationale of such a process is that the heat of the crucible breaks up the chloride compound, liberating the chlorine in the nascent state; which in turn combines with the metals lead, tin, and silver contained in the gold to form their respective chlorides. These are either volatilized or taken up by the flux, the gold remaining free of them. IMercuric chloride is sometimes used when the contamination of the gold with lead or tin is extensive, or where it is desired to remove a quantity of silver. But its use is so dangerous on account of the fumes evolved it is rarely employed. Sulphur or antimonic sulphide is used to abstract large quantities of silver from gold, by combining with the former to form the fusible sulphide of silver, leaving the gold free, or if the antimonic sulphide has been used, contaminated with antimony, which may be removed by fusing with borax and potassium nitrate, as previously described. In the process of refining by fluxes, the first step should be to deter- mine, as far as possible, the nature of the debasing elements; this being known or reasonably approximated, the process may be confined to the particular flux most likely to free the gold from its contamination. Iron, steel, zinc, copper, antimony, and bismuth are perhaps, best removed by oxidation through the agency of potassium nitrate. Lead, tin, and silver are removed by chlorine. If, after such treatment, the alloy is found to be malleable, but stiff or elastic, or dull in color, it very probably contains some platinum which cannot be removed by this means, but which may be gotten rid of by a wet method. When desired, such an alloy may be made direct use of as clasp gold. When the object is to produce pure gold from which to subsequently prepare desired carats by alloying the results, it is best and most con- veniently attained by the process known as " parting gold." Parting Gold. — A wet method for refining gold by inquartation, or "quartation," as it is more commonly called, is known as the process of parting gold. This is accomplished by digesting the thinly rolled or granulated alloy of silver and gold in either nitric or sulphuric acid. In the choice of metal for tliis operation, an endeavor should be made to obtain gold containing as much silver as possible, and, as this 8 114 MKTM.S AXD ALLOYS USED fX PROSTHETIC DEyTlSTRY. will rt'(juirc an additional (jnantity of the latter metal fused with it in order to carry out the operation, it is of course an object, if possible, to employ silver which contains small quantities of gold, and thus, to carry on a double refining process at once. As the actual separation of the two is effected by digesting the mix- ture in hot nitric acid, which, while it is a ready solvent for other metals, is inactive upon gold, it may be asked: Why not at once treat the alloy with acid without such alloying? Such would be quite useless, for, the foreign metals being in so small a relative proportion, the acid would only remove the alloy at or near the surface, the metal being sufficiently close in texture to mask all the rest from the action of the acid. The sulphuric acid process is doubly reconmiended, especially when large quantities of the alloy are to be digested, as it is less expensive, and the gold is obtained of a greater degree of fineness. The oxidizing action of the nitric acid is of especial value, however, when tin or antimony is ])resent in the batch of metal. Preparation of the Alloy. — The impure gold is first weighed and the approximate weight of the silver, if it contains any, subtracted; silver is then added in the proportion of three to one, less the amount already contained in the alloy, thus when melted forming an alloy of three parts silver and one part impure gold. Hence the term "quartation." These proportions are then fused together in a clean and boraxed cruci- ble, well mixed, and either poured into warmed and oiled ingot molds, to be subsequently rolled, or dropped while molten from the crucible into a wooden tub or tank of cold water for the purpose of granulation. The latter is unquestionably the simplest method of preparing it for the digesting process, for, if poured into the ingot molds, the alloy will require rolling to a very thin ribbon (No. 35 gauge), after which it must be cut into small pieces. Rolling it many times is impossible because the gold that it is desired to refine is exceedingly brittle. The alloy being thus prepared, is ready for the acid. Nitric Acid Process. — Yov this process the prepared alloy is placed in a Florence flask and nitric acid to the amount of about one and one- half times the weight of the alloy poured on. The acid should always be tested for chlorine by adding a drop of a solution of silver nitrate (AgXOg) to it, which, if chlorine l)e present, will instantly be rendered milky from the precipitated chloride of silver. Ileat the flask gently in a sand bath over a Bunsen or alcohol flame. Copious red fumes of the oxides of nitrogen and ammonium will be given off, showing vigor- ous action on the alloy, and the silver and other metals will be dissolved, leaving the gold in a spongy mass of a blackish-])rown color. When this evolution has entirely ceased and the flask is clear, carefully decant the solution of the nitrates of silver, etc., thus formed and preserve it, adding a fresh portion of nitric acid and boil until all fumes cease to rise, which marks the termination of the digesting process. The acid is now replaced by distilled water two or three times, for the purpose of washing the gold remaining. At length filter the contents of the flask, catching the gold on the filter paper, add a sufficient quantity of potas- sium carbonate, fold the ])aper over the whole, and ]>lace in a previoush' boraxed crucible, melt and pour into warmed and oiled ingot molds. GOLD. 115 Gold thus refined may reaeh -{\;\f'(j fineness, and is ready for any desiraV)le alloying. For the recovery of the silver, see that subje(t. Sulphuric Acid Process. — The use of sulphuric acid for the operation is preferred by many. For, as was stated, it is more economical; and the gold so refined is more thoroughly freed from silver; indeed, it is said that gold having been previously refined by the means of nitric acid may be freed of still more silver by this acid. In operating the metals are so mixed that the gold amounts, at most, to not quite half the weight of the silver; and if copper is contained (which in small pro- portions facilitates the operation), it should be under 10 per cent., for if too much be present, a large quantity of sulphate of copper will be formed, which latter is insoluble in the strong acid liquors. The process may be employed for silver containing very small cjuantities of gold Thus, in France, it was found very profitable to separate the gold from old five-franc pieces, which contained only Yihr ^^ ToVo ^f gold. The alloy having been granulated, as before described, is introduced into a digester (Florence flask) with about two and one-half times its weight of concentrated sulphuric acid. This is allowed to boil, during which strong action is evidenced by copious evolution of sulphur dioxide, while the silver and copper are simultaneously converted into sidphates. This first boiling is continued as long as sulphur dioxide is evolved, which in large quantities of metal will commonly go on about four hours. The liquid is then removed and a smaller quantity of acid added, the boiling being further carried on for a short time, after which the digester is allowed to remain at rest, in order that the gold may subside. Sometimes it may be necessary to make even a third addi- tion of acid. Repeated washing of the gold with boiling water is now necessary, as the sulphate of silver is a very insoluble salt, and sulphate of copper, when contained in so acid a menstruum, is alsc somewhat so. The gold is then dried, melted, and poured, as described before. This process affords gold as pure as f-ff^. The Preparation of Chemically Pure Gold.— The metal, either in the form of powder, granulations, thin plate, or "cornets" from the purest gold that can be obtained, is dissolved in chemically pure nitro- hydrochloric acid.^ The best material to operate on is gold which has been refined in the ordinary way; this may be used in the form of a powder, as it is precipitated in the last process, as granulations or as plate. The acid for small quantities is best contained in an evaporat- ing dish placed in a sand bath upon a tripod, over the flame of a Bunsen burner, beneath a chimney or near an open window. The action will be tolerably energetic when the metal is first introduced; hence, it is not necessary to ignite the burner at the start, but as the action slackens a moderate heat may be applied. Instead of previously mixing the acids, the hydrochloric acid may first be poured over the metal, and the nitric acid afterward gradually added in small portions, the function of the nitric acid being to oxidize the hydrogen of the hydrochloric acid, converting it into water, while 1 Ooe volume of nitric to two of hydrocbiloric acid (or any proportion, so the latter is in excess). 116 METALS AND ALLOYS USED L\ PROSTHETIC DENTISTRY. the chlorine, which is the active solvent, is Hl>erate(l in the nascent state and unites with the gold, converting it into auric chloride, which dissolves. Each ounce of gold will require about three and one-half ounces of mixed acid for its solution. During the processs of solution a sediment will be noticed in the bottom of the evaporating dish, which will be recognized by the operator as a silver chloride, formed by the union of the silver contained in the gold and the liberated chlorine. It must not be expected that all the silver will l)e directly precipitated to the bottom as a chloride, for the liquor is strongly acid, and some may be held in solution. Therefore, this must be taken into consideration, and subsequent pains taken to throw it down by the thorough evapora- tion of the nitric acid. The gold having been dissolved, the solution is now best transferred to a clean dish bv decantation, leaving the chloride of silver in the first and the solution contained in the second dish heated to further evaporate. When about one-third is evaporated more chloride of silver will be found to have been separated from the solution and precipitated. It is well, therefore, to again transfer the solution to a third dish by decantation and evaporate as before, care always being maintained during the heating not to apply so great a temperature as to decompose the auric salt which adheres to the sides of the dish above the fluid. As the bulk is reduced over the gentle heat by evaporation, small quantities of hydrochloric acid are to be added from time to time, which has the effect of liberating nitrous anhydride by decomposing the remain- ing nitric acid in the liquor; these additions must, however, be made very cautiously, for the action produced is very energetic, and, without due precaution, considerable portions of the now rich liquor will be thrown out of the dish and lost. When the liquor has become of a deep- red color, and of the consistency of syrup, it is to be withdrawn from the heat and permitted to rest for a time, when the whole of the auric chloride will crystallize, forming a mass of prismatic crystals. The bottom of the dish is now carefully wiped off to remove any sand or dirt that may have collected there from the sand-bath, and the dish and its contents immersed in about a half pint of distilled water, acidulated slightly with hydrochloric acid. It is better now to let this solution stand a week, for chloride of silver, although slightly soluble in a very strong and hot acid solution, is separated by dilution, and by allowing this rest, it will completely subside in the vessel. At the end of this time the solution must be filtered to remove any foreign substance, together with the silver chloride. The filtrate will then be seen to be a rich straw-yellow, and the gold it contains is ready for precipitation. Precipitating the Gold. — The solution is now best contained in a large glass flask, and the precipitating reagent added. As gold is one of those metals which, as a base, combines with very feeble affinities, it is consequently not only very easily separated, but the physical con- ditions of the precipitate may be much modified and controlled by the nature of the precipitant, as also by the mode of operating. Thus gold may be thrown down in a powder, in scales, in more or less of a crystalline state, in a tolerably compact sheet or foil, or, lastly, in a GOLD. 117 spongy condition, mucli rosonibling so-called "solila" or "moss (ihre." And these states may be attained with some degree of certainty, although the circumstances determining the more compact forms are more difficult. Spontaneous precipitation may take place to some extent in a vessel of trichloride of gold when exposed to the air; and thus the sides of the vessel containing it will slowly become covered with the deposit. This is probably due to the action of the nitrogen of the air. Many element- ary substances will precipitate gold from the trichloride. Most of the lower metals reduce it, some metallic salts throw it down, and many organic bodies readily precipitate it. Thus sugar when boiled in it gives at first a light-red precipitate, which afterward darkens in color. Practically, how^ever, ferrous sulphate or oxalic acid are the only precip- itants used. The oxalic acid is preferred, and is an excellent precipitant. The gold salt, being in solution, is broken up by the addition of a strong solution of oxalic acid, and the gold is precipitated to the bottom as either a crystalline mass or a leafy foil. It is necessary to add a slight excess, and the whole should be kept at a gentle heat in a sand bath over a flame. Soon after the application of heat some slight bub- bling is noticed, a copious evolution of gas takes place, and at the same time the body of the liquid appears filled with most delicate spangles of metallic gold, which become coherent as they descend, and in con- sequence assume most any one of the forms above mentioned. The gas seen to escape is COj, from the compound, oxalic acid. The reaction is of the simplest — an acid on a binary salt — 2AuC]3 + 3C2H2O4 = 6HC1 + 6CO2 -\- 2Au. The action of this pecipitant is gradual, and capable of much regulation, by the amount and nature of heat employed, while it is also peculiar in being attended throughout by this evolution of gas which rises quickly through the solution, there is produced from the former cause a tendency in the metal to deposit in a crystalline or crys- tallogranular state; while from the latter a more or less spongy char- acter is given to it; hence it will be readily seen that inasmuch as we are able to modify these conditions, so we can in the same degree influ- ence the molecular nature of the result. Where ferrous sulphate is used about four times the w^eight of the gold will be necessary for precipitation. This may be dissolved quickly in hot distilled water and added to the gold solution. The precipitate thrown down is of a brown color, and will, on being gently burnished with the finger-nail, assume that metaUic golden lustre characteristic of the metal. The following is the reaction — 2AUCI3 + 6FeS0^ = Fe^Clg + 2Ye^{S0^)i + 2Au. After the solution has fully subsided from the disturbance caused by addition and precipitation a quantity of hot hydrochloric acid may be added, and much of the supernatant liquor removed, either with a siphon or by decantation, and the remainder of the solution and pre- 118 METALS AND ALLOYS USED /.V PROSTIIKTIC DES'lUSTRY. cipitate poured ujxmi the filter pajxr. The precipitate is afterward washed with hvch-ochloric aeid, (Ustillcd water, a(|ua aiiiinonia, and a<:jain witli (hstilled water. The necessity of this is apparent, es{)eeially in the use of ferrous sulphate, as the precipitate will become more or less contaminated with the iron. In the use of oxalic acid this is to remove the cojij)er, as goV\ precipitated hy oxalic acid from an acid solution containing copj)er is always contaminated with cupric oxalate. It is then also advisable to heat the solution with a slight addition of potassium carbonate, a soluble double oxalate of copper and potas- sium is formed, and the gold is left in the pure state, (iold may also be precipitated from its acid solution in a state of purity in the form of brilliant spangles by means of hydrogen dioxide, thus — 2AUCI3 + SHjO, = 6HC1 + 60 + 2Au. When the precipitated gold has been carefully washed and rewashed with distilled water and the above-mentioned reagents, it may be dried and placed in a new crucible, previously boraxed, with some potassium carbonate and potassium nitrate, melted, and cast into an ingot. If iron ingot molds are used the gold should be washed after molding in hot hydrochloric acid to remove any trace of metallic or oxide of iron that may by chance have adhered to its surface during the process of casting the ingot. Properties. — Pure gold is of a rich, beautiful, yellow color, of strong metallic lustre, unalterable in air. It is the most ductile of all metals, but ranks onlv fifth in point of tenacity. One grain, however, if covered with a more tenacious metal, like silver, forming a composite wire, may be drawn into a wire 550 feet in length, and only ^-gVo" o^ ^^i inch in diameter. It is also the most malleable of all metals. One grain of it may be beaten into leaves so thin as to cover an area of 75 square inches, being of but ^ttoito ^^ ^" ^^^"^ ^" thickness. Very thin leaves of gold appear green in color by transmitted light; but when heated, the light transmitted is ruby-red. Gold possesses the property of welding cold. Thus, thin leaves, foil, and other forms of gold are more especially adapted to the use of the dentist as a filling material. The small particles are welded together in one perfectly homogeneous mass as the filling is inserted. The finely divided metal, such as that thrown down in the preparation of pure gold from the chloride solution, may be compressed between dies in the form of disks or medals. The pure metal fuses at 1100° C. or 2012° F., and its alloys at much lower temperatures. When heated much abo\e its melting point it slowly volatilizes and is readily dissipated in vapor by the oxyhydrogen flame. Pure gold is nearly as soft as lead, in consequence of which articles of jewelry, coin, etc., made from it are alloyed with copper, silver, platinum, etc., to give them the recpiisite hardness, durability, and elasticity. The specific gravity of gold cast in an ingot is 19.2G5; when stamped, 19.31; and that of the precipitated metal from 19.55 to 19.72. OOLD. 119 Graham has shown that gold is capal)le of occhidino; 0.48 of its voliimc of hy(h'oi2;cMi, and 0.2 of its vohime of nitrogen. Alloys.. — (lold very readily unites witli most of the metals, form- ing alloys of varied qualities. When in the pure state gold is too soft for any great use other than for filling teeth; consecjuently the greater (juantity of gold is alloyed with some metal that will increase its hard- ness and durability, without greatly impairing its more vahiahle cjuali- ties. The metals usually employed for this purpose are silver, platinum, and copper. Silver and gold are easily mixed together, but do not seem to form definite compounds. Such alloys are more fusible, more ductile, harder, more sonorous and elastic than gold, and are generally of a greenish- white color. One-twentieth of silver is sufficient to modify the color of gold. The alloys of gold and silver are known to jewelers as yellow, green, and pale gold, according to the content of silver. Copper and gold imite much more readily than silver and gold ; indeed, it is reasonable to believe from their behavior that a chemical com- bination is formed wdth 76 per cent, of gold and 24 per cent, of copper. Alloys of copper and gold are much harder, tougher, and more easily fused; less malleable and ductile, and greatly changed in color, being of a decidedly reddish tint, depending upon the proportion of copper with which the gold is debased. An alloy of gold 76, and copper 24, as referred to above, is distinctly crystalline and quite brittle; but a larger proportion of either gold or copper restores the malleability of the alloy. Standard Gold. — The standard alloy of most nations is one of copper and gold. Some contain small quantities of silver, but this is due to imperfect parting of silver and gold, or it may be contained in the copper used for the alloy. The proportion of copper to gold varies slightly in different countries, and such proportions are stated in thousandths; thus, pure gold is one thousand (1000) fine. The following table gives the composition of standard gold, as fixed by the nations mentioned : Nation. Gold. Copper. United States . France .... Germany Belgium Italy .... Switzerland Spain .... Greece . . . . China .... Austrian Crowns Great Britain . Ducats, Hungarian . Ducats, Austrian Ducats, Dutch . The first United States gold coins were ten-dollar pieces, coined in 1795; they weighed 270 grains each, and were of 916.666 (22-carat) fineness. Their weight was reduced in 1S34 to 258 grains, with 899.225 (21.581-carat) fineness; and in 1837 the present standard of 900 (21.599- carat) fineness was established. 900 100 (Carat 21.6—) 916 84 989 11 986 14 982 IS 120 METALS AM) ALLOYS I'Shl) L\ PROSTHETIC DENTISTRY. Alloys of golfl with copper, or with silver, or with hoth, are much used in the manufacture of jewelry. When the gold contains copjxr only it is termed red gold; when, silver only, white gold ; if the gold con- tains both metals, the caratation is termed mixed. In many countries a legal standard of fineness is fixed for gold ornaments and jewelry. In England gold is stamped, or Hall Marked, 10, IS, and 22-carat; in France, 18, 20, and 22-carat; in Germany, 8, 14, and 18-carat, and, also, under the term joujou gold, a G-carat gold used for electroplated jewelry. The purpose of the stamping is to protect the purc-haser, who is enabled to know the carat of the gold he is buying. The following alloys used by jewelers are also of much interest to the dentist : Table of Mixed Caratatiox. — Brannt. Parts. Carats. 23 22 20 18 15 13 12 10 9 Gold. .Silver. Copper. 23 1 h 22 1 1 20 2 2 18 3 3 15 3 6 13 3 8 12 3h 8J 10 4 10 9 4J 10* 8 5i m 7 8 9 Colored Golds. - -Brannt. Parts. Color. Gold. Silver. Copper. Steel. Cadmium. 2 to 6 1.0 Green. 75.0 16.6 8.4 (( 74.6. 11.4 9.7 4.3 (( 75.0 12.5 12.5 « 1.0 2.0 Pale yellow. 4.0 3.0 1.0 Dark " 14.7 7.0 6.0 a tt 3.0 1.0 1.0 Pale red. 10.0 1.0 4.0 " " 1.0 1.0 Dark red. 30.0 3.0 2. D Grav. 1 to 3 1. 3 Blue. Higher Carat Colorei J Gold.s. Parts. Color. Carat. Gold. Silver. Copper. 15 dwt. . 2d wt. 18 grs. 2 dwt. 6 grs Yellow tint. 18 K. 15 " 1 " 18 " 3 " 6 " Red " 18 K. 1 oz. 16 dwt. 6 11 12 " Reddish spring gold. 16 K. 1 " 7 11 5 " Yellow tint 16 K. 1 " 2 " 8 " Re d " 16 K. Jewelers usually make their solders from the gold upon which they are to be used by the addition of small cjuantities of copper, silver, oi GOLD. 121 brass, the latter greatly increasing the fusibility and fluidity. The following are examples: Jewelers' Soldeks. For 18-carat gold. For 16-carat gold. 18-carat gold 1 dwt. 16-carat gold . . 1 dwt Silver 2grs, Silver . . .10 grs. Copper Igr. Copper . . . 8 " Carat. — The fineness of gold is also expressed in carats, a twenty- fourth part, formerly the twenty-fourth part in weight of a gold marc. It is now assumed that there are 24 carats in unity; whether the unit be one pound, one ounce, or one pennyweight, it is divisible into 24 equal parts, and each of these parts is called a carat to express fineness. If a quantity of gold is chemically pure, in other words contains no alloying elements, it is, as we have previously explained, 1000 fine; or as each -^ part is gold, it is said to be of 24-carat fineness. If. however, 2 carats, or -^ of the imit quantity are composed of one or more alloying metals, the gold is said to be 22 carats fine; or if 6 carats or 2^^ of the alloy is debasing metal, the carat is 18 fine, etc. The following table shows the equivalent of each carat in thou- sandths : Carats. 1 2 3 4 5 6 7 8 9 10 11 12 Thousandths. Carats 41.667 13 83.334 14 125.001 15 166.667 16 208.333 17 250.000 18 291.666 19 333.333 20 374.999 21 416.667 22 458.630 23 500.000 24 Thousandths. 541.667 583.333 624.555 666.667 707.333 750.000 791.666 833.333 874.999 916.666 958.333 1000.000 Gold Plate. — Pure gold is rarely employed in the dental laboratory, except for soldering continuous-gum cases, and in some branches of crown and bridge-work. Its extreme softness and flexibility make alloying absolutely necessary. The latter must be accomplished, how- ever, without practically impairing either its malleability, or pliancy, and at the same time endow it with that degree of hardness, elasticity and strength necessary to resist the stress and wear to which an artificial denture is exposed in the mouth. Copper and silver are much used to debase or alloy pure gold. It is questionable, however, whether copper should be used as almost universally as it is; indeed, some regard it as exceedingly objectionable. A plate made from a gold alloy containing a large percentage of copper is more easily tarnished, and has a disagreeable metallic taste. Silver exercises a very benign influence over copper contained in gold plate, controlling the tendency to the disagreeable redness. Equal parts of silver and copper have little or no effect upon thig color of gold. 122 METALS AXn ALLOYS VSF.D IS PROSTHETIC DEXTISTRY. Silver assists in iinjnirtiiig Iiurdiiess, elasticity, and durability to the alloy, without so far debasing it as copper alone. Platinum and silver are sometimes used to endow pure jujoid with the qualities necessary for a dental base; but the labor of swaging is very greatly increased when platiinmi is contained in the ])late. In order to secure the best results, alloys intended for plate should not be less than 18-oarat in fineness, and the alloy should contain as little copper as possible. The following are some of the formnhe in use for the preparation of allovs for dental bases: Number Carat. Parts (if Formula. Gold. Silver. Copper. riatinum. 1' 18 18 dwts. 2 dwts. 4 dwts. 2 18 18 " 3 " 3 " 3 18 18 " 4 " 1 dwt. 1 dwt. 41 19 19 " 2 " 3 dwts. 5 19 19 " 3 " 1 dwt. 1 " 6' 20 20 " 2 " 2 dwts. 7' 21 21 '• 1 dwt. 2 " 8' 22 22 " 18 grs. 1 dwt. 6 grs. 9^ 18 Ml dwts. ($60). 13 dwts. 10' 18 20 " 2 dwts. 2 dwts. IP 18 510 grs. ($20). 96.45 grs. (25c. coin). 12' 19 20 dwts. 40 + grs. 25 grs. 13' 20 20 " 20 + " IS " 143 20 516 srs. ($20) 10c. coiii. 15' 21 20 dwts. 13 + grs. 16' 21 20 " 6 " 7? grs. Clasp Gold. — Gold for clasps, elastic wires, backings, stays, posts, pivots, etc., usually contain a small amount of platinum to give it greater strength and elasticity. The following formnlcTe are recom- mended by. Professor Chapin A. Harris: No. 1— 20-Carat. No. 2— 20-Carat. Pure gold 20 dwts. Coin gold . 20 dwts. " copper . . 2 " Pure copper . . 8 grs " silver . . 1 dwt. " silver . . 10 " " Platinum . 1 " " platinum . . 20 " A content of ))latinum in gold renders the alloy more liable to oxida- tion, and, says Professor Harris, "This effect is so marked that such an alloy is readily acted upon by nitric acid." It is not probable, how- ever, that the small amoimt contained in clasp gold would affect its integrity. Crown Gold. — Gold for crowns should combine strength with good color. Those alloys of a large copper content make exceedingly un- sightly crowns on account of their deep-red color. Professor C. L. Goddard recommends the following for alloys the color of pure gold: No.l— 21.6-Carat. No. 2— 21.6-Carat. Pure gold . 90 parts. Coin gold . ' . 50 parts. " silver 5 " Pure "... 45 " " copper . 5 " " silver . 5 " ' Richardson's Mechanical Dentistry, p. 56. 2 Johnson Bros. * Professor C. L. Goddard. GOLD. 123 Gold Solders. — These are usually alloys of gold, silver, eop))er, and zinc, and are designed to be a trifle more fusible than the pai-ts to be soldered; this property is conferred upon them princij)ally by the con- tent of zinc (or brass). They should also possess considerable strength; too much base metal, therefore, should not be added, as it will, by oxidizing, tend to very materially weaken the alloys. Their carat should be as high or nearly as high as that of the plate, and their color as nearly as possible the same. The following formuhie have yielded satisfactory results as gold solders : Parts. No. Carat. , ^ U. S. Coin. Pure Pure Pure Pure Spelter Gold. Gold. Silver. Copper. Zinc. Bras.s. Solder.* 1 14 SIO. 4 dwts. 2 dwts. 2 14 16 dwts 5 " {iig;; } 3 15 6 " 30 grs. 20 " 10 grs. 4 16 .... 11 dwts. {3 dwts.} I 2 dwts.} 5 16+ .... {}1^;;. } 3 dwts. {,\%\-} 12 grs. 6 IS 30 parts. 4 parts 1 part. .... 1 part. 7 18 .... 27 parts. 4 " 4 parts. .... 1 " 8 20 110. 20.64 grs. 9 20 5 dwts. 12 grs. 6 grs. 6 ( (18 K. gold plate for-) 10 14 <^ mulaNo. 9.) 20dwts. [- 2.5 dwts. 20 grs. 35 grs. ( Johnson Bros. J A simple method for making a good solder suitable for the plate upon which it is to be used is : 5 parts of the plate and 1 of brass or of silver solder. In the case of coin gold, or the crown alloy given above, a solder thus made will be exactly IS carat.^ Rules for Computing and Compounding Gold Alloys^ and Examples.^ PART I. To ascertain the carat of any given alloy, the proportion may be expressed as follows: As the weight of the alloyed mass is to the weight of gold it contains, so is 24 to the standard sought. Example. — Gold 6 parts, silver 2 parts, copper 1 part, total 9 parts. 9:6::24:? 6 9)144 16 Answer. x^nother method when alloyed gold is used in forming the mass, instead of pure gold, is to express the proportion as follows — As the weight of the alloyed mass is to the weight of the gold alloy used in its composition, so is the carat of the latter to the carat of the former — 1 Composed of equal parts copper and zinc. - Professor C. L. Goddard. * Rules by Professor George Watt. * Examples by Professor C. L. Goddard. 124 METALS AM) ALLOYS USED IN PROSTHETIC DENTISTRY. Example. — Harris No. 1 solder: 22-carat gold ....... 48 parts. Copper . . . . . 16 " Silver 12 " Total 76 " 76 : 48 : : 22 : carat. Ans. 13.9 carat. PART II. To reduce pure gold to any required carat, the proportion may be expressed as follows: As the required carat is to 24, so is the weight of gold used to the weight of the alloyed mass when reduced. The weight of gold subtracted from this gives the quantity of alloy to be added. Example. — Reduce 6 ouncesof pure gold to 16-carat, 16:24::6 ounces: 9ounces. 9 — 6 = 3 ounces alloy to be added. To reduce gold from a higher carat to a lower carat, the proportion may be expressed as follow^s: As the required carat is to the carat used, so is the weight of the mass used to the weight of the alloyed mass when reduced. The weight of the mass used, subtracted from this, gives the quantity of alloy to be added. Example. — Reduce 4 ounces of 20-carat-gold to 16 carat: 16:20::4 ounces:? 16)80 5 ounces 5 ounces — 4 ounces = 1 ounce alloy to be added. PART III. To change gold from a lower to a higher carat, add pure gold or a finer alloy. As the alloy in the required carat is to the alloy in the given carat, so is the weight of the alloyed gold used to the weight of the changed alloy required. The weight of the alloyed gold used siibtracted from this gives the amount of pure gold to be added. Example. — Change 1 pennyweight of 16-carat gold to IS-carat. First subtract 16 and 18 from 24 to find the amount of alloy in each carat. 24 24 6 8 :: 1 pennyweight:? J 6)8^ IJ pennyweights. IJ — 1 ^ § pennyweight of pure gold to be added. To change gold from a lower carat to a higher carat, by adding gold of a still higher carat. Substract the lower carat and the required carat each from the highest carat (instead of from 24) and proceed as before. GOLD. 125 Example. — Change 2 pennyweights of 16-carat gold to IS carat, by adding 22-carat gold. First subtract 16 and IS from 22. 22 22 IS 16 4 6:2 pennyweights : 3 pennyweights. 3 — 2 = 1 pennyweight of 22-carat gold to be added. Tests for Gold in Solution. — Sulphuretted hydrogen or ammo- nium hydrosulphide throws down a brown precipitate of auric sul- phide (AujSg). The second precipitant is not used, however, as the precipitate is soluble in it, as it is also in the alkaline sulphides. Auric sulphide is insoluble in nitric or hydrochloric acid taken separately, but soluble in aqua regia. Ferrous sulphate and oxalic acid precipitate the gold in the metallic state; it is a browTi powder, darker in the instance of the former than the latter, but develops the color and lustre of gold by being burnished with the finger-nail or instrument. Stannous and stannic chloride give probably the most delicate test for gold by the formation of the purple of Cassius. If" the precipitate formed in the experiment above be dried and heated on charcoal a metallic globule results. Gold is reduced from many of its compounds by sunlight, and from all of them by more or less heat. Electrodeposition of Gold. By Simple Immersion.— From an acid solution of gold chloride, the base metals, and silver, platinum, and palladium deposit gold in the metallic state. In the double cyanide of gold and potassium zinc will quickly become gilded; copper, brass, and German silver, slowly, and antimony, bismuth, tin, lead, iron, nickel, silver, gold, and platinum not at all. Deposition by a Separate Current. The Solution. — There are many solutions prepared for electro-gilding, some being formed by chemical means, others by a separate current from the battery; but whether they are made by chemical or electric process, the best for a thick reguline deposit is the pure double cyanide of gold and potassium. A cyanide solution may be prepared as follows: Dissolve 120 grains of pure gold in one ounce of chemically pure aqua regia, thus preparing the chloride of gold as described pre- viously.^ Dissolve the chloride obtained in 32 ounces of warm dis- tilled water and add to it 1^- ounces of magnesia; the gold is precipi- tated. Filter and wash with pure distilled water; digest the precipitate in 10 parts of distilled water mixed with 0.75 part of nitric acid to remove magnesia; then wash the remaining oxide of gold with distilled "water until the wash water exhibits no acid reaction with test paper. Next dissolve 3 ounces of ferrocyanide of potassium and 6 drachms of caustic potash in 34 ounces of distilled water, add the oxide of gold prepared, and boil the solution about twenty minutes. When the gold is dissolved there remains a small amount of iron precipitated, which 1 Preparation of Chemically Pure Gold. 126 MFTALS AND ALLOYS USED L\ PROSTHETIC DESTIsritV. may be removed by filtering the .solution. The lifjuid, a fine, elear, golden eolor, is then ready for use, to be employed either hot or cold, but a better and quicker deposit is nearly always obtained from the warm solution. In electroplating objects the first essential is a finished surface, which must be made just as it is desired to be when completed. The next is cleanliness. If it be a silver denture or any other metallic object it should first be cleaned of all surface combinations, as oxides, sulphides, etc., by polishing in the ordinary way; then scrubbed with a solution of hot water and soap by means of a brass or steel scratch brush on the lathe; then washed or boiled in a strong solution of caustic potash, afterward washing in distilled water, and finally in an acidulated water to remove all traces of the alkali. The apparatus is exceedingly simple, consisting of a single battery cell and a glass bowl (preferably of perpendicular sides) to contain the solution. The latter may or may not be adjusted in a water bath, according to whether the operator desires to work his solution hot or cold. Aside from these, connecting and guiding wires, cathode and anode hooks, together with an anode, a thermometer, a scratch brush, etc., are all that will be needed. The article to be plated is suspended by a hook in the solution from the cathode, while a piece of pure gold is hung from the anode to keep up the strength of the solution, the latter electrode being easily determined by the fact that gas is liberated there by the passage of the current through the solution. When a sufficient coating has been formed the object is to be removed from the bath and burnished by the scratch brush or agate biu'uisher moistened with a solution of warm water and soap, until the surface is finished as desired. SILVER. Argent um. Symbol, Ag. Atomic weight, 107.12. Malleability, second rank. Melting point, 1040° (1904° F.). Tenacity, fourth rank. Dvu'tility, second rank. Specific gravity, 10.53. Conductivity (heat), 100. Conductivity (electricity). 100. Occurrence. — Silver is widely diffused throughout the earth's crust. It is found chiefly in the United States, iSIexico, Peru, and Chile; Aus- tria, Hungary, Norway, and Australia also furnish considerable amounts. Of the varieties of silver ores the following chiefly are metallurgically important: (1) reguline silver, (2) horn silver, (3) silver glance, (4) silver-copper glance, (5) pyrargyrite, (6) stephanite, and (7) polybasite. Silver is also frequently met with in base metallic ores, as in lead ores and many kinds of pyrites. Chemically Pure Silver. — Small fpiantities of this may be easily prepared in the laboratory by dissolving conmiercial or coin silver in pure dilute (50 per cent.) nitric acid contained in a Florence fla.sk, hastening the action by gentle heating over a sand bath. After the silver has been dissolved, and the solution somewhat cooled, add an equal bulk of distilled water, and filter into a second flask. To the SILVER. 127 filtrate add a saturated solution of sodium chloride (common salt) until no more white precipitate of silver chloride is formed — AgNOs + NaCl = AgCl + NaNOs. The flask should then be stopped and shaken for several minutes when, on being allowed to rest, the chloride will quickly fall to the bottom, leaving a clear, supernatant liquid above, which, if copj)er be present, will be colored a bluish-green. If to this clear supernatant liquid the salt solution be added, the operator is enabled to determine instantly whether all of the silver has been thrown down as the chloride, or not. If so, the clear liquid is decanted off and the chloride washed until the wash water does not assume the slightest tinge of blue upon the addition of ammonia. The chloride is now best transferred to a beaker, or some other wide-mouthed vessel, and about twice its bulk of water, acidulated with about 10 per cent, of sulphuric acid, added. Several small pieces of iron in some form, preferably lath-nails, may now be added to the mixture, and the whole stirred with the closed end of a test- tube. The following reactions then take place, during which ferrous sulphate and hydrochloric acid are formed and silver liberated, thus — Fe + H2SO4 = FeSO, + H^, and 2H + 2AgCl = 2HC1 + 2Ag. The completion of the reaction is recognized by the changing of the precipitated mass from white to a dark gray, which is the color of the finely divided silver. The small pieces of iron are now removed, the precipitated silver w^ashed and rewashed with dilute hydrochloric acid, then with distilled water, dried, mixed with about an equal bulk of potassium carbonate, and melted in a well-boraxed crucible. Properties. — Silver is the whitest of metals, very brilliant, tenacious, malleable, and ductile, in the last two qualities being inferior only to gold; if considered weight for weight, it is superior to gold, for while one grain of gold may be beaten so thin as to cover an area of 75 square inches, a grain of silver may be made to cover 98 square inches, though the foil of the former is much thinner than that of the latter. The extent of the malleability of gold and silver has never been absolutely deter- mined, as the means employed have invariably failed before the prop- erty in either was exhausted. In tenacity silver is superior to gold. It is also harder than gold, but softer than copper, and is the best-known conductor of heat and electricity. It fuses at 1040° (1904° F.), far below the fusing point of either gold or copper. It volatilizes appre- ciably at full red heat; in the oxyhydrogen flame it boils, with the formation of a blue vapor. The fused metal readily absorbs oxygen gas (when used under potassium nitrate it takes up as much as twenty times its volume). As the metal cools the oxygen escaping through the semisolid crust on the surface of the fused mass produces very beauti- ful effects. Pure silver retains a trace of the absorbed oxygen per- manently. It is unaltered in the air at any temperature, but is readily acted upon by sulphur, phosphorus, or chlorine. Nitric acid is the proper solvent for silver, and is most efficient when diluted about 50 per cent., but active whether concentrated or dilute. 128 METALS AND ALLOYS USED L\ PROSTHETIC DENTISTRY. with the profhiction of nitric oxide (X/)-..) and silver nitrate (AgXOg). Sulphuric acid, hot and concentrated, acts upon silver, forming a sul- phate which is sparingly soluble. Hydrochloric acid, hot and concen- trated, forms argentic chloride. Fused alkaline hydrates or nitre are without action upon silver; hence it is usetl for the manufacture of crucil)lcs for the fusion of caustic alkalis, etc. Alloys. — Pure silver is too soft for coinage or comnicrcial purposes; it is, therefore, alloyed variously for its different usages to increase its hardness. Gold. — Formerly silver was much used to alloy gold. The metals are easily mixed together, but do not appear to form definite c-ompounds. With certain proportions of the metals the resulting alloys are more duc- tile, harder, more sonorous and elastic than either metal considered singly. Copper. — The alloys of copper and silver are the most useful of the alloys of silver. In most countries the silver coins are made of these two metals. In the United States the silver for coinage is alloyed with 10 per cent, copper, the proportion of each being stated in the thou- sandths; thus, pure silver being 1000 fine, the coin or "standard silver" is 900 fine, with 100 parts copper added. The German and French silver coins are of the same grade, those of Great Britain are 925 fine, with 75 parts of copper added, being known as "sterling" silver. Most silverware is of "sterling" fineness. The presence of copper does not modify the color of silver so long as the proportion of the former does not exceed 40 or 50 per cent. Copper imparts to silver greater hard- ness, tenacity, and strength. Comparison of the silver dollar of the United States with that of Mexico : United States Mexican dollar. dollar. Pure silver 371.25 grs. 377.14 grs. " copper 41.2-5 " 40.65 " Total weight .... 412.50 " 417.79 " The Mexican dollar weiglis 0.866 of a Troy ounce. Zinc and silver have a great affinity for each other, and are conse- quently readily alloyed. Silver solder for soldering the metal is usually composed of an alloy with copper and zinc. The following are well adapted for the porpo.se: No. l.> No. 2.2 Silver . 66 parts. Silver 6 parts, Copper . . 30 " Copper . 2 " Zinc . 10 " Brass 1 part. "When the material to be united is composed of pure silver and plat- inum, silver coin alloyed with one-tenth zinc may be used as a solder." So-called "Standard" silver is also an excellent solder for high fusing brass and German silver. If the article is to be soldered tw^ice, this may be used first and the silver solder afterward.' ' Richardson'.* Mechanical Dentistry, p. 78. "- Ibid. » Professor C. L. Goddard. SILVER. 129 Dr. Kirk' recommends the following compositions: Fine silver. Copper Brass. Zinc. 4.0 3.0 . 2.0 1.0 19.0 V.6 10.0 5.6 66.7 23.3 10.0 50.0 33.4 16.6 11.0 4.0 V.o These may be used for soldering the surfaces of standard silver. Electrodeposition of Silver. — Silver is the most important and prominent metal in electroplating processes. The solution generally used is the cyanide, and it may be prepared by either of two methods — the battery or the chemical process. The method of procedure in the former is simple, when thoroughly understood. First, the percentage of actual cyanide in the cyanide salt used must be ascertained. If it contains about 50 per cent., dis- solve each ounce in about one quart of distilled water; or if it contains more, add more water and vice- versa in proportion. Suspend a large anode and a small cathode of silver in the liquid, and pass a strong current of electricity through until the required amount of metal is dis- solved from the anode. As this process produces some caustic potash in the liquid, some of the strongest hydrocyanic acid may now be added to form this into the cyanide, and more of the anode dissolved in the mixture by the battery. Solutions for deposit are made by the chemical process as follows: Take four parts of pure grain silver and reduce it to argentum nitrate by mixing with nitric acid. Dissolve this in distilled water, in the proportion of one quart to every one-half ounce of silver used. At the same time make a solution of from two to three parts of cyanide of potassium in twenty or thirty parts of distilled water. This is to be added gradually to the solution of nitrate of silver as long as it produces a white precipitate. If too much be added, however, it will cause some of the precipitate to be redissolved and wasted. In such a case the liquid should be stirred and then allowed to settle clear. A small amount of nitrate of silver dissolved in distilled water should be added as long as it produces a white cloud. This may be better observed by using a separate glass vessel to see the precipitate as it dissolves. The liquid should now be left to settle until quite clear, and the clear portion then decanted, and the precipitate washed four or five times in a large quantity of water by simply adding the water, stirring, and allowing it to settle again, and decanting as before. Next dissolve from six to eight parts of cyanide of potassium in twenty parts of distilled water, adding it a portion at a time, with free stirring, to the w^et cyanide of silver, until the whole is barely dissolved; then add about three parts more of cyanide of potassium to form free cyanide, and sufficient dis- tilled water to reduce the whole to the proportion of about one-quarter of an ounce of silver to the quart; finally, when all the free cyanide is 1 American System of Dentistry, vol. iii. p. S79. loO MKTALS AND ALLOYS USED IS riKJSTUKTlf DENTISTRY. dissolved, filter the solution uiid it is ready for use. The speeifie gravity of the solution should be maintained at between l.S and 1.15. Deposit solutions are very numerous, V)ut, in the author's judgment, the above is best adapted for a good, reguline solid deposit. Knowledge of the management of solutions is essential. There are varying cireumstances which must be noted in order to keep them in good condition for a reguline deposit. New solutions do not work a> well, usually, as old ones, provided the latter are not too old. Solutions of two or three years of age work probably the best. They change from many causes; they become dirty and concentrated from exposure; increase or decrease in their relative proportions of cyanide and metal; they acquire other metals in solution, dissolved from the anode and corroded from the cathode; plaster and plumbago accumulate in them, in consequence of which they should be filtered ; they grad- ually decompose, become brown, discolored, and evolve ammonia by exposure to light, especially if they contain too much free cyanide; therefore, all these deviations from the proper condition should be cor- rected. The specific gravity should be maintained, and the proper amount of metal and cyanide kept in solution. To determine any dis- proportion in the latter, place 25 grams of the solution in a test-tube of proper size, and add to it, at first freely, and afterward gradually, at last, drop by drop, with constant stirring, a solution of 1 gram of crystallized nitrate of silver in 10 grams of distilled water. If the precipitate formed is dissolved rapidly, with but little need of stir- ring, there is too much cyanide. If, however, it does not dissolve, even after much stirring, there is too little cyanide; but if it wholly dissolves (the latter part (juite slowly) the proportion of silver to cyanide is about correct. Many other minor troubles not mentioned are encountered, which must be corrected by means gathered only from experience in working the process. The processs for making dental bases by electro-deposition on the plaster cast of the mouth was patented February 5, 1889, by Joseph G. Ward, of Irvington, X. J. The author has had some experience in the work; in fact, was engaged in perfecting a process for the same result when ]Mr. Ward secured his patent. The method of proceedure in the preparation of a dental base is as follows: A true impression of the mouth is secured, and from this a cast is obtained by casting in the usual manner. After the cast ha.s become thoroughly dry it should be soaked in hot fluid paraffin, until saturated, and before cooling the surface is wiped clean of all superflous adhesions which might in any way destroy the exactness of the surface. The cast is then coated freely, A\here the deposit is desired with a mixture of ecjual parts of pure, finely ])ulverized plumbago and the finest tin-bronze powder or any other conducting substance which would be suitable for the purpose. This is applied with a thick, short-haired camel's- hair pencil. The cast is now so wired that perfect connection is made with the palatal, buccal, and labial surfaces. From these guiding PLATINUM. 131 wires a cathode hook suspends the cast in the solution. After the metal has been deposited to a sufficient thickness, the cast, with its deposit, is to be taken from the bath, the deposit removed from the cast, trimmed and polished; but if it is desired to have the plate of increased thickness at any part to give the appearance of a turned rim, etc., the cast, with the deposit adhering to it, may be removed from the bath, and all the exposed surface of the deposit, except the portions to be thickened, may be covered with a coating of wax or some other non-conducting substance, and re-submerged in the bath and left there until the required thickness of deposit is secured in the parts desired. It may then be taken from the bath, burnished, trimmed by scraping, burring, and filing to the proper shape and thickness, then polished, and spurred. A thick plating of gold should now be added to the properly shaped plate, or the rubber for the attachment of the teeth will not harden and adhere to the plate during the process of vulcanization (the sulphur of the vulcanite combining with the silver). After the teeth have been attached and the vulcanite and all properly finished, a second coating of gold should be electroplated over it all to cover portions that had been made bare in finishing the vulcanite. The denture may be made by depositing the metal directly on the teeth as in cheoplasticwork, and, where necessary, clasps maybe formed. Broken dentures have been soldered with 18-carat gold solder. Crown and bridge work may also be made in various ways by this process. PLATINUM. Platinum. Symbol, Pt. Atomic weight, 193.3 Malleability, sixth rank. Melting point, oxyhj'drogen flame, Tenacity, third rank. 1 770° C. Specific gravity, 21.46. Ductility, third rank. Conductivity (electricity), 18.8. Conductivity (heat), 8.4. (Silver being 100.) Occurrence. — ^The ore of platinum, " polyxene," which is a most complex mixture of a number of heavy reguline species of platinum, osmiridium, iron-platinum, platin-iridium, iridium, palladium, gold, and a number of non-metallic species, notably chrome-iron ore, mag- netic iron oxide, zircone, corundum, and occasionally also diamond, is found in the province of Choco, South America, wdiere it was first discovered in 1736, in New Granada, Barbacos, California, and Aus- tralia, but chiefly in alluvial deposits in the Ural district in Russia. Fusing- Platinum. — By means of the oxygen blowpipe the spongy platinum is easily reduced to a compact mass, a result formerly only fairly well obtained by the very tedious and laborious means of welding. The furnace for fusing platinum (^Fig. 114) is at once a cupel aiid furnace, consisting of two thoroughly burned lime blocks with a Ixisin- like concavity in each, and fitted one over the other. The concavity in the lower block forms the bed of the furnace, and is provided with a gutter leading from the basin to the outside. Through the top are 132 METALS AND ALLOYS USED IS PROSTHETIC DENTISTRY. passed the oxyhydrogen blowpipes. These each consist of two con- centric tubes, a small inner and a larger outer tube. Through the outer, or larger, tube the hydrogen or illuminating- gas is passed, and through the inner, or smaller, the oxygen is forced into the centre of the flame. The tubes are of copper, tipped with platimnn. Platinum scraps are melted by first heating up the furnace and then introducing them through an opening in the side. In the case of plat- inum sponge the mass is introduced before heating up the furnace, and it is here that the furnace acts as a cupel; the impurities remaining in the metal are oxidized and volatilized or absorbed by the lime of the furnace. The temperature produced is supposed to be about 2870° C. Osmium does not melt at this point, but if present it is volatilized with palladium and gold. Fig. 114. c^;^-^ Furnace for fusing platinum. Properties. — The metal is bluish silver-white, about as soft as pure copper, and has a specific gravity of 21.46. It is tough, ductile, and malleable, and may be rolled or iSeaten into foil or drawn into wire of almost microscopic fineness. Dr. Arendt states that a cylinder of platinum one inch in diameter and five inches long may be drawn into a wire sufficiently long to encircle the earth at the ecjuator. The fine wire used in the micrometer eye-piece of microscopes suggested by Wollaston is made by drawing a composite wire of platimnn coated with silver to its greatest attenuation, and then dissolving off the silver in nitric acid. Platinum possesses tenacity in a high degree, being slightly inferior to iron and copper. The fusing point, according to Violle, is 1779° C. When heated much above its fusing point, it soon begins to volatilize. The fused metal, like silver, absorbs oxygen, and consequently "spits" on freezing. At a red heat it "occludes" hydrogen gas. The volume of hydrogen absorbed by a unit volume of PLATINUM. 133 metal at a red heat, under one atmosphere's pressure, was found, in the case of the fused metal, to vary from 0.13 to 0.21, volume measured cold; in the case of merely welded metal from 2.34 to 3.8 volumes. Oxygen, though absorbed by the liquid, is not occluded by the solid metal at any temperature, but when brought in contact with it at moderate temperatures, suflFers considerable condensation at its sur- face, and in such a state exhibits a high degree of chemical affinity. When a jet of hydrogen gas strikes a layer of spongy platinum it causes it to glow and the gas takes fire. The most striking example of the property of the metal for absorb- ing gases is demonstrated in the finely divided state known as "plat- inum black." This state is produced by dropping platinum chloride solution into a boiling mixture of 3 parts of glycerin and 2 of caustic potash. Platinum black is said to absorb 800 times its volume of oxygen from the air, and is, therefore, a most active oxidizing agent, acting catalytically — i. e., after having given up its oxygen to the oxi- dizable substance it takes up a fresh supply from the atmosphere. Platinum is not dissolved by any single acid, its proper solvent being nitrohydrochloric acid. The caustic alkalis, the alkaline earths, nitrates and cyanides, and especially the hydrates of barium and lithium, attack platinum at a red heat, although the alkaline carbonates have no effect at the highest temperatures. Sulphur, in the absence of alkalis, has no action, but phosphorus and arsenic attack platinum when heated with it. Direct contact of platinum with burning charcoal should be avoided, since the silicon reduced from the charcoal ash unites with platinum, making it brittle and liable to fracture. Heating platinum with spirit lamps is preferable to the use of ordinary gas. When gas is used, care should be taken to have the supply of air sufficient to ensure complete combustion, since, with the flame con- taining free carbon, the platinum suffers deterioration by the forma- tion of a carbide of platinum. For this reason, also, the inner cone or reducing flame should not come in contact with the metal. Platinum was introduced in France as early as 1820 for a base in continuous-gum work. Its low rate of expansibility under increased temperature, its coefficient being about equal to that of glass, and its very high fusing point make it most useful as a base for porcelain work, and for pins for artificial teeth. Its comparatively great resistance to chemical agents ensures it against corrosive action, and places it on an equality, in this particular, with gold for dental bases, crowns, and bridge work. Coils of platinum wire are useful in dental offices in various forms of electric heating devices. The heat is free from products of combus- tion and can be most accurately controlled. A device of this nature is especially valuable in annealing gold. Alloys. — Platinum alloys with most of the metals. With mercury spongy platinum unites to form an exceedingly unctions amalgam. It does not unite readily, and its union is best accomplished by con- tinuous rubbing: in a warm mortar. l.']4 Mh'TALS AXD ALLOYS USED J.\ rnosTIIKTIC DF.STLSTRY. Iridium from 10 to If) per cent, added to plntimim greatly increases its hardness, elasticity, infusihility, and resistance to chemical action. Platinum alloyed with iridium can he made very useful in dentistry to strengthen weak parts of {)artial continuous-gum, partial vulcanite dentures, and crown and bridge work. An alloy of 7S.7 platinum and 21.3 iridium will withstand the action of acjua regia. E(jual parts of the metals form a very l)rittle allov. (iold and })hitinum form an alloy of great value for the construction of dental bases. Platinum gives to gold a greater hardness and elas- ticity. Two parts to one of gold forms a brittle alloy, while with equal parts the alloy is malleable. Prinsep found that 7 parts of gon the extent to which the di\ision is carried. It is prepared in a wet way l)y intimately mixing 100 parts of mer- cury with 38 parts of the flowers of sulphur, and the l)lack sulphid of mercury thus obtained digested, with constant agitation, in a solu- tion of 25 parts of caustic potash in 150 parts of water at 45° C. (the water lost by evaporation being constantly replaced), until the pre- MERCURY. 137 paratioii has come up to its maximum of fire and brilliancy, which takes a good many hours. Purely sublimed vermilion has a compara- tively dull color, and must be manipulated with an alkaline (jjotassium) sulphide solution to give it the necessary fire. The action of the alkaline sulphide consists probably in this, that it dissolves successive installments of the amorphous preparation and redeposits them in the crystalline form. Properties. — It is a fine, bright-scarlet powder, permanent in air, odorless and tasteless, insoluble in water, alcohol, dikite nitric, con- centrated hydrochloric, or sulphuric acids. Nor is it acted upon by boiling potassium hydrate, sulphide of ammonium, cyanide of potas- sium, or sulphite of soda. It is slightly acted upon by concentrated hot nitric acid, and completely soluble in a solution of potassium sul- phide in the presence of free alkali or a solution of sodium sulphide. Nitro-hydrochloric acid decomposes it into mercuric chloride, which is readily soluble. It may be completely sublimed, as has been seen, without decomposition, but if exposed to a temperature of 315.5° C. (600° F.) it is decomposed into metallic mercurv and sulphur dioxide. It is frequently adulterated with red lead, dragon's blood, chalk, ferric oxide, realgar (As^S^), and brickdust. If lead be present it will yield a yellow precipitate when digested with acetic acid and potassium iodide added. Dragon's blood may be detected by alcohol, which will take up the coloring matter of that substance if present. Chalk is detected by an effervescence on the addition of an acid. INIost other impurities may be detected by subliming a small portion of the com- pound. The non-volatile substances used for adulteration will remain behind. Uses. — When pure it is much used as a pigment, on account of its brilliancy and color. Its unalterableness and resistance to chemical action render it particularly valuable in giving the red color to vulcan- izable rubber used in the construction of artificial dentures of red and pink vulcanite, in the composition of which it forms, in some cases, about one-third of the entire weight of the compound. Notwithstand- ing the poisonous character of mercurial compounds in general, and the frequency of troubles of an inflammatory nature of the mucous membrane in mouths fitted with rubber dentures, it is obviously very improbable, when we consitler the properties of pure vermilion, that such conditions can be in any degree attributable to the presence of this substance per se. It is quite possible that impure vermilion may contain from the start free mercury; be contaminated with arsenic bisulphide, or poisonous adulterations. Again, the practice of dis- solving tin-foil off of the surface of plates with nitro-hydrochloric acid just after vulcanization mav possibly decompose some little vermilion, forming soluble bichloride. It Is highly improbable that any of these conditions can be found, yet it is possible. It is said that free mercury has been observed with the microscope in finished \T.ilcanite. The occurrence of oral inflammatory conditions, under black rublier den- tures, precisely similar to those under red rubber, practically relieves vermihon of the responsibility. Such inflammatory troubles are directly 188 MF/r.\LS AM) ALLOYS USKI) IN I'llOSTII lyi'lC DF.yTISTIlY. attrihiitahlo to its r()ii<^li and jjorous surface, lack of clcaiiliiu'ss on tlie part of the wearer, and the fact that rnhher is a non-conductor (*f heat. ALUMINUM. Aluminum. Symbol, Al. Atomic weignt, 2()9. M;illeal)ility (said (o equal silver). Mcltiiifi point 625° (1157° V.) Tenacity, fonrth rank. Ductility, eifihtli rank. Specific gravity, 2.583. Conductivity (heat), 100. ("onducti\itv "(electricity), 100. (Silver heiufr 1 ()().) Occurrence. — With the exception of silicon and oxyfi;en, aluminum is tlie most al)undant element in the earth's crust. It is never found in the free or metallic state, l)ut occurs combined with silicon and oxyo;en, as marl, clay, slate, pumice-stone, feldspar, mica, and nearly all rocks, with the exception of limestone and sandstone. As the cry.s- tallized oxide — alr.mina — it occurs as corundum (AI2O3), emery, ruby, sapphire, emerald, topaz, and amethyst, which are used as gems. The metal i.s further found in combination with nearly two hundred difTerent minerals. Properties. — Aluminum is a bluish-white metal, somewhat resem- bling silver in appearance. It is also said to be as malleable, of the same tenacity, and equal to that metal in the conduction of heat and elec- tricity. It is harder than tin, but softer than copper. By hammering in the cold it may be made as hard as soft iron, but is softened again by fusion. It is remarkably sonorous, and has been used for making- bells. It is one of the lightest of metals, being approximately only two and a half times heavier than water, and four times lighter than silver. It fuses at ()2o° C, or about 1157° F. ; do€\s not oxidize in air, even at a red heat; has no action on water at ordinary temperatures, nor is it acted upon by the compounds of sulphur, thus preserving its lustre where silver would be tarnished and blackened. It is without odor or taste. Aluminum may be melted in an ordinary clay crucible, no flux being needed. Borax is not only useless, but is actually harmful, as aluminum readily attacks the glasses. Biederman recommends dipping the scraps which are to be melted together in benzine before putting them in the crucible. Should any be contaminated with solder this may be removed by nitric acid, which does not act u])on aluminum. Alloys. — Aluminum alloys with nearl}^ all metals, except lead; in- deed, tiie wonderful alloys it is capable of producing gives it, perhaps, its greatest value. Aluminum may be melted in a graphite crucible without flux, but great care must be taken not to heat it too hot. On account of its high specific and latent heat, aluminum requires a long time to melt; but, unlike some other metals, it soon becomes fluid after tlie melting point is reached. Cold and aluminum unite, forming a hard and brittle alloy. One per cent, of aluminum in gold destroys the ductility of the noble metal and gives it a greenish cast; 5 per cent, of ahimimnn with gold yields ALUMINUM. 139 an alloy hrittlr as j^lass, and 10 per cont. of aluiiiimiiii ])ro(ltice.s a white, frvstalliiu\ and brittle alloy. Niirnberg gold, an alloy, for cheap goldware, very much resembling gold, antl unchanged in air, is composed of aluminum 7.5, gold 2.5, and copper 90 parts. Silver and aluminiun readily unite, forming alloys of beautiful white- ness, and unchangeable on exposure to air. Their hardness is con- siderably greater than aluminum, but they are more easily worked. An alloy of 100 parts of aluminum and 5 parts of silver diflfers but little from piu-e aUuninum, save that it is considerably harder and takes a beautiful polish. An alloy of aluminum 169 parts and silver 5 parts possesses considerable elasticity, and has been recommended for watch springs, dessert and fruit knives. Equal parts of the two metals pro- duce an alloy equal in hardness to bronze. Copper and aluminum for some exceedingly important alloys, differ- ing according to the quantity of aluminum they contain. Those of a small content of copper cannot be used industrially. With 60 to 70 per cent, of aluminum they are very brittle, glass-hard, and beautifully crystalline. With 50 per cent, the alloy is quite soft; but under 30 per cent, of aluminum the hardness returns. The usual alloys are 1, 2, 5 and 10 percent, of aluminum. These are known as aluminum bronze. The 10 per cent, bronze has a bright golden color and keeps its polish in air; it may be easily engraved, shows a greater elasticity than steel, and can be easily soldered with 18-carat gold solder. When first made it is brittle, acquiring its best qualities after three or four meltings, after which it may be melted several times without sensible change. It casts well in sand molds, but shrinks greatly. It has a specific gravity of 7.68, about equal to soft iron. Its strength when hammered will equal that of the best steel. Anneahng makes it soft and malleable. It does not clog a file, and may be drawn into ware. It melts at about 1700° F. Aluminum bronze as a base for artificial dentures: "In the propor- tion of aluminum 100 and copper 900 it oxidizes but superficially in the mouth, and is as strong and resistant to attrition as 18-carat gold; it may be swaged as easily as 20-carat gold, but it must be annealed fre- quently, and it is necessary to carry the heating almost to whiteness, for if the bronze be merely heated until it assumes a dark-red color it remains as hard as before." (Professor Saner.) The alloys of copper and aluminum are prepared in the Cowles electric furnace by fusing together the oxides of aluminum and metallic copper with enough carbon and flux to reduce them. The oxides and other materials thould be as finely divided as possible. Solders. — The difficulty of soldering aluminum prevented the metal from being applied to many useful purposes for some time. The solder recommended for general use in the manufacture of articles of orna- mentation is composed of copper, 4 parts; aluminum, 6 parts; zinc, 90 parts. The use of this requires some skill and experience; no borax is used and the adhesion is induced by the friction of small aluminum tools. The following alloys may be used as solders for articles of jewelry mad/? of 10 per cent, aluminum bronze: 140 METALS AND ALLOYS I'SKD IS I'liOSTHETIC DENTISTRY. Gold Silver Copper Gold Silver C'opp(>r Hard Solder. Medium Hard Solder. 88.88 per cent. 4.68 6.44 54.40 per cent. 27.00 18.00 Mr. Wm. Frismiith, of Philadelphia, recommends the following solders for aluminum, with vaselin as the flux: Soft Solder. Pure block tin .... from 90 to 99 part.s. Bismuth . Pure block tin Bismuth . Aluminum Hard Solder. 10 " 1 from 98 to 90 parts. " 1 " 5 " 1 " 5 " Schlosser recommends the following for dental laboratory use : Pl.\tinum-Alu.minum Solder. Gold .... Platinum Silver .... Almninum Gold . Silver . Copper . Aluminum Gold-.\luminum Solder. 30 parts. 1 part. 20 00 parts. 50 parts. 10 • ii 10 " 20 " O. M. Thowless has patented the following solder for aluminum and the method of applying it: Tin Zinc Silver . Aluminum 55 parts. 23 " 5 " 2 " First melt the silver and aluminum together then add the tin and zinc in the order named. The surfaces to be soldered are immersed in dilute caustic alkali or a cyanide solution, and then washed and dried. They are next heated over a spirit lamp, coated with the solder, and clamped together; small pieces of solder being placed at the points of union, the whole is then heated to the melting point. No flux is used. The following are useful as solders: Zinc Aluminum The flux used in soldering is composed of 3 parts balsam of copaiba, 1 part Venetian turpentine, and a few drops of lemon-juice. The soldering iron is dipped into the mixture. So far, the soldering of aluminum in the dental laboratory is very difficult and unsatisfactory. I. II. III. 80 parts. 85 parts. 92 parts 20 " 15 " 8 " ALUMINUM. 141 Another solder for aluminum, recommended by the Scientific Ameri' can, is composed of the following: Cadmium ....... 50 parts. Zinc 20 " Tift 30 " The zinc is first melted in a suitable vessel; then the cadmium is added, and then the tin, in small pieces. The proportions of the vari- ous ingredients may be varied, in accordance with the use to which the article is put. For instance, when a strong and tenacious solder- ing is required, a larger proportion of cadmium can be used; where great adhesion is desired, a large proportion of zinc should be used, and where a nice and durable polish is desired a greater per cent, of tin should be used. An alloy of zinc, copper, and aluminum has been introduced as a dental base. (See also Carroll's alloys for cast dentures, Chapter XV.) It is said to be unaffected by the oral fluids. Tin and aluminum form alloys little affected by acids. With 100 parts aluminum and 10 parts tin an alloy is produced much whiter than aluminum and but little heavier. It can be welded and soldered like brass. Iron and aluminum unite readily. Ostberg, a Swedish inventor, discovered that an exceedingly small content of aluminum (two" of 1 per cent.) in wrought iron served to lower its fusing point about 500° F., so that castings may be made from it as readily as fiom the highly carburized cast iron. Iron may be coated with aluminum much as it is with tin. Zinc and aluminum unite to form alloys very useful for soldering the latter. They are prepared by first melting the aluminum and adding the zinc gradually, after which some fat is introduced to prevent oxida- tion, and the alloy is stirred rapidly with an iron rod. Aluminum may be frosted by immersion in a solution of potassa. IRON. Ferrum. Symbol, Fe. Atomic weight, 55.5. Malleability, ninth rank. Melting point, 1600° (2912° F.). Tenacitj^ first rank. Ductility, fourth rank. Specific gravity, 7.844. Conductivity (heat), 11.9. Conductivity (electricity), 16.81. (Silver being 100.) Occurrence. — Iron is widely and abundantly distributed throughout nature, being found in nearly all forms of rock, clay, sand, and earth; its presence in these being commonly indicated by their colors, for iron is tke commonest of all natural mineral coloring ingredients. Properties. — Pure iron is a hard, malleable, ductile, and tenacious" metal, of a grayish-white color, and of fibrous texture, slightly styptic taste, and has a sensible odor when rubbed. Its strength and tenacity are very high. In magnetic properties it is superior to all other sub- stancesl! nickel and cobalt being next; when it is almost pure, the mag- netic influence produced, owing to induction, by the proximity of a 142 METALS AM) ALLOYS USFP fX PROSTnETK' DEXTLSTRY. permanent ninf^net or of an electric current, disappears entirely on removal of the magnet or current; if, on the other hand, carbon be present (as is usually the ca.se to some small extent even in the softest malleable iron), there remains after the remo^'al of the magnet or cur- rent a greater or less amount of permanent magnetism, according to the circumstances, hard steel exhibiting the greatest power of becoming permanently magnetized under given conditions. In thermic and electric conductivity iron is rated at 11.9 and 1(1. SI, respectively. Its specific gravity is 7.844, its specific heat 0.113(S, and its melting point is variously estimated from 1500° to 1000° C. (Pouillet) to 1<)00° to 2000° C. (Deville). The presence of minute fjuantities of carbon, sul- phur, etc., very sensiWy lowers the fusing point, while 1 per cent, of the former furnishes a steel melting at several hundred degrees lower than pure iron. Cast iron, containing more carbon, melts very much lower. It possesses the remarkable property of becoming plastic just before fusion, so that two hot masses may be pressed or scjueezed together into one by the process of welding. So by forging, rolling, hammering, or other analogous operations, it can readily be fashioned into shapes which its rigidity and strength when cold will enable it to maintain. Modifications of Iron. — There are three distinct modifications of iron, viz., cast iron, wrought iron and steel. Other intermediate varie- ties are recognized technically, but all are closely related and impercep- tibly shaded into each other, due to various percentages of carbon, etc., contained in the metal. Cast iron is an impure carburized iron. The melted metal drawn off from the furnace below is conducted into a large main, called the "sow," and thence into lateral molds called "pigs;" hence the term pig iron. This iron is found to have combined with a considerable (juantity of carbon, aljout 4.5 per cent, being the maximum; a portion of which exists as a chemical combination, the carbide of iron, the remainder having been simply dissolved in the form of graphite. Other substances in the furnace are also found dissolved and combined in the iron, and have an important bearing upon its physical properties. These are principally phosphorus, silicon, sulphur, manganese, etc. Pig iron may, therefore, be recognized as a crude form of cast iron. It is assorted and classed by the iron masters as Nos. 1, 2, and 3, differ- ing in the amount of carbon contained. No. 1 is most highly carburized, No. 2 less, and No. 3 contains the least carbon. The first melts and becomes so fluid that it is used for ornamental castings of fine pattern, and furnishes cast-iron cutlery from which the carbon is subsequently extracted. Cast iron, which contains the most carbon, is the most fusible variety, melting at about 1200° C. It is hard and l)rittle. Though some kinds admit of beintj made hard or soft nearlv in the same manner as steel, and, like steel, assume different degrees of hardness, according to the rapidity with which the pieces are allowed to cool; but unlike steel, when once hardened, will not admit of that hardness being reduced by various gradations to any specific degree, called tempering. To FRON. 143 softcMi niivteviallv it nv.ist hv siil)inittcd for some time to a whitish licat, and thcMi very gradually cooled. Wrought iron is the cast, or pig, iron, freed from carbon, and may be considered a nearly pure decarburized iron; at least, it is the purest form of commercial iron, containing the least amount of car})on — less than { per cent. The decarburization is effected by first remelting the pig or cast iron, and refining by exposing it to an intense heat and forcing a blast of air over its surface, in order to remove some of the impurites of the metal; it is then run out into a large flat mold, and acquires the name of plate metal. The next process is called "puddling," the object being to free the metal of its carbon. The operation is conducted in a reverberatory furnace, where the metal is again reheated and converted into wrought iron by keeping it in a state of fusion with a certain amount of black oxide of iron, FcgO^, which gives up its oxygen after a time to the car- bon, and other impurities of the melted mass, leaving the latter nearly pure iron. As the process approaches termination the fusing point of the mass grows higher, until it loses nearly all its fluidity. It is then divided into several parts and formed into balls, which are removed from the furnace and subjected to intense pressure through a series of powerful rollers, which squeeze out the more fusible slag entangled in it and convert it into bars or "bloom." A number of these blooms are then raised to a welding heat and repeatedly passed through rollers, until all the remaining slag is forced out and the metal becomes tough and fibrous. Thus the process is repeated usually once, and some- times twice or three times, to produce a superior iron. By this process the metal is converted from a fusible, hard, and brittle substance, as cast iron, into a tough, elastic bar ; in fact, it has been rendered malle- able, ductile, more closely compact, and of a fibrous texture, and is less fusible. It is also very tenacious, and added to its properties is a new and remarkable one, by virtue of which two pieces being heated similarly may be forged or welded together. For purposes where light- ness, strength, and durability are wanted, it is more extensively em- ployed than cast iron. In this state it is known in commerce as bar, or wrought iron. Steel is composed of iron and carbon, and, generally speaking, it is prepared by either one of two ways: by adding a certain percentage of carbon to a lightly carburized iron, such as wrought iron; or by abstracting an amount of carbon from a heavily carburized iron, such as cast iron. This fact gave rise to two typical methods of preparing steel, viz., the cementation process and the Bessemer process. While technically there are a variety of methods by which steel is made, yet all methods are more or less modifications of these two typical ones. The proportion of carbon varies, of course, in the different qualities of steel; but in that used ordinarily the carbon rarely exceeds IV per cent.; for some purposes it is as low as 1 per cent. Good ordinary tool steel contains about 1 V per cent, of carbon. Different kinds of iron produce steel of different properties, and different qualities of steel are used for different purposes. 144 METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. Bessemer's Process. — By this process steel can be maiuifactured of anv degree of hardness directly from the cast iron, without the inter- mediate operation of making it malleable by puddling, etc. The prin- ciple of the process consists in directing a blast of cold air upon molten cast iron contained in a "converter." The oxygen of the blast combines with the carbon, silicon, and manganese. Sulphur and phosphorus are difficult to remove by this process; hence the necessity of emj)loying ores as free from these as possible. The intense combustion of the carbon in th". iron is attended with great elevation of temperature, so that the metal is maintained in a fluid state throughout the whole operation, solely by the energy of the reaction in the converter. Thus the cast or pig iron is decarburized, or converted into tool steel, or to mild welding steel, or to the state of malleable iron, according to the length of time the combustion is continued. It has been found, how- ever, that a better (piality of steel can be produced by contimiing the decarburizing and purifying process until all, or as nearly all as possi- ble, of the carljon and impurities are removed, and then adding to the fused wrought iron a certain quantity of a peculiar kind of white cast iron known as spiegel-eisen^ ("looking-glass" iron), containing a known quantity of carbon and a little manganesium and silicon. Bessemer steel is largely used in the construction of railroads, bridges, armor plates for vessels, girders, etc., in the construction of edifices, the manufacture of machinery, tools, etc., and there is good reason to believe that steel of an excellent cpiality for numerous purposes will, at no distant period, be manufactured cheaper than wrought iron is now produced by the operation of puddling. The Cementation Process. — The furnace in which the iron is cemented and converted into steel, called a converting furnace, has the form of a large oven, constructed so as to form in its interior two large and long cases, commonly called troughs or pots, and built of good fire-stone or fire-brick. Into each of these pots layers of the purest malleable iron bars and layers of pulverized charcoal are packed hori- zontally, one upon the other, to a proper height and (juantity, according to the size of the pots, leaving room every way in them for the expansion of the metal when it becomes heated. After the packing is completed the tops are covered with a bed of sand or clay. This is to confine the carbon and exclude the atmosphere. The whole is then heated for eight or ten days, according to the degree of hardness recjuired. Then the mass is left to cool for several days. The properties of the iron are remarkably changed by this process: it acquires a small addition to its weight, becomes much more brittle and fusible than originally, loses much of its ductility and malleability, but gains in hardness, elasticity, and sonorousness. The texture, which was fibrous before, has now become granular; and its surface is found to be covered with blisters, and it presents, when broken, a ' Spiegel-eisen is composed of the following: Iron 82.86 Manganesium .......... 10.71 Silicon 1.00 Carbon 4.32 IRON. 145 fracture much like inferior iron. Iron under this process has been shown to have taken up about 1 per cent, of carbon. It is, however, far from beino- homogeneous in composition, and is called blister steel. Uniformity of composition is secured by subjecting bundles of the carburized bars to repeated blows from a steam hannner at a welding heat, striking in rapid succession, until it closes the seams and removes the blisters. It is then termed shear steel. After this treatment is re- peated it is called d()ul)k -shear steel. Homogeneity is best ol)tained, however, by fusing the blister steel in crucibles, covering the mass with clay or some other substance to exclude air, and casting it into ingots. It is then designated as cast steel or crucible steel. Spring steel is blister steel simply heated and rolled. Case hardening is accomplished by heating such articles of forged or bar iron, as it is desired to harden superficially, in contact with some substance rich in carbon, and afterward chilling them in water. Gun- locks are thus treated. Harveyized steel, which is employed for armor plate on account of its extremely hard and resistant surface, is prepared by heating the steel plate to the melting point of cast iron and then tightly packing its surface with carbon; after it has taken up about 1 per cent, of the car- bon the plate is dropped into water and cooled. Nickel Steel. — In 1889, M. Henry Schneider, of Creusot, France, patented an alloy of steel and nickel. The alloy usually contains about 5 per cent, of nickel, and is especially suitable for use in the construc- tion of ordnance, armor plate, gun-barrels, and projectiles. It is said that ordinary steel is more readily acted upon by sea-water than are the more impure grades of iron, but nickel steel is less liable to corrode in salt water than ordinary steel. Manganese Steel. — When about 15 per cent, of manganese is added to steel it produces an alloy of great strength and toughness, and so hard that it is almost impossible to work the product by ordinary methods. The alloy is usually prepared by adding manganese iron to molten Bessemer, or open-hearth steel. From 4 to 5 per cent, of manganese gives to the alloy its extreme brittleness. Extremes of atmos- phere, heat or cold, do not appear to affect the properties of manganese steel. When a piece of it heated sufficiently to be seen red hot in a dark room is plunged into cold water, it becomes soft enough to be easily filed. Hardness is then restored by reheating to a bright red and cooling in air. The presence of manganese in proper proportions in nickel steel is said to very much improve it. Indeed, the best results are only obtained by the admixture. Chrome Steel. — Chromium gives greater hardness, tensile strength, and elasticity to iron, but decreases its weldability. It is also stated that chromium steel is more susceptible of oxidation than ordinary steel. Chromium is added to iron by heating the mixed oxides of iron and chromium in a brasqued crucible with pulverized charcoal and fluxes. Chrome steel is then produced by melting chrome iron with wrought iron or steel in graphite crucibles. Copper Steel. — This alloy usually contains from 5 to 20 per cent. 10 146 METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. of copper, according to the })iirp().se for which it is to be used. It pos- sesses remarkable streiif^th, tenacity, and malleabihty, and these prop- erties are still further developed by tempering. Aluminum Steel. — In amounts not greater than 1 per cent, alum- inum is said to slightly increase the tensile strength, and proportionally the elastic limit, of rolled and cast steel. Tungsten in small (juantities produces an exceedingly hard steel, without the necessity of tempering. Carburized Iron. — As has been previously hinted, carbon may be present in iron under two conditions. AVhen iron is fused in contact with carbon it is capable of combining with nearly G per cent, of the latter element to form a white, brilliant, and brittle compound, which may be represented pretty nearly as Fe^C. Under certain circum- stances, as this compound of iron and carbon cools, a portion of the carbon separates from the iron and remains disseminated throughout the mass in the form of minute crystalline particles very much resem- bling natural graphite. Iron containing the least possible carbon, and otherwise compara- tively pure, is called wrought iron. Iron containing from 1.04 to 4.81 per cent, of carbon is designated as cast iron. Iron containing from 0.15 to 1.04 per cent. (Bloxam) is considered steel. "The portion of combined carbon within certain limits bears a direct relation to the tensile strength of the metal, variations as minute as y^Q- of 1 per cent, making a considerable alteration in this quality. The same is true of hardness, the effect of carbon up to a certain point being to increase tenacity and decrease ductility, and also to cause the metal, when heated and suddenly cooled, to become more or less hard, the hardening being in direct proportion to the amount of carbon present and the rate of cooling."* 1.4 per cent, of carbon in iron produces a highly carburized steel that must be worked with great care. It should not be heated above a cherry-red, for fear of burning. Such steel is used for the manufac- ture of razors and tools for cutting hard metals. Steel containing from 1 to 1.25 per cent, of carbon is used for making most tools. Steel containing about 1 per cent, of carbon can be welded readily, and a portion of a tool made of it can be made tough, so as to stand a blow from a hammer, without chipping, while another part can be hard- ened, as in the case of a cold-chisel. Hardening and Tempering Steel. — After soft steel has been shaped into the form of instrument desired, it may be made very hard by first heating to redness and then immediately chilling by plungi-^g into cool w^ater, oil, or mercury. If, however, the hardened steel be heated to redness again and allowed to cool slowly, it returns to its soft condition. Any desired variation between these two extremes may be obtained by heating the steel to redness and (juickly chilling it, thus obtaining the full hard state. If this be polished and heated > Kirk, American System of Dentistry, vol. iii. p. 900. IRON. lAl gradually aiul carefully, it will be foiuid to take ou a sucee.s.sion of shades and colors, owing to the formation of a film of oxide which grows thicker and of deeper shade and color as the heating progresses. The temperature at which given degrees of temper are produced has been carefully determined, and the experienced operator knows by the shade or color of the film of oxide the temper of the instrument operated upon, provided the piece is known to be steel and to have been full hard. The following table shows the approximate temperatures corre- sponding to the various shades and colors : Temperature. Color. Temper. 430° to 450° F. . . Very faint yellow Lancets, razors, surgical instru- to pale straw. ments, enamel chisels. 470° . ■ . . . Full yellow. Excavators, very small cold- chisels. 490° .... Brown. Pluggers, scissors, pen-knives. 510° .... Brown with pur- Axes, plain irons, saws, cold- pie spots. chisels. 530° .... Purple. Table knives, large shears. 550° .... Bright blue. Swords, watch-springs. 560° .... Full blue. Fine saws, augers. 600° .... Dark blue. Hand and pit saws. Since the amount of hardness which can be developed in steel is directly in proportion to the amount of carbon and rate of cooling the article from the heated condition, and as pieces of steel vary greatly in their content of carbon, the temperature at which it is necessary to heat them before chilling must be determined by actual experiment, in order to produce the greatest hardness. The piece should never be overheated. It is better to err upon the side of underheating instead of overheating, for under the latter condition the steel is burned, presents a blistered, scaly appearance, and is incapable of taking a fine tempr. Wlien small instruments, such as burs, excavators, etc., are to be hard- ened, it is best to protect the surface of the steel with some substance to prevent a loss of carbon by oxidation in the heating. "Common soap answers admirably for this purpose," says Dr. Kirk. The means of applying the heat to articles when they require hard- ening will, of course, depend upon the size, shape, and use of the article. They may be heated in the flame of the Bunsen burner, alcohol lamp, open fire, and sometimes it is best to enclose them in a sheet-iron case with carbon, and heat in a suitable furnace; but for a more uniform degree of heat red-hot lead is probably better than any other means. In chilling, water is by no means essential, as the sole object is to extract the heat as rapidly as possible by good conduction; and the more suddenly the heat is extracted, the harder the steel will be; but if the hardness is not carried to an extreme, a certain amount of tenacity is also obtained with the hardness. Water with a small amount of acid or salt is sometimes used, the former to aid in removing the oxide, and the latter to increase the con- ductivity. For extreme hardness mercury is used, which, on account of its superior conductivity, chills the piece immediately. 148 METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. Tempering. — A rod of good steel in its hardest state is broken almost as easily as a rod of glass of the same dimensions. This brittleness can only be diminished by decrecising its hardness; and the manage- ment of this is ealled tempering. The snrfaee of the steel is brightened and tried with a fine file to make sure of its full-hardness, and is then exposed to the heat, which, upon the appearance of the color desired and previously determined upon, is discontinued, and the article cooled by instantly plungmg into cool water. Tlie methods for applying the heat for tempering are as varied as those for hardening. The heat for this purpose should be slowly applied; indeed, it is said that the slower the heating, the tougher and stronger will be the steel. The article may be placed upon a hot iron plate, upon the surface of melted lead, or in a bath of a more fusible alloy; in hot sand, a gas stove, or in almost any place where sufficient temperature may be gradually obtained, without injury to the steel. The following table of alloys of lead and tin may be conveniently used to secure a imiform temper: MeltinK points. Degrees F. -120° 430° 4.50° 470° 490° -510° 530° 550° 560° 600° (Compare tjie degrees with the colors of the previous table.) When instruments are only partially dipped and afterward tem- pered by the heat from the back, they must be cooled in water, or other substance, instantly on the cutting part attaining the desired color; otherwise the body of the instrument will continue to supply heat, and the cutting part may become too soft. In the case of excavators, enamel chisels, and cutting instruments with slender, tapering shanks, ter- minating in a fine cutting edge, the edge must be protected from the heat while tempering the shank, the latter being drawn to a blue, a state much too soft for the former. The point or edge may be protected by placing against a large piece of cold iron or other subtance which, on account of its conduction, prevents the heating of the end of the instrument. Rubber-dam clamps are best tempered a blue spring by what is known as blazing off. This is accomplished by dipping them in oil, and then burning the oil off. Action of Acids on Iron. — Iron dissolves in the acids, and the car- bon which it almays contains, so far as combined in the carbide of iron, passes oflP as carburetted hydrogen, and so far as uncombined will remain undissolved, as graphite. Composition Lead. Tin. 7.0 4 7.5 4 8.5 4 10.0 4 14.0 4 19.0 4 30.0 4 48.0 4 50.0 4 Boiling oil COPPER. 149 COPPER. Cuprum. Symbol, Cu. Atomic weight, 63.1. Malleability, third rank. Melting point, 1200° (2192° ¥.). Tenacity, second rank. Ductility, fifth rank. Specitic gravity, 8.94. Conductivity (heat), 73.6. Conductivity (electricity), 99.95. (Silver being 100.) Occurrence. — This exceedingly interesting and useful metal ha.s been known and used by the human race since the most remote periods. Its alloy of tin-bronze was the first metallic compound used by man. It is found in the metallic state in the Lake Superior region, and in Mrginia, the southwestern portion of the United States, and many other parts of the world. Properties. — Copper, or cuprum, in name is derived from Kupros, the Greek spelling of Cyprus, an island where it is extensively mined. Its symbol is the planet ^>nus, as the isle of Cyprus was sacred to that goddess. It is a peculiar red-colored, brilliant metal, differing in this respect from all other metallic elements, except, perhaps, titanium. Its atomic weight is 63.1, and its specific gravity 8.94. It takes a brilliant polish, and is very malleable and ductile, being second to iron in point of tenacity. It may be rolled into thin sheets or drawn into very fine wire. A copper wire, hard draw'n, having a sectional area of a square millimetre, sustained a weight of 90.20 pounds at the moment of rupture. The same wire, annealed, broke under a weight of 69.52 pounds,^ The melting point of copper is probably best stated at 1200° C. or 2192° F., and it expands slightly on passing from the molten to the solid state. It is unaffected by dry air, but in a moist atmosphere it becomes coated with a green carbonate, malachite, which is also found native in most beautiful shades, takes a high polish, and is used for ornamental articles. When heated or rubbed with much friction, it emits a peculiar, disagreeable odor. In the conductivity of heat (73.6) and electricity (99.95) it is second only to silver (100). Copper does not dissolve in acids with evolution of hydrogen. In nitric acid it dissolves most readily, chiefly with the evolution of nitric oxide, forming copper nitrate. In sulphuric acid, hot and concentrated, it also dissolves readily, with evolution of sulphurous anhydride, forming copper sulphate- blue vitriol. In hydrochloric acid copper is slowly soluble. Alloys. — The preparation of copper alloys is generally attended with many difficulties, on account of the high fusing point of the metal and the almost invariable presence of small cjuantities of other elements. Gold and copper alloy readily, the latter giving a desirable hardness to gold and deepening its color. If, however, any considerable portion of copper be added to gold, the alloy is apt to be brittle, especially if the copper be not absolutely pure. For United States gold coins 10 ' Ganot, Elements de Physique. loO METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. per cent, of ('()])j)er is lukled to pure gold, fj;ivin<^ it Ji carat fineness of 21. G and a proper degree of hardness for dnrahility. Silver and copper also alloy readily, and the copper again gives hard- ness with a slight change of color. Ten per cent, of copper is added to silver for United States coin. Platinum and copper alloy at an intense white heat, giving an alloy much resembling gold in color and specific gravity. I^ad added to copper from yijoo-to ^^-^ somewhat increases its duc- tility and malleability, but the presence of y-i/fnj renders the metal unfit for preparation of malleable or ductile brass. Iron to the amount of ywqq ^^^ ^as an injurious effect upon the properties of copper, rendering it hard and brittle. Antimony, bismuth, and arsenic in small quantities have a very in- jurious effect upon copper. Babbitt metal consists of copper 4, tin 12, and antimony 8, melted separately. The antimony is added to the tin, then the copper, and 12 parts more tin after fusion. Metal. Type Metal — Table of Composition.' Parts. I. II. III. IV. V. 3 10 70 6 100 1 2 18 30 2 4 8 Lead .... Antimonj^ Copper .... Bismuth 1 . . . . 2 Zinc 90 Tin 10 . . 20 Nickel 8 Britannia Metal (Wagner's) consists of tin 85.64, antimony 9.(56, copper 0.81, zinc 3.06, and bismuth 0.83. Queen's Metal consists of tin 88.5, antimony 7.1, copper 3.5, and zinc 0.9. Zinc alloys with copper in any proportion, all of which alloys are included in the term brass. Alloys of copper and zinc were known in the time of Aristotle, and the manufacture of brass was first introduced in Germany in 1550, but was probably not produced by the direct union of the two metals until 1781 in England, as the art of obtaining zinc in the metallic form became known but a short time previous to that period. Notwithstanding copper and zinc may be alloyed in any proportion, the product is always serviceable. "Generally speaking, it may be said that with an increase in the percentage of copper the color inclines more toward a golden, the malleability and softness of the alloy increasing at the same time. With an increase in the per- centage of zinc, the color becomes lighter and lighter, and finally shades into a grayish-white, while the alloy becomes more fusible and brittle and at the same time harder."^ Alloys containing from 15 to 20 per cent, zinc are the most ductile. Those of 36 to 40 of zinc can be worked cold as well as hot, while those containing 60 to 70 of zinc are so brittle that they cannot be worked at all. Raising this percentage to from 70 ' Table from Brannt. ^ Brannt, Metallic Allois. COPPER. 151 to 90 of zinc, the alloy again l)econies chictile, and can he worked quite well when hot, but not when cold. An alloy of copper 75 and zinc 25 fuses at 1750° F. Good sheet brass may be made according to many fonnulic; two are cited : Eosthoru (Vienna") — Copper GS.l and zine 31.9 parts. Eoniilly— Copper 70.1, zinc 29.26, lead 0.3S, and tin 0.17 parts. For wire, the following:"" England — Copper 70.29, zinc 29.26, lead 0.2S, and tin 0.17 parts. Neustadt — Copper 71.5 and zinc 2S.5 parts.'' Alloys containing as high as 37 per cent, of zinc are used as ductile and malleable products. Fine cast brass usually contains from 20 to 50 parts of zinc to 100 parts copper, together with lead, or tin, or both in the proportion of 0.25 to 3 per cent, of each.^ Oreide (French gold) is a brass allov much resembling gold. It takes a fine polish and is very ductile, malleable, and much used for the manufactiu'e of cheap jewelry, on account of its beautiful color. Form- ula by analysis. — copper 68.21, zinc 13.52, tin 0.48, and iron 0.24,' "The most malleable of the brasses is Dutch metal, composed of copper 11, zinc 2 parts; it can be rolled out into thin sheets and after- ward beaten into leaves of extreme tenuity, and is used in this form for decorative purposes under the name of Dutch leaf-gold or, reduced to powder by levigation with a small quantity of oil or honey, it is sold as bronze powder."^ Pinchbeck, an alloy of copper 88. 8 and zinc 11.2 parts, very much resembles gold; is very ductile and malleable; used for cheap jewelry. INIosaic gold, a term sometimes applied to tin sulphide, is composed of about equal parts of copper and zinc. Copper Coins. — Those of the Ignited States are composed of copper 95, tin 3, zinc 2 parts. Nickel and copper unite in all proportions, the color varying from the red of copper to the blue-wliite of nickel, according to the propor- tions of the respective metals: Copper with 10 per cent, of nickel gives a light copper-colored alloy, verv ductile; with 15, the color is a very pale red, but the alloy is still quite ductile; with 25, a nearly white alloy, and 30, a silver-wliite alloy. United States nickel coins are composed of copper 75 and nickel 25 parts. Nickel, copper, and zinc alloys are called German silver, argentan, etc. They are in reality brasses with nickel added, which gives them a white color and much hardness. ' Figures from Brannt, Metallic Alloys. 2 Ibid. 3 Ibid. * Kirk, American System of Dentistry. 15-2 MI<:TAJ,S AM) ALLOYS USED /.V mOSTIIKTir nFXTfSTnV. Tlicse C'ompDsitioiis vary greatly, as may l)e noticed: Copper . . . . . . . 50 to 0(3 parts. Zinc 19 " 30 " Nickel 13 " IS "« White Metal. — A variety of alloys consisting of copper and a large proportion of zinc. They are very white, or, tlepending npon the pro- portion of copper, may be a pale yellow; melt at a low point, may be cast, and are somewhat malleable and ductile. Aluminum alloys easily with copper, producing aluminum bronze, the alloys showing different properties, according to the quantity of aluminiun they may contain. With 00 to 70 per cent, aluminum, a very brittle alloy is produced; with 50 per cent., one quite soft, but less than 30 per cent, of aluminum, the hardness returns. The bronze composed of copper 95, aluminum 5, is a beautiful gold color, takes a fine polish, casts well, is malleable hot or cold, and is very strong, espe- cially after hammering. With 7.5 per cent, aluminum, the color is a greenish golden. The most common alloy is 10 per cent, aluminum, which yields a bright golden, is not tarnished in air, may be engraved, possesses, it is said, greater elasticity than steel, and may be soldered with 20-carat gold solder. It melts at about 1 700° F. Tin and copper form a very important series of alloys termed bronze. (See Tin.) Brazier's Solder. — An alloy composed of copper, zinc, tin, and lead in a variety of proportions, according to color and fusibility. Dr. Kirk gives the following; table: Copper. Zinc. Tin. Lead. A. Golden yellow . 53.50 43.34 2.12 B. Medium liglit . 43.75 .50.5.S 3.75 1 C. White . 57.50 27.90 14.90 trace It is used in soldering brass and copper, which may also be soldered with the ordinary soft solder, spelter (zinc), or silver solder. Copper is a constituent of most hard solders, its proportion varying according to the purpose for which they are to be used. (See Silver and Gold.) ZINC. Zincum. Symbol, Zn. Atomic weight, 64.9. Malleability, eighth rank Melting point, 415° (779° F.). Tenacity, sixth rank. Ductility, sixth rank. Specific gravity, 6.915. Conductivity (heat). Conductivity (electricity). (Silver being 100). ' Brannt, Metallic .\lloys. ZISC. 1 53 Occurrence. — Zinc is a somewhat abundant metal, but never occurs in the nati\e state. It is found as a carbonate, sulphide, silicate, etc., associated with lead ores in many districts ; large supplies are obtained from Silesia and from the neighborhood of Aachen. Properties. — Zinc is a bluish-white metal, which but slowly tar- nishes in moist air, usually forming a superficial carbonate; it has a specific gravity of 6.915, and is, under ordinary circumstances, quite brittle, but when heated to 100° or 150° C. it may be rolled or ham- mered into thin sheets, or drawn into wire; and, what is very remarka- ble after such treatment, it retains its malleability when cold; the sheet zinc of commerce is thus made. If the temperature be carried to 205° C. it again becomes so brittle that it may be easily powdered in a mor- tar. Care should be exercised in handling hot zinc dies, for if by acci- dent one be dropped upon a hard surface it is likely to be ruined. The metal melts at 415° C. or 779° F. It boils and volatihzes at 1040° C. or 1904° F., and, if air be admitted, bums with a splendid greenish incan- descence, forming the oxide. In boiling water zinc is said to be attacked appreciably , forming the hydroxide, Zn2HO, with evolution of hydrogen. Zinc has long been very extensively used in the dental laboratory for making dies. Its comparatively low fusibility, hardness, and other properties eminently fit it for this purpose. Pure zinc dissoh cs very slowly in acids (or alkalis), unless in contact with copper, platinum, or some less positive metal. Any metallic impurity in zinc renders it quite soluble in the acids or alkalis. Alloys. — Zinc readily unites with gold. The malleability, brilliancy, and color of gold are impaired by a content of zinc. Small pieces of platinum may be dissolved in molten zinc, and the union is attended with considerable energy, owing to the formation of a definite chemical compound. The alloy is hard and brittle. An alloy may be prepared of platinum, 16; copper, 7; and zinc, 1; which very much resembles gold in color, specific gravity, and ductility. Silver and zinc have a great affinity for each other. This fact, with the knowledge that zinc and lead are comparatively so incompatible, led to the process of desilvering lead by the assistance of zinc. The alloy of silver and zinc is best obtained by throwing the required quan- tity of zinc wrapped in paper into molten silver, stirring thoroughly with an iron rod, and pouring the fused mass at once. The alloy of two parts zinc and one part silver is flexible, ductile, and has nearly the color of pure silver. Larger proportions of zinc produce brittle alloys. Iron plate and ware when perfectly cleaned may be immersed in molten zinc and the sm^ace alloyed slightly, forming what is known as "galvanized iron," the name being derived from the circumstance that the coating is analogous to that produced by electric means. Zinc alloys \vith the iron melting pots of the laboratory, the admLxture rendering the zinc less fluid when molten and more difficult to fuse. This contamination may be prevented by coating the pot with whiting. ^Yith lead zinc does not alloy, except to a very slight degree. "Mat- 154 METALS AND ALfJ/YS CsKD fN PROSTUETIC DENTrSTRV. tliiesseii found' that on incltiiiff ('(|iial ])art.s of zinc and U-ad, and, after well mixini;, allowing the alloy to eool slowly, they separate, but the heavier lead on subsidini^ retains l.O per cent, of the zinc alloyed with it; while on the other hand the uj)per layer of zinc thrown out retains 1.2 per cent, of lead." It often occurs that lead and zinc will become mixed in the laboratory, and is seldom discovered until the molten mixture is poured. Then the lead, owing to its greater specific gravity, falls to the bottom of the mold, forming the alveolar ridge of the die, rendering it worthless. Many times the counter-die is poured before the mistake is noticed, resultins: in a union of the die and counter-die. Tin and zinc alloy in almost any proportion. Mr. Fletcher recom- mends an alloy of zinc 2 parts and tin 1 part for making dies for swag- ing, claiming the impression from the sand is much finer, and the shrinkage on cooling is greatly reduced. It melts much lower than zinc alone, hence some care must be exercised in pouring the counter- die. The die should be perfectly cold and the lead should be just hot enough to pour, but not sufficiently heated to char a slip of paper. BISMUTH. Symbol, Bi. Atomic weight, 206.9. Malleability, brittle. Melting point, 264° (507° F.) Tenacity, brittle. Ductility, brittle. Specific gravity 9.823. Conductivity (heat), 1.8. Conductivity (electricity), 1.24. (Silver being 100.) Occurrence. — Practically the only ore of this element is the native metal found disseminated in veins through slate rock associated with the ores of copper, iron, cobalt, nickel, silver, gold, and arsenic. Properties. — Bismuth is a highly crystalline, iT^rd, and very brittle metal, having a grayish-white color, with a decided reddish tint. Its specific gravity is 9.823, and it fuses at 264° (507.2° F.). It expands about 3^ of its volume upon cooling, and imparts this property to its alloys. The metal volatilizes at a high temperature, and has a specific heat of 0.0308. It is the most diamagnetic of all substances. Exposed to the air at ordinary temperatures, it is unaffected, but when heated to a red heat it rapidly oxidizes, producing a beautiful play of colors. Sulphuric and hydrochloric acids have but slight action on bismuth, while nitric acid dissolves it very energetically. Alloys. — Bismuth has its greatest use in the preparation of low fusing alloys. With tin bismuth alloys in any proportion. A very small quantity of the metal imparts to tin more hardness, sonorousness, lustre, and a fusibility lower than either of the metals taken separately possesses. An alloy of equal parts of the two metals fuses at 212° C. ' Makins' Metallurgy, p. 62. Bi. Sb. Sn. Pb. 9.0 10.5 Type-metal. 48.0 32.5 8.0 1.0 4.0 5.0 1.0 3.0 8.0 BISMUTH. 155 With k'lul bismutli alloys very easily, producino- an alloy which is inalleahle if the proportion of bismuth does not exceed that of lead. The specific gravity is greater than the mean of the two taken separately. Its alloys are whit(>, lustrous, harder than lead, and more malleable up to a certain proportion. Bismutli 1 and lead 2 gives a very ductile and malleable alloy fusing at 330° F. With antimony it produces a grayish, brittle, lamellar alloy. Lead and tin added render it malleable, l3ut its fusibility is increased rather than decreased. Such alloys are very frequent and much used in the preparation of Britannia and Queen's metal. Cliche-metal Alloys of bismuth, tin, and lead are known as the triple alloys, and are very numerous and useful. Newton's alloy, sometimes called "Melotte's metal," consists of bismuth 8, lead 5, and tin 3 parts, and fuses at 202° F. Rose's fusible alloy is composed of Bismuth . . - . Tin Lead ...... The first fuses at 200.75° F. and the second at 174.2° F.^ They w^ere used as safety plates and inserted in the top of steam boilers, intended to prevent the explosion of boilers by allowing the steam to escape at a certain tension. Wood's metal consists of lead 4, tin 2, bismuth 5 to 8 and cadmium 1 to 2, melts at 140° to 161.5° F., in color resembles platinum, and is, to a certain extent, malleable.^ Onion's fusible alloy contains lead 3, tin 2, and bismuth 5 parts, and melts at 197° F. La Nation describes a new fusible alloy, of which the follownng is the formula: Bismuth 48, cadmium 13, lead 19, and tin 26. It melts at 158° C. and resists great pressure. • Hodgen's fusible, alloy, for making dies and counter-dies by the dipping process, is composed of the following: Bismuth 8, lead 5, tin 3, and antimony 2. It is a light, lustrous alloy, very hard, slightly malleable, expands slightly on cooling, copying the finest of lines, takes a high polish, and resists great pressure, melting at 224° F, Dr. Mathews' Fusible Alloy. — This alloy is composed of bismuth 48, cadmium 13, and tin 19 parts. It melts below the boiling point of water and may be packed with the fingers. It may be poured into plaster impressions immediately after they have been taken, producing 1 WilUam T. Brannt. " Ibid. I. II. 2 8 parts. 1 3 " 1 8 " Bismuth. Tin. Lead. 7 4 2 16 7 4 8 2 6 156 METALS AM) ALLOYS USED IN I'llOSTIIETIC DEM'ISTRY. .sharj), bright, hard dit'.s, with which .sliot niav 1)0 ii.sed for the counter- die. Darcet's fusible alloys are a series of proportions of bismuth, tin, and lead, and their melting ])oint varies a,s per the following table: Parts. Melts. . 212° F. . 212° F. . 205° F. Most of these fusible alloys are of much value in the dental laboratory m the hands of a practical, resourceful man. 'J'he cleaner ones may when lack of time will not permit of a more perfect repair, be used to mend a denture or replace a tooth or block of teeth on a vulcanite plate, and the more fusible ones may be used for the same purpose, even though the base be celluloid. In replacing teeth undercuts may be made with a file, or preferably with a large bur in the engine, the tooth placed in position, and the alloy packed in with warm instruments, smoothed, and afterward polished. These alloys are also valuable baths for tempering steel instruments. They give a very exact temper- ature, which may be adjusted to the purpose intended. They are used, according to Thurston, by placing the articles on the surface of the unmelted alloy and gradually heating until fusion occurs and they fall below the surface, at which moment their temperature is right; they are quickly removed and cooled in water. TIN. Stannum. Symbol, Sn. Atomic weight, 118.1 Malleability, fourth rank. Melting point, 228° (442° F.). Tenacity, .seventh rank. Ductility, seventh rank. Specific gravity, 7.29. Conductivity (heat) 14.5. Conductivity (electricity), 12.36. (Silver being ino.) Occurrence. — Tin occurs chiefly as tinstone, cassiterite, or native oxide, SnOj. The pure ore is colorless and very scarce. Another native form known as "wood tin" occurs in roundisli masses. The metal is rarely, if ever, found fi-ee. Properties. — Pure tin is white (except for a slight tinge of blue); it exhibits considerable lustre, and Is not subject to tarnishing on expo- sure to normal air. It is soft and exceedingly malleable; indeed, it is said it may be beaten into foil -^-^ of a mm. in thickness; at 100° C. it may be drawn into wire, but is almost devoid of tenacity. That it is elastic, within narrow limits, is proven by its clear ring when struck with a hard body under circumstances permitting free vibration. Though it is seemingly amorphous it has a crystalline structure, hence the crackling noise known as the "tin cry" which a l)ar of tin emits on being bent. The crystalline structure must also account for the TIN. 157 strange fact that an ingot, when exposed to the temperature of — .'iO° C. for a siiffieient length of time, becomes so l)rittle that it falls into powder under pesde or luimmer. At some temperature near its fusing point it again becomes brittle. Tin fuses at 228° (442.4° F.).^ At a red heat it begins to volatilize slowly; at 1600° to 1808° C. it boils^ and may be distilled. Tlie hot vapor jjroduced combines with the oxy- gen of the air, forming the white oxide, SnOj. The specific gravity is 7.29. Its specific heat is 0.0562. Casts of tin are used to vulcanize upon, and plaster casts are often covered with tin-foil to give a clear and finished appearance to the denture after the process of vulcanization. Tin dioxide under the name of "polishing putty" is used for polish- ing glass, ground porcelain surfaces, hard metals, and similar sub- stances. The three mineral acids and boiling solutions of the caustic alkalis act on tin. Alloys. — Gold and tin form a malleable alloy, provided the tin be pure and does not exceed in quantity 10 per cent. Platinum and tin in equal proportions form a hard, but brittle, alloy, fusing at a comparatively low temperature. Palladium, says Mr. Makins, forms a very brittle alloy with tin. In view of the fact that gold, platinum, and palladium so readily unite with tin to form alloys whose fusing points are so comparatively low, and in view of the behavior of tin with other metals, and of metals in general toward each other, there is little reason to doubt a chemical affinity of tin for these metals. The affinity of tin for gold in particular has been clearly demonstrated by Dr. Matthiessen. Into a crucible of molten tin a rod of gold and one of copper were dipped, the latter having been previously tinned to ensure perfect contact. The gold united readily and rapidly with the tin, while the copper rod remained unaffected. A gold wire which has been superficially tinned will melt like one of tin when held in the flame of a Bunsen burner. A wire of tinned copper exposed to the same heat, under like circumstances, remains unaffected, except that the tin is burned off. The affinity of tin for platinum is so great, states Clarke, that if tin and platinum foils be rolled together and heated before the blowpipe, combination takes place explosively. The affinity of tin for gold is uncpiestionably an interesting subject for the dentist, in view of the place these two metals occupy in operative dentistry. Silver alloys with tin, and, in the proportion of 80 of the former to 20 of the latter, it is said produces a very tough alloy. Dr. G. F. Reese's alloy for artificial dentures, constructed by the cheo- plastic process, is composed of tin 20, gold 1, and silver 2 parts. Other alloys much used in cheoplastic work are composed largely of tin. Bean's alloy, intended for casting lower dentures, is composed of tin 95 and silver 5 parts. Antimony 1 and tin 16 parts form another alloy, which is intended 1 Rudberg. ^ Williams. 158 METALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. for the same purpose, and was introduced by the hite Dr. William li. Kiufjshury. Britannia metal is made under a great variety of fc^rmuhe; one known as English is composed of antimony 7.8, tin 90.7, and copper 1.5. It sometimes contains lead or bismuth. Tin is easily deposited upon small articles of brass or copper by simple immersion, as by the following experiment: Place the articles in layers between sheets of grain tin in a saturated solution of potassium })itartrate and boil. A little stannous chloride may also be added, if necessary. LEAD. Plumbum. Symbol, Pb. Atomic weight, 205.35. Malleability, seventh rank. Melting point, 325° (617° F.) Tenacity, Jowest (eighth) r-ink. Ductility, eighth rank. Specific gravity, 11.25. Conductivity (heat), 8.5. Conductivity (electricit}-), 8.32. (Silver being 100.) Occurrence. — This abundant and very useful metal Ls almost wholly obtained from its native sulphide (PbS), or galena, and is rarely, if ever, found free. AMien found associated with silver, the ore is termed argentiferous galena. Properties. — Pure lead is a feebly lustrous, bluish-white metal, endowed with a high degree of softness and plasticity, and almost entirely devoid of elasticity. A wire j^ of an inch in thickness is ruptured by a weight of about thirty pounds. It is said to he the least tenacious of all metaLs in common use. Its specific gravity is about 11.25. It melts at 325° C. or 617° F. At a bright-red heat it vaporizes and at a white heat boils. Its specific heat is 0.0314. Lead exposed to ordinary air is rapidly tarnished, forming it is supposed a suboxide. The same supposed suboxide is formed upon lead kept in a state of fasion in the presence of air, when at the same time the metal rapidly absorbs oxygen; then the monoxide (PbO) is formed, the rate of oxid- ation increasing with the temperature. Its chief use in dentistry is in the laboratory as a counter die. Action of Acids on Lead.— The presence of carbonic acid in a water does not affect its action on lead. Aqueous non-oxidizing acids gener- ally have little or no action on lead in the absence of air. Sulphuric acid, when dilute (20 per cent, solution or less), has no action on lead, even when air is present, nor upon boiling. Stronger acid does act slowly, in general, but appreciably, the more so the greater its concentration and the higher its temperature. Pure lead is more readily acted upon than that contaminated with antimony or copper. Boiling concentrated sulphuric acid converts lead into the sulphate, with evolution of sulphurous oxide. LEAD. 159 The mrtal is readily dissolved in dilute nitric acid, nitrogen dioxide being evolved and plumbic nitrate formed. Strong and hot hydrochloric acts but slowly upon lead, forming the dichloride and liberating hydrogen. Water when pure, has no action on lead per se. In the presence of free oxygen (air), however, the lead is quickly attacked, forming a hydrated oxide, Pb2HO = PbOH20, which is appreciably soluble in water, rendering the liquid alkaline. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbonate — PbCOg (which is slightly soluble in water containing carbon dioxide) — so there is room made, so to say, for fresh hydrated oxide, and the corrosion of lead progresses. Now, all soluble lead compounds are strongly cumulative poisons, hence the danger involved in using lead pipes or cisterns in the distribution of pure waters. We emphasize the word "pure," because experience shows that the presence in water of even small proportions of bicarbonate or sulphate of lime prevents its action on lead. This little sulphate, almost invariably present, causes the deposition of a very thin but closely adherent film of lead sulphate upon the surface of the metal, which protects it from further action. Alloys. — Pure lead unites with almost all metals. Very small quan- tities of lead admixed with the noble metals destroy completely their malleability, and hence render them unworkable. It is said that YWTo P^rt of lead in gold will greatly impair its coining property, and that gold containing g-jy-Q part of lead is "rendered unfit for coinage." The gold drawer in the dental laboratory is often so situated that it is almost impossible to prevent particles of lead from accumulating with the gold scraps and filings. These, however, may be easily removed by roasting with potassium nitrate and sulphur.^ Tin unites with lead in almost any proportion with slight expansion.^ The following table gives an idea of the melting points of alloys of lead and tin: An alloy of — Lead 1, Tin 2 . " 1, " 6 . " 2, " 1 . " 4, " 1 . ". 17, " 1 . With tin 1 part and lead 5 parts^ Dr. Haskell makes counter-dies to be used with Babbitt-metal dies. It fuses at a lower temperature than the die alloy, and also has the advantage of being harder than lead, which he claims is too soft for counter-dies. Tin-lead alloys are used largely in soldering. The following are compositions and melting points of frequently used compounds:* Grade. Tin. Lead. Melts at— Fine solder .... 2 1 340° F. Common " . . . .1 1 370° F. Coarse " . . . .1 2 442° F. > See Gold. ^ Kuppfer. 3 The author has found the fusing point of this alloy to be 378° F. * Tomlinson. Fuses at — . 340° F. . 382° F. . 442° F. . 498° F. . 557° F IGO MFA'ALS AND ALLOYS USED IN PROSTHETIC DENTISTRY. Pewter may be said to be substantially an alloy of the same two metals; but small (|uantities of copper, antimony, and zinc are fre- ([uently added. Common pewter contains about 5 parts of tin for 1 of lead. In France a tin-lead alloy, containing not over 18 per cent, of lead, is recognized by law a,s being fit for measures for wine or vinegar. "Best pewter" is simply tin alloyed with a mere trifle {\ per cent, or less) of copper. liCad contaminated with small proportions of antimony is more highly ])roof against vitriol than the pure metal. An alloy of S3 parts of lead and 17 parts of antimony is used as type metal; other propor- tions are used, however, and other metals added besides antimony — e. g., tin, bismuth — to give the alloy certain properties. Arsenic renders lead harder. An alloy made by the addition of about Jg- of arsenic is used for making shot. Lead forms a very important part in " fusible alloys."^ ' See Bismuth. CHAPTER III PORCELAIN TEETH. By Charles J. Essig, M.D., D.D.S. Revised by Ellison Hillyer, D.D.S. The general use of porcelain teeth in dentistry began about 1825. Previous to that date the materials and means employed in the con- struction of artificial dentures were confined to the various methods of setting human teeth, the teeth of animals, and carving dentures of hip- popotamus tusks, walrus, and elephant ivory, etc. The part of the human tooth used by the dentists, except in transplantation, was the crown portion. MATERIALS USED IN THE FORMATION OF BODIES AND ENAMELS. These are feldspar, silica, and kaolin or clay. The pigments employed to imitate the various shades of color of the natural enamel, dentin, and gums are titanium oxide, platinum, cobalt, iron, gold, and tin. The Body. — ^The body represents the dentin of the natural tooth, and is composed of feldspar, silica (in the form of finely ground quartz), and kaolin (clay), and the yellow-white or ivory-like color of that portion of a tooth is imparted to it by the addition of finely ground titanium oxide. The Frits. — A frit is an imperfectly vitrified mass, formed by the partial fusion of sand and fluxes, from which glass is made by melting. The frits which enter into the composition of teeth are crude colors composed of metallic oxides, such as those of gold, tin, cobalt, etc., ground exceed- ingly fine, in combination with feldspar and certain fluxes, which will hereafter be described. These are burned in a suitable crucible, and then powdered for use in imparting tints to enamels. Enamels. — Enamels are composed chiefly of feldspar, to which is added sufficient quantities of the different frits to produce as nearly as possible the colors of the natural teeth and gums. Bodies and enamels especially prepared for the manufacture of por- celain teeth should possess, after burning, translucency and natural color, together with strength and heat-conducting qualities to a degree that will admit of soldering without danger of fracture from unequal expansion. Translucency and the power of withstanding high tem- perature in soldering depend largely on the feldspar, which forms four- fifths of the bulk of the body. Silica is next in importance as a constituent of the body : its function is to add density and the strength required for masticatory purposes, and, being highly infusible, to assist in retaining the teeth in shape dur- ing the burning process. Without silica the teeth when near the fusing 11 161 362 POliCKLAiy TKKTII. point woukl evince a tendency to a.ssunie tlic .s])lierical form, and their lines and characteristic features would l)e lost. Kaolin, according to S. Welles Williams, " Middle Kingdom," ol)- tained its name from Kao Ling, a])lace in China where it was first ob- tained. The name has been ado])te(l for all varieties of fel(ls])athic com- ponents of porcelain. Kaolin and the clays in general give plasticity to the body, by which the workman is enabled to mold and handle the unburned teeth and blocks without danger of breaking them; it also imparts strength to the porcelain mixture. Feldspar. — This is generally spoken of as a double silicate of alu- minum and potassium, and is represented by the formula A1/)3.K2(), .GSiO,. The best quality of feldspar is found in the neighborhood of Wilmington, Del. It possesses a distinct cleavage, and when broken splits into plates of more or less magnitude. It is of an indefinite color, between yellow and pink, but when fused in the furnace it becomes transparent and colorless, and if not exposed to a too prolonged or an excessively high temperature it retains its original form without round- ing at the corners : this is one of the tests of good feldspar. Feldspar from different parts of the same quarry has been observed to differ in quality. In selecting spar for the preparations of enamel a number of pieces broken from the most perfect-appearing specimens should be fused in the furnace to determine the quality. The crude pieces from which the samples were taken, if found satisfactory, are then broken into small fragments with a steel hammer until they become of a size to admit of its being ground in a large Wedgwood mortar; at intervals the powder is sifted through a No. 10 bolting-cloth sieve, placed iix covered jars, and kept dry for future use. In the preparation of feldspar for enamels the grinding should not be carried too far, as transparency may be greatly lessened, or even entirely lost, by its being ground too fine. The effect of a complete obliteration of the crystallization of the feldspar by too much grinding may be ob- served in a test suggested by Dr. William R. Hall in the chapter on "^Moulding and Carving Porcelain Teeth," American System of Dentis- try: It consists in placing on a slide covered with coarse silex a small piece of crude feldspar of the best cjuality; then taking another piece from the same specimen, grinding it very fine, and fusing the two in the furnace; when cold the difference in appearance will demonstrate that when ground into a very fine powder loss of transparency is appa- rent, and that to preserve its beauty feldspar must be ground only to a certain fineness, beyond which opacity is the result. In preparing feldspar in large cpiantities for extensive use by the manufacturers of molded teeth it is customary to calcine the spar by heating to redness and dropping it into water while hot. This is done to facilitate the reduction of the large masses into small fragments suit- able for grinding in the mortar, which in the large factories consists of a tub with a burr-stone or quartz bottom. The pestle, which is generally formed of a piece of the same mineral, is arranged to revolve by machin- ery. The grinding is done under water. While this plan is less laborious than dry grinding, it probably never affords as good results, in conse- quence of the excessive fineness of the powdered spar. MATERIALS USED. 163 Silica (Si(\,).— This body, sometimes called ,c[uartz, occurs in crystal- line and amorphons forms; it is colorless, infusible at ordinary tempera- tures, insoluble in water and in all the acids excej)t hydrofluoric. The amorphous and gelatinous varieties are partially soluble in alkaline car- bonates, but quite soluble in caustic alkalies. Silica combines with the bases to form silicates. A variety of cjuartz well suited for use in the manufacture of porce- lain teeth is found in great abundance in Pennsylvania and other parts of the United States. It occurs in large irregular masses, white in color, and very difficult to powder. It is used for the purpose of giving sta- ' bility and firmness to porcelains, and its infusibility stiffens and keeps the other materials in shape, so that an object made of porcelain may preserve its molded form while exposed to the high temperature dur- ing the fusing process. For these reasons it is incorporated with feld- spar and clay, and is looked upon as the "main prop in tooth body," in which it is just as essential for the purpose of lessening fusibility as flux is essential in enamels, which are required to fuse more readily. Quartz must be ground under water to an impalpable powder. The proper degree of fineness may be ascertained by placing a small portion of the powdered quartz on the end of the tongue: if it is found to be without grittiness when rubbed against the teeth, it may be dried for use. The preliminary steps in the reduction of this hard mineral to a fine state of division consist in heating to a bright redness as large pieces as the muffle of a furnace will admit, and dropping them into cold water; this causes the quartz to crumble into pieces the size of a pea, which are further reduced in a Wedgwood mortar by the succesive blows of a heavy pestle until fine enough to pass through a No. 10 bolting-cloth sieve, after which it may be brought to the state of an impalpable powder by grinding with water either in an ordinary hard porcelain mortar, or, when it is prepared in very large quantities, by one of the powerful grinding mills, turned by steam, in use at the large manufactories. Clay. — This is a hydrated silicate of aluminum, and when pure may be represented by the formula (2Al203,3Si02) + SHgO. It is formed by the long-continued action of air and water upon granite rock, the dis- integration of which is probably due to both mechanical and chemical causes. Mechanically, the rock is continually broken down by variations of temperature and by the congelation of water witliin its minute pores. Chemically, the action of water containing carbonic acid tends to remove the potash from the feldspar and mica in the form of carbonate of potash, whilst the silicate of aluminum and the quartz are separated by the action of the water: the former, being the higher, is separated from the heavy quartz, and, wlien again deposited, constitutes clay. Kaolin is a pure quality of clay from which such impurities as sand and mica have been carefully excluded by washing, which is accomplished by mixing the clay with a large amount of water in a basin-shaped vessel. It is at first thoroughly stirred, and then, after sufficient time has been allowed for the sand to settle, the upper or lighter layer is poured or run off into another vessel. It is then permitted to stand until the kaolin subsides to the bottom of the vessel: the water is siphoned off; 164 PORCELAIN TEETH. the kaolin is tlicMi dried, when the mass may be turned out and the l)ottom scraped free from any sand found adhering to it. Chiy is infusible in an ordinary furnace when heated alone, but readily unites with feldspar, at high temperature, when incorporated with it, and is an element of strength in porcelain compounds. Kaolin should be thoroughly mixed with the other ingredients of the body while in the dry state, and complete admixtures may be attained by passing the dry body through a No. 9 bolting-cloth sieve. German clay is imported from Europe, and is used to manufacture various articles which require an infusible silicate of aluminum. Formulas for Body. The formulas for bodies and enamels used by manufacturers to-day are, of course, trade secrets. The following, taken from the American System of Dentistry, Vol. II, p. 962, are the well-tested standard bodies of Dr. W. R. Hall, and give an idea of the proportion of the ingredients : Bodies for Molded Block Teeth. Kaolin . 1 oz. German clay . ; Silica 3 " Silica .... 3 Feldspar. . 18 " Feldspar . 18 Titanium oxide . 65 gr. Titanium oxide , 65 Starch, 10 gr. to each oz. Starch, 10 gr. to each oz. II. i oz. gr. It was formerly the custom of makers of molded teeth to at first par- tially burn or biscuit-bake the teeth or blocks. The addition of starch or gum tragacanth to the body does away with the necessity of the "first burning:," as it gives the teeth sufficient firmness to allow of their being safely handled during the process of trimming, which must be done before the final burning. The titanium oxide and the starch are placed in a mortar and ground, at first without water; kaolin and silica are then added, ground together, and sifted through a No. 9 bolting- cloth sieve: the feldspar is then added, and after sieving a second time the mixture is ready for use, and should be kept in a covered glass jar. As it is often desirable to have in stock a small variety of bodies of dif- ferent shades, it will be found of great convenience to have attached to each jar a test sample of the body which has been burned in the furnace, so that the color and texture may be ascertained without loss of time. In preparing bodies and enamels for use in molds they are mixed with water, and then dried to the consistence of dough, when they are placed in the molds with small spatulas. The enamels being laid in the face side and the body in the pin sitle of the molds, these two halves of the mold are then atljusted to each other, placed in a strong press until in complete contact, secured by a strong clamp, and exposed to a heat sufficient to bake the starch or gum, which so hardens the teeth or blocks that they will withstand a very consiilerable amount of force without danger of breaking. During the biu-ning of the teeth the starch or gum burns out without injury to either the body or enamels. COLORS USED IN MANUFACTURE OF PORCELAIN TEETH. 165 At tlio prosont time in most maiiii factories, no difference is made in the composition of the enamels and l)odies save in the matter of their coh)rinif constitnents. 'I'lie body is generally somewhat yellower, due to the titanium oxide. The body and enamel are so placed and prtj- portioned on the face side of the mold as to give the same gradations of color to the artificial tooth which are observed in the natural teeth. We have dnis " point " and " base" enamels to give the correct colors to these portions of the porcelain tooth. In bodies used for carved blocks no starch need be used. The work being done entirely by hand with small knives, it is essential that the material should be plastic enough to cut with facility, while it possesses sufficient toughness to permit of careful handling. These conditions are obtained by simply mixing the body with water, the kaolin present furnishing the desired plasticity. COLORS USED IN THE IVIANUFACTURE OF PORCELAIN TEETH. The colors used in imitating the tints of the natural enamel, dentin, and gums are produced by thoroughly incorporating titanium oxide and preparations of gold, tin, platinum, iron, and cobalt with the mineral substances of which porcelain bodies and enamels are composed. Color frits are made by grinding the metal or its oxide with feldspar and a fine cjuality of glass, which serves as a flux to lower the fusibility of the enamel. The levigation is continued until a very fine state of di^^sion is reached, after which they are biscuit-baked in the muffle of a furnace. ^Mien cool the frit is removed and pulverized in a Wedg- wood mortar, which has first been thoroughly scoured out by grinding with coarse silica to effectually remove traces of any coloring pigment previously prepared in the same mortar. In grinding prepared platinum or gold-foil the feldspar and flux are added by small portions at a time until the greatest degree of fineness is attained. The shade of the enamel will depend largely upon the state of minute division of the metal or oxide. As an example it may be stated that distinctly different shades may be made from portions of the same mixture by reducing one lot to extreme fineness and leaving the other comparatively coarse. Frits. William R. Hall's formulas : Platinum Frit. Blue. Platinum Frit, Gray. Platimmi (dissolved in aqua regia) 1 dwt. Feldspar. . . .1 oz. Plate glass . . . 20 gr. Cobalt Frit, Azure Blue. Smalt (cobalt) Titanium oxide Gold frit . Feldspar ... Platinum frit . Titanium oxide Gold frit 60 gr. 6 t( 60 li 1 oz. Iron Frit, Gray. Iron scale Titanium oxide Gold frit . Feldspar 30 gr. 10 " 100 " 4gr 1 " 60 " 1 oz. Pure gold-foil Plate glass . Feldspar Gold Frit, Reddish-brown. 12 gr. 20 " 1 oz 166 roncELAiN ti:i:tii. Dr. William U. Hall's directions for ])r('])aniio; the ])lalinum and gold frits are as follows: " The metal for the pldfiinnn frit is dissolved in boil- ing nitro-muriatic acid, care being taken not to nse more acid than is just sufficient to make a saturated solution. ^Yhen cold the spar and glass are added and mixed with a glass rod, and ])hice(l in a clay crucible pre- viously washed inside with ])owdere(l (juartz mixed with water. A cover must be closely fitted to the inner edges of the crucible, the joint being carefully closed or luted with clay and (piartz, and burned as has been described." "The metal of the (/old frit is diss( lv<>d in cold nitro-hydrochloric acid; with this exception it is treated in the same way as in the directions for the platinum frit." Prof. Wildman, w^ho called the gold frit " silicdtc of gold," directed that "coarse felds])ar 120 gr., gold-foil 10 gr., flux S gr. be placed in a mortar and ground until the gold is entirely cut up; it is then made into a ball, placed on a slide, and fused in the muffle; then made fine, ready for use." His gold " mixture " was made by dissolving 8 grains of gold-foil in aqua regia, to which were added and well stirred 300 grains of very finely pulverized feldspar. \Mien nearly dry the mixture was formed into a ball and fused on a slide in the muffle of the furnace, after which it was pulverized and kept dry for use. Sponge platinum is a gray, loosely coherent form of finely divided plat- inum that readily absorbs certain gases, as oxygen, and is used as an oxi- dizing agent. Sfonge 'platinum is made by dissolving pure platinum filings or scraps in six times their weight of nitro-hydrochloric acid composed of 1 part of nitric to 3 parts oi hydrochloric. The platinum and mixed acid should be placed in a clean Florence flask and heat gently applied by means of a sand bath, for the purpose of facilitating the action of the acid on the metal. The heat should not be too great, otherwise the effer- vescence will be so violent that a portion of the mixture may be ejected from the flask. Shoidd effervescence cease entirely before all the metal is dissolved, the fluid must be decanted and more acid added until the last particle of the platinum disappears. The solution is then poured into an evaporating dish and evaporated on the sand bath until the mass is nearly dry an. 1 1 gr- 1 oz. 15 gr. 3 gr. 1 oz. 15 gr. Gold-yellow, No. 2. Titanium, pure Gold frit . Starch Feldspar ^Igr. 4 " 15 " 1 oz. Iron-gray, No. 4. Iron-gray frit 4gr. Feldspar loz. Starch 15 gr. No. 6. Iron-gray frit 6gr Feldspar 1 oz Starch 15 gr Iron-gray, No. 8. Iron-gray frit . S gr. Feldspar Starch 1 oz. 15 gr. Gold-yellow, No. 3. Titaniiun, pure Gold frit . Starch Feldspar Broivn-yellow, No. 1. Titanium, pure Platinum frit . Gum frit Feldspar Starch 1 gr. 1 " 4 " 1 oz. 15 gr. Brown-yellow, No. 2. Titanium, pure Platinum frit . Gum frit Feldspar Starch .... 3gr. 6 " 15 " 1 oz. 2gr. 2 " 8 " 1 oz. 15 gr. GUM FRIT (PURPLE OF CASSIUS). Gum frit, as its name implies, is for the purpose of imparting the pink color to the gum portion of a gum section. It derived its name of " Pur- ple of Cassius " from the Gernmn chemist, Andreas Cassius. The dry method, originated by the late Prof. ^Yildman, is the one now employed by manufacturers of porcelain teeth in the preparation of this frit. Pure' silver 240 gr., pure gold 24 gr., and pure tin 17^ gr. are placed in a crucible, with sufficient borax to cover the mass, and meltetl. In order to ensure a thorough mixture of the diff'erent metals the melted mass .should be poured from a height into a vessel of cold water, and this process of granulation should be repeated at least three times; but at every melting the alloy should l)e well covered with borax GUM FRIT. 169 to prevent loss of tin by oxidation. The vessel into which the molten mass is ponred shonld he a woodcMi one. The component parts of the alloy have now been thoronghly incor- porated, the next step is to collect the granulated mass and separate from it any adherent particles of glass or borax. The metal is then put into a glass or porcelain evaporating dish (the Berlin porcelain is the best), and sufficient chemically pure nitric acid is added to cover the rnetal. The dish is now placed over a sand bath, and gentle heat applied and continued until chemical action ceases. If at this point it is found that all the metallic particles ai-e dissolved, the dish may be removed from the bath. Should any solid particles be found in the solution, a little more nitric acid must be added and the operation con- tinued until all are dissolved. The silver having been entirely dissolved by the nitric acid, the solution should be poured off and the remaining oxide carefully washed until the last trace of silver is removed. After several washings with a large quantity of pure warm water the latter should finally be tested with a clear solution of common salt, and if it remains clear, without show of milkiness, the silver is all removed. When the oxide is sufficiently washed the purple of Cassius should be dried by gently heating, after which it is ready to be incorporated with the silicious materials. The process of making gum enamel is divided into three stages: first, the preparation of the oxide; second, fritting, or by the aid of heat uniting the metallic oxide with the silicious base; third, diluting the frit so as to form the desired shade. The frit is formed by mixing 8 grains of the metallic oxide (purple of Cassius) with 700 grains of feld- spar and 175 grains of a flux. The oxide is placed in a smooth Wedg- wood mortar and ground as fine as it is possible to get it. The flux is then added in small quantities and the levigation continued, after which the feldspar may be added and treated similarly. It is of the highest importance that the mass be reduced to the utmost degree of fineness, and an expert workman will spend six or eight hours at least in levigating the quantity given in the formula. While the mass is being ground in the mortar foreign substances, such as small particles of wood, etc., must be carefully excluded; otherwise during the vitrefying process these will be converted into carbon, which will be sure to reduce a por- tion of the gold in fine metallic globules distributed throughout the mass. The vitrefying or fritting process consists in packing the mass, after the most thorough levigation, in the whitest sand crucible that can be ob- tained. (Dark-colored crucibles are liable to injure the frit by contami- nation with iron.) This must be provided with an accurately fitting cover made of the same material, or a suitable top may be formed of a piece of slide such as is used in burning continuous-gum work. Before placing the frit in the crucible the interior surface of the latter should receive a thin coating of very fine cpiartz, made into a paste with water, to prevent the frit from adhering to it during fusion. The frit in the dry state is then packed in, and the cover tightly luted to its place wdth 170 PORCELAIS TKKTH. kaolin. The rrucil)l(> is then buried in a stronfr anthracite coal fire, remaining there until the contents are fused. The time recjuired to do this will depend u])on the size of the crucible and the intensity of the heat. Anv ordinary coal stove provided with a good draught will answer, but the fuel must i)e packed around and over the crucible, and the heat carried to the highest attainable ]>oint. Usually about two hours will be retjuired to thoroughly fuse the mass, after which it is removed from the fire and j)ermitted to cool. The vitrefied mass is removed from the crucible by breaking the latter. Every particle of adhering ({uartz or portions of the crucible should be cleared from the surface. It is then pulverized to a fineness which will allow it to pass through a No. 10 bolting-cloth sieve, and is ready for the third stage in the preparation of gum enamel, which consists of diluting the frit with the proper amount of feldspar. As the strength of the coloring pigment varies according to the degree of fineness attained during the levigation, it is usually necessary to make several tests in order to arrive at the desired shade. This is accomplished by mixing separately several different lots in the following proportions: Gum frit . . 1 part ; Gum frit . . 1 part; Gum frit . 1 part ; Feldspar . . 2 parts. Feldspar . . 3 parts. Feldspar . . 4 parts These are applied to marked pieces of porcelain body and fused in the usual wav, the result determining the proportions necessary to pro- duce the desired shade. Formula for Flux (Glass) for Reducing tfie Fusixg Point OF Enamels. White bottle-glass, which does not contain lead or iron, may be used as "flux" to reduce the fusing point of enamels, but owing to the un- certainty of the composition of glass, most of the manufacturers of porcelain teeth make a fine glass for this purpose after the following formula, the ingredients of which are first ground separately, then thoroughly mixed, and placed in a white crucible provided with a cover (which must be tightly luted) and thoroughly fused. If perfectly pure materials are used, the result will be an exceedingly brilliant, colorless, and transparent glass: Finely pciwdered silica " " .tr;lass of borax Potassium car])oiiate . 12 oz. 3 " 3 '• FORMULAS FOR CONTINUOUS-OUM WORK. 171 FoKMl'LAS FOR CoNTINUOTTS-CrM WoRK. Bodies and tMiaincls intended for u.se in the " eontinuous-guni " pnx-- ess nuist necessarily be more fusible than the materials of which teeth are composed, in order diat the latter may not be affected by the three heatings the denture must be exposed to i)efore it is completed. It will therefore be noticed that an unusual amount of flux enters into their composition. The formulas herein given are those of well-known experts in continuous-gum work. Continuous-Gum Formulas of Dr Hunter. Flux: Quartz 8 oz Calcined borax 4 " Caustic potash 1 " Granulated Body: Spar . 2 oz Quartz ^ " Kaolin 1 ii Body: Flux, as above 1 oz Asbestos 2 " Granulated body . n " Gum Enamel: Flux, as above 1 oz Fused spar 1 " English rose-re d 40 gr Form ULAS OF Grannlated Body: Quartz 20 gr Spar . 24 " Caustic pot£ ish 1 " Titanium 2 gr.-l oz. Flux: Quartz, very fine 18 dwt. Spar 10 " Glass of borax . 2 " Cryolite . 1 " Caustic potash . 10 gr. Titanium . 1; gr.-l oz Gum Enamel: Gum frit of (S. S.White) . 4J dwt Flux without titanium . 16 " Gfanulated body . 11 " Cryolite 7 " Fuse in crucible to form glass; when cold reduce to powder. Fuse in crucible, and powder to pass through No, 50 wire sieve. Grind the first two articles very fine, then add granulated body, which is mixed with the fine without grinding. Grind very fine and -semi-fuse in crucible; powder coarsely for use. Grind fine and fuse on slide in furnace; powder coarsely for use. Fuse same as above, and grind very fine. Fuse and grind for use. Dr. Moffit's Formula for Continuous-Guai Body. Body: Spar . . . 12 oz. Quartz . . -42 " Bohemian glass . 60 gr. [ Grind coarsely. French china . . 35 " German clay . . 2 dwt No gum-enamel formula came with this. Dr, Smith's formula for gum enamel will do for the above, minus the cryolite. 172 PORCKLA IN TKKTII. THE PROCESS OF MANUFACTURE OF PORCELAIN TEETH. Brass Molds for Porcelain Teeth. — The mamifacture of molded teetli as carried out ii])on tlie extended scale of the present day, to meet the (hMnands of tlie trade, recjuires the use of a large number of hrass molds of various sizes and forms. In the l)est factories a " pattern " mold of each of these is kept as a standard to preserve the uniformity of the product. It is not used in molding teeth, but serves as a pattern, from which are secured the " duplicate " molds actually employed in manufacture. Mold-making includes the carving^ of the plaster blocks, making the plaster pattern of the mold, casting it in hard brass or bronze, and the " cutting " or finishing of the mold. The plaster blocks constitute the design, and are a complete set of block teeth carv^ed in plaster with allowance made for shrinkage and other changes which take place in the vitref}ing process. This part of the work must be done by an artist, and one who has know- ledge of the several classes of hu- man teeth. These designs should always be made from natural teeth. Dr. J. Leon ^Villiams, of Lon- don, is at the present time pre- paring a set of molds, 36 in num- ber, providing 12 t\'pes, with 3 sizes in each tx'pe. These molds, it is hoped, will provide a range of selection for any but the most extraordinary cases. The original models are being carved with the greatest attention to anatomical form and are, in the sets for edentulous cases, to be articulated in the original to provide for a minimum amount of grinding in articulation of the porcelain re- productions. The plaster blanks are arranged on a rim of wax similar to articulating wax; the gums are formed of paraffin or the pink combina- tion of paraffin and wax; broad spaces are left between the centrals; the cuspid and first bicuspids and the second bicuspids and first molars, as sectional blocks for an entire upper denture, are divided into six pieces. Each block must be provided with a slight excess of material at the joint, to afford sufficient latitude in fitting them together. A wall of plaster about one-fourth of an inch in thickness is rim on the outside of the model, so as to include the entire set of l)lanks ; when hard enough it is trimmed so as not to exceed in height the cutting edges of the tooth blanks, varnished, and oiled; an inside wall is then made of plaster of the same height as the outside one. The inside wall is removed when hard in one piece; the outside one fs cut into six pieces with a thin saw Tooth blanks arranged on cast. THE PROCESS OF MANUFACTURE OF PORCELAIN TEETH. 173 blade, the cut being made between the centrals, the canines and l)icus- pids, and the bicuspids and molars at the spaces shown in Fig. 115. The sections tluis made are then separated from the model. Both the outside and inside walls are trimmed, varnished, and laid aside to dry. The removal of the blanks from the model is next in order, and both walls are given a coating of shellac varnish. Fig. 11 G shows the walls made for a lower set of plaster blocks. The walls are now to be placed on the model and secured in position with twine or wire; they are then oiled, and plaster mixed to the consistence of cream is first painted over the surface, as representing the teeth, w^ith a camel's- hair brush, when the residue is run in betw^een the inside and outside Fig. 116 View of the walls and cast, separated. walls and allowed to set thoroughly before removal. If the plaster has been carried into all depressions and interstices between the walls, a continuous set of plaster blocks will be the result. These are separated into six sections by means of a thin saw blade, as shown in Fig. 117, the six front teeth in two sections of three each, the first and second bicus- 174 PORCELAIN TEETH. pids of each side in two otiier sections; the molars are (Hvided in the same may. These phistcr blocks now recjuire trimming on the inside, the carving of the masticating surfaces of the bicuspids and molars in imitation of the natural organs, and the cutting of a recess for the pins, as seen in Fig. 117. The ends of the blocks, or those parts technically called the joints, must be trimmed so that they will taper sufficiently to ensure their safe Fig. 117 k,^, liiii^lietl delivery from the plaster mold, as shown by the pin sides of the plaster blocks in Fig. 117. If not properly bevelled at all points, so that no undue retention will occur, it may be necessary to remove the pattern blocks piecemeal. The plan of such a mold would then be found to be defective and, as may readily be surmised, it would not be possible to Fig. 118 B- % 1 , -; -3 " ~ J fro O^ m] -' M "31 = -^ w _J ^^ ^ Foundation plate. obtain a brass mold from a pattern in which so serious a fault existed. After these details have received careful attention and the blocks have THE PROCESS OF MANUFACTURE OF PORCELAIN TEETH. 175 been triinined and carved to the satisfaction of the workman, they should receive at least two coats of shellac or sandarac varnish. The Plaster Mold. — ^I'he preparation of the plaster mold is the second part of the process of mold-making, the first part being the modelling of the designs in the form of plaster blocks. These designs are really the foundation of the whole system, and require in their production artistic talent and knowledge of the forms of the different t;y'pes of human teeth. With the exception of the cutting or finishing of the brass mold, . the rest of the process is purely mechanical. By referring to Figs. 122 and 123 it will be seen that the finished mold consists of five pieces — the face side, the pin side, two end or crown pieces, and a key piece. These pieces have all to be made in plaster to serve as patterns from which to cast fac-similes in brass. The plaster blocks are arranged on a foundation plate. This plate can be Fig. 119 Brass frame. made of brass or zinc, as shown in Fig. 118, 5j inches long, 3 inches wide, and \ of an inch thick, with oblong recesses to receive the blocks — those for the front blocks 1 inch long and I an inch wide; those for the bicuspids, f of an inch long and h an inch wide; and those for the molars I of an inch long and % an inch wide. The black lines on the plate are used as guides to measure from in placing the blocks ; the round holes at the ends are to receive the pins seen on the frames in Fig. 119. The plaster blocks are placed on the foundation plate faces upward, with the cutting edges opposite to each other, as seen in Fig. 120, and are then secured in position with beeswax, clay, or putty, which also marks the correct line of division between the two halves of the mold. This is a very important detail of mold-making; upon its correct man- agement depend the successful application of the enamels and the safe delivery of the molded porcelain blocks. This line of division should extend along the middle of the cutting edges of the incisors and canines 176 PORCELAIN TEETH. and the fjjum cd^Q, but should include but little of that portion of the block called the joint. By referring to Fig. 121 the reader will see that the face side of the mold gives merely the distinct outline of the entire face of the block, and that the bulk of the block is rej)resented in the pin side; yet the edges of the face side of the blocks should be sufficiently well defined to assist in holding the enamels in position when the mold is pressed together. The front blocks are secured to the plate -j^ of an inch from its centre line; the bicuspid blocks, -^ of an inch from the line; the molar blocks, yg- from the line. The side blocks are arranged farther apart at the end near the molars than at the other, to allow for a tapering key, as shown by C in Fig. 123. If all spaces between the block and the foundation plate have been stopped with clay or putty, the plate with arranged blocks is ready for the frame. The frame is in two sections made of polished brass, one section made to articulate with the other by means of pins and corresponding holes, as seen in Fig. 119. The inside tapers so that the plaster mold when hard may deliver without difficulty. These frames measure 4f inches in length FiG. 120 Pin side of plaster mold, with plaster blocks and wax in position. and 2f inches in width, each section being f of an inch high with a thickness of \ of an inch. The part with pins is well oiled on its inside and placed on the plate. The plaster blocks having been oiled, the plaster is mixed by dropping it into water and allowing it to settle without stir- ring, so as to exclude air; the excess of water is then poured off; the blocks carefully coated with thin plaster by means of a small camel's- hair pencil; the rest of the plaster is then poured in and levelled off even with the top of the frame with a spatula. When the plaster becomes hard the plate is removed and the face side of the pattern is secured. The blocks are then carefully taken from the face side of the plaster mold, and, if any of the edges are broken, they must be repaired with wax or plaster and made smooth. After the removal of the blocks the mold must be varnished with shellac or sandarac and allowed to dry thoroughly, when the blocks are replaced. The space between the bicuspid and molar blocks is filled by a piece of wax, as seen in Fig. 120, to form the depression intended for the end pieces and key in the pin side of the mold. The whole fixture is then thoroughly saturated in clean water, the surfaces coated lightly with oil, and the other section THE PROCESS OF MANUFACTURE OF PORCELAIN TFFTlf. 177 of the brass fnunc [)lace(l in position and filled with j)laster mixed and applied in the manner previously deseril)e Gum sections for partial cases. partial ones, gum teeth may be used to great advantage. They are made in a variety of forms and offer a wide selection (Fig.?. 154 to 159). The forms of porcelain teeth ase determined by three factors. The most important of these is the anatomical characteristics of the teeth they are to substitute. As only the crown is represented, the labial or buccal surfaces, the morsal surfaces, a.nd such portions of the approximal surfaces as are presented to view are patterned after the natural teeth. Teeth c^uite satisfactory in this respect are manufactured to-day, although the market contains many made according to old designs which are poor imitations of the natural organs. The form of the incisors and cuspids is in general much better than that of the molars and bicuspids, the occlusal surfaces of many of which are too narrow for the best masti- 190 PORCELAIN TEETH, catoiy results, the cusps are too poorly defiuod, and no attempt is made to have those of opposing sets fit together. The sha})e of the other ])()rtions of the teeth is determined by con- siderations reUitive to their attacliment to the l)ase u])on which they are mounted, and by the mechanical re(|uirements which the shape and relation of the jaws impose. Teeth for vidcanite and celluloid work are similar in design (Figs. lOO to 169). WTien the latter came into use the artistic ])()ssibllities of die new^ material created a demand for more natural forms of teethe and so-called " celluloid " teeth were designed Fig. 160 Fig. 161 Fig. 162 Fig. 163 Fig. 164 FiQ. 165 Plain rubber teeth, upper and lower incisors, and cuspids. Fig. 166 Fig. 167 V\G. 168 Fig. 169 Plain rubber teeth, upper and lower bicuspids, and molars. to meet it. Teeth of this form may also be used with a cast-metal base, but they are all designated rubber teeth. Plain rubber teeth are provided with two-headed pins to secure their attachment to the vulcanite. In the incisors and cuspids there is the so-called " pin " guard situated between the pins and the morsal edge to afford a shoulder to which the vulcanite may be finished. CLASSES OF POIiCELAiy TKF.TH. 191 Countersunk pin teeth were introduced about 1885. Their hngual surface corresponds in shape to that of the natural teeth, the attaclinient to the molded base being by means of pins located in a (lc])ression in their base. Their close conformity in contour to the natural organs makes them more acceptable to the tongue than teeth backed in the ordinary way, and renders articulation easier and more distinct. Inasmuch as they must be mounted almost over the alveolar ridge, they cannot be used in cases with a short bite. Plain plate teeth are designed for use on a metal plate or in crown and bridge-work. The incisors and cuspids, in either instance, are similar in form (Figs. 137 to 139), but those for use in the bicuspid and molar region for crown and bridge-work represent only the buccal surface of the tooth, and are sometimes known as " veneers " (Fig. 170). Two pins project from the flat back of the tooth to afford attachment to the metal backing which is brought into contact with the back of the tooth for its support. These pins are arranged crosswise in very short Fig. 170 Veneers for crown and bridge-work. teeth, but where there is space they are arranged longitudinally because of the greater strength thus obtained. Saddle-back teeth (Figs. 146 and 147) are suitable for vulcanite dentures in which the space between the jaws posteriorly is very slight and where it would not be possible to get in a plain rubber tooth. They may also be used for bridge-work. Continuous-gum teeth are illustrated in Fig. 148. It will be seen that they have only one long pin, and that the buccal and labial portions of their roots are represented in porcelain. This contributes to the firmness of their attachment to the base, the porcelain body fusing upon the roots and uniting them to the plate. It also maintains the contours of these regions by reducing the amount of porcelain body to be baked, and hence the contraction in this locality. English tube teeth (Fig. 145 to 149) are little used in this country except for crowns, but they are more or less extensively employed in England. A platinum tube baked in the centre of the tooth affords lodgment for the pin. which attaches them to the plate or root. CHAPTER IV. THE HUMAxN DENTAL MECHANISM; ITS STRUCTURE, FUNCTIONS, AND RELATIONS. By Charles R. Turner, D.D.S., M.D. The oral cavity is situated at the beginning of the aHmentary canal; it serves for the intake of food, and is provided with the means of its preparation for the subsequent stages of digestion and for assimilation. In the production of articulate speech, it serves as a resonating chamber for the vowel tones, and by alterations in its shape, through the action of the muscles of the tongue, lips, cheeks, and palate, it determines the char- acter of the consonant sounds. The face is the principal seat of the expression of emotions in man, and as the mouth is one of its most mobile features, it participates prominently in the performance of this function. The loss of the teeth and the changes in the surrounding tissues which result therefrom arefollowedby an interference with the functions above enumerated, and by alterations of facial contour which greatly affect the personal appearance of their losers. As it is the purpose of dental prosthesis to prevent or remedy these changes, as far as possible, by the substitution of artificial apparatus for the lost tissues, a knowledge of the normal operation of the several functions thus affected is of fun- damental importance to the study of the subject. This chapter aims, therefore, to treat of the functions of the dental mechanism from this point of view. It will discuss the manner in which they are affected by the loss of the teeth, so that the problems arising in tooth replace- ment may be solved the more satisfactorily. The most important function of the mouth, from the standpoint of the body economy, is the preparation of food for the subsequent stages of digestion. Food stuffs of various degrees of physical consistence are introduced into the cavity, their solid portions are cut, crushed or ground, mixed with the saliva, and are then carried into the stomach. As this is largely a mechanical process, we shall first take an analytical view of the mechanism by which it is accomplished. THE HUMAN DENTAL MECHANISM. Architecturally considered, the masticating mechanism consists of a fixed base, the upper jaw, supporting the upper teeth, against which is operated a movable arm, the lower jaw, also equipped with teeth so placed as to oppose those of the upper jaw. The teeth serve as the armament of the apparatus, affording hard surfaces between which the food is crushed. Muscles extending between the fixed and movable elements of the apparatus, and so disposed as to be capable of exerting 192 THE FIXED BASE. 193 great pressure in approximating them, furnish the active forces of the mechanism; while the walls of the buccal cavity, which contains the apparatus, serve to confine the food, and the lips and cheeks together with the tongue further assist the process by keeping it between the crush- ing surfaces. The Fixed Base. — The axial skeleton of the body consists of the ver- tebral column, the skeletal basis of the trunk, which supports upon its upper extremity, the skull. The cranial portion of the skull receives direct support at the atlo-occipital articulation, the facial portion being in turn suspended from the anterior surface of the cranium. The superior maxilla?, united in the median line and lodging the upper teeth, are sup- ported upon the cranium through articulations by the frontal, lachry- mal, ethmoid, palate, vomer, and malar bones, and constitute the fixed base of the masticating mechanism. Fig. 171 k 9 i^B ' ^flB ■^H ^H ^wl I'P M ^^|H H| J/ .1 ^j^MMHH^^^Hj^^H j^M SS( jUi m ' c 4t. P 3>r-' 1 Architectural construction of skeletal portion of masticating apparatus: the fixed base, and moveable arm. Columns, arches, and buttresses of the fixed base; fronto-nasal column, A B; zygomatic column, C M D; pterygoid column (only partly visible), supra-orbital arch, B F D; infra-orbital arch, BID; upper nasal half arch, B G; palatal arch (not shown); lower nasal arch, A H; large molar arch, A C; molar buttresses (descending from M); pterygoid arches (not shown). Columns and arches of the moveable arm; mental column, N K; coronoid column, P Q O; and condyloid column, J L; external oblique column, Q N. (From a photograph of specimen No, 4237, Wistar Institute of Anatomy.) The arch shaped alveolar process (Fig. 171), containing the teeth which receive the impact of mastication, is supported upon the cranium by three pairs of columns or buttresses, reinforced by several secondary arches and braces. (Burchard.) The columns are the fronto-nasal, as- cending from the position of the canine tooth to the inner margin of the orbit, and reaching the frontal bone; the zygomatic, extending from 13 194 THE HUMAN DENTAL MECHANISM. the position of the first molar tooth through the outer margin of the orbit; and the pterygoid, ascending from the molar region upward and backward to the sphenoid. These columns are strengthened by braces as follows: the supra-orbital arch, the infra-orbital arch, the upj)er nasal half arches, the palatal arch, the lower nasal half arches, the large molar arch, the molar buttresses, and the pterygoid arches. It will be noted that the articulations of the maxilhie, notably those with the malar bones, are disposed to resist stress operating from below. This fixed base is admirably constructed to receive, resist and dissipate force received from below so that the brain and delicate sense organs contained in the face will not be unduly shocked during mastication, and is "erected to endure the shock of impact from all sorts of biting, i.e., cutting, tearing, crushing, triturating with longitudinal and trans- verse motions, and to resist and stand firm during all the varieties of movements incident to mastication." Fig. 172 The lower jaw as a lever of the third class. (Burchard.) The Movable Arm. — The lower jaw consists of an arch-shaped horizontal portion, the body, which supports the lower teeth by means of its alveolar process, and terminates posteriorly in two vertical branches, the rami, by which it articulates with the cranium. It may be re- garded mechanically as a lever of the third class (Fig. 172), the fulcrum being located at the temporo-mandil)ular joint, the power being repre- sented by the levator muscles, while the weight consists in the resistance to the elevation of the jaw offered by the food between the teeth. The muscular apparatus by which the jaw is raised is attached in part to the cranium and in part to the strong zygomatic arch, and is not concentrated upon one point on the mandible, but its attachment thereto is distributed between the fulcrum and the weight. The resultant line of its action when the teeth are in contact passes sliglitly behind the coronoid process of the mandible and approximately through its anatomical angle. (Fig. 173, M.) The lever is bent at this point so that the general plane of the teeth wdien in occlusion is perpendicular to tlie line of action of the levators. By this arrangement the teeth may best oppose the action of the levators, since they resist force best which acts THE MOVEABLE ARM. 195 at a right antrle with the general plane of their occlusal surfaces. The resistance offered by the food between the teeth (which represents the weight) varies for any given food with its position on the teeth. For a food of given resistance the farther back in the mouth it is placed, the nearer it is to the fulcrum, and the shorter the weight-arm of the lever, and hence the less will be the muscular force necessary to overcome the resistance. The point of the origin and insertion of the muscles is fixed, but as the position of the fulcrum changes in its relation to the former of these during the movements of the jaw, the length of the power-arm of the lever is affected, and the resistance which may be overcome by the muscles varies in accordance therewith. The more nearly the jaws are together Fig. 173 Diagram illustrating line of action of levators of lower jaw. the longer is the power-arm, and hence it will be found that the more nearly they are approximated, the greater will be the power which the muscles may exert. From these several facts it will be seen that the lever is of the type best suited to the lifting of a heavy weight slowly through a short distance. To resist the stress put upon it in these operations the jaw is well de- signed structurally. (Fig. 171.) It may be considered as composed of several pairs of bony columns. The vertical columns are the mental column, extending perpendicularly upward from the lower margin of the body to the position of the canine tooth, the coronoid, ending in the coroiioid process, and the condyloid parallel to it and ending in the condyle. These are bound together by the long body columns of the HjQ THE HUMAN DENTAL MECHANISM. external and internal oblique lines. Cryer' has pointed out the archl tectural strenjjjth of the body of the bone, the cortical U-shaped por- tion of the exterior being bound together by the internal cancellated tissue. The fulcrum of the lever rests upon the base of the skull where the bone is thick and dense and well dcsi(2;ned to resist the force put upon it, and the joint itself is provided with intervening soft tissues as a means of lessening the shock of activity. The Temporo-Mandibular Articulation. — This is a condylarthrodial joint, the structures taking part in it being the glenoid fossa of the tem- poral bone and the condyle of the lower jaw, together with the proper and accessory ligaments, and the tissues interposed between the bones. The Glenoid Fossa. — This is an ol)long cavity (Fig. 174) on the under Flc 174 Left glenoid fossae of four skulls, showing differences in form. surface of the squamous portion of the temporal bone, its concavity being directed downward. It is bounded anteriorly, by the eminentia articu- laris (the anterior root of the zygoma), externally, by the middle root of the zygoma and the auditory process, and posteriorly, by the tympanic plate of the petrous portion of this bone. The Glaserian fissure which runs across the cavity divides it into two unequal portions, the posterior third being rough and lodging a portion of the parotid gland, the anterior two-thirds being smooth and covered in the recent state with a dense fibrous tissue and receiving the condyle of the lower jaw. Of the articular portion of the fossa the distal part is the most concave and is also the most elevated. From this point it slopes downward and forward to the crest of the eminentia articularis, furnishing a surface over which the condyle glides in the forward excursions of the mandible. The shape of the cavity varies with different nationalities, with different individuals, and sometimes on both sides of the same individual. The principal variations are (1) in size and general concavity, in corres- pondence wath the shape of the condyle; (2) in extent of surface from the most concave portion to the eminentia articularis; (3) in its inclination. ^ Internal Anatomy of the Face. THE GLENOID FOSSA. 197 The outlines in Fig. 175 show the curve of this cavity obtained fnnn skulls after the method of Tomes and Dolamore. The fossa alters frequently in old aner teeth. Fig. 188 It Tm 1 1 1 I 1 ^' J ^^Hki'^' 1 ■ ■ ■ "^ '^ M ! ! i 1 Occlusal surfaces of the lower teeth. The outlines of the upper and lower arches correspond in shape in order that the teeth may be opposed throughout their series, except that the upper is slightly larger and Qverhangs the lower externally. This difference in size is necessary in order that the fixed base shall present a sufficientlv large surface for contact durino; the excursions of the move- able arm and it is extended in the direction of these movements. This is due to the greater size of the upper teeth and the greater segmental form of the arch : both the overhanging and difference in size decrease from the median line backward. THE OCCLUSION OF THE TEETH. 213 The teeth of each arch (Figs. 1S7 and 188) are seen to present an unbroken series of occlusal surfaces extending from the terminal molar on one side to that on the other. Besides giving greater surface area for masticatory purposes, this approximal contact provides a mutual support for the teeth which is of great value. When the character of man's food is remembered, it will be seen that this provision is also of some importance for the protection against injury of the soft tissues of the interproximal space. The teeth are divided into four classes anatomically, and their func- tions as mechanical instruments more or less correspond to this division. As viewed from their occlusal surfaces, (Figs. 187 and 188), it will be noted they are well-shaped for their several offices. The incisors are designed to cut off definitely sized masses of food, and their morsal sur- face is in the form of a broad blade, the labial and lingual surfaces meeting at an angle at the incisal edge. The canines in the lower animals serve largely for prehension — to pierce and hold the more resistent food, and among the carnivora to guide the lower jaw into place. In man these functions are largely rudimentary, the tooth being intermediate in function between the incisors and bicuspids. It has a double blade ending in a well defined point. The bicuspids, as their name implies, have two cusps, and are intermediate between the canines and molars. While their func- tion is largely to crush and press the food, they participate in trituration, which is the characteristic office of the molars. The greater sharpness of their cusps fits them more for piercing than for grinding. The molars are the grinding teeth proper, for which their tuberculated surfaces are well designed. The cusps alternate with fossae and grooves, and are joined by ridges which afford a surface most effective for trituration as will be seen later. It will be observed of the molar and bicuspid series of cusps and depressions of each jaw, that as they are higher and larger anteriorly, they are smaller and shorter as the distal end is approached, so also there is a diminution in the distance between them both mesio-distally and bucco-lingually. The Occlusion of the Teeth. — When the ideal or typical denture is viewed in occlusion, (Fig. 189), it will be seen that there is a definite mutual relation of the occlusal surfaces of the teeth. It will be observed that there are two different types of occlusion — first, that correspond- ing to the incisors and canines, in which the morsal edges do not meet end to end, but those of the lower teeth rest upon the lingual surfaces of the upper; and second, that corresponding to the molar and bicuspid teeth, where considered collectively their cusps are either received into fossae or depressions in the occlusal surface of the opposing series, or overlap the buccal or lingual surface of their opponents. It is notable that with the exception of the lower central incisors and the upper third molars, each tooth is opposed by portions of two others. This provision serves to dissipate the force of impact in occlusion, and tends to preserve the integrity of the denture; for with this 214 THE HUMAN DENTAL MECHANISM. arrangement the loss of a tooth in one arch does not mean the loss of a tooth in the other througli lack of antagonism. Fir,, is'l Upper and lower teeth in occlusion. (From jihotograph of specimen in the Wistar Institute of Anatomy.) The Occlusion of the Incisors — The typical occlusion for the incisors is shown in Fig. 190, in which it will be seen that the upper incisors overhang the lower for about one third their labial surfaces. This Fig. 190 fV ' I^^^^^^^^^^^^^M i < ^ Occlusion of the incisor teeth. (Fnjm photograph of specimen No. 4237, Wistar Institute of Anatomy.) relation is spoken of as the normal overbite of the incisor teeth and ob- tains in a large percentage of cases. The angle formed by the long axes of the upper and lower incisors differs much in individuals, as will be seen under Temperament, and the contact between the two varies THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 215 in consequence. In some the angle is so obtuse that tliere is con- siderable contact between the labial surface of the lower and the lingual surface of the upper incisors. The size of the arcli of the upper may be so great that the lower teeth are not in contact with the up])er in the position of occlusion, only coming in contact when the lower jaw is protruded in incision. The so called "edge-to-edge" bite of the incisors, which is seen in those temperaments in which the over- bite has been short and the teeth have worn, or have been originally erupted in this position (Fig. 191, B), is within the range of the normal but is a less effective mechanical arrangement for incision. Fio. 19; A B Occlusion of the incisor teeth. A. Normal overbite; B. Edge-to-i incisors distal to lower. (Grevers.) bite; C. Upper The condition presented by Fig. 191, C, in which the upper in- incisors occlude lingually to the lower is less effective still, because the lower jaw cannot move backward to bring the teeth into contact. In a typical denture BalkwilF has pointed out with regard to the scissors-like action of the front teeth, that as they are wedge shaped, he "expected to find the angle of the wedge equally divided by the circleof motion, which would give the greatest dividing power," but this is not the case. The angle of the wedge points more outward in the upper and inward in the lower, and he reminds us that in closing the teeth, there is a backward as well as an upward motion. Burchard has pointed out the effect of this motion upon the direction of the im- pact of the teeth during closure of the jaw; that unless there were a back- ward movement at the same time the effect would be to drive the upper incisors forward. Constant^ has shown that the direction of impact in ordinary closing is almost vertical. The Occlusion of the Bicuspids and Molars. — The bicuspids and mo- lars on each side viewed collectively (Fig. 194), may be considered as consisting of a series of cones or cusps alternating with fossae or de- pressions, and so fitted together when the teeth are in occlusion that the fossae receive cusps of the opposing teeth. The inner line of cusps of the upper are received into the fossae between the outer and inner cusps 1 Transactions Odontological Society of Great Britain, Vol. v., p. 133. 2 Naked Eye Anatomy of the Human Teeth, p. 187. 21() THE HUMAN DEXTAL M ECU AX ISM. Fio. 102 Occlusion of the molar and bicuspid teeth, external view. (From photograph of specimen in posses- sion of Dr. F. A. Peeso.) Fi.;. I'j:-; Occlusion of the molar and bicuspid teeth, internal view. (From photograph of specimen in posses- sion of Dr. F. A. Peeso. Same specimen as Fig. 192.) THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 217 of the lower (Fig. 193), the outer line of cusps of the lower being corres- pondingly received into fossae in the upper (Fig. 192). Thus the lin- gual cusp of the first upper bicuspid (Fig. 193) is received, between the buccal and lingual cusps of the lower bicuspids, the point of the cusp corresponding to the line between the mesial marginal ridge of the sec- ond and distal marginal ridge of the first. Similarly, the lingual cusp of the second upper bicuspid is received at the line of contact between the marginal ridges of the second lower bicuspid and first molar. The mesio-lingualcuspof the first upper molar fits into the central fossa of the first lower molar, while the disto-lingual cusp occupies a position in relation Fig. 194 Occlusion of the molar and bicuspid teeth, occlusal view. Lines are drawn from the lingual cusps of the upper teeth and buccal cusps of the lower to the corresponding depressions into which they fit. (From photograph of specimen in possession of Dr. F. A. Peeso. Same as Figs. 192 and 193.) with the adjoining marginal ridges of the first and second lower molar teeth, i.e., in the depression between the disto-lingual and buccal of the first lower molar and mesio-lingual and mesio-buccal of the second lower molar. The second upper molar occludes similarly with the lower second and third molars, while the third upper molar varies in this re- spect as it varies in form. The large lingual cusp, usually found on this tooth, however, normally occupies the central fossa in the third lower molar. The buccal cusps of the lower are received into the fossae and depres- sions of the upper teeth as follows (Fig. 192) : The buccal cusp of the first lower bicuspid is in relation with the distal marginal ridge of the canine and the mesial marginal ridge of the first bicuspid (Fig. 194). This is less like a cup-shaped depression than any which succeed in this description. The second bicuspid has its bucca;l cusp received in the depression between the buccal and lingual cusps of both upper bicuspids, 218 THE HUM AX DENTAL MECHANISM. its point being in definite relation with their udjaeent marginyl ridges. The mesio-buccal cusp of the first lower molar rests in relation with tlie adjoining marginal ridges of the first upper molar and the second bicuspid, while the large buccal cusp is received in the central fossa of the first upper molar. The disto-buccal cusp of the first lower molar, which is present in 50 per cent, of cases, is usually so distaliy located that it shares with the mesio-buccal cusp of the second lower molar the space between the cusps of the first and second upper molars and is in contact with their adjacent marginal ridges; or it is forced to the lingual to be lost in the distal marginal ridge. The mesio-buccal cusp of the second lower molar has just l)een located, while the remainder of the tooth is similarly related to the upper first and second molars as the first molar is to its two antagonists. The third lower molar varies so in form that it is difficult to say what is its typical occlusion. The two types most commonly seen (Broomell)* are those with four or five cusps respectively, the occlusion being similar to the molars already described with regard to the mesio-buccal and buccal cusps in either case, while where the fifth cusp is present, it occludes simply with the distal portion of the upper molar. The buccal cusps of the upper molars and bicuspids are sharper than the corresponding lingual cusps (Fig. 195). They are related to Fig. 195 Section through upper and lower teeth in occlusion, showing relative height and sharpness of buccal and lingual cusps. (Cryer.) the lower teeth as follows: (Fig. 192.) That of the first bicuspid is re- ceived in the groove between the buccal cusps of the lower bicuspids and to the buccal side of the teeth. That of the second corresponds to the space between the buccal cusp of the second lower bicuspid and me.sio- buccal of the first lower molar. The mesio-buccal of the first upper molar is received in the buccal groove of the first lower molar, the disto- buccal cusp occupying the disto-buccal groove, where the lower molar "^ Broomell: Anatomy and Histology of the Mouth and Teeth. THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 21!) has five cusps, or the space between the second molar and the first, where it has four. The buccal cusps of the second upper molar occlude so sim- ilarly to those of the first that they recjuire no mention, while the two buccal cusps of the third likewise similarly occlude, except where the lower molar has no disto-buccal cusp when only its last cusp is in re- lation with the distal surface of the lower tooth. As to the lingual cusps of the lower teeth (Fig. 193), they are in general smaller and more pointed (Fig. 195) , than the buccal. A marked excep- tion exists in the case of the first lower bicuspid whose lingual cusp is frequently so rudimentary as to be represented only by a ridge of enamel; the second is sometimes similarly formed. BonwilP has pointed out the lack of function of this cusp of the first bicuspid, which fact will be seen more clearly when the functions of the teeth are described. When present this cusp is in relation only with the mesial slant of the lingual cusp of the first upper bicuspid. The lingual cusp of the second is lingually placed to the space between the lingual cusps of the two upper bicuspids, while the mesio-lingual cusp of the first lower molar is similarly placed between the first molar and second bicuspid. The disto-lingual cusp corresponds in position with the lingual groove of the first upper molar. The lingual cusps of the second molar are similarly related to the first and second upper molars, and this is likewise fre- quently true for the third lower molar, with the exception of its disto- lingual cusp, which, having no groove with which to be in relation, touches only the distal incline of the single lingual cusp of the upper wisdom tooth. It will be noted that the cusps received into fossae are the more rounded, that the cusps which overlap are the sharper and the smaller. (Fig. 195.) In the upper jaw it is noted that the fossse are separated antero-pos- teriorly or mesio-distally by the transverse ridges of the bicuspids and by the mesial marginal and oblique ridges of the molars. These ridges are received by the grooves separating the buccal cusps of the lower teeth. The lower fossse and depressions which receive the lingual cusps of the upper series are similarly separated, and these ridges are received into the grooves separating the lingual cusps of the upper teeth (Fig. 194.) It is evident that in order to move the jaw in any direction it must be depressed to disengage the cusps from these fossae. The actual height of the cusps and the corresponding depth of the fossse vary greatly in different individuals and will be discussd later under the head of Temperament. It may be mentioned here however that this variable factor is one of that related group which will be con- sidered later in this chapter. Bonwill ^ has stated that there is an almost constant relation between the overbite of the incisors and the length of the cusps of the bicuspid and molar teeth. (Fig. 196.) Wliere the overbite is considerable, the molars and bicuspids will usually be found to possess high cusps, the overhanging of the buccal cusps of the upper being an index of their length, while short cusps are associated ^ American System of Dentistry, Vol. ii., p. 495. _ ' Ibid., p. 488. 220 THE HUMAN DENTAL MECHANISM. with a small amount of overbite. In either event the cusps grow proportionally shorter from before l)ack\vard. AMiilc this proportion Fia 196 Diagram showing fj-pical proportion between cusp length and overbite. Modified from BonwUI. is fairly constant for typical dentures, many instances of a departure from it are found. A denture conforming closely to the description of the typical denture in every particular except this, is shown in Fig. 197. The canine tooth occupies a position between the incisors and the Fig. 197 Denture typical in other respects exhibiting disproportion between cusp length and overbite It will be noted that the cusps of the first molar are much worn as this is oldest tooth in mouth (From photograph of specimen from Dr. Cryer's collection.) masticating teeth and hence is intermediate in the character of its oc- clusion. "VSTiere from the shape of the arch it continues the line of the THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 221 molars and bicuspids, it partakes more of their type of occlusion, and its overbite corresponds to the overlapping of the molar and bicuspid cusps and is proportioned to them in this regard. When its position is more in the line of the incisors, it partakes of their type of occlusion, its overbite corresponding with theirs, while if it is intermediate in posi- tion, it participates in the character of both types of occlusion. This relation of cusp length and overbite is another of the related variations to be discussed presently. When the overbite and cusp length are not thus proportioned the condition may be referred to as abnormal. We next come to discuss some peculiarities of the occlusion of the bicuspid and molar teeth, which are so closely related to the manner of movement of the mandible and have so important a bearing upon the efficiency of these teeth as masticatory organs, that it will be necessary to bear in mind the movements of which the lower jaw is capable in order to understand the functional significance of these characteristics. If a curved line be drawn touching the summits of the buccal cusps of the upper teeth from canine to third molar, it will more or less accurately correspond to the arc of a circle with its convexity downward. (Fig. Fig. 198 Diagram illustrating the "compensating curve," or the "Curve of Spee." 198.) The upper lingual cusps will be found to occupy a similar line and both series of cusps of the lower teeth correspond also to the arc of a circle. This condition is to be attributed to differences in the level of the teeth and in the direction of their long axes as they are placed in the alveolar process. The long axes of the upper teeth anterior to the second bicuspid are inclined toward the median line of the denture, that of the second bicuspid being practically vertical, while the teeth distal to it are placed at a higher level in the bone and have their long axes increasingly inclined away from the median line. In the lower jaw a corresponding condition is found, successive cusps of the series being placed at higher levels in each direction from the mesial cusps ooo Tf"-^ II I'M AX DEXTAL MECHANISM. of tlic first molar wliiflimarks the lowest portion of the curve. (Fig. 19.S.) This is frequently spoken of as the "eonipensatinireurve" of the molars and bicuspids, aiid also as the "Curve of vSpce," by whom it is described.^ This curve varies considerably in difl'erent individuals. In its most ideal form, if continued in a projection of the jaw upon the vertical sagittal plane, it touches the anterior face of the articular surface of the condyle. (Fig. 199.) It more frequently passes posterior to this than anterior. In typical dentures its form has a definite relation to two of their characteristics which have already been mentioned, viz., the length of the cusps of the teeth, and the path which the condyles pursue in the forward excursion of the jaw. The relation existing between the curve and these two factors may be stated thus: the longer the cusps of the molars and bicuspids the shorter will be the radius of this curve, and the shorter the cusps the longer will be its radius; also, the greater Fig. 199 The "Curve of Snee." Line passing through anterior face of condyle. (From a photograph of a specimen in the Wistar Institute of Anatomy.) the inclination of the glenoid fossic and hence the greater the angle be- tween the path of the condyles and the horizontal, the less will be the radius of this curve, while the more nearly horizontal is the path of the condyles in their forward movement, the longer will be the radius of the compensating curve. To understand the bearing of this feature of the occlusal surfaces of the bicuspid and molar teeth upon the forward excursion of the jaw, let us see what takes place if this move- ment occurs and the lower teeth maintain contact with the upper, simply sliding forward over their occlusal surfaces. It must be evident that the path of the jaw would be determined during this movement by the condyles and fo.ss.ne po.steriorly and by the teeth anteriorly. The object which this arrangement serves is that all of the bicuspid and molar teeth shall be in contact within a certain range of the forward and back- ward movement of the mandible. It likewise provides that when the mandible is elevated into contact with the upper jaw not too far for- 1 "Die Verschiebungsbahn des Unterkiefers am Schiidel." F. Graf v. Spee; Arch. f. Anat u. Physiol, 1890. THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 223 ward of the position of occlusion, the lower teeth may simultaiie(ni.sly strike their opponents and be able to preserve a sliding contact with them in the retraction of the jaw to the position of occlusion. We have already seen that the position of the lower jaw at any time when the teeth are in contact is determined anteriorly by this contact of the teeth, and posteriorly by the glenoid fossa upon which the condyle rests. Its path, therefore, during a sliding contact of the teeth, would be determined anteriorly by the teeth and posteriorly by the fossse over which the condyles move. That there must be a correspondence be- tween these is evident. In order to understand how it is possible for this sliding contact of the teeth to take place, and for sake of simplicity in description, let us suppose that the opposed surfaces are smooth in- stead of being broken up into cusps and fossas. (Fig. 199.) In order that the lower teeth may slide upon the upper and the con- tact be interrupted at no point, the sliding surfaces must be either per- fectly flat or represent a curved plane, a section of which would be the arc of a circle. These are the only two kinds of surfaces between which a sliding contact could take place. In the former case the sliding body moves in a straight line, in the latter in the arc of a circle. That the condyle must move in a path harmonious with that pursued by the teeth of the lower jaw becomes evident when it is remembered that the jaw moves as a whole. Where the sliding surfaces are planes and the jaw moves in a straight line, the condyle moves in a line parallel to this or identical with it. When they correspond to the arc of a circle, the condyle moves also in the arc of a circle, its path concentric with or identical with it. If this did not occur the sliding contact between the teeth would be interrupted. We find therefore that when the general line of the teeth is that of the arc of a circle, that portion of the fossa over which the condyle slides is likewise an arc which is either identical or concentric. And the more nearly the plane of the teeth approaches a straight line or an arc with an infinite radius, the more nearly straight is the floor of the fossa. These two associated and related characteristics vary therefore in individuals. With a well defined curve of the molars and bicuspids there must be a corresponding slant of the glenoid fossae to permit the condyles to descend as the jaw sweeps round this curve, while where they are more nearly in a straight line and the jaw may move the more bodily forward, the fossae do not incline downward so much but permit the condyles to go more horizontally forward. The addition of cusps to the surface of the teeth complicates very much this sliding contact. In fact the surfaces do not slide as such, but the points of the cusps of the lower teeth glide upon the fossae of the upper. We have already seen that from before backward the cusps get proportionately shorter and, of course, the fossae into which they are received are proportionately shallower. As the mandible is moved for- ward these cusps have the effect of separating the jaws, or of rotating the mandible about a horizontal axis passing through the condyles. This is produced by the lower buccal cusps sliding upon the anterior walls of 224 THE HUMAN DENTAL MECHANISM. their fos.sjie, while the fossae in the lower teeth containing the lingual cusps of the upper teeth slide upon them. This provision keeps the cusps in contact during the forward movement, until the incisors come into action, and where the normal overbite exists, the lower centrals then slide down the lingual surface of the upper incisors and separate the distal teeth. Usually when the incisors are edge-to-edge, all the teeth distal to them are out of contact. From what has been said it wall be seen that the buccal cusps of the lower teeth and the lingual cusps of the upper are the ones which it is most important should conform to the compensating curve, since they are in contact with the fossae. One of the commonest variations from this typical arrangement which will be observed, is that in which the distal cusps of the upper molars are below the curve mentioned, the long axes Fig. 200 Fig. 201 BPK^^^ 1 BBr^ f> ^^^^H QP ^^^^^^^^ ■ ^J The "Curve of Spee." Short cusps and long curve. Line passing through anterior face of condyle. (Specimen No. 4237, Wis- tar Institute of Anatomy.) The "Curve of Spee." Short cusps and long curve. Line passing distal to anterior face of condyle. (From photograph of specimen No. 800, Wistar Institute of Anatomy.) of the teeth being almost vertical (Fig. 189). In such cases if the jaws are t\'pical in other respects the mesio-buccal cusps will be found more or less worn down to conform to the general plan, the disto-buccal re- maining unworn, because they occupy space between the lower teeth and not in their buccal grooves. Another characteristic of the molar and bicuspid teeth may be observed in Figs. 202 and 203, in which it may be seen that their buccal and lin- gual cusps are not on the same level, the buccal cusps occup\ing a higher relative position as we proceed backward from the first bicus- pid. In the upper jaw the lingual cusp of the first bicuspid is usually higher than the buccal, the cusps of the second bicuspid being either on the same level or the buccal being a little higher (Fig. 202). The buccal cusps of the first molar are successively higher than the lingual, and this continues until we find those of the third molar relatively highest of all (Fig. 202). This condition obtains also in the lower jaw (Fig. 203). It is pardy due to an actual anatomical difference in the height of the cusps as the tooth is viewed out of the mouth, and partly due to the increasing inclination of the long axes of the teeth in the THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 225 alveolar process. This is another of the related factors subsequently Fiii. 202 Upper and lower bicuspid and molar teeth, side \-iew, showing relative height of buccal and lingua] cusps of upper teeth. (From photograph of a specimen in the Wistar Institute of Anatomy.) Fig. 203 Lower bicuspid and molar teeth, front -v-iew, showing relative height of buccal and hngual cusps. Same mandible as Fig. 202. (From photograph of a specimen in the Wistar Institute of Anatomy.) to be considered and varies with the path of the condyle. That this condition provides for the contact of the cusps in the lateral excursion 15 226 THE HUMAN DENTAL MECHANISM. of the jaw has been well brought out l)y Walker' (Fig. 20')). When the jaw is moved to one side with the teeth sliding in contaet it rotates about a vertical axis passing through the condyle on that side, the opposite condyle moving inward, forward, and downward. On the side toward which the movement is taking place, the rounded buccal and the lingual cusps of the lower teeth slide upon and come in contact with the buccal and lingual cusps, respectively, of the upper teeth. Fig. 204 Diagrammatic view of the relative height of the buccal and lingual cusps of the molar and bicuspid teeth. (W^alker.) On the opposite side the high buccal of the lower teeth slide up and come in contact with the high lingual of the upper teeth, which is rendered possible by the fact that the jaw^ is depressed on that side. If the condyle simply moved forward instead of downward as well, the buccal and lingual cusps might be of the same height and this same re- lation of the cusps would obtain, but as it moves downward the outer cusps have to be higher in order that there shall be compensation for Fig. 205 Diagram illustrating contact of cusps in lateral excursion of the mandible. Section through jaws at position of second molar. OP, line touching lingual cusps of upper molars; L R, line touching buccal cusps of upper molars; S T line touching buccal cusps of lower molr>.'-=. showing the downward movement of the mandible on the right side necessary for contact of the cusps. the rotation of the jaw about a horizontal axis passing through the stationary condyle. This provision serves to balance the masticating force and prevent overstrain when the jaws are in occlusion in the lateral position. It provides contact of the teeth on both sides to resist the strain exerted by the pairs of levators which have simultaneously contracted. Witb ^ The Dental Cosmos. Vol. xxxix., p. 789. THE OCCLUSION OF THE BICUSPIDS AND MOLARS. 227 artificial dentures this condition may be imitated to advantage for a purpose to be discussed in a later chapter. Thus it will be seen that in the movement of the jaw forward or from side to side there are a series of cones which may be applied to the food, the series of cones or depres- sions so alternating and being so arranged in each jaw that within a certain range of movement the whole series of cones may be in contact with opposing surfaces of some sort. The immense functional value of this is obvious, and it is also apparent that this provision tends toward preventing overstrain and undue shock upon the denture. In exerting the force by which the food is crushed, the jaw moves to what has been called the position of occlusion, from the various positions which it has assumed in biting through the food placed between the teeth. There are several ways in which the food may be crushed in such movements. It may be done by the action of a cone-shaped point not closely fitting its opposing surface, which is applied to the food and acts as a dividing wedge; or the cone may be received into a depression which it closely fits, the food being simply crushed as between two plane surfaces, or the cone may not fit the depression, there being space or spaces for the crushed food to escape, the cone in the depression acting somewhat as a pestle in a mortar. Two closely fitting plane or curved surfaces, when the force between them is exerted at a right angle to the surf aces, do not act well for crushing except for rather brittle substances. But if in addition to their approximation they slide upon each other, the crushing effect is increased. A cusp received into an accurately fitting depression does not possess the greatest efficiency for crushing. Clearance spaces must be provided. During the operation of the denture as the food is pressed toward the buccal and lingual surfaces of the teeth, grooves between the cusps and the interdental spaces are provided for clearance. Grooves run down to the bottom of the depressions in the bicuspid and molar teeth, and the chief clearance channels are located between the buccal cusps of the upper and the lingual cusps of the lower. This carries the food lingually above the tongue so that it may be manipulated more easily, while bucally is carried downward into the sulcus between cheek and teeth, where, because of the peculiar mus- culature of the cheek and because the lower jaw is depressed below this point when the mouth is opened, the food is again carried in between the teeth. This overhanging of cusps also serves to protect the cheek on the outside and the tongue on the inside from being caught between the surfaces of the teeth. In addition to the fact that the forms of the occlusal surfaces of the teeth peculiarly fit them to act as the crushing organs of the mechanism we find that they are adapted for this function in other particulars. They are provided with the means of resisting the wear and stress which the constant activity of the apparatus entails. The enamel of the teeth, the hardest tissue of the body, forms their external covering. It gives them a hard, resistant, and highly polished surface, and offers its greatest thickness to those parts most exposed to 228 THE HUMAN DENTAL MECHANISM. wear. The enamel masses are also arranged to give the best mechani- cal support in resistance of the force exerted upon the teeth. The hulk of the tooth is composed of dentine which confers the necessary strength. Resistance to the stress of mastication is provided for by the form and location of the roots of the teeth. These are all modified cones fitting into conical sockets, and as their long axes are generally in line with the direction in which stress is exerted upon them, the mechanical advan- tage is evident. The force upon the incisors does not always act to force them into their sockets (Figs. 206, 207, and 200). For the upper it serves to drive them forward as well as upward, and is resisted by the Fig. 206 Fig. 207 Fig. 208 Line of resistance to force offered by upper central in- cisor. (Burchard.) tine or resistance to force offered by upper lateral in- cisor. (Burchard.) Lines of resistance to force offered by upper canine, (Burchard.) Fig. 209 Lines of resistance to force upon the lower incisors and canine teeth. (Hurcliard.) flattened labial surfaceof theirroots. The canineis forced u])ward and outward and the labial surface of its root also serves to prevent displace- ment in this direction (Fig. 208). The six lower anterior teeth are forced downward and inward, the inclination of their roots and their arch-like arrangement resisting the strain upt)n them. The principal strain upon molars and bicuspids is vertical, their roots l)eing well dis- posed to resist it (Figs. 210 and 211). The lateral stress upon these teeth, which is largely determined by the height of their cusps, is resisted by the direction of their roots. As the princi})al lateral strain upon the upper molar and bicuspid teeth i.s' inward, we have their long axes in- clined in the direction to resist this (Fig. 210). The opposite is true of the lower molars whose long axes resist a TIIK OCCLUSION OF THE BICUSPIDS AM) MOLARS. 229 force tending to displace them downward and outward — the direction in which strain in mastication is apphed to them. (Fig. 211.) The character of the retentive tissues of the teeth tends to prevent and resist strain upon them. The pericementum is composed largely of connective tissue fibres extending from the tooth to its socket, and so disposed as to support the tooth even under great pressure on its long axis, but by their elasticity permitting slight movement in any direction. The membrane is also highly vascular, particularly in youth, which doubtless also contributes to its resiliency. This mol)ility enables the tooth to resist far greater force exerted upon it than if it were solidly attached to the bone, and the independent mobility of theteeth makes the denture capable of resisting greater strain than if they were a united mass. The teeth mutually support each other because of their approxi- mal contact, so that each tooth is thus able to resist greater force. The Fig. 210 Fiq. 211 Lines of force upon an upper molar. Lines of force upon a lower molar. (Burchard.) (Burchard.) pericementum has upon the tooth, a vital protective influence as well as a cushioning effect. Black^ has reminded us that its tactile sensi- bility is a constant safeguard against overstrain. It has been stated that there are a number of characteristics of the typical denture which are variable, and that they are all more or less directly related, and that they vary harmioniously and proportionately. Those which have been mentioned are: 1. The size and shape of the individual teeth. 2. The overbite of the incisors and the cusp length of the molars and bicuspids. 3. The shape of the dental arch. 4. The curve of Spee or " the compensating curve." 5. The inclination of the long axes of the teeth. 6. The relation of the buccal and lingual cusps of the bicuspid and molar teeth. 7. The shape of the condyle and the inclination of the glenoid fossa. In what may be called typical dentures variations in any of the above may and do occur without the creation of a departure from the mechanical • The Dental Cosmos. Vol. xxxviii., p. 476. 230 THE HUMAX DENTAL MECHANISM. design, provided they are assoeiated with variations in other eharacter- istics to which thty are directly rehited. The mutual relation of these has been pointed out as they were described. The only item mentioned which is not a characteristic of the denture is the last, which of course determines the path and the manner of movement of the jaw. The relationship existing between this and the anatomical form of the teeth is the most important one to bear in mind. It has been well stated by Walker^ that "there is a certain definite co-relation between the morphology of the morsal surfaces of the teeth" and the path pur- sued by the condyle. The foregoing description has been that of the ideal or typical mas- ticatory apparatus rather than the normal or usual. Few dentures absolutely perfect in every particular exist, it is indeed doubtful if any do. A number which are approximately perfect have been used to illustrate this chapter. Nature has been prolific in her provision for man's needs in this regard, and while she has furnished few with a perfect mechanism, yet within the bounds of what may be termed the normal, many dentures exist, which although they fall short of the mechanical design described in many particulars, nevertheless serve their posses- sors for purposes of mastication in an entirely satisfactory manner. The human organism frequently displays its ability to adapt itself to serious shortcomings in the work of this apparatus, but it must not be forgotten that the greater the departure from this typical design the less must be the efficiency of the mechanism. THE PREPARATION OF FOOD. Incision. — Having studied the characteristics of the masticating mechanism, we are prepared to discuss the method by which it func- tionates. Prehension, or the seizing of the food, is a function of the teeth of some of the lower animals but is unnecessary with civilized man. His first act is that of incision,although with the development of cutting instruments for food and the cultivation of a conventional use of them, even incision is confined to few articles. In the performance of this act the lower jaw is depressed from the position of occlusion and carried forward, the condyles moving approximately evenly in their fossae in this direction, rotation of the jaw about them occurring suffi- ciently to permit a grasping of the substance to be incised. The food is carried through the lips in contact with the upper incisorteeth, when the jaw is elevated and partly retruded, the edges of the lower and upper incisor teeth being approximately opposite during the movement. The more resistent the food, the more nearly will the teeth be opposed in their course through it. As soon as their cutting edges come in con- tact, the retraction of the jaw is so combined with elevation that the edges of the lower incisors slide up the lingual surfaces of the upper, the incisal edges passing each other somewhat after the order of shears. In some cases where the normal overbite of the incisors described ^ The Dental Cosmos. Vol., xxxviii., p. 576. MASTICATION. 231 on page 214, does not exist, the curve of the occkisal surfaces of the molars is such that the third molar in its advanced position due to the protrusion of the jaw, is in contact with the second molar dur- ing incision, thus tending to relieve the strain upon the incisor teeth. This is usual where there is an edge-to-edge bite, but with a normal overbite it does not occur. It will be noted (Fig. 190), from the curve of the occlusal edges of the upper incisors, the laterals usually being lower than the centrals, that their edges are first opposed in the centre, and thus the shearing action is carried on in both directions from this point. It is also noted that when the edges come into contact the food has been practically severed, and that until this time, the direction of the stress upon the upper teeth is almost that of the long axes of the teeth, while for the lower incisors this is not true until the cutting edges begin to slide upon the upper incisors, the stress up to this time being such as to displace them downward and forward. Mastication. — Man's diet consists of food of various degrees of physical consistence, and while the tendency of civilization is toward such preparation of the food as to lessen the necessity of mastication, some of it, indeed, requiring none at all, yet the bulk of it demands a thorough trituration to best subserve the body's needs. The principal articles of human diet requiring mastication are meats (animal fibre), vegetable fibre, and cereals, and foods made from them. Some of the other vegetable products have a hard protective covering which must be broken through to give access to the digestive juices, for it has been demonstrated that some of them, as grain for instance, might pass unaltered through the alimentary canal if this were not done. From a mechanical standpoint the essential feature of mastication is to crush these various articles of food in order to break up their physical organization and reduce the size of their separate particles. The object of this operation is to facilitate the action of the digestive fluids upon the food and render its passage through the alimen- tary canal easily accomplished. After the food has been incised or after an appropriately sized portion has been introduced into the cavity of the mouth, it is passed back by the tongue to the bicuspid and molar teeth to be reduced to small particles. In accomplishing this the jaw executes two more or less distinct kinds of movements and there are various combinations of them. The first of these is the direct up and down motion of the jaw in which it moves in the sagittal plane ; the morsal surf aces are sepa- rated and the food placed between them is crushed when they are approximated. The great crushing ability of tuberculated surfaces is a well-known mechanical principle, the cones acting as wedges to divide the food. The efficiency of the denture in this movement is propor- tional to the height of the cusps, sufiicient clearance spaces being nec- essary, as has heretofore been shown. This motion is pecuhar to the carnivorous animals, their masticating efforts being practically limited to it. Black has stated that this kind of masticating motion is used 232 THE HUMAN DENTAL MECHANISM. almost exclusively in the mastication of meats by persons with fairly normal dentures. The ultimate position attained by the jaw in this motion is, of course, as nearly that of occlusion as the crushed fibre in- terposed between the surfaces will allow. The jaw does not always close so that the cusps are exactly opposite their respective fossae: hence a small sliding takes place to bring the teeth togetlier, but in the main the movement is toward the position of occlusion, the food being squeezed in a pulpy mass buccally and lingually. The other motion, which in its simplest form is similar to that charac- teristic of herbivorous animals, is produced during the lateral excursion of the jaw. The mandible is depressed and carried to one side, the con- dyle on the side toward which it is moving usually remaining in the distal portion of the fossa, that of the opposite side l)eing pulled forward. The jaw is then elevated in this lateral position, the food being inter- posed, and the rounded buccal cusps of the lower teeth on this side come into relation with the sharp buccal cusps of the upper, the inner cusps likewise touching; the food is crushed and cut, a portion of it being left to occupy the foss{e between the inner and outer cusps. The jaw is then pulled upward and inward toward its occlusal posi- tion, the food being crushed between the rounded and strong lower buccal cusps and upper lingual cusps. With this movement is fre- quently combined a slight protrusion, though this is largely a matter of habit, and is not by any means necessary for the most efficient action of the teeth. While this is occurring on one side, on the other the teeth are not brought into the same functional relations. The high cusps above and below are usually in contact at one or more points, which serves to prevent strain and offers resistance to the elevator muscles of this side which contract simultaneously wath those on the opposite side. This side does not functionate during the excursion of the jaw to the other side,, because only the upper lingual and lower buccal cusps are in contact. This motion is used in the crushing of cereals and food made from them, and is by far the more effective of the two. It is also used in crushing most of the brittle and very hard substances. In the normal denture mastication of this nature takes place instinctively upon the two sides alternately, although this is largely a question of habit. The force required in crushing various articles of human diet, by an up and down motion, has been investigated by Blacks He used an instrument, consisting of two molar teeth carved from brass, which are forced against corresponding teeth by the direct thrust of a sliding bar. Head^ has reduced the figures thus obtained by Black by the use of a device for producing a triturating as well as a crushing movement. He utilized " a natural skull with practically perfect molars of average size." This was inverted, arranged with weights suspended from the lower jaw, and "so tipped that the force of gravity would, during * The Dental Cosmos, Vol. xxxviii., p. 484. 2 The Human Skull used as a Gnatho-dynamometer to Determine the Value of Trituration in the Mastication of Food. Paper read before Union meeting, Washington, 1900. MASTICATION. 233 mastication, give a sliding or triturating motion." The comparative table, appencUnl below, gives the results in pounds obtained bv these two investiirators. Dr. Head's Dr. Black'8 Raw cabbage 16 lbs. 40 — 00 lbs. Raw onion 4 " Head lettuce 8 " 25 — 30 " Radish, whole broke 20 — 25 lbs 20 — 25 " Pieces radish, pulverized 10 — -25 " 3.5 — 40 " Corned beef 18 — 22" 30 — 35 " Boiled beef 3 " Tongue 1 — 2 " 3 — 5 •' Lamb chop 16 — 20 " Roast lamb 4 " Roast lamb kidney 3 " Tenderloin of beefsteak (very tender) 8 — 9" 35 — 40 " Sirloin steak 10 — 20—43 " Round of beefsteak, tough 38—42 " 60 — 80 " Roast beef 20 — 35" 35 — .50 " Boiled ham 10 — 14" 40 — 60 " Broiled ham 10 — 13 " Pork chops 25—30" 20—25 " Roast veal _ 16 " 35 — 40 " Veal chops 12 Roast mutton 18 — 22 '' Mastication is a voluntary act, but the co5rdinating mechanism once having been set in motion by the v>'ill, it continues reflexly and auto- matically, and is independent of conscious action. The food, having been passed into the mouth, is carried back to the molar and bicuspid teeth and crushed after the manner already described. The tongue is the principal agent in keeping it between the crushing surfaces. The tonic contraction of the orbicularis oris in the lips and the buccin- ator in the cheeks opposes the tongue, active action of either occurring w'hen necessary, while within the arch the tongue shifts the food from side to side between the morsal surfaces as may be necessary, the rugae affording a rough surface upon which it may be rolled. The saliva, which is constantly secreted in the mouth and which flows in greater abundance under the stimulation of the presence of food and of masti- cation, is mixed with the food. It softens the food, dissolving some of its soluble constituents, adds a digestive ferment, and its mucin assists in agglomerating and lubricating the mass for deglutition. It also lubricates the soft tissues which play about the teeth. During the process of mastication the teeth not only crush the food, but also func- tionate as organs of exquisite tactile sensibility, giving instant knowledge of the location of the alimentary particle and of its physical consistence. Deglutition. — After the food has been masticated and mixed with saliva, it is gathered into a bolus upon the tongue, the edges of w^iich are curved upward to form a gutter. The anterior portion, being lifted by its intrinsic muscles and the stylo-glossus, is in contact with the anterior portion of the palatal vault, the rugae affording here also a roughened surface against which it is placed. The soft palate is then lifted by the levatores palati to touch the posterior wall of the pharynx, which has been bulged forward to meet it by the action of its 234 THE HUMAN DENTAL MECHANISM. superior constrictor. At the same time the funnel-shaped pharynx is brought up by the palato-pharyngeus and the stylo-pharyngeus to cover the mass, which is then shot past the pillars of the fauces by the pressure exerted upon the tongue by the contraction of the mylo-hyoid and of the hyoglossi (Kronecha and Metzger); the opening into the larynx is closed by the contraction of the lateral crico-arytenoids and the constrictors of the glottis, by the elevation of the larynx, and partly by the epiglottis, although the part taken by the latter structure is of small importance. The peristaltic action of the oesophagus then carries the food to the stomach. THE LOSS OF THE TEETH. It is beyond the purpose of this work to discuss the causes which re- sult in the loss of the teeth. SuflBce it to say on this subject that the absence of the teetli is so frequent an accompaniment of old age as to be looked upon as one of its usual features. An edentulous condition is not the result of natural physiological processes analogous to those by which the accomodation fails from a weakening of the ciliary muscle and a hardening of the crystalline lens, or the hair turns gray in old age from a disappearance of its pigment. The teeth are lost from patho- logical processes which either receive no treatment or are unsuccess- fully treated; from accidental causes; or are removed surgically in the treatment of diseased conditions. Unfortunately it frequently happens that they are lost from these causes before the period of old age. Effect upon Mastication. — The effect of the loss of the teeth upon the masticatory function may be readily understood since their part in it has been described. As each tooth performs a definite portion of the work of the mechanism, its loss is followed by a definite interference with that work. The loss of one tooth deprives the opposed series of one of its antagonists, and renders it functionally useless at this point. The loss of a tooth also deprives the adjacent teeth of the support of approximal contact. WTiile some teeth are more important than others and their loss is followed by more serious consequences, yet in gen- eral, the loss of teeth having antagonists increases proportionately the deficiencies of the apparatus. The function peculiar to any class of teeth is affected by their loss. Whenthe incisors are missing, the incisive function suffers; when the molars and bicuspids are absent, the trituration of the food is interfered with. The function of the lost teeth is partly taken up by those which remain. The incisors are frequently called upon to perform the work of the molars and bicuspids, a service for which they are in no degree suited, and one which ultimately causes them to be unduly abraded, and also results in an approximation of the jaws distally , and establishes con- ditions which complicate the subsequent insertion of artificial dentures. The molar and bicuspid teeth remaining on one side of the mouth may CHANGES IX JAWS FOLLOWING THE LOSS OF TEETH. 2:35 have to perform all the mastication which should have been divided between the two sides. This condition is also followed bv unnatural consequences, because the apparatus is designed for symmetrical operation. The usual result is a distortion of the normal relation of the jaws, and a movement of the remaining teeth under the unnatural masticatory force. The assumption of the whole masticatory function by a portion of the denture mav continue satisfactorilv for a while, and the digestive process may not suffer, because in the alimentary tract there occurs a large amount of adjustment to the conditions in the mouth . The more slowly the teeth are lost, the more readily will this adjustment take place. The food habit usually alters naturally in the course of the process, articles requiring little trituration or those previously sub- divided being utilized in increasing proportion; this being especially true as the period of old age is reached. There is also an alteration in the secretions to harmonize with the changed conditions. Lefoulon has pointed out the compensatory increase in the flow of saliva which takes place at this time. It is evident however that the metabolic balance must sooner or later be disturbed by the decline of the masticatory apparatus. Oefele' has shown that there is a marked failure in the digestion of starches by those who have lost their molar teeth, and Richard' has called attention to the fact that in some animals, vegetable particles may go through the alimentary canal practically unchanged, unless their natural pro- tective covering had been broken by the teeth. When mastication is defective, not only is there a failure to assimi- late the food, but since particles of too great size for gastric digestion are swallowed, peristalsis is delayed, fermentation takes place, and pathological processes ensue in the stomach and intestines. The gas- tritis and enteritis which are frequently observed in edentulous patients give clinical evidence of the truth of this assertion; and the removal of the cauise^that is, the restoration o-f the masticatory function by the insertion of satisfactory artificial dentures, is usually followed by a disap- pearance of these conditions. CHANGES IN THE JAWS FOLLOWING THE LOSS OF THE TEETH. The principal changes which occur in the jaws after the loss of the teeth take place in the alveolar process. This structure is developed with the teeth, furnishes them with support, and is largely resorbed after they are lost. When a tooth of the permanent denture is ex- tracted, its socket in the process of repair is partially filled up with can- cellated bone tissue, which in turn becomes covered over with cortical bone tissue and mucous membrane. There is a resorption of the mar^ gins of the socket, particularly of those corresponding to the outer and 1 The Dental Record, Vol. xxv., p. 160. ' De la prothese dentaire. Th^se pour le doctorate en medicine. Paris, 1866. 23G THE HUMAN DEXTAL MECHANISM. inner plate, and a general rounding and lowering of the alveolar process, and a lo.ss of contour at this point. This change is repeated each time a tooth is lost, so that after the loss of all the teeth, the alveolar ridge persists in the modified form described. The process of resorp- Vi'.. 212 Edentulous U7)per jaw, showing thin alveolar ridge. Fi(.. 2i:i E;lc of the jaw to make this possible. The average angle made by the body and ramus of the jaw in an adult is about ]20°. While the natural teeth are in position, no change occurs in the rela- tion of the jaws except a slight approximation due to the wear of the teeth. If the teeth remained there would be no a])preciable alteration of the Fig. 220 Stull showing relation of edentulous jaws: profile view. (From photograph of a specimen in the collection of Dr. M. H. Cryer.) r Fig. 221 4 "a ] k i Skull Hhowinj: relation of edentulous jaws; views from below. From photograph of a specimen in the collection of Dr. M. H. Cryer.) angle of the jaw. As age advances and as the back teeth are lost, the powerful traction exerted by the muscular apparatus, upon the ana- tomical angle of the jaw, the forward end of the jaw being in occlu- sion through the anterior teeth, causes a flattening of the angle and at the same time the symphysis is carried forward. After all the teeth are lost this becomes still more evident. Slisrht cliange occurs also at VOICE AND SPEECH. 241 the condyle where ti general flattening takes place. There is also an alter- ation of the glenoiti fossa corresponding to this. Its pronounced mar- gin is lostandit becomes in generahnore flattened. (Fig. 175 D and E.) The Relation of the Jaws. — "As the resorption of the alveolar process goes on, the vertical distance between the body of the lower jaw and that of the upper is lessened while the natural difference in their width is in- creased. The area of the upper jaw becomes smaller in proportion to that of the lower; the axes of the mandible extending further outward. ' In the endeavor to close the jaws under these circumstances, the lower is projected further forward as it rises to meet the upper, until in extreme cases, it may pass absolutely outside of the upper.^" Figs. 220 and 221 give two views of an edentulous skull and show the relations of the jaws. VOICE AND SPEECH. Voice. — Voice is the audible sound originating in the vibrations of the vocal cords and reinforced by the resonance of air cavities situated in the head and chest. The apparatus by which it is produced consists of (1) the lungs, chest walls, and muscles of expiration, which furnish the motive power; (2) the larnyx, in which are situated the vocal cords; and (3) the chest cavity below, and pharynx, mouth and nose above, which constitute the resonating chambers. When the air is forced out of the lungs by the contraction of the chest muscles and at the same time the vocal cords are approximated and made tense, their edges are set in motion and a sound is emitted. The cords themselves are capable of producing only a feeble sound/ but when they vibrate close to self-sounding bodies as the air in the cavities above mentioned, this air is thrown into sympathetic vibration and the volume and character of the original sound are altered. The sound made by the cords is not a simple musical tone but is a complex "note made up of a funda- mental tone combined with upper partial tones," overtones, or harmon- ics, of which as many as sixteen in some instances accompany the fun- damental.^ When the air contained in the resonant cavities is thrown into vibration, it is capable of emitting a musical note, the pitch of which depends either upon the size of the cavity or upon the size of the opening by which it communicates with the external air. The larger the cavity or the smaller its opening, the lower wall be the pitch and vice- versa. The resonant cavities reinforce either the fundamental tone, or the harmonic of the laryngeal sound, which corresponds to the pitch to w^hich they are tuned, so that alterations in their shape largely determine what is known as the quality of the voice. The voice possesses in common with other musical sounds three characteristics — (1) pitch, (2) loudness, and (3) quality. The pitch is determined by the tension of the vocal cords; the tighter they are stretched the higher will be the pitch. Loudness is proportional to the strength of the expiratory blast, and is also related to the resonance i M. H. Cryer. Internal Anatomy of the Face. p. 168 ^ Sewall: American Text Book of Physiology, Vol. II., p. 421. ^ Helmhoitz— Quoted by Sewall: American Text Book of Physiology Vol. II. p. 435. 16 242 THE HUMAX DENTAL MECHANISM. of the cavities above and below the larynx. The quality of the voice is dependent upon the charaeter or form of the sound wave, and is re- lated to the number and relative intensity of the overtones or harmonics which accompany the fundamental tone. The form of the wave is determined by the state of tension of the vocal cords, and by the form and size of the air cavities, which act as the resonating cham- bers. The air cavities, whose pitch may be changed at will, contribute by their resonance, now to reinforce this tone and now that, so that any overtone or the fundamental may be intensified by change in the shape of the resonant cavities. The sesthetic value of a human voice depends upon the number, character, and relative intensity of the overtones which accompany its fundamental tone. This tone-quality is determined by the power of adjustment of the larynx, in which the tones are produced, and by the precision of the muscular adjustments regulating the resonant pitch of the air cavities in which the tones are accentuated. The capability of the larynx is the more important of these factors, but it is evident that ability to correctly attune the resonating chambers is a necessary adjunct. The mouth is one of the most important of these resonant cavities. Its form is altered by the depression of the mandible and by the move- ments of the tongue. The soft palate also assists in this process, its chief function, however, being to separate the mouth and nose cavities. Increase in the size of the mouth causes a corresponding lowering of its pitch. The lips serve to increase or diminish the opening communi- cating with the outside air; the larger the opening, the higher will be the fundamental note of the cavity; the smaller the opening, the lower will be the note. The pharynx is changed in shape by the rising and falling of the larynx, while the resonance of the nasal chambers and the air cells communicating therewith cannot be altered at will. The resonance of these cavities may be controlled only as they are added to or sepa- rated from the mouth space by the action of the soft palate and tongue. Those portions of the air cavities which are uninfluenced by muscu- lar action are but passive factors in regulating their resonance. It is by the adjustment to these of the soft and moveable parts that variations in their resonance are produced. Therefore the change in the fixed parts of the mouth caused by the loss of the teeth, necessitates a change in the muscular adjustments, and requires new co-ordinations on the part of the tongue. The lips and cheeks, which are no longer supported by the teeth, fall in and complicate the process of adjust- ment. This change in the fixed portion of the mouth is almost al- ways succeeded by changes in the qualities of the voice, although the speaking voice is altered less than the singing voice, in which more precise muscular co-ordination is required. Speech. — Articulate speech by means of which man communicates his thoughts, has for a long time been divided by scholars into vowel sounds, or those produced in the larynx and modified by the position of the various mouth parts, and consonant sounds, which are noises ac- VOICE AND SPEECH. 243 companying the other sounds and are largely made in the mouth. Language consists in the regular progression of these sounds, which either singly or in combination represent ideas. Vowels.— The vowels are the true laryngeal sounds, originating in the vibration of the vocal cords and determined by their state of tension and by the peculiar resonance of the pharynx, nose and mouth. For any given vowel the posture of the mouth parts is the same, that is, this cavity is tuned to a definite pitch, the difference in the actual pitch of the vowel being determined by the tension of the vocal cords and by the size of the pharynx. In tuning the cavity for the sounding of ah (father), o (oivti), and oo (shoot), the tongue does not touch the palatal vault, but in a (ate) and e (met), its sides touch the molar and bicuspid teeth and adjoining alveolar process. The vowels are closely related, for the position of the mouth parts for one vowel easily changes to that for another. It is evident therefore that the changed conditions of the mouth incident to the loss of the teeth require new muscular adjust- ments to enable the mouth cavity to be tuned to the proper pitch for each vowel. Consonants. — In the production of most of the consonant sounds, a current of air is interrupted or stopped at some point during its exit from the lungs, the noise resulting therefrom being the consonant sound. Some consonants however may be sounded only in conjunction with a vowel, in which case the consonant is only a superadded noise to the laryngeal sound and is due to the peculiar resonance imparted by the mouth and nose cavities when they are properly disposed for the for- mation of the consonant. In other consonants the sound originates in the mouth, from vibration of the mouth parts caused by an in- complete obstruction to the air current, while in others there is an ex- plosive sound due to the sudden stoppage or starting of the air current. The following table^ gives a classification of the consonants accor- ding to the place at which they are formed : Oral Nasal Place of articulation Momentary Continuous Continuous Surds without voice Sonants with voice Surds without voice Surds with voice Sonants with voice b w m f th (in) s, c (ity) V th (y) Tongue and hard palate (forward).. t d z, r n Tongue and hard palate (back) .... ch J sh z, r Tongue, hard palate and soft y.l k, c (at) g ng * Sewall: American Text Book of Physiology, Vol 11.. p. 437. 244 THE HUMAN DENTAL MECHANISM. A careful study of this table will show that the factors determining any given consonant are the strength of the expiratory blast, the pres- ence or absence of voice, and the position of the mouth parts. Mechanism of the Production of the Consonant Sounds. — In the for- mation of the P and B sounds, the air blast meets no interference in the mouth because of a narrowing of the channel, but is interrupted at the lips by their sudden opening or closure (Fig. 222). In P there is greater air pressure and greater suddenness in the stoppage or startin g of the current. In K and (i also the channel is unaltered, stoppage of the blast occurring at the posterior portion of the palatal vault by the application of the base of the tongue to the soft palate. In G the air pressure is not so great as in K and the tongue touches a larger area. In L and ^I and NG the channel is not changed by the appli- cation of the sides of the tongue to the vault. In L the tip of the tongue touches the process back of the incisors, while the air current divides and escapes around its sides. In M and NG the air current escapes through the nose. Fig. 222 T&DTh > K&e 1. LMagraramatiu drawing showing plac« of articulation of the consonant sounds. 2. Drawing showing contact with the tongue with molars and bicuspids in the formation of certain consonants. In all the other consonant sounds except H, an air channel is made by means of the tongue to direct the blast either upon some point at which there is a partial obstruction or upon some point at which it is stopped. For this purpose the tongue is applied to the sides of the palatal vault touching the molar and bicuspid teeth and the process ad- joining, and forms a sort of gutter through which the air is expelled. The position and area of this contact varies with the different letters. In the formation of T and D the lips are open and the tongue touches the alveolar border with its tips and sides, the tip being depressed as the current of air is ejected. In T the pressure of the blast is greater VOICE AND SPEECH. 245 than in D. In CII and J the tongue is pressed upon the alveolar border at its sides and tip, and the air current is forced between the tip and the anterior portion of the palate. The noise is made between the palate and the tip of the tongue. In N the oral cavity is closed by the placing of the tip of the tongue against the palate instead of by the closure of the lips as in M. The air escapes through the nose in these consonants, and a peculiar nasal resonance is added to the voice sound. In articulating the sounds S, Z, Th, Sh, Zh, F and V the air current produces a blowing sound due to its impingement upon some portion of the channel. In the F and V sounds the lower lip is in contact with the incisal edges of the upper teeth, the tip of the tongue being pressed against the lower incisors, while its sides touch the last molar teeth and process, and guide the air between the lip and teeth. In the formation of S the air is forced past an incomplete obstruction, the edges of the adjacent tissues being set in vibration by the current. The teeth are almost in contact, the lips slightly open, the tongue curls upon the sides to touch the molar and bicuspid teeth as far forward as the canine and the adjoining palatal vault, leaving a narrow opening in front for the egress of the air. The end of the tongue touches the lower incisor teeth, and the sound is caused by the impingement of the air upon the edges of the closed teeth. Z and C (soft as before E and I) are made in the same way, the difference in the intensity of the blast differentiating them. In Sh and Zh the air is forced by two obstructions, one caused by the arching up of the tongue to nearly touch the palate, the other caused exactly as in S. X is a combination of theS and K sounds. In Th the tip of the tongue is placed betw^een the upper and lower incisors, the air channel being formed by its sides as in some other consonants. The edges of the upper teeth are set in vibration by. the air. "Lingual R" is produced by having the sides of the tongue in contact with the molar teeth and palate, to direct the current upon the tip of the tongue w^hich is curved upward toward the palate and thrown into irregular vibrations by the blast. In H^ the air current meets no obstruction in the mouth, the vibra- tions producing the sound of the consonant being those of the separated vocal cords. As W is a combination of H and V, its production needs no separate description. The above description is that of the method of producing these sounds when the apparatus is in its normal state. A change in any of the parts concerned in this process must result in an interference with the correct articulation of the sounds. It will be seen that so far as the part taken by the mouth in the pro- duction of these sounds is concerned, interference with the mechanism by w^hich the air channel for a given consonant is formed, or v^dth that by which the air is obstructed or stopped, must necessarily be followed 1 Sewall, op. cit., p. 438- 246 I'H^ HUM AX DENTAL M ECU AX ISM. by an interference with the proper articulation of that sound. The channel is formed by the adjustment of the tongue to the teeth and the sides of the palatal vault, and the obstructions and interruptions are made through contact of the tongue with the teeth, or the hard or soft palate, or of the lips with each other, or with the teeth. The loss of the teeth, the resorption of the process, which alters the form of the palatal vault, and the change in the relation of the jaws must all have a serious influence upon articulation. This is also comj)licated by the fall- ing in of the lips and cheeks which are supported by the teeth and al- veolar process. The tongue is the principal organ of articulation and may learn to accommodate itself greatly to the change in the parts to which it is applied in the production of the sounds, but it is evident in some instances that satisfactory adjustment cannot take place. The loss of the incisor teeth aft'ects the sounds articulated in this region. When a single upper incisor is missing, at thl.^ point there is an escape of air in some of the co-ordinations in which it should be con- fined, and the sound of S seems to be made. One of the commonest defects from absence of the incisors is in the formation of Th, the air escaping and giving an S sound instead. When the upper incisors are lost, the F and V sounds are difficult to make, the lower lip having to accommodate itself to the alveolar process or to the upper lip, and the sound approximating P or B. In some cases the S sound itself is diflicult, especially if both upper and lower incisors are missing, Sh and Zh being similarly affected by this condition. T and D are also sometimes difficult to enunciate, especially if there has been much resorp- tion of the alveolar process against which the tip of the tongue presses. As the molar and bicuspid teeth and adjoining process form part of the lateral walls of the air channel in the enunciation of a number of consonants, deficiencies here affect speech also. The tongue cannot close the sides of the air passage, and the current escapes into the cheeks. T, D, Ch, T, Th, Sh, Zh, S, Z and C are principally affected and especially so when followefl by the vowels A or E, con- tact of the tongue with the sides of the vault being also necessary in the making of these vowel sounds. F and V may occasionally suffer, and sometimes the tongue cannot shut off the air in N and it escapes into the mouth, destroying the full nasal resonance character- istic of this letter. THE EXPRESSION OF IDEAS AND EMOTIONS BY THE FACE. The expression of thought and feeling in man is accomplished for the most part in three ways: by gesture, by speech and by wTiting. Speech is the means in most universal use, gesture being chiefly employed to supplement language, while the utility of WTiting is naturally re- stricted because of the conditions required for its performance. Be- side containing the organs of articulate speech, the face performs ex- pressional services which are not included under any of the above headings. These consist of the movements of associated groups of facial muscles which occur simultaneous with the existence of cer- EXPRESSION OF IDEAS AND EMOTIONS. 947 tain emotions or which express some idea of the mind. The muscu- hir niovemcnts and the etl'ect they produce upon the countenance are the accompaniment and the outward manifestation of the psychic state. Accorchng to Mantegazza^ they have two diverse functions; they may replace or conn)lete hmguage, or they may defend the nerve cen- tres or the parts of the body against dangers of different kinds. The forms of facial expression are the same throughout the world ^ and are too familiar to require description. As one of the most mobile features of the face, the mouth partici- pates largely in its expressive movements. The loss of the teeth in- terferes with the performance of this function of the mouth; it is pro- posed therefore to give a brief account of the mechanism by which the facial movements are effected, to point out the part taken by the mouth, and to indicate in what way the absence of the teeth interferes with the muscular action. Before discussing this mechanism, however, it will be necessary to clearly differentiate between the terms "facial expression" and "the expressive movements of the face." The former alludes to the coun- tenance, its form, contours, surface, muscles — in short, its anatomy; while the latter refers to the contraction of the facial muscles which produces the movements expressive of ideas and emotions. Facial expression will be treated under a separate heading. The Facial Mechanism of Expression. — The mechanism of the expres- sional movements of the face consists of (1) a bony framework,the facial skeleton; (2) the muscular apparatus, technically referred to as the muscles of expression; (3) the connective tissue, fat, and over- lying integument which complete the face. The facial portion of the skull gives form to the countenance. It also contains some of the special sense organs which are features of the face, and it affords points of origin for the muscles of expression. The skin of the face is very thin, elastic, and loosely adherent to the underlying structures. Over the nose, however, it closely adheres to the bone and cartilage, and over the chin it is closely united to the mandible through the medium of the intervening tissue. The super- ficial fascia underlying the skin is intimately united to it. It is very loose and cellullar, contains large amounts of fat, and permits great mobility of the integument. The facial muscles, illustrated in Fig. 223, are numerous. For the most part they aie superficial and are closely attached to the under surface of the skin. This arrangement permits them to produce in the skin the folds and depressions associated with their movements. The facial muscles of expression (Fig. 223) may be divided into three groups — those centering about the eye, about the ala of the nose, and about the mouth. They are all concerned in the expressive move- ments of the face, but as we are interested only in the part played by the mouth, we shall discuss only the muscles operating in this region. ^ Physiognomy and Expression, p. 80. " Charles Darwin: The Expression of the Emotions in Man and Animals. 248 THE HUMAN DENTAL MECHANISM. The orbicularis oris which surrounds the aperture of the mouth is the common meeting ground of the muscles of this group. It consists of two portions — the labial, which occupies the red border of the lip, and is narrow, thick, and regular, and the facial, which iswiderand thinner, antl spreads out into a wide band just beneath the skin around the oral orifice. Joining these two portions are a large number of mus- cular fibres which run antero-posteriorly from the skin on the outside to the mucous membrane on the inside. This muscle constitutes the larger portion of the substance of the lips, and gives them their Fig. 223. The facial muscles of expression. fleshy characteristics. The labial portion of the orbicularis has no attachment to the bone beneath it. At the angle of the mouth it is deeply situated and intimately connected with the buccinator muscle, decussating fibres of which pass above and below this point to be lost in the upper and lower lip. The facial portion has only three small bonv attachments on each side. The largest of these are the naso- labial slips, which are slender bands passing up to be attached to the anterior nasal spines, and which correspond on the external surface to the philtrum. Two other slips are attached to the incisive fossa? in the upper jaw, and two smaller slips are inserted in the incisive fossse of the mandible. The orbicularis is not truly speaking a sphinc- EXPRESSION OF IDEAS AND EMOTIONS. 249 ter muscle as that term is applied to orificial muscles of other parts of the body. It is doubtful if any of its fibres are continuous around the aperture. When the labial portion alone contracts, it everts the lips and diminishes the width of the mouth. The facial portion serves to press the lips against the teeth and adjoining process. Its two por- tions oppose the other mouth muscles. Arranged radially about the mouth and united to the orbicularis are the other muscles of this group. They are the levator labii superioris aheque nasi, the levator labii superioris proprius, the levator anguli oris, the zygomaticus major, the zygomaticus minor, the depressor anguli oris, the depressor labii inferioris, the levator labii inferioris and the risorius. The points of origin from the bones of the face and the points of at- tachment to the orbicularis indicate the line of action of these muscles. The anterior teeth and alveolar process form a base upon which the tissues of the lips may be moved by them. The mucous membrane is lubricated by the saliva and permits an easy sliding of the lips upon the underlying structures. The canine tooth and eminence are especially important as a base of support for these movements. They underlie the corner of the mouth and afford a prominence over which the lips may be pulled by the muscles attached to the angle of the mouth. The action of the individual muscles constituting this group has been carefully investigated by Duchenne ^ who has given a detailed account of their effect upon the countenance. It is beside the purpose of this work to discuss the significance of these movements in great detail. The reader is referred to the literature on the subject for more minute particulars. In order, however, that the reader may understand the usual meaning of the contraction of these separate muscles the following account of their action is appended. The levator labii superioris alsequse nasi is the principal muscle in the expression of contempt and disdain. It dilates the nostrils and raises the upper lip and draws it slightly forward, and with the other levator muscles increases the prominence of the cheek below the orbit. It helps to develop the naso-labial fold. When it contracts with the other levator muscles of the upper lip, an appearance of sadness and grief is produced. The levator anguli oris draws up the angle of the mouth, pushing up the lower eyelid as in crying. The zygomaticus major is the muscle of joy or laughter. It draws the corner of the mouth back- ward and a little upward. The zygomaticus minor assists in drawing upward and outward and backward the outer corner of the upper lip but not the corner of the mouth. It produces an expression of sadness. The depressor anguli oris (triangularis menti) draws the corner of the mouth backward and downward and is necessary in the expression of sadness or grief. The depressor labii inferioris (quadratus menti) draws the lower half of the lower lip downward and a little outward. With its fellow of the other side it draws the lip directly down and slightly 1 Mecanisme de la physioenomie humaine ou analyse electro-physiologique de I'expression des passions. Paris, 1876. 250 THE HUMAN DENTAL MECHANISM. everts it. It is used in the expression of irony. The levator hil)ii in- ferioris acts with its fellow in raising the lower li]). It protrudes the chin, as in pouting, and ])ro(hiees dini])les in the skin of the chin. It is used in the expression of doubt and disdain, especially when aided by the triangularis menti. The risorius or muscle of Santorini is ordi- narily spoken of as the smiling muscle. It draws the corner of the mouth directly backward and produces what is known as the "sar- donic grin." The platysma myoides muscle must be mentioned in this group because a few of its fibres really constitute the risorius muscle. It is attached above mainly to the subcutaneous periosteum of the lower jaw from the symphysis backward. Its middle and anterior parts assist in depressing tlie jaw, while its posterior or lower parts are largely attached to the corner of the mouth, and act to draw the lower lip and angle of the mouth downward and backward, and this assists in the expression of grief and fear. It will be noted that the buccal orifice and its surroundings are molded to the teeth and alveolar process by the tonicity of the facial portion of the orbicularis and the muscles which connect with it. The loss of the teeth and the subsequent resorption of the process are followed by a falling in of these tissues. Inasmuch as they are no longer supported upon a solid basis, their movement is restricted in amount and in direc- tion. The muscles which serve to carry the lips upward or downward are not so much hampered in their action as those which draw the lips and the mouth backward. This limitation is more particularly due to the absence of the canine tooth and its eminence than to anything else. All expressive movements of the face in which part is taken by the oral muscles are limited after the teeth have been lost. In a succeeding chapter the restoration of the various functions of the face and mouth by artificial dentures will be discussed. It is desirable that the student should be familiar with the effect of the contraction of each of the muscles of expression, in orderthat he may not impartto the countenance any unnatural look caused by too great fulness of the plate at anypoint. A denture which produces the same effect upon the countenance which the contraction of any muscle or muscles causes will impart to that countenance the expression associated with th.e contraction of the muscles so distorted. This will be discussed more in detail in Chapter XII. FACIAL EXPRESSION. The human countenance is made up of the features of the face. Facial expression may be regarded as consisting; first, of the features of the face, which are considered from a purely anatomical standpoint, and are the result of natural endowment; and second, of a certain impress made upon these features by the thoughts and actions of the individual. The natural endowment of the face is the resultof hereditaryinfluences, principally those of race and parentage. As the individual grows and develops after birth, the impress of thought, of action and of experience IS made upon the countenance. The frequent use of certain of FACIAL EXPRESSION. 251 the muscles of expression, attendant upon the existence of some thought or emotion, cultivates a tonicity of those muscles, and develops, folds in the integument of the face, which impart a certain cast or ex- pression to the countenance. These two influences taken together, the one prenatal and determining the anatomical form of the features the other postnatal and influencing the expression — establish the char- acter and indentity of a face. In discussing the human countenance Mantegazza gives five verdicts which may be taken upon it; physiolog- ical, ethnological, aesthetic, moral and intellectual. The ethnological and aesthetic are based solely upon anatomical characteristics, while the physiological, moral and intellectual are more largely founded upon expression. The term facial expression in its broadest sense is intended to include all the data upon which these several judgments are based. In this chapter it will be used with that meaning. Facial expression is altered by the loss of the teeth and alveolar process in several ways : First, Absence of the Teeth. — -The teeth are displayed in the move- ments of the lip in laughing, in smiling and in speaking. Cigrand^ says, "Artists tell us that when patients speak w^ords as "at" or "ate" with the short or long sound of a, the lips should disclose about one-half of the labial surface of the anterior teeth, upper and lower, while with words having the long sound of o, as in "oral" or "open" the lips should hide the teeth to their edges." A larger proportion of the denture is displayed in smiling or in laughter. The relation of the lips and the anterior teeth during these acts constitutes one of their chief elements of beauty, and it is during their performance that a beautiful or unsightly denture imparts beauty or the reverse to the countenance. Second, Changes of Contour. — The alteration of the contour of the mouth and lips by the loss of the teeth is characteristic. While it is less marked in those of the lymphatic temperament whose short teeth and process underlie lips which are thick and sufficiently rigid to undergo little change after the teeth are lost, yet in the nervous temperament where the lips are thin and require support from the teeth, the alteration in appearance is striking. The changes consist of alterations in the contours of the lips and in their relations. The lips fall inward instead of incfining outward as they do when supported by the teeth. The proportion of mucous membrane which is displayed is diminished. The line of separation between the lips, which may be an element of great beauty in a mouth becomes changed, being altered in most instances from a graceful curve to a characterless straight line. The corners of the mouth, which are supported mainly by the canines, droop, and an expression of weakness is imparted to the face. There is a disappearance of the sulcus mento-labialis, which is formed at the highest point of attachment of the soft tissues of the chin, when the 1 Facial Guide Lines as Taught by Artists and Sculptors. Paper read before the Fourth International Dental Congress, 1904. 252 THE HUMAN DENTAL MECHANISM. lower lip is supported and slightly everted by the teeth (Figs. 22o and 227). Third, Changes in the Surface. — The changes which occur in the sur- face are the obliteration of some of the normal folds in the skin and the establishment of additional ones. Before discussing, however, those which result from the loss of the teeth, it will be necessary to point out those which result from old age, in order that no confusion between the two shall arise. The changes which ensue in old age are the result of two conditions: the absorption of the fat contained in the substance of the lips and cheeks, and the atrophic changes which occur in the skin. These result in the obliteration of contours at points sup- ported by cushions of fat and in the establishment of wrinkles in the Fig. 224 Photograph slinwiriK the wrinkles commonly observed in old age. skin at points in the line of frequent muscular action. Normally there are masses of fat located in the hollow of the cheek around the buc- cinator and zygomatic muscles. There is usually also some beneath the levator labii superioris muscle and some at the .symphysis of the lower jaw. These may be absorbed early in life because of defective nutri- tion. They are frequently absorbed after middle life in certain tem- peraments, notably the nervous. In the sanguine or lymphatic they are more likely to persist through old age. In most individuals past forty, wrinkles of the skin are apt to be established, although as is true in the case of the absorption of the fat, they are likely to appear earlier in a nervous face than in that of other temperaments. Camper has called attention to the fact that they are established at right angles to the line of muscular action. Tho.se normally seen in an individual after forty are the transverse wrinkles of the forehead, vertical wrinkles of the forehead, crow's feet or wrinkles at the external canthus of the eye, the naso-labial fold, which extends FACIAL EXPRESSION. 253 downward and outward into the check from the base of the ala of the nose, and the genio-mental wrinkles, extending from the cheeks to the chin (Fig. 224). Fig. 225 ^g| ^^^^^^^iJr w IHHi ^ ^E|^^ ^^^^^^^■j^^j^^g^l L Photograph showing effect of the loss of the teeth upon the mouth, and the wrinkles established thereby. Fig. 227 Photograph showing the effects of the loss of the teeth upon the profile. Fig. 228 Photograph showing effect of the loss of tne teeth. Front view of patient in Fig. 228. Photograph showing the effect of the loss of the teeth upon the profile. The loss of the teeth is followed by additional changes in the coun- tenance. The naso-labial fold is accentuated because the upper lip falls in, and frequently becomes two lines, one of which descends from 254 THE IIUMAS DENTAL MECHANISM. the ala of the nose to the corner of the mouth (Fif^;. 225). The phihrum is usually obhterated (Fig. 227). This is due principally to the increase of the sphincter action of the orbicularis, which becomes necessary to keep the mouth closed in order to confine the saliva and food. Usually a groove is established extending from the corner of the mouth in the direction of the chin. In addition to these, small wrinkles placed radially to the oral orifice are established. While these last normally exist in old people, they are much accentuated when teeth are lost because of the marked sphincter action of the orbicularis above alluded to. Fourth, Changes in the Relation ot the Jaws. — The removal of the points of contact between the jaws results in their approximation. This is attended by a shortening of the soft tissues extending between the two; the muscles and connective tissues being actually decreased in lent'th. As far as the lines of the face are concerned, this effect is particularly noticed in the decreased distance between the nose and the chin. In some instances this effect is marked, the nose and the chin coming so close together, that a characteristic deformity is noticed. Fifth, Alteration of the Profile. — This is always changed to a greater or less extent. In those cases in which only a small amount of absorp- tion of the alveolar process has occurred, there is only a flattening of the mouth (Fig. 228), while in others the falling in of the lips may be very marked (Fig. 226). Sixth, Alteration of Expression. — From what has been said con- cerning facial expression, or that which gives character and identity to the countenance, it must be seen that a condition which is succeeded by so pronounced a change in the features must greatly alter the charac- teristic expression of a face. One has but to remember the striking change in appearance which has succeeded the extraction of all of the teeth of an acquaintance to appreciate how marked an effect it has upon this means of identification. CHAPTER V. THE HUIMAN DENTAL MECHANISM AS MODIFIED BY TEMPERA^IENT, AGE, AND USE. By a. H. Thompson, D.D.S., and CmiRLES R. Turner, D.D.S., M.D. The functional relations of the human denture having been discussed in the preceding chapter, it is proposed in this to treat of those consider- ations relative to its appearance which are important from the stand- point of dental prosthesis. xAji-tificial dentures are recjuired not only to restore the functions of the natural organs which they substitute, but they must also restore as far as possible the former appearance of the mouth and face. In nature there is a certain harmony in the various physical charac- teristics of each individual. There is, for instance, a pleasing proportion in the various parts of the body, and while this proportion varies some- what among indi\iduals, as they are not all constructed upon the same physical plan, normally these variations occur within fairly well-defined limits, and a marked departure from the plan is at once recognizable as abnormal. The size and shape of the teeth bear in general a certain ratio to the size and shape of the body, or more especially to the size and shape of the head or face, but this is by no means fixed and varies some- what within the limits of the normal. The mensural proportions of the teeth of different individuals differ vastly, but for the most part they cor- respond to the bodily dimensions of the individuals owning them. This is also true of their surface form and contours, which similarly correspond to these characteristics of the physique of their owners. There is also a general relation between the color of the teeth and that of the other pig- mented tissues of the body. ^^^lile this harmonious correspondence between the physical character- istics of the teeth and those of the body is not absolutely universal, it is the general rule, and any departure from it marks the individual as un- usual. This bodily harmony is taken as the basis for the selection of teeth in dental prosthesis. Familiarity, therefore, with the physical char- acteristics of the teeth and of the body and their mutual association is necessary to a judicious selection of artificial teeth to replace the natural ones. With a view to acquainting the student wdth this, the dental ap- paratus in its relation to the general physique and the characteristics of each will be discussed in this chapter. The denture will be described from the standpoint of its natural endowment first; that is to say, its variations in the various temperamental t\-pes of mankind. Secondly, the changes which occur in it under the influence of age and use will be described. 255 256 THE HUMAN DENTAL MECHANISM. TEMPERAMENT. Temperament according to Flagg is a term used to express differ- ences in the mental and physical constitution of individuals. It has been used with this designation since the days of Hippocrates, the ear- liest systematic writer upon the subject. It is an attempt at the classi- fication of mankind according to his physical characteristics, and has as its basis the relative proportion of his mechanical, nutritive and nervous systems, and the relative energy of the various functions of the body. "Each temperament is the result as well as the indication of the preponderence of one or another of these systems and of their relative functional activity." Despite the antiquity of this classification it is commonly utilized in dental prosthesis at the present time for lack of a better. Descriptive anthropology and ethnology have not as yet presented a classification of the physical attributes of man which is a satisfactory substitute for that according to temperament. Ethnic peculiarities are sufficiently fixed to identify individuals whose racial characteristics are not mixed, but pure races exist in so few quarters of the globe that this basis of classification is valueless and a more fundamental one is necessarv. In its ordinary use temperament refers equally to mental and physi- cal attributes. It does not divide mankind into well-marked natural groups which may be distinguished anatomically, but it offers groups of associated physical characteristics, the possession of a majority of which by an individual assigns him to that group. When the pre- dominating temperament of an individual has been determined ac- cording to this plan as a preliminary to the restoration of lost parts Hke the teeth, the result will be harmonious if these are selected to ac- cord with the physical characteristics of that temperament. According to the present generally accepted classification there are four fundamental divisions of the temperaments. The bilious, in which there is marked activity in the function of the liver; the san- guine, in which the circulatory system is most prominent; the nervous, characterized by a highly developed central nervous system; and the lymphatic, in which the lymphatic system has a preponderating influence. The following is a description of the general mental and physical characteristics of the basal temperaments, and while these are to be considered chiefly from the physical standpoint, the mental attributes are appended for assistance in diagnosis. The Bilious Temperament. (Fig 229.) — Physical characicristics: the pulse is hard and strong; frame muscular; movements without grace;angularity of features and physique; stature is medium or tall; body has a firm pose; countenance is severe and serious; complexion ordinarily sallow or swarthy; the hair dark brown or black; eyes black or brown. Mental characteristics: strong susceptibility and constancy of feeling; quick perception and precise judgment; capacity for reason- ing; firm decision and will power; great violence in anger; stubbor- ness; great pride and ambition; usually generous and magnanimous. TEMPERAMENT. 257 Fig. 229 Fig. 230 Man with characteristics of the biliou'i tem- perament in preilominenee. Fig. 231 Individual with characteristics of the san- gnine tonip;^rame:it in prodaminence Fig. 232 Individual with characteristics of the ner- vous temperament in predonainence. Individual with characteristics of the lym- phatic temperament in predominence. The Sanguine Temperament. (Fio-. 230.) — PJiysical cJiaracteristics: the highest manifestation of physical life; active circulatory system; pulse excitable and irregular; stature above the medium; body well proportioned; movements graceful, regular and easy; athletic type; voluptuousness in females; vivacious and beautiful; complexion fresh and ruddy; red lips; hair light or auburn, rarely dark, sometimes red; eyes are light in color, usually blue. Mental characteristics: Great 258 THE II (MAX DENTAL MlX'IIAMSAf. liopefulness and elasticity of iiiind, ()])tiinisin; enthusiastic, hut have litde perseverance; fondness for achniration or display; fickleness; great liberality of sentiment; little al)ility for self-denial; seldom undergo mental sullering. Nervous Temperament. (Fig. 231.) — Physical cluirddcrtstics: Quick movements; great sensitiveness; muscular system small; body refined and slender; angular; head proportionately large; the skin is soft and of fine texture; complexion generally light, sometimes sallow; hair is fine and dark; not plentiful; eyes gray or very dark. Mental charactcrisfics: very highly organized; great fluctuation of feeling of exhaltation or depression; keen sympathy; great anxiety or fear; ability to endure physical suffering in excess of what would be expected; predominates in highly civilized countries. The Lymphatic Temperament. (Fig. 232,) — Physical characteris- tics: tends to heaviness and roundness of the body; medium or low stature; cellular repletion due to lymph; slow circulation; pulse weak; large rounded head; features heavy and expressionless; hair light and coarse without lustre; complexion heavy and muddy; eyes dull gray and small. Mental cl i ar a -t eristics: Little passion or ambition, great self satisfaction; contentment and good nature marked; judgment calm, and slow but sure; plodding and industrious; individuals success- ful in business; safe to trust. The physical characteristics associated with the four basal tempera- ments are well described and may be easily studied in the comparative table on page 259 prepared by Dr. A. H. Thompson. It must be remembered that these are basal temperamental types which are more ideal than real, and that pure types seldom occur. Few indi- viduals exist who possess all the characteristics of a single tempera- ment. The description applies therefore more largely to principles than to individuals, but the tables of binary temperamental compounds on pages 260 and 201 describe combinations of physical characteris- tics which may frequently be ob.served in individuals. Combinations of three basal types exist in many persons, ■ and in addition there is a large class in whom the temperamental indication is so obscure as not to be discernible at all. In the diagnosis of the temper- aments or the predominating temperamental indication in any in- dividual, constant observation of people and keen perception are necessary and good judgement in selecting the characteristics which are most significant. No rule can be given which will completely cover the ground, but the proportion of individuals who do not possess char- acteristics which indicate the predominance of one temperament over the others, is small. In America because of the larger number of races which have contibuted to its population, the diagnosis of temper- ament is particularly difficult. The sanguine and bilious are the usual basal temperaments in this country although these are not so distinctive as is the nervous of the Latin races and the lymphatic of the German and Dutch races. TEMPERAMENT. 259 Ph "^ C8 " S rO ^^ (^ M (H X >- S 1) .C G ■" CD o o n a ;i ." (JO r/l ^ ^ a o; o >^ W h ti o Oi a, o-o m ■" a "C^ 03 n 2 s 2'C I— 1 fc, OiC— =3 ^ a •^ o cs o y 9 " 3 C.2 t^ Eh O SrS ^ "^ ig p,Cl.o3 03 o5 CC C,2 03 g 03 P.+j'O C S IS 03 o5 0.5 g c o aj;q O tn^ icsa t)-i U Oc fl i aj 03 tH >■ §^| ^ -a. £ !3 1^ ftps ,a a H sj 3 M « O H P. ■^^^ ^c ■£ i 03 g S . 3 0°lg J m fti3 C - „ H o o °^ f=^ o g c a)-r (H fccy„ a> o3t3 3 OJ s ft o o 3 CD o> « f=S el o3 cS'O oj "3 3 IJ cS oi a i^^ t^ <^ CI ■ u^ '"" C P. Eh CO OS rt ft i-Orrt i ^ iH =3 a> 0) o"^ a'o "S^ ) o OS J3 03 'O "H M 58- g.2j3s.g-gfl g oj Ct!'0 Oi . ft--g^g.S§>. 1^.2 73 C-S-^ c " P ^ O) X2 a (D a 03 TS a^ « a ^^ s^ as- |gfta 2 a fl '^ ^ al E! 2 3 a "^"^ M >• f^'3 c ^ ^ >^^ ^? 2 a> r !-, t,_< ^ bo g§S°^^ft^ ^ — D CJ c3 P* rf bofttttSP a s S.S.C a o tiO> *^ a> 2en 3 03^ a .2 ^ ^ " o -be 2 > xn^ 0=^0.2 r and ough ions e ex- • - CO -O CD -H a> =3 Z; °^ >>'3 ula d;r ilat arg ties "" o "S ■ lllii ti 03 o3i3 ^- -M ^ -1^ m O J2 ^ac 'a-sv X a> o3 2 ^ oi >-3 3 g S ft^ g 2 § *.aaftfi !S C S^ S2S" c ft cuga . H 5-- S 260 TIIE JICMAX DESTAL MK('UAMS.}f. K ^ s Full, and usually dark and lustrous. Eyes dark, large, lus- trous, and expressive ; sometimes deep blue. Eyes dark- brown or gray. Large, dark, or dark gray ; weak and ex- pressionless. Dark-brown ; rather ex- pressive. ■3 oQ C S a)J3 J; S 3 OS j>^Wi ^g"aX!a'g « >% -y S^S33"§ -O 3'Ov,2 !«3'5;5'"t, p 3 3 c s "•£; OS as .1^ ,3 a) as 3 t; 3 >•. Ml . cS,g as aj'S >> WS35£l ^"M ?J2 a! OJ3ga>-»' g- t- ^ r— > ? it— 03 .a 'a3g'& M-c i 3 0.0 cj fl-2 •^ ..t, a) m oaS.2 £aS§ a t^o 3*^ 1*^0 <-"a^,3 Oaj- oj bti'O . aj "^ y 3 ^ ^ f-^Q t- as 0*J a) 3 S a; -lit «1 •^35 -C as^TJ 0) 03-3 3 «;=i -ooS * 3 *^ Ei)o33,2 ^"-C ^'3 S;3 » -g eS ^ C2 3.S"? ^j:"^ .,a T^ 'Sa)--="'a-^--S , oi bJOe > a) » J3 ^ t* t- i5 5 ,bC-2 J.-'C M 35 '"'O "3 =« 5-3 ^c 5^.S/=c-os F\ <1> " "w " r^-^ ■2.2 S 25^2 CO t • - 03 • ' " "^ o o; — ' rl a; 0^ Q ^-. ^ (- gj J_ a) --fc 5-^ Oj OJ ^ m ti*^ .3 Ui^ ^-c S ■» "5^l§a;t ^ CO -2 SgS|ca-3 ,3^.2,8! cnO. 3 G 3^ |!n.S 3 --tocp --; >- K C > 3 as tH (H pa a! 3^ 3 c ws --« oj =;; .^ ~ — -3.35 OOP. 1 --D, I' Cm 3 P-SX'S «^^|a t3 a mS ■6 e-i 2^2c a> S 3 -d-d . - ^ g2 .S '* 00 3 to 0)^ 3 p- 3^2 ID a> 3,3 p a) Sb •l-i i --^ a, . M OS a> 3 a) ,Ct3 bca It a^ ? a; tTa; OS ^ ".2 'd'-^'O 3 M S a> .« ai— ' u 3 a> n p. a; -co 50 3 « :: gro) Hi ^3i SS;-3a; t. be 0; cS" 3^5 ^§3Z: 000 « c3 as .S>3 :3 c; 3 a! "in a> * . ^1 ap<-; t- ° . a) 03 .3 •3 ^S OS'S 3 ^ «4J t: M ^oa) fi.Q6c cS cs 0) s la :; 0) a) .S as .3 B P-StR J<'3 M£ a) • ^ !=i';:*j-3 « ■S.S.S s i^s ;n 3 a ^"O ^-' oS.2;«2 -ct: a) M a >- c *- *^ cs ~ 3 S m '-S & 3i£ OJ.S S.2 c a a 1 3 as i "- a) s-s SI ,0 s S.2 6 ^1 J OI ■ ^1 ti<:mi'kil\mknt. 201 cc o S C Ol^ o : tH t- t: biijD >.__ ^."3 'O'CrO ji; OS -O O as'p S_, tu^ j; "(Ho" •' -< s- a ^4 03 Ml &5* 5 R cu 3 O M 0) IH Sji t, . 1) C OS j^ti-o r'.S QJ ^ 03 5 "lllall ^ ♦; 0) ■ --« ^ ft b£ j3 a odd s="axH O-^ O 3 ai 3 « a 2 «* ■" S m r^ ^ (u' s> I .S 2 X! 2 a MS o .0 9 • „ o ttrz N '.ii OS ■ C >.,=" lis? a -.^_ a^ > C-S O O £ cS > p, O T.o >< of.^S^ _tac— — _bt^'^ eS m a; . P.IH S u 4^ ° d 1^ bB '2 -. rH >-■ "- ' ■— ^ cd "^ "^ Vi CD— T^ •^^ .S . S ^H s " P, « la's os frt "T^ • ' fH (h rt " ^ ,i (U " to C 0) B >^ " r? Wi^ flc S ?3 • w cS vP be -O P §2, rt'Sc 03 p,P< 03 > pr ""' oj P-o3 p a a p^p'^g ^^tuoa 2 OJ tH poo 03 o3_W] 5b'? (H a> CD o !^ • ^a'2"!^^ ■3 S ° - '^ c ?'p p.;:: o) ^ OS o ^. r S So f^ 03 p p ri'i'2 3 *; S W3 >,o rPi'oS ^Si 032 ^& a'2 S'i>.o3a . OS-P OS ■ea ^ 03 to to l-CoS 5 S f^p^ ^ P of-r. g T3 03 P S • o a; >.^oS^ ^ P CD o 2 os^ ^■S^: r/i - :« P OJ ¥ !-, " - p. -a tH 03 !h o ^ «» m cS 03 «5.2 o3^ P tuo O 03 SB'S 5tBod p PS §> P 03 O 01 S'7^-^ K 1 r" > P ( pa a'a O.P -p 5 > PS o S w p a'a Z p^ "iJ . .S55g,bj) 03 CD q.g oS S p 2 0— I p °JU n^ oi S 3^ Q> (U.i, •I .25 a 2 bctcd ^'Oa«-;3 c« 0)4=; P p ft .2 hca 0) "S 03 -P bc^ to OJ.rf .2 !='^ P '^ > '2 ax's ?^ • C7 p P t-< sli a s^^^c t->^ a -w ••-' '^piiSl 03 -M*^ P.^ O P r^ ^ > fto O 2 l^i&a »>>piajo r-H Q) P-O »o MS £ el 60 03 > 03 S 5 g-bc CBg ?p ^p.> 12; J3 P. 262 77/A' Irr^fAX dental mfj-jiamsm. > ^ < Gums. Rugse. Heavy, square, and rugged. Numerous and grace- ful in out- line; well- rounded. c . S3 "^ £ Sparse and flat. 73 fc. s»> m OS P- S S S3 O-gosaiSO (4 U S3 o!i: = a S3 ^ a 01 M 03 c 0: ^ .a" < OJ t- S w 5 a - ^-S M go pi W CJ< CO O O a c5 c § P^ p. ■3 --^J t. t- P.O "S-cd J"2 ^ 03 1 *"* ■ to ^1 o 5=1 g illl TEMPERAMENT. 263 TIk' illustrations, Figs. 233, 234, 235, and 230, show characteristic dentures of tlie four basal temperaments. Fig. 233 Denture whose characteristics are chiefly those of the bilious temperament. (Photograph of a specimen in the Wistar Institute of Antomy.) Fig. 234 Dentuj'e whose characteristics are chiefly those of the nervous temperament. (Photograph of skull in collection of Dr. V. Walter Gilbert. 1) Fig. 235 Denture whose characteristics are chiefly those of the sanguine temperament. (Photograph of a specimen in the Wistar Institute of Anatomy.) 264 Till-: lll'MAX DKSTM. MKcllAMs.V. ^ i 1 Heavy, numerous, and angular. Well- marked and numerous. Sparse and snapeless. Close set, but narrow. Thin and very tortuous. 1 o ft* a C «^ g = -x 2 sltl ill Vault. Round and high. < 5 OS < sis' o .73 . -2 = 2 o 5? gs-ga i p 1 1 o o 3 > - o > o ..a PI h3 P, Ss&a j c3 an) o 2 S s 3 'i c^-^ £ a * 2 S3 0) { O Cusps ana edges. 'H 1 "2 ■r 2 c ~. i ^ s ^ ~ £ " a .C2'5) 1111 ■a le sj a> I cS O o£'r a-^i bco >» ^H o o3 S-og. e8 S MS s3 ^^i^ ! ^-H ^«2 r- ..C S ?^ * »-' •c i O. s =! 01 --, - J= X •rt o -. So"?; = r o Cop So c;s «? 00.0 I e B TEMPERAMENT. 2G;j Long and well- marked ; numerous and o Numerous and sharply de- fined. O to 1-1 dS-o o 0) a; gd «H !^ a CO 03 o *? •1-1 Soft and ir- regular; sometimes quite low. 1 ^ ^i fH . ^ S ?; ft -- del •-r-^ -d-S fibo . «Sb ^ as OS lA*^ o u S£ . Deep pin and clear margin wel marked; fe toons d5 ■p.M Eh S's 3;S "a ^ 7 cS'^ e -^ a ^ ai bo--S .-f'd—i Medium and well- arched. o-d 4^ d iD.d'— ' -d . W Q bJJoj >g . o , « -d f- O , ^ d sh °>i o1 ^ 05 "3 d< a 'S n 2 -oja d 9 03 ■d d cj ■d"^ +^ s a cs S d d ^ "S ° 2 S 1^ s-o |2^ o o* 3 p U 0) d,0 P5^a =* ? d ^^ ^ bo > ^ o3 Close, deep, and firm ; the plane cjirved. lis © ho ■d •d •d ^. d ;;'S 03 ojt« (D d !u M bpc d-g 03 .,3 boftho sp Ogjoi Close and shapely, but strong- ly disposed to mal- C o o p. ■^ « ho . -d ^ -d -rt 1 ^^ bb S'«S§ .d - +^ hi Sd^i-S d od§ G;: ft 3 X! -« -d « ■d "d ell- r k e nd pely. TJ P. o OS d "^ 03 .^ , "d -r-l OI d Std 3 P 0) o -d " ft^S* 13^ -S bO§^ a " "3 1 'O O oj §< t-''^ d mooth, i 1 1 i an and trong. a-als r- 03 is a> 8" rH 03 d . i3 OJ CS rt 5^ id CO f-. " CO rt . M dO «2 d ^, ^ ,Q "^ 03 ,Q 03 O OQ d .# ^ o d ^ fto* •d ■S'^S bo -d . 02 ■" oa 03 o^ _o ,d >. ^ A ^"^^A ,G o .2 9 s>, QQ ■ ■">, 5aS CO CS 2=^ 03 cS ■^ hO !^5 o o -o o O o m -d .- d-d fe to ^ ft d OS ., -d d d •-m'O d t; , ,0_c3 'S'Od -d 'O d S?d S o d d O tH oj o ,2 «u ho ^11 s |a ^ a fi - A. a X3 0) . ten 0) hJD . > m 0) OJ — o3 ?r P ho O OS It 03 0) I'. > S <5 (7) Nervo- san- guine. , O 3 6 ft - o3'-"S i^ d. ^a.2§ ftd ^ => • 2 2(JG THE HUMAN DENTAL MECHANISM. Denture whose characteristics are chiefly those of the lymphatic toiiipfrarneril. (Cryer.) AGE. From infancy to old age a gradual succession of changes in the bodily tissues occurs. In common with the other tissues of the body, the teeth are influenced by the increasing years. From the time of the establishment of the permanent denture until the time it is lost, these changes occur in regular progression, so that for the most part the physical characteristics of the teeth are constantly in accord with the physical characteristics of the individual at all times during his life. In some persons these alterations are more discernible than in others, but after the age of thirty-five or forty the marks of time may be found in almost every instance. The chief alteration which may be attributed to the influence of age is a deepening of the color of the teeth. This is a molecular change in their tissues which is physiolgic in nature. It occurs most strikingly in individuals of the bilious and saguine temperaments, the yellow shades of whose teeth are markedly deepened. This change amounts to two or three shades as measured by the shade-guide for artificial teeth. It also occurs to a lesser degree in the teeth of the nervous and lymphatic temperaments. Due regard, therefore, should be taken of this fact in the selection of artificial teeth for individuals past forty, and a propor- tionally darker shade of the color suitable for each temperament should be selected to correspond with the age. Beside the wear of the teeth, which is to be discussed later and which is a frequent accompaniment and an indirect result of old age, another influence of age must be mentioned. This is the physiologic recession of the gums about the crowns of the teeth which occurs as life goes on. It must not, of course, be confounded with recession of the gums due to pathological processes. In individuals past forty there is a recession of the gum tissue from about the necks of the teeth, exposing the cementum. The round full festoons of the gingiva^ characteristic of youth disappear, and instead, the line between the gum and the teeth becomes accentuated. This is more strikingly evident in some tem- THE USE OF THE TEETH. 2(57 peraments than in others. It occurs particularly in individuals of the lyiupiuiticjsanguincundhilio-lymphatic temperaments. In the arrange- ment of the gums of artificial dentures due cognizance should be taken of this peculiarity. THE USE OF THE TEETH. The use of a denture through the course of a human life must neces- sarily leave some evidences of v^ear upon the teeth. Although the enamel covering the teeth is the hardest structure in the body, yet it is unable to resist the wear which occurs in the performance of the masticatory function. T he character and the amount of wear of the teeth depends upon three factors: the food, the manner of movement of the jaw, and the character of the dental tissues themselves. According to J. H. WassaP and A. H. Thompson" the teeth in the skulls of pre- historic man show evidences of great wear. They wore out in the performance of their daily duties, because of the rough character of the food. For the same reason skulls of American Indians display similar evidences of attrition, large quantities of finely divided sand having become mixed with their corn meal in its manufacture. Modern conditions of civilization are not so conducive to wear of the teeth. Individuals are frequently seen who have reached the age of thirty-five or forty years and yet exhibit little evidences of wear. The amount of wear bears no relation to the age of the individual. It is more directly related to the temperament, or in other words, to the physical resistance of the teeth in certain temperaments. Wear occurs most commonly in those of the lymphatic temperament, and in the combinations of this temperament and the bilious with the san- guine. Harrison Allen says, "the wear is not related to the form of the condyle of the jaw," but inasmuch as worn conditions of the teeth are most frequently found in cases characterized by lateral movement of the jaw% it is believed that they must be connected in some w-ay. The existence of certain habits, as that of tobacco chewing, as pointed out by A. H. Thompson^ is responsible for wear of the teeth, the silica mixed with the tobacco exerts an abrasive influence upon the teeth. According to I. B. Davenport*, in a perfect denture wear proceeds evenly upon all its morsal surfaces. Thompson describes the pro- cess as follov/s: "The points of the tubercles of the molars and the cusps of the bicuspids and canines are first worn off, exposing the den- tine, which wears out in little cups surrounded by a wall of enamel. The edges of the incisors are worn to show a fine line of dentine." In those temperaments in which the cusps are short, as the wear proceeds further, the lower jaw moves forw^ard, so that the incisors are brought into the position of an edge-to-edge bite. Dentures wnth long pene- trating cusps exhibit a different kind of wear from those in which the 1 The Dental Cosmos.Vol. xliii., p., 977. * The Dental Cosmos. Vol. xlii., p. 519. " The Dental Cosmos, loc. cit. * International Dental Journal. 268 THE ITlUrAN DEXTAL MECHANISM. cusps are short. The following degrees are noted by C. H. Ward"; 1st degree, enamel somewhat worn; 2d degree, cusps disa})jiear and dentine exposed; 3d degree, teeth reduced in height; 4th degree, teeth worn off to the necks. It is evident from the standpoint of dental j)rosthesi.s that the imita- tion of extreme degrees of wear is undesirable. It will seldom be Fig. 237 Fig. 238 'ryp'cal arch outline of bilious temperament. (Fhotograpti of a specimen in the Wistar In- •stitute of Anatomy.) Typical arch outline of nervous temper.i- ment. (Photograph of a specimen in the col- lection of Dr. M. H. Cryor.) Fig. 239 Typical arch outline of sanguine temper- ament. (Photograph of a specimen in the collection of Dr. M. H. Cryer.) Typical arch outliiieof lymphatic tempera- ment. (Photograph of a specimen in the Wis- tar Institute of Anatomy.) found wise to imitate more than the first and second degrees, inas- much as the third and fourth are seldom seen excepting among savage people. Of the first degree two t\^es of wear are noted as worthy of imitation so far as the incisors are concerned. They are, first, that observed in cases in which there is a pronounced overbite. (Fig. 197.) In this in the forward and backward movement of the jaw, ' The Dental Cosmos, Vol. xl. p. 261. THE USE OF THE TEETH. 269 tlie occlusal ends of the upper and lower incisors are worn, the wear upon the up{)er occurring- at the expense of the lingual surface, that upon the lower beini; at the expense of the labial. This results in a straightening of the line of occlusal edges of the incisors in both jaws. Fig. 241 Denture exhibiting first degree of wear. (Photographed from living subject) Fig, 242 Denture exhibiting second degree of wear. (Photograph of skull in collection of Department of Dentistry, University of Pennsylvania.) Fig. 243 .i '^^^!^^ '--'^'^ ■ a '■■^*^^-^ ^- TUhi Denture exhibiting second degree of wear on the right, third degree on the left. (Photographs of skulls in collection of Department of Dentistry, University of Pennsylvania.) the labial surfaces of the upper and the lingual of the lower ending in a sharp edge which is more or less continuous from side to side. (Fig. 24L) After a while some little chipping of the enamel edge of the upper may occur making this line irregular. The second type oi wear in the incisor region is that in which the overbite is not so pro- 270 ^^^^' I^UMAX DEXTAL MECHANISM. nounced, where the incisal edges of the lower teeth move more nearly directly across the edges of the upper. This results in the wearing of grooves at such places as the teeth come in contact. (Fig. 242.) This is not so extensive a degree of wear as occurs in the first type, and is more frequently observed. The second degree of wear in the incisal region is common to those dentures in which an edge-to-edge bite exists. Here the enamel is ex- tensively worn through and the dentine is exposed. ^Fig. 243.) As the attrition proceeds the length of the crown is gradually reduced,- the dentine is frequently stained and a characteristic appearance results. The margins of the enamel are chipped and an irregular incisal line is produced. CHAPTER VI. THE EXAMINATION, PREPARATION, AND STUDY OF THE MOUTH PRELIMINARY TO THE INSERTION OF ARTIFICIAL TEETH. By a. DeWitt Gritman, D. D. S. A CAREFUL preliminary examination of the mouth is a matter of \'ital importance to the successful insertion of substitutes for the natural teeth. Two objects are held in view in this examination: The first is to determine the state of health of the tissues of the mouth, particu- larly of those to be in relation with the artificial denture. As the latter is at best a foreign body, and does not tend to improve the general hygiene of the mouth cavity, it is important that the tissues with which it comes in contact shall be perfectly healthy and not the seat of patho- logical processes. It is also an evident necessity that the tissues which are to afford support to the denture, whether it be plate or bridge, must be sound and strong. In this connection must also be considered the means of bringing these tissues to a state of health and usefulness in cases where the latter condition does not exist. The second object has to do with plans for the prospective denture. At this examination it wdll be necessary to determine the type of denture which is to be employed, the general plan of its construction, its means of retention, and whether the mouth is favorable thereto, together with any means necessary to prepare the mouth for the reception of the den- ture. In \dew of these facts, it is evident that thoroughness in this preliminary examination is desirable. The condition of the soft tissues of the mouth may be ascertained by visual examination, either directly or by means of a mouth mirror and by palpation wdth the finger. Familiarity ^\ith the general appearance of healthy mucous membrane, its color and consistence, and ability to recognize aberrations from this should be part of the equipment of the examiner. If the mucous membrane, which in the normal state is a firm resistant structure capable of giving adequate support to the denture, is not in a state of health, it is evident that it must be brought to this condition before the impression is taken. This T\dll presently be discussed. If the mouth is edentulous, and inspection shows its tissues to be healthy, further examination and study have only to do with the plans for the denture. A careful digital exploration of the entire surface to be covered by the future plate should be made to determine the loca- tion of hard or soft areas, as account must be taken of these in providing for the retention of the plate. Special attention must be paid to the retention of the plate in the lower jaw, and as this \\ill mainly depend on the action of the muscles attached to the mandible, the location •271 272 THE STUDY OF THE MOUTH. of the margins of the future plate must be carefully noted, as well as the positions of the labial and lingual frsena. These should be accu- rately marked ui)()n the cast, for if these be neglected in all probability the lower plate will not rest undisturbed upon the jaw. If there are teeth in the mouth the examination must determine if they are strong and healthy, and if not, whether they may be made so. A judgment of their probable tenure of life and usefulness must also be formed in this preliminary survey. The presence of teeth and their location must be accurately mai)i)ed and recorded, and if these be .scat- tered in the front of the mouth their color and position must be care- fully noted. MORBID CONDITIONS OF THE GUMS. The condition of the gums in the mouths of many patients presenting themselves for prosthetic work is not always favorable to the immediate insertion of artificial dentures. It is inadvisable to ])lace an artificial denture in a mouth until all the tissues, especially those in direct relation with the plate, are in a healthy condition. A general hypenemia of the mucous membrane is frequently present, produced by the presence of roots or fragments thereof, or of more or less diseased teeth. This is frequently aggravated by a general lack of hygienic care on the part of the indi\ idual l)ecause of a hypersensitive condition of the mucous membrane and tenderness of the remaining teeth. The removal of the useless roots and teeth, and the use of an astringent antiseptic mouth-wash, is usually all that is necessary to render the mouth in suitable condition to receive the denture. Long-continued sickness is frequently responsible for gingival inflam- mation, which is increased generally by lack of proper care of the oral cavity during the period of illness and even during convalescence. It may be remarked that proper care of the oral cavity during illness is not yet one of the sohed problems in nursing. Stomatitis is not infrefjuently observed. As its name indicates in a general way, it is inflammation of the mouth, but this term embraces a large number of subdiseases, such as a])hthous stomatitis, ulcerative sto- matitis, mercurial stomatitis, syphilitic inflammations, etc., divisions too numerous for the average prosthetic worker to treat satisfactorily, and they had best be referred to a specialist for proper treatment. Stomatitis simplex, or ordinary inflammation of the mucous membrane, is so common that it should come under his sjx'cial care. If this exists, there must be careful hnage of the mouth three or four times a day with an antiseptic solution. Potassium chlorate, 15 grains to the ounce of water, is an effective mouth-wash. A 2 per cent, solution of hydro- naphthol in alcohol is another excellent mouth-wash for all forms of gingival inflammation. If this condition is the result of gastric disturb- ance, much help may be gained by the use of the following for internal administration : H. Acidi hydrochlorici dil fo'J- Sig. — Five drops well diluted with water after ineala. EETEyTIOX OE EXTRACTION OF NATURAL TEETH. 27.] iNIuc'h of the gingivitis found contiguous to remaining sound teeth is due to calcuh at the free margin of the gums. This may result in ex- tensive infiammation of the peridental membrane and finally in pyorrhea alveolaris. A discussion of the treatment of the latter properly belongs elsewhere. Suffice it to say that all teeth must be thoroughly scaled, and the patient advised to use the following mouth-wash: B. Zinci chloridi 5-10 gr.; Aquse menth. pip 5'J- — -^I- Sig. — Use as a mouth-wash for a week. Another formula will be found excellent: B. Hydronaphthol gr. xx; .\lcoholis oi : Aquae 5j. — M. Sig. — Twenty drops to a small tumbler of water, as a mouth- wash, to be used once daily. Unless teeth so affected are placed in a healthy condition, it is useless to attempt to insert a plate while they remain as centers of disease. It is unnecessary to enlarge further in this direction, as it would carry this chapter too far afield to attempt to cover all the pathological condi- tions interfering with the insertion of artificial dentures. Siiffice it to say that all pathological lesions of the tissues of the mouth of whatever natiu-e must be treated and the entire mouth restored to normal health before anything is done in the way of substituting artificial for the nat- ural teeth. KETENTION OR EXTRACTION OF NATURAL TEETH. It would be difficidt to overestimate the value of a thorough prelimi- nary examination of the mouth for which an artificial dentiu-e is required, if it only served to settle the various questions arising in relation with any natural teeth and roots which remain in place. These teeth and roots must be considered carefully in the light of the future comfort of the patient, as well as in their bearing upon the work of the operator. There is probably no greater problem in prosthetics than that presented in such cases, nor one reciuiring the exercise of more judgment. In some mouths there T\-ill be presented sound roots and those in all stages of decay, teeth with vital pulps and those with pulps no longer li^■ing. If the roots are healthy, are they suitable for crowning, and will crown- ing be of advantage to the patient ? Will the roots if properly treated and crowned be of se^^-ice in mastication, or will the artificial crowns themselves eventually produce pathological conditions ending in destruc- tion of the roots? Are the vital teeth present of prospective value suffi- cient to force their retention in the mouth? Will the \-ita\ teeth aid in the retention of the plate or prove an unending annoyance to the patient ? These are queries which must be carefully considered and answered in all preliminary examinations. It is clearly good practice to first extract all the teeth and portions of teeth that cannot be restored to health. AMiether teeth and roots which are healthy or may be made healthy are to be retained or not must . 18 274 THE STUDY OF THE MOUTH. be decided by two considerations, viz. : whether they will assist in the retention of the denture or whether they will interfere with its adjust- ment. ^Vhile the retention of artificial dentures is not the subject of this chapter, it assumes preliminary importance here in considering the extraction of roots and vital teeth. The retention of a plate is a matter of so great importance that the operator who fails to take this into con- sideration invites defeat. It is well understood that a full upper denture is more satisfactory in the ease with which it is maintained in place than a partial plate. The adhesion is, as a rule, adequate in the former, but in the latter it is subject to many disturbing elements. Very dry mouths interfere with retention, but this is not a common condition and does not need extended notice. Where it does exist, it is not an insuperable obstacle. ^Mouths in which the alveolus has been entirely resorbed, as is frequently the case in both jaws, present more serious problems, es- pecially in the case of the lower jaw. Properly constructed plates can, however, be made of rubber for such cases that will exhibit remarkable adhesion. While these are frequently made heavy to overcome dis- placement, this is not absolutely required, provided care is taken to adapt them by wide flanges to meet the loose tissues subjacent to the alveolar ridge. Greater liberty should be allowed in extraction in a mouth favorable to plate retention, while great conservatism should be observed in cases in which stable retention of the plate is doubtful. ^'ital teeth may be of value as supports either in bridge-work or in plates, but in some instances they interfere with the stability of the artificial substitute. It will probably be wise to retain these as long as possible, but too much should not be expected in the way of their reten- tion. The idea prevails, not only with patients but ^^^th some dentists, that artificial dentures cannot be used with the same satisfaction in crush- ing articles of food as natural teeth. Greater force can be applied wit\\ the latter it is true, but the extremity of force is not needed in mastica- tion, and all food, of whatever kind, can be comminuted satisfactorily with a properly arranged set of artificial teeth. The permanency of all isolated teeth and roots is, and must always be, a question of doubt. With adjoining teetli lost and resorption of the alveolus taking place continually, the tissues surrounding the isolated tooth must eventually give way, the pericementum becomes detached, and the result is eventually the loss of the tooth. This apj)lies to all teeth and is one great obstacle to the permanency of bridge-work. There are cases, however, when an isolated molar is of great service to the operator, for a well-advised clasp upon it may stay the plate and add materially to the patient's comfort. It is difficult, therefore, to give any opinion except in general terms as to the advisability of the extraction or retention of indiN'idual vital teeth. Each one must be judged by existing conditions. If canine teeth can be retained, they have a distinct value in giving form to the jaw which is not easily obtained by artificial substitutes. Even the RETENTION OR EXTRACTION OF NATURAL TEETH. 27 o canine roots may be better treated and crowned than extracted. It is time wasted to attempt to save a single anterior tooth. Any isolated incisor is useless for appearance or for mastication, and it is extremely difficult to align artificial teeth mth it satisfactorily. Roots crowned, except in rare instances, are of doubtful value beyond temporary expedients. They may have a life of a few years, but e\'ent- ually they are brought to the forceps. This is due to positive patholog- ical conditions engendered by undue strain on the peridental membrane, or by direct irritation through the bands of the crow-ns and sometimes from imperfect root treatment. There are not infrequently cases, however, where roots should be retained, especially in the anterior portion of the mouth. With care in their preparation for crowning these may be made of great value as a support to the artificial plate in mastication. Judgment is, however, necessary here, for the root or roots must be of the best quality and capable of being placed in perfect hygienic condition; otherwise their retention would be a mistake and result unhappily. Whatever may be decided upon in this preliminary stage must be thoroughly done. If extraction is chosen, this leaves a socket of tempo- rary bone tissue with sharp and friable edges, which becomes inflamed and may become infected if pains are not taken to prevent it. All the gums should be carefully bathed with an antiseptic wash, the sockets of extracted teeth painted with tincture of iodine or a 1 per cent, solution of formaldehyde. This latter should be carefully prepared to insure its being strictly 1 per cent., for beyond this strength it is very irritating. In case of excessive bleeding tannic acid may be effectively used, either in pow^der or in solution, the latter being preferred. Some operators recommend cutting away the exposed portions of the alveolus, but as this is a painful operation and is not absolutely necessary, it is usually better to leave the resorption to nature. A mild antiseptic wash should be used by the patient three times a day until healing of the tissues has occurred. Length of Time to Supervene After Extraction. — The length of time it is necessary to wait before a so-called permanent denture can be prepared is a matter that can only be discussed in general terms. If the insertion of the denture is* deferred until the last spicula of alveolar process is disposed of by resorption, it may be years before the final settling of the artificial denture upon the bone of the jaw or jaws is attained. It is not only necessary to wait for this final resorption, but it is not advisable to wait at all, for the process of resorption proceeds more satisfactorily if a temporary plate or plates be inserted. It Mill be necessary to wait if possible a week after the extraction of useless teeth has been completed to permit partial healing of the irri- tated tissues. The exact time must depend upon the judgment of the operator. There are cases where shock supervenes after extraction, and it is not unusual to be obliged to defer all operations for a series of weeks. As a general rule a temporary denture may be prepared in the time first named. 276 THE STUDY OF THE MOUTH. The question of a temporary plate having been settled between the dentist and the patient, further proeedure must be left to the judgment of the operator. A temporary denture is not supposed to be of mueh value in mastieation, the sharp edges of the alveolar processes forbidding this. This condition frequently suggests the surgical operation of re- moving these projecting points with bone-cutting forceps. It is not uncommon, however, for temporary sets to be so satisfactory to the wearer that they are continued in use for years. They are not supposed to be in use for more than one year, when a more comfortable plate can be inserted; but even this must assume a semi-temporary character, for this resorption of the process is continuous until all the alveolus is re- moved, and this covers an uncertain period. The method of taking an impression for these temporary dentures does not differ from that ordinarily employed, })ut while some use other materials in general prosthetic practice for this purpose, plaster alone should l^e used here, and for the excellent reason that the tissues sur- rounding the extracted teeth are left in a soft, flabby condition which any impression material requiring force will displace. Aside from this, it is well to have the sockets of the extracted teeth produced on the cast when the artificial teeth are ground to fit the gum. It is sometimes demanded by patients that the natural teeth all re- main in place until the temporary set is completed. ^Yhen this is the case it will be necessary to take an impression of the mouth before the teeth are extracted, when the plaster teeth are cut ofi" the cast, and their sockets carved out to represent resorption. This is freciuently resorted to, but cannot be recommended for all cases, although it may be used to advantage, for example, in a case for which a partial plate is to be inserted after the removal of the six anterior teeth. An impression in plaster should first be secured. This will give the operator the exact form and position of the natural teeth and enable him to reproduce them with greater satisfaction to the patient, who is always naturally sensitive to any marked change of appearance. After the length, size, and position of the natural teeth have been carefully noted, they may be cut from the plaster reproduction, and then the cast carved to represent the possible resorption certain to follow after extraction. A better way, however, is to take another impression after extraction and to use this to prepare the base for the temporary set. Plain teeth will be required here, and these must be arranged in anticipation of the great amount of resorption sure to follow. Such a plate belongs entirely in the category of temporary dentures and should be replaced inside the year. All temporary plates of the kind described have a value besides inviting by pressure more rapid resorption of the alveolar process, and that is their binding effort upon the tissues, retaining them in position and protecting them from the continued irritation which would other- wise occur in mastication. It is possible that in rare instances surgical interference may be necessary in removing adhesions of cicatricial tissue, but these are not likely to occur where temporary sets are inserted soon after extraction. SURGICAL COMPLICATIONS. 277 and even should such take place, it has been the experience of the writer that the plate soon finds its proper bed, removing at the same time all trouble from adhesions. Patients are very sensitive to the thought of a surgical operation, and this should be adopted only as the last resort. SURGICAL COMPLICATIONS. The complications that may precede or follow the preparation of the mouth for an artificial denture, while briefly alluded to on previous pages, demand a more detailed discussion. After the removal of teeth, whether firmly implanted or where inflammatory conditions have pro- duced a loosening of their attachment, there is more or less swelling of the gum which interferes with the correctness of any impression taken immediately. This necessitates delay to permit these abnormal condi- tions to subside before attempting the insertion of an artificial substi- tute. Inflammation. — Where many teeth are to be removed, the possi- bility of subsequent inflammation is increased in direct proportion to the number of teeth involved. It is important that the tissues surrounding the teeth should be carefully protected from injury. The careless handling of the forceps may not only produce unnecessary sufi^ering to the patient, but may be the cause of subsequent com- plications in the preparation of the denture. After the extracting has been done, care should be taken to discover if any portion of the alveolus is fractured and all loose pieces should be carefully removed. Pendant portions of the gum, the result of previous inflammatory condi- tions, or brought about by the operation, should be removed, as their presence delays the healing process. The removal of portions of the alveolus by a subsequent operation has pre\dously been alluded to, and while not imperative, may sometimes be resorted to with advantage to both patient and operator. Before attempting this, however, the con- sent of the patient should be obtained, for while not a serious operation, it involves some pain and subsequent soreness. For this reason many prosthetic practitioners prefer to await the slower process of resorption. If, however, a surgical operation seems necessary, it must be performed under a local anaesthetic such as cocaine, or under nitrous oxide and oxygen, and be followed by the use of an antiseptic mouth-wash. Whether the surgical procedure be adopted or the slower process of resorption be allowed to take place, this antiseptic washing of the mouth should be continued by the patient during the period of healing. Temporary Denture. — Should it be necessary to insert a temporary denture immediately after the teeth are removed, and this not infre- quently happens, it will be necessary to follow a somewhat difterent procedure from that described on a preceding page. To prepare such a denture immediately after extraction involves many difficulties. Any attempt to secure an impression from which a plate at all permanent in character can be made must necessarily be futile, as the inflamed condition of the gums renders this impossible. A plate made at this 278 THE STUDY OF THE MOUTH. time will, therefore, as a rule, be of a strictly temporary nature, and this should be distinctly understood by the patient. To secure a correct impression, plaster alone should be used. While opinions may differ as to the use of this agent under other conditions, there can be but one opinion as to its use for these cases, for it will, if properly prepared, cause little or no displacement of the parts, and re- produce them with an exactness not possible with any other material. It is very important that protruding portions of the gums should not be disturbed, and those remaining in place, in cases in which it has not been deemed wise to remove them surgically, should be reproduced on the cast upon which the plate is to be made. The insertion of the temporary denture is not ordinarily productive of discomfort to the patient. The close adaptation of the base to the gum tissue serves as a binding bandage and will, by its continuous pres- sure, hasten the healing of the irritated areas. The time such a denture may be worn will vary and cannot be predetermined with accuracy ; but when properly prepared, may serve for a long time with comfort to the wearer, and even in some rare instances take the place of a permanent plate. It is not, however, advisable to continue its use for more than a year, as complications may result which will interfere with the more perfect adaptation of the permanent denture. Adhesions. — There is occasionally found a cicatricial adhesion be- tween the mucous membrane of the cheek and that of the aheolar wall. This may be a cord-like attachment which marks the site of the fistulous opening of a previous aheolar abscess. The adhesion may be the result of extraction, the loosened tissue falling into the wound and becoming attached as a result. This may not be so situated as to call for surgical interference, but if it does interfere with the proper setting of the plate and endangers its stability, it should be removed. Wounds of the mouth, whether incised or from the use of caustics, may in healing cause extensive attachments between the cheek anfl alveolus, rendering the wearing of a plate difficult or impossible. This condition must also be treated before a denture is inserted. The surgical principal invohed in the procedure is to produce a separa- tion of the parts and to continue this separation by the insertion of a plate which will prevent contact of the parts and consequently a reunion of the tissues. This operation may require local anesthesia for its per- formance unless it be of minor character; the use of a 4 per cent, solution of cocaine will be found satisfactory for the purpose. The incised parts should be washed with an antiseptic solution. The surface may be painted with styptic collodion. If the adhesions are extensive, it will be found preferable to first take an impression of the mouth and prepare a cast for stud>'. The cast is then cut away as is proposed by the surgical operation, and a plate with- out teeth prepared from it. The plate should be inserted in the mouth immediately after the operation to prevent the readhesion of the parts. A styptic should be applied after the operation; a spray of hydrogen peroxide usually checks hemorrhage. If this does not suffice, a solution CHOICE OF BASE. 279 of tannin will usually be found effective. The cut surfaces may be dried with pieces of lint and painted with styptic collodion; as soon as this is dry the plate should be inserted. The patient must be directed to wash the mouth several times a day with an antiseptic, such as veruas, listerine, or 1 per cent, solution of hydronaphthol. The irritatinji; character of the latter must be carefully noted and its use discontinued if the irritation is marked. This plate is simply placed in position to promote healing and prevent adhesion of the cut parts, and it should not be required to be in place more than two weeks. Should the granulated surfaces show an unhealthy appearance, they should be painted with a solution of nitrate of silver, 4 grains to the ounce. Tincture of iodine may also be found effective painted upon the gums. CHOICE OF BASE. The importance of the selection of a proper base for the artificial denture cannot be overestimated. Each case under consideration calls for careful study, and each presents a problem not easy of solution. Vulcanite, celluloid, gold, silver, platinum, aluminum, certain alloj^s of tin, known as "fusible alloys," and porcelain have all been used as a base for the denture. A consideration of the merits of each of these materials will now be in order. Vulcanite is the material in most com- mon use for full sets, and, while not a perfect material, justly deserves a prominent position in the list if care is taken to overcome its disad- vantages. Under the early conditions of its use a serious objection to vulcanized rubber developed. In some cases it was liable to produce inflammation of the mucous membrane, or what was known as "rubber sore mouth." This pathological condition, manifesting itself shortly after vulcanite was introduced as a base for artificial teeth, became a problem to dentists and a source of much complaint on the part of the medical profession. So serious did this objection become that it threatened to destroy the usefulness of this material in dentistry. The question was taken up by the Pennsylvania Association of Dental Surgeons in the early sixties, and it was given over to Professors Wildman, Buckingham, and Tru- man to find, if possible, any ground for the charge that the inflamma- tion was the result of mercuric action arising from some chemical change in the vermilion used in coloring the rubber. This research committee in its final report proved conclusively that mercury had nothing to do with the pathological condition in question. In their opinion it was due in part to the non-conductivity of the rubber, and possibly, in addition, to the rough surface left after vulcanization, which retained food particles in close proximity to the mucous membrane, exciting by their presence inflammatory processes. This latter conclusion was con- sequently confirmed by Black, who demonstrated that this condition invited microbic development, and the presence of toxic matter derived from this source produced the result complained of so universally. It had been demonstrated that metal as a base could be tolerated by the 280 THE STUDY OF THE MOUTH. membrane without producing undue irritation, and this was explained by its superior thermal conductivity and the fact that its polisiicd surface prevented accretions. This was so positively manifest in the use of metal plates tiiat pros- thetic workers adopted the use of tin foil placed over the cast before vulcanization. This became imbedded in the rubber and upon removal left the surface highly polished, requiring only the revolving brush to complete it. This was found largely to overcome the trouble heretofore experienced, and vulcanite as a base has become almost universal for ordinary sets. Its superior quality of lightness, comparative ease of manipulation, and adaptability to all conditions of the mouth, and, above all, its hygienic possibilities brought it into general use. The introduction of rubber as a base for artificial teeth resulted in a revolution in prosthesis. The old methods of soldering were practically abandoned, with the result, natural to such sudden changes, of much inferior work. The merits and demerits of the material were not under- stood, nor were the proper methods to be observed in its use fully known. The material is still much abused at the present time. The committee of the Pennsylvania Association, previously alluded to, demonstrated by careful research, microscopical and chemical, that rubber if properly vulcanized would not absorb the fluids of the mouth. Whether absorption would take place under a greater pressure than is present in the vulcanizer remains yet to be proved ; but no plate exam- ined gave any evidence of moisture, nor was imbibition possible under ordinary mouth conditions, the vulcanite being absolutely impervious. This applies, however, only to properly vulcanized rul)ber. There is doubtless much vulcanite inserted in mouths so porous as to become positively foul from the absorbed fluids and the decomposition of organic matter, resembling in this respect the early walrus carved specimens which are interesting now only as exhibits in dental museums. Aside from its value for entire sets, it is even more important in attach- ing teeth to gold or platinum plates, completely relegating the older processes of backing and soldering to ancient history, and adding there- by not only to the comfort but to the health of the wearers. While the non-conductivity of vulcanite is the main objection to its use for entire plates, this does not apply to rubber attachments on metal. The gold base is still extensively used for a superior class of work. For full dentures it is usually employed as the base-plate proper with the teeth attached by means of vulcanite. This type of denture ranks next to that of continuous-gum in its hygienic qualities, its aesthetic possibili- ties, and its general acceptability to the mucous membrane. Its ability to transmit thermal changes, its highly polished and hence easily cleaned surface, and its freedom from interstices in which foreign matter might find lodgment, account for its general hygienic value. The relative thinness of the base-plates and its strength, its unalterable character in the mouth, and the possibility of any desired arrangement of the teeth in the vulcanite attachment are all advantages associated with this type of denture. CHANGE OF BASE. 281 Because of the relatively high cost of the material it is not customary to insert full gold dentures until at least a year after the last teeth have been extracted, to allow the greater portion of the resorption to occur. Gold offers many advantages for partial plates and is to be pre- ferred in most instances. Where much strength is required, as, for ex- ample, when some of the teeth to be replaced are isolated, it is impossible to secure the desired strength in vulcanite. Sih-er was the ordinary base for temporary dentures in former years by reason of its cheapness ; it was also used for permanent plates for the same reason. Where cheapness in a full metal plate is now desired, swaged aluminum has come into general use for this purpose. With the development of exact methods of casting, cast aluminum base-plates have also become a useful addition to our list of cheaper metal base-plates. Opinion is divided as to whether they are as satis- factory as the swaged plate, but it is likely that if careful attention is given to the technic of making, they will be equally useful. For the lower jaw, as before stated, weight in the denture may be necessary. Base-plates cast of any of the fusible alloys recommended for this purpose will usually prove quite satisfactory. Where something better than the metals ordinarily used is desired, the porcelain continuous-gum denture may be employed with great satisfaction, combining as it does artistic beauty, cleanliness, and weight. The high aesthetic quality of the continuous-gum denture, its freedom from crevices in which foreign matter may be retained, and its highly polished surface, unalterable in the mouth, recommend it strongly in suitable cases. Celluloid has practically become obsolete as a material for dental plates because of inherent defects in the material. CHAPTER VII. IMPRESSIONS OF THE MOUTH. By a. DeWitt Gritman, D. D. S. The first step incident to the construction of a successful denture, and one that does not always receive proper consideration, is to obtain a perfect plaster reproduction of the jaw for which the denture is intended, and this requires that an accurate impression of the same be taken. The impression of the edentulous mouth, or of one requiring a partial substitute, has been defined variously at the several stages in the development of prosthetic dentistry. In the earlier periods it was re- garded as quite sufficient to have a counterpart of the mouth made with softened wax (beeswax), and the impression was ordinarily taken without much regard to the conditions relating to the retention of the plate. The present aim in all impression taking is the ultimate perfect adap- tation of the plate, and in order to accomplish this a thorough study of the mouth is a prerequisite; without this adaptation a comfortable denture is not to be expected. Each mouth requires separate study, and no absolute rule can be laid down which will be of general value. The condition of the soft tissues, the various abnormal growths, the possibility of retention, especially in the lower jaw, the difficulty of overcoming in some persons the nausea consequent upon a foreign body being placed in the mouth — all these details must be considered; in fact, no impression should be attempted before every unfavorable condition has been studied and provision made to overcome it. An impression of the mouth, in the modern acceptation of the term, means the first step in the successful retention of the denture, and in order that this may be accomplished there should be a careful study of the appliances and materials used for this purpose. ^ Impression Materials. — In order that the impression shall be an exact counterpart of the jaw, a suitable material to eft'ect it is demanded. To answer the requirements of such usage, the material must readily become plastic and pliable at a temperature not injurious to the tissues of the oral cavity. It must copy accurately the irregular surfaces of the jaw, and with equal accuracy retain its form after the impression has been removed from the mouth. It must also harden in less than five minutes. It should neither contract nor expand. The perfect impression material has never been produced, and prosthetic operators are compelled to make use of present materials — viz., plaster of Paris, modelling compound, beeswax, and gutta-percha. These substances group themselves into two classes of materials commonly used for this purpose: one made into a plastic mass with water, and the other through 282 IMPRESSION MATERIALS. 283 the action of heat. The first, plaster (phister of Paris), is generally considered to have the greatest number of qualities recommending it to use. In proportion to the difficulty connected with the impression will plaster be found the best material to use. IMPRESSION MATERIALS. "Wax. — This is a solid of animal and vegetable origin. Of wax from the animal sources there are beeswax, Chinese insect wax, and sperma- ceti. The vegetable waxes are Japanese wax, myrtle-berry wax, palm wax, etc. Beeswax is secreted by all honey bees, and by them formed into the cell-walls of their honeycomb. It is this that was formerly the only material suitable for impressions, and the older practitioners of dentistry were forced, in the absence of better material, to prepare this from the mass furnished through the general markets. It is now prepared by the supply houses in convenient cakes ready for manipulation. Beeswax is softened either by dry heat or in hot water, and is then kneaded into a doughy mass and applied as an impression material in proper trays. It is not a satisfactory material for this purpose, as it does not make a sharp impression, leaving a degree of uncertainty as to the final result. This fact has, therefore, placed it among the obsolete impression materials, but it has not been entirely driven from the prosthetic laboratory, where it still fills a proper place in many of the minor operations arising in practice. While springs were used in full dentures, wax served imperfectly to give a reproduction of the jaw. Since' springs have become obsolete, and dependence on adhesion is the main reliance, beeswax has been abandoned as an impression material except in rare instances. Various combinations have been prepared to take its place as a heat-softened material. The most prominent of these is what is termed Modelling- Compound. — This is a material prepared to take the place of beeswax, as described. It is composed of a resinous gum, such as copal, dammar, or kauri, with stearin, French chalk, colored and flavored. A formula of Mr. E. Lloyd Williams, of London, England, is as follows: "French chalk, one and three-quarter parts; kauri, one part; stearin, one and a half parts. j\Ielt the stearin in an enameled pan and stir in the gum. When these are thoroughly incorporated, stir in the chalk. It may be colored with carmine and flavored as desired." The Use of Modelling Compound. — A proper tray having been selected, a sufficient quantity of the modelling compound is taken and softened by dry heat and then placed in the tray, previously warmed. ^ The surface of the compound is then slightly softened by passing it over the flame before it is inserted in the mouth. It is then placed in the posi- tion desired and pressed well into place, care being taken that all parts of the compound on the lateral margins are pressed by the index-finger into all the irregularities of the jaw. This completed, the compound 284 IMPRESSIONS OF THE MOUTH. is left in the mouth to harden. This process may he hastened hy the use of cold water ejected from a syringe or by the contact of an aseptic napkin saturated with water. When hard it is removed in the usual way, by slight pressure downward if in the upper jaw, and upward if in the lower. The removal is effected with far less difficulty than with ])laster. Advantages Claimed for the Compound. — It is claimed that this model- ling compound takes a sharp impression, that is not subject to fracture on removal and that it compresses the soft tissues. These are true, but it must be -remembered that a sharp impression does not always mean a perfect impression. Disadvantages. — The force required to insert the compound necessarily disturbs the relation of the soft tissues with the jaw, and as these are moved from what may be termed the normal position a more or less imperfect impression of the mouth is obtained. The operator should aim to secure a perfect duplication of the tissues in situ, and this is always a question of uncertainty when a material like modelling com- pound is used. It is true that in skilled hands this material may suc- ceed in giving apparently excellent results, but the general principles governing all operations of this kind must be carefully adhered to if the best results are to be attained. The following quotation gives the method in use adopted by the Green Brothers, and, as this seems to meet fully the requirements of the material, it is given, in part, as quoted by George Henry Wilson, D. D. S., in his "Manual of Dental Prosthetics": "The best position in which the operator may stand while taking this form of impression is directly back of the patient. The partially taken impression is removed from the mouth, thoroughly chilled, and any excess removed. The peripheral labial and buccal borders are warmed over a small weak Bunsen flame and quickly returned to place in the mouth. It is securely held in place with one hand, and while the patient is muscle trimming, the operator makes interrupted compres- sion over the lips and cheeks with the disengaged hand. It may be necessary to rewarm and muscle trim several times, or an addition of soft dry compoimd may be needed upon portions of the rim of the impression. This addition may be made by " tracing on" with a stick of compound. When the operator is satisfied that the rim of the im- pression is perfectly adapted backward to the anterior border of the molar processes, the impression is thoroughly chilled, dried, and a small roll of soft modelling compound placed along the palatal border, upon the maxillary surface, and extended around the tuberosities to the malar processes. This addition is made quite soft in the small Bunsen flame, carried into the mouth, and quickly forced into place, removed, and dried. If it has not been sufficiently compressed, it is again softened and pressed in the mouth. The impression may now be considered complete. It is placed in cold water until thoroughly cold and, with- out dr\'ing, it is replaced in the mouth and very firm pressure made upon the hngual surface of the tray while the hp and cheeks are lifted IMPRESSION MATERIALS. 285 and drawn into position over the rim of the impression. The finjijers are removed from the mouth and the patient requested to remove the impression." The various uses to which modelhng compound may be appHed will be described under the proper headings. Gutta-percha (Isonandra Gutta).— This name is applied to the con- creted or inspissated juice of various plants. The geographical dis- tribution of the trees producing this is very restricted. The yield of a •well-grown tree is from two to three pounds of gutta-percha. Its use in the arts is very important, but is much restricted in dentistry, al- though many attempts have been made to use it as an impression material. It makes a sharp impression, and on account of this has been much used in the taking of molds for electrotypes, etc. Its intractable nature has relegated it to other purposes than that of impressions in prosthetic work. Plaster of Paris and modelling compound have entirely superseded this as they have other materials. The base-plate gutta-percha is furnished by the supply houses. It is not the pure gum, but other ingredients are mixed with it to give it a firmer body. This is used principally for temporary trial plates in the mouth. Impression Plaster. — Plaster of Paris has already been considered as to its general qualities, but for use in this connection it must now be given more detailed attention. This is a special grade of plaster, very finely ground, and setting quickly. Properly mixed, it should set in about three minutes, but is not nearly so strong as casting plaster, which is used for the cast and flasking. When employed for impression taking it should be colored and flavored. The advantage of flavoring it is that this gives greater comfort to the patient, overcoming the disagree- able taste of plaster and reducing the tendency to nausea, which fre- quently interferes with absolute accuracy in the impression. Coloring of Plaster. — By coloring the plaster the operator is enabled to remove it more readily from the cast, thus reducing the danger of mutilating the latter. The work can also be completed more quickly, and time is an important factor in all dental operations. The preparation of this coloring fluid, as used by the writer, is as follows: To one ounce of carmine add five ounces of ammonia, and, when thoroughly dissolved, remove the. cork and allow the fumes of ammonia to pass off. To one ounce of this solution add one ounce of Roger and Gallet's violet water. While impression plaster sets in a comparatively short time, it is often desired to hasten this in order to reduce the period during which the impression must be held in the patient's mouth. Each batch of plaster should be tested as to time of setting, as this varies in different lots of the same brand. Several methods are employed to hasten the setting of the plaster. Potassium sulphate in powder or, preferably, as a saturated solution of crystals is effective. Chloride of sodium is used also for this purpose. Warm water also hastens the setting. Each of these methods may be 286 IMPRESSIONS OF THE MOUTH. used, depending upon the grade of plaster and the time desired for the completion of the setting process. The method used by the writer is, first, to place five drops of the coloring material in a bowl, then a half teaspoonful of a saturated solution of potassium sulphate. The amount of water necessary for the average impression, from one to one and a half ounces, is then added. The plaster is now sifted in carefully until the water has been completely taken up by the plaster. Then the plaster spatula is cut back and forth through the mass to eliminate any air bubbles. Stirring the plaster will hasten its setting. The proper consistency of the mix for taking the impression is attained when the batter on the tray is thick enough to permit inserting the latter without danger of the plaster dropping. PLASTER IMPRESSIONS. Plaster Spatula and Bowl.- The form of spatula shown in Fig. 244 was designed to serve two purposes. It presents at its point two corners, one round and the other square. The round corner is used in mixing the plaster, and the other is best adapted for shaping up the cast on Fig. 244. Plast atula. the glass slab. The plaster can be so shaped with it that there is very little need of trimming after it has hardened. The square corner is also useful and a great time saver in flasking, and will leave the plaster in proper shape to be varnished. The plaster bowl made from rubber (Fig. 245) is now more generally used than'that of porcelain or metal. The advantages of the porcelain Fig. 245. Rubber plaster bow bowl consist in its firmness and smooth surface, there being less resist- ance to the spatula when stirring. Its great disadvantage is in the adhesion of the material to the porcelain, and the difficulty in its removal when allowed to harden in the bowl. The rubber bowl is flexible, but TMPEESSION TRAYS. 287 is not as agreeable to the operator in handling. Its surface not being smooth, more resistance is offered to the movement of the spatula. The great . ad vantage of the rubber bowl over the porcelain is the ease with which the plaster can be removed after it has hardened. It is also possible to squeeze its sides together and thus make a spout for pouring. This, combined with the fact that it is unbreakable, seems to have brought it into general use. IMPRESSION TRAYS. An impression tray is the receptacle by means of which the impres- sion material is conveyed to the mouth, and by means of which it is Fig. 246. Fig. 247. Full upper impression tray. Full upper tray with deep flange for use with pla.ster. brought into relation with the tissues of which the imprint is to be made. It also serves to retain this material in place until it hardens, Fig. 248. Fig. 249. Full lower fmpression tray. Tray for partial upper impressions. and in some instances to remove it from the mouth. When an impres- sion has been broken in withdrawing it, the tray serves as an accurate matrix into which these fragments may be fitted. 288 LVPEESSfOXS OF THE MOUTH. There are two kinds of im|)ression trays in f^jeneral use. The first is the cast tray, which is made of brittania metal or tin. The main ad- vantage of this type is that it can very readily be bent and shaped to fit the case. The other style of tray is that made of German silver by Tray for partial lower inij'ressions. swaging, and this is very strong and rigid. For typical mouths, where no bending or shaping is required, this type of tray is ideal. In selecting a tray for an edentulous mouth great care should be taken to have it properly adjustefl to the case in hand. It should be Fig. 251. Full upper tray with wax added at the posterior edge. about three-sixteenths (y\) of an inch larger in each direction than the mouth for which it is to be used. For an upper impression, in addition to conforming the tray to the case, a small roll of yellow beeswax should be placed on its posterior edge, and while this is still soft it should be placed in the mouth and pressed up into proper position. The wax will CLASSES OF I3IPRESSI0NS. 289 then receive an impression of tlie parts posterior to those to which the denture will extend, and should then be trimmed down to leave a narrow rim of wax at the posterior edge (Fig. 251). The object of this wax is to prevent the surplus of plaster from being forced beyond the posterior edge of the tray and thus nauseating the patient. With the wax properly fitted to the tray equal pressure upon the tissue may be obtained at the posterior surface, as at the buccal and labial surfaces. If wax be not placed on the tray, as described, an im- perfect impression is almost certain to result. A Method of Making a Special Tray.— In very difficult cases it is sometimes necessary to make a tray, as one suitable for the case in hand may not be found. An impression is taken in wax and a cast produced, and over the cast a sheet of wax one-eighth (|-) of an inch in thickness is placed. A zinc die is then made, using the enlarged cast as a model, and a counter-die, with which a tray is swaged from German silver of about 22G. A handle is then soldered on with silver solder, and the tray should exactly fit the case. CLASSES OF IMPRESSIONS. Cases of -which impressions have to be taken for the making of plate dentures may, for purposes of simplicity, be divided into four groups: 1. Simple edentulous cases (upper and lower), in wdiich the jaw is typical and regular in shape, such as Figs. 252 and 253. Fig. 252. Fig. 253. Cast of upper jaw of regular form. Cast of upper jaw, of which impression is easily takeii. 2. Complicated edentulous cases with jaw irregular in shape, such as upper cases with high palatal vaults, with cleft palates, or with very high maxillary tuberosities ; cases in which part of the alveolar ridge has been removed; upper or lower cases in which there has been great re- sorption of the alveolar ridge, and w^here the muscular and fibrous attach- ments have become in consequence abnormally prominent (Figs. 254, 255). 19 290 IMPRESSIONS OF THE MOUTH. 3. Simple partial cases in which the remaining natural teeth are parallel and stand in columns, and from which the gum has not receded. Cases, Fig. 254. Cast (if upper jaw witli large maxillary tuber- osities. Cast of upper jaw with median bony tumc^r. in short, in which it is possible to remove the impression from the mouth upon the tray (Fig. 256). 4. Complicated partial cases exhibiting dovetailerl approximal spaces where the teeth have been lost, and cases in which tlie teeth themselves Fig. 2.5G. Cast of upper jaw, showing teeth in a column and having parallel long axes. are bell shaped. These are the most difficult cases of all, from the taking of the impression to the insertion of the finished plate (Figs. 257 and 258). Taking- a Full Upper Impression in Plaster.^When the patient is seated in the dental chair for taking an upper impression the chin should be on a level with the operator's elbow when the latter's arm is dropped at the side (Fig. 259). This gives full control over the field of operation and is the most convenient position for the various manipulations. A towel or bib made for the purpose is placed over the front of the patient's clothes to prevent injury from the plaster. For an upper impression the plaster should be placed higher on the CLASSES OF IMPEESSIONS. 291 anterior third of tlie tray (Fig. 260) and slope back to the posterior edge of the tray. With the left arm around the patient's head, the index- FiG. 257. Fig. 258. Cast of lower jaw with isolated teeth not Cast of upper jaw exhibiting several inter- parallel, dental undercuts. finger is placed in the left angle of the mouth. The right distal corner Fig. 259. Proper relative positions of patient and operator for taking upper impressions. of the impression tray is introduced into the right angle of the mouth (Fig. 261), and then by a rotary motion the left distal corner is carried 292 IMPRESSIONS OF THE MOUTH. into the left angle, and the tray brought in so the handle occui)ies the median line. Then, holding uj) the lip so that the alveolar border is visible, the tray is pressed up so that the plaster will touch the anterior Fir.. LljlJ. Full n))pLT tray with c 'rirct anion nt uf plasti-r fur taking an ini]ii-(_-^ioii. portion of the alveolar l)order first. Pressure is continued until the entire alveolar ridge has been covered with j)laster and the air has been f inscrtiiiR the tray in taking an npper impression. forced" out at the rear, the plaster beginning to show over the posterior edge of the tray. Then, with the posterior edge of the tray held firmly up against the tissues, the anterior part of the tray is pressed up into CLASSES OF IMPRESSIONS. 293 proper position witli a slight oscillating motion, and the surplus plaster will be forced over its anterior and buccal edges. The cheeks and lips are then manipulated in such a manner that the plaster will be pressed well up on the buccal and labial sides of the aheolar process. The tray is then to be held firmly in position by pressure with the fingers upon its centre until the plaster sets (Fig. 262). While the plaster is in process of hardening the patient is instructed to lean the head forward, and, should signs of nausea be manifested, the request is made to breathe hard through the nose and to hold the tongue down quietly. Fig. 262. Correct method of holding an upper impression tray in place during the setting of the plaster. If these suggestions are given to the patient, ordinarily there will be little trouble on the score of nausea. After the impression has been properly hardened, if it cannot be easily removed by slight pressufe upward on the handle of the tray, the finger is passed up under the lip to admit air over the edge of the impression, and in this way the adhesion is broken up and the impression can ordi- narily be removed with ease. If, however, it should have been left in too long and found difficult to remove, then by the use of a water syringe, placing the nozzle over the edge of the plaster impression and injecting a small quantity of water on both sides of the mouth, the impression may be removed without further difficulty. A good impression should appear as in Fig. 263. 294 IMPRESSIONS OF THE MOUTH An accurate impression is absolutely essential to obtain satisfactory adhesion in the phite denture. Fi- to enable the operator to press the soft tissues out of the way and still have plaster left to secure a perfect impression. Standing in front of the patient, the thumb of the left hand presses back the right angle of the patient's mouth. The left side of the tray is carried well back into the mouth, and with a rotary motion the right side of the tray is carried to its place (Fig.- 265). The lower ho Fig. 265. Method of inserting the tray in taking lower impressions. should be pulled down to enable the operator to view the alveolar border, and with an oscillating motion (Fig. 266), produced by placing the index- fingers on the tray, with the thumbs under the chin, the tray is forced down into .the desired position. The tray must then be held firmly in position until the plaster is sufficiently hardened to allow it to be safely removed (Figs. 267, 268). In edentulous lower cases the jaw often exhibits at various places areas of hard and soft tissues. In such instances after the tray is fitted, an impression is taken either in wax or in modelling compound: this is removed and slightly enlarged and thus a perfectly fitting tray 296 IMI'IiESSIONS OF THE MOUTH. Fio. 266. Operator iioldiug down lower lip mid bringing the plaster in contact with alveolar ridge. Fig. 267. Holding lower impression tray in place during setting of plaster. CLASSES OF IMPRESSIONS. 297 is secured. A thin coat of impression j)Iaster is then placed over its surface, the tray returned to the mouth, and pressed into position. By this means an impression of excellent quality is obtained (Fig. 270). The adhesion of the lower plate may he greatly increased by extending Fig. 26S. Full lower impression. the lateral flanges to press upon the muscles of either side, thus increasing the possibility of its adhesion and adding firmness to the fixture. Taking Partial Impressions in Plaster. — Partial trays differ in shape from those used tor edentulous cases, as they are deeper and are designed to fit over the natural teeth. Each tray must, of course, be carefully Fig. 270. ^^V!&P^^ S 11 ^m£''M(^ ^ t'"^^ r J^tb ^^^^^ p ■ L wf 1^ Jj H ^mII Cast of lower jaw with good alveolar ridge. Cast of lower jaw -with great resorption of the alveolar ridge. fitted to the mouth before the impression is attempted. This should be done even at the risk of mutilating the tray and destroying its future value. The tray and plaster must be regarded as simply a means to secure a perfect cast, and should be treated as such, and there should be no hesitation in mutilating a tray by cutting or bending it to fit the indi- vidual case. When fitted the tray should be three-sixteenths (y\) of an 298 IMPRESSIONS OF THE MOUTH. iiicli lar' to which it fits, as accurate joining of the parts would then be impossible. Nor should any wax be allowed to flow upon the surface of the impression which is to give form to the cast. Great care also should be taken to have the impression firmly waxed to the tray, so it will not be displaced while the plaster is being thrown out of the impression at the time the cast is poured. 302 SEPARATING MEDIA. 303 Considerable care and accuracy are required to assemble the parts of a badly broken impression, and much patience is necessary to obtain satisfactory results, \yhen it is remembered that only by attention to the miiuitest details of assembling can a perfect cast be i)roduced, the profitable employment of time necessary to do this is Tnade manifest. SEPARATING MEDIA. Before pouring the cast it is necessary to coat the surface of the plaster impression with some medium which will prevent the aflhesion of the plaster of the cast to that of the impression. This medium must be of such a character as not to obliterate any of the details of the surface of the impression and yet permit its separation from the cast. Soap has been used as a separating medium for this purpose, being employed as a solution of one ounce of Castile soap in a pint of water heated until the soap is dissolved. A lather made by dipping a brush in water and rubbing it upon a cake of soap may also be applied to the impression. Thin liquid silex is also used as a separating medium. Collodion may also be used. The most satisfactory method employed at the present time is that of double varnishing the impression, using shellac and sandarac varnishes. By this method the impression is first varnished with a solution of gum shellac in alcohol, which serves to stain the plaster by soaking into its substance about a sixty-fourth of an inch, and establishes a line of demarcation between the impression and the cast, and thus reduces the danger of marring the cast when separating. The shellac serves also as a filler, infiltrating the surface of the porous plaster and rendering it non-absorbent for the sandarac var- nish which is to follow. It should all sink into the plaster and should never be thick enough to glaze it. After two or three coats of shellac have been placed on the impression, and have thoroughly dried, the surface is treated to a thin coat of sandarac varnish. This serves to glaze the surface of the plaster, which in turn imparts a smooth surface to the cast. The sandarac is also the separating medium proper. One of the most satisfactory ways to use shellac and sandarac var- nishes is the method employed by the author, and that is to use them mixed in equal parts for the first coat and to use sandarac alone for the second coat. The advantage of this method is in the saving of time, as shellac and sandarac when mixed in equal parts will dry as quickly as sandarac alone. Great care should be taken to have the varnishes of the proper consistency. A very safe rule is to have them so thin that when placed on the impression the first and second coats will sink into the plaster, and not until after the third coat will there be any gloss on the surface of the impression. Varnish that is too thick will obliterate the fine lines of the impression and give an imperfect cast. Two varnish bottles such as are shown in Fig. 280 will be found very useful containers of the media. The glass cap largely prevents the evaporation of the alcohol, which would result in a thickening of the varnish. Bottle No. 1 should contain sandarac and shellac in equal 304 THE MAKISG OF PLASTER CASTS. parts, while No. 2 contains sandarac alone. P^ach should be supplied with a larjje and a small brush for ap])l ying the varnish ( Fig. 281 ). These brushes should be hung, by means of wire hooks inserted in their shafts, upon a wire support stretched across the mouth of the bottle. The suspension of the brushes prevents their bristles being bent out of shape b\- the weight of the brushes upon them. Very soft bristle brushes will be found most satisfactory. The large brush is to be used for eden- tulous cases and for partial cases, except at the j)ortions exhibiting the impressions of teeth. In these cases the small brush will be found very advantageous to carry the varnish down into the tooth impressions. Fig. 280. a r Vanilsli bottle witli brushes iiantjing mi wire support. Varuish hruslies. From five to ten minutes after the impression has been rem()\ed from the mouth it may be varnished, first with No. 1 (shellac and sanda- rac), and five minutes later it will be dry enough to varnish with No. 2 (sandarac), and in five minutes more it will be dry enough for the next step. Before pouring a plaster impression it should now be thoroughly soaked in water for not less than from three to five minutes. The objects of this soaking are threefold: first, it enables the plaster to flow freely over the surface of the impression, as plaster flows poorly over a dry surface; second, the impression does not absorb the water from the cast, thus allowing the plaster to harden naturally and to develop its greatest possible strength (a dry impression would absorb the water from POURING THE CAST. 305 the cast and make it worthless); third, soaking the impression softens it, and it can be removed with greater ease and with less danger of break- ing the cast. Impressions of modelling compound receive no treatment preparatory to pouring the cast, save to be thoroughly wetted and to have the excess of water carefully shaken out. Wax impressions receive a very thin coat of sandarac varnish, which glazes the surface and permits an easy separation from the cast. Plaster for Casts. — Casting plaster should be much coarser and stronger than that used for impressions, and should set in about twenty to twenty-five minutes. It should be the strongest and hardest plaster that it is possible to obtain. The author has been recently using a plaster made by mixing a very coarse builder's plaster with a finer grade of casting plaster, and the casts obtained with it have been exceptionally hard. POURING THE CAST. The technic of pouring the cast is the same, irrespective of the material of the impression, so one description will suffice for all. After the impression has been made ready the proper amount of water is placed in a plaster bowl and the casting plaster is sifted in slowly, so that each particle becomes saturated with water and settles to the bottom of the bowl. Nothing should be added to the water to hasten the setting of the plaster, as this will be at the expense of hardness in the cast. The proper amount having been added to absorb the water, the spatula is used to cut through the batter a few times to make the mix homogeneous. If the bowl is jarred on the table any air bubbles present will rise to the surface. It is infinitely better to do no stirring at all than to stir too much. The operation of pouring the cast may be divided into two stages to correspond with the two objects in view in this procedure. In the first the surface is to be perfectly covered with the soft plaster. Ac u- racy in this step will, of course, determine largely the trueness of the surface of the cast, as any air confined next to the impression, or any water left in the tooth depressions, or any failure to have the surface covered with plaster free of air bubbles, will result in a corresponding defect in the cast. The second stage is simply to add enough plaster to give the cast proper bulk. In pouring a full upper or lower cast, plaster is placed in the impres- sion at one distal corner, and should follow the alveolar border around to the opposite corner. By slightly jarring the impression, as it is held in the hand, by striking the latter upon the work-table, the plaster is easily made to flow over the surface of the impression in a thin coat. Most of the plaster is then thrown out of the impression by inverting the tray and by jarring it slightly, leaving only a thin film co^'e^ing the surface. INIore plaster is added. This is also largely thrown from the impression. This is repeated a few times until all traces 20 . . 306 THE MAKING OF PLASTER CASTS. of air bubbles have been removed, and the thin fihn of plaster re- maining on the impression is smooth. In an edentulous case the plaster is now added very carefully with the spatula, jarring each por- tion into place, until it is at least one-half inch thick in the thinnest place. A mass of plaster batter is then placed upon a glass slab, and the partly poured impression is inverted o\'er it and pressed down on the glass slab until the cast is of the desired thickness, the size and shape of the cast being determined by the purpose to which it is to be devoted. The bottom of the tray should then be parallel to the glass, in order that the alveolar ridge of the cast may be parallel to its base (Fig. 282). The square edge of the spatula, shown in Fig. 244, is then used to shape up the cast for metal or for vulcanite work, as desired in accordance with the shape later to be advised ff)r these two classes of work. A slab 3j by 4 inches of thick window-glass will be found very convenient for Fig. 282. Lower impression poured and inverted on glass slab. this work. When the cast has hardened, glass of that thickness can be bent by placing the index-finger in the centre of the glass and the thumb and the middle finger on opposite edges of the slab. By pressing down with the finger and raising up with the thumb the glass can be bent enough to break up its adhesion and separate it from the cast. The object of the use of glass for this purpose is to give a smooth glazed finish to that surface of the plaster which in the completed cast will be denominated its "base." In pouring partial upper and lower impressions great care must be taken to remove all of the water and air from the tooth impressions. This is done by flowing the plaster carefully into the impressions of the teeth, and then by throwing it out by jarring the inverted impression. This should be repeated until all the air and water have been removed, leaving a thin film of plaster over the entire surface of the impres- POURING THE CAST. 307 FiQ. 283. angmQ sion. Then plaster is added to the depth of one-half of an inch, care being taken that no air is confined in the impressions of the teeth. Steel brads, such as are shown in Fig. 283, are then placed in the soft plaster, from one to four in the impression of each tooth, with their points toward the incisive and masticating surfaces of the teeth. Then the proper amount of plaster is added and the cast is inverted on a glass slab and shaped up with a spatula, as already described. Steel brads for strengthening the teeth in partial cases are about fourteen or fifteen gauge, and about three-fourths or one inch in length. These are to be preferred to brass brads, as they rust in the plaster and its attachment to them is very secure. Removing the Impression from the Cast. — After the plaster has hard- ened from twenty to thirty minutes (depending upon the quality and the manner of mixing of the plaster) the glass slab is removed by bending it slightly. The edges of the tray are freed of the overhanging plaster, and then, by tapping the handle of the tray on its upper surface, this may be easily removed from the impression. The next step is to remove the impression from the cast. If it be an edentulous upper or lower case, the operation is a very simple one. Fig. 284. Brads used in reinforc- ing partial casts. Tray removed from impression, and plaster of latter cut down to shellac stain. First, the outer edge of the plaster of the cast is trimmed down so as to expose the outer edge of the impression all the way round and to release all undercuts. The impression is then cut down over the alveolar bor- der until the warning yellow stain made by the shellac is reached (Fig. 284). A vertical groove is cut at each canine eminence. By tapping the impression ^^ith the handle of the knife gently over the entire sur- face, then by placing the point of the knife at the edge of the impression and pressing away from the cast, the buccal and labial portions of the 308 THE MAKING OF PLASTER CASTS. impression are removed. In a full upper case the palatal portion can be removed in a single piece. In a full lower case one or more grooves are cut on the lingual surface of the impression, and that portion can then be easily removed in sections. In partial cases greater care should be taken in this operation, as the plaster teeth are very easily broken. The fact that the im])ression has been fractured in removing it from the mouth will be of great assistance in remo\ing it from the cast. After the tray has been removed, and the border of the impression freed of the overlying plaster of the cast, the plaster over the incisive edges and masticating surfaces of the teeth is trimmed away very carefully until the shellac stain appears as an in- dication that the teeth are near at hand. Then the work should be directed to removing the plaster from about the teeth, so they will not be caught in the large masses of the impression and broken off when the latter are removed. The plaster should be chipped from about them until this danger is completely avoided. Especially about isolated teeth Fig. 285. Plaster casts trimmed; -s a quantity of zinc in immediate contact with the walls of the pot. This more promptly melts than would a mass not in contact, and the pot does not become overheated. He begins to pour the metal when sufficient is melted to partly fill the mold, then returns the pot to the fire until another portion is melted; this he pours, and continues to melt and pour until the casting is complete. By this pro- cedure the zinc is never overheated, but is poured at as low a temperature as possible, and may be used for years without deterioration. He claims also that there is less shrinkage on the face of the die. Care is needed that the portion poured does not chill before another portion is added. 332 DIES, COUyTEIi-DIEJS AND MOLDISU. time and labor are saved. Wlien cold, the die is critically examined, and any corrections needed before making the counter-die arc lunv made. If for a partial plate, the teeth may now be cut from the least desirable die, or this may be deferred until after the counter-die has been made. Any places where wax has been added to facilitate making the mold should now be corrected by cutting out the zinc to make it conform to the model. Inaccuracies due to imperfect molding on that portion of the die to be covered by the plate will also need attention. Spaces between teeth, undercuts, and the margin of vacuum-cavities, should be carefully compared with the model, and made to correspond as nearly as possible. In making these corrections the cold chisel may be used where much has to be removed; as a rule, however, the files and gravers, and occasionally a bur in the dental engine, will prove effective tools. It is just as important not to remove too much as it is to remove enough, and very important not to encroach upon any portion of the die that does not need correction. When these corrections have been made, casting the counter-die will be next in order. CASTING THE COUNTER-DIES. in making a counter-die three points are to be considered regarding its office during swaging: — First, it is to force the plate into contact with the die; second, to accomplish this it must hold the die in position; and third, it must by its shape and mass make for the die a solid foun- dation. It must take in all the face of the die that is to be covered by the plate, and sufficient beyond this to prevent the die changing its position. This usually requires that it shall extend about half an inch over the sides of the die. In mass, it should be a full half inch thick over the most prominent portions of the die. Convenience of handling and an eye for neatness will suggest that neither die nor counter-die should be unduly massive; they should, however, be properly propor- tioned to the size of plate to be swaged, and well able to bear without breaking or change of form, the impact of a heavy hammer. The first step in making a counter-die is to place the die in a casting- ring, face up, elevated by packing sand beneath it until the plate line is about one-fourth of an inch above the edge of the ring. In cases in which the plate is confined to the face of the die, as in partial plates, the dies should be from one-half to three-quarters of an inch above the ring. The die will extend into the counter-die just as far as it extends above the ring. While it is necessary that it should extend into the counter-die sufficiently far to be firmly held in position, if this is overdone, it will prove embarrasing in separating the die and counter-die and in removing the plate from the counter-die after swaging. After the die is adjusted as to height, see that its face is quite level, and then firmly pack molding- sand between the die and casting-ring, filling the space solidly. With a point or small molding-trowel, smooth off this sand level with the top of the ring. xAfter carefully removing any sand that may have fallen on LOW FUSFNG ALLOY DIES AND COUNTER-DIES. 333 the face of the die, place over it, large end down, a casting-ring just large enough to leave about one-half of an inch of space all around the die. The die is now ready for casting the counter-die. While there is a fair margin between the fusing temperature of lead and of zinc, thev will unite if the lead is poured too hot. If all the lead in the pot is molten, it is safe to conclude that it is too hot to be safely poured over the die. It is best to wait until, on tilting the melting pot, the metal is seen to cling to its sides. When this condition has been reached, the metal is poured into the ring until it is about half an inch thick over the most prominent portion of the die. As in making dies, time is saved by arranging on the molding bench as many dies as are to be covered and pouring a number in succession. After the counter-die has set, the die and counter-die are removed from the sand, the casting-ring removed, and after brushing from them the loose sand, if needed im- mediately, they may be chilled in cold water. Before separating the die from the counter-die, if two have been made from the same model, one set should be marked by a hammer blow on the edge of both die and counter-die so as to know to which die each counter-die belongs. The die and counter-die are readily separated by a few sharp blows on the die with the swaging hammer in a direction to drive the die out of the counter-die. In most cases thty fall apart readily; in some cases, however, where there are undercuts, they must be cautiously coaxed apart by light blows first on one side and then on the other, otherwise the die may be broken and ruined. The dies should now receive a careful inspection, and the final prepa- ration for use. Where two have been made from the same model, the best is reserved for the final swaging. If not previously done, the teeth should be cut from the first die used for partial plates, and any carving necessary to correct inaccuracies completed. LOW FUSING ALLOY DIES AND COUNTER-DIES FOR SPECIAL USE. Now and again, the so-called fusible metals can be advantageously used for making dies and counter-dies. Dies for small plates to be made of thin, soft, high carat gold, may be made of these low fusing alloys cast into plaster or moldine impressions of that portion of the model which the plate is to cOver, the counter-die being also made of the same alloy, saving not only time, but the expense and trouble of melting the less fusible metals. In repair work, the low fusing alloy may be cast upon a gold, silver, or vulcanite plate with perfect safety, when a die is needed to fit a re-enforcing piece to an irregular surface. It may, in some cases, be cast upon a plaster model direct, thus mak- ing the counter-die first, the die being made by using it as an impres- sion. Wliile dies and counter-dies of low fusible alloys are not hard enough for serious work in swaging, and are too brittle to withstand heavy hammer blows, they, nevertheless, serve a useful purpose in forming 334 DIES, COUyTEIl-DIES AND MOLDING. soft and pliable nu'tals. Instead of usinji; a liammer, the swaging may be done l)etween the jaws of the bench vise, or in tlie "shot-swage." To get the best results with low fusing allovs thev should not be over- ^^ Oil heated, nor yet poured when quite fluid. Just before setting they assume a plastic condition, and then make a harder and smoother die. In order to prevent the two dies from uniting when both are made of low fusing alloy, the first cast should })e (|uite cold, and the alloy used in making the second casting should be as cool as it can be poured. Painting the surface with whiting is helpful, but unnecessary, if proper care is observed. CHAPTER X. SECURING THE VARIOUS DATA TO BE USED IN CONSTRUCTING ARTIFICIAL DENTURES. TAKING THE BITE. ARTICULATORS. By Charles R. Turner, D.D.S., M.D. In addition to the plaster casts which represent the jaws of the patient, other data are, of course, necessary for the design and con- struction of artificial dentures. These casts must be placed and main- tained in the same relative position to each other during the subsequent stages of arranging the artificial teeth in occlusion as the jaws they represent are to occupy when the finished dentures ha\'e been inserted and are brought into occlusion. We have seen in Chapter 1\. that when the natural teeth are in the mouth the so called "position of occlusion" is fixed by a certain definite fitting together of the occlusal surfaces of the teeth. When the natural teeth have been lost, the position of occlusion no longer exists, and it is necessary to determine for the mandible by considerations presently to be discussed a position in relation with the maxilla which will answer the requirements of an occlusal position for the artificial dentures. The operation of securing a record of this relationship in accordance with which the casts may be molmted upon an articulator, an instrument designed to maintain them in it, is commonly called "taking the bite." Besides the securing of a record of this relationship, it is necessary to obtain also an estimate of the fulness which the artificial dentures must possess to restore the external contour of the lips and cheeks, and to record both the location of the median line of the mouth and the amount of the dentures which will be displayed during the ordinary movements of the lips in laughter and speech. But it is not enough that the artificial teeth shall occlude properly; they must also be arranged to be capable of functionating to the best advantage during the various mandibular movements in which the food is masticated. This necessitates during their arrangement the use of a so-called "anatomical articulator," an instrument which is capable of reproducing the movements of the mandible of the person for whom the dentures are being constructed. The instrument must be adjusted so that its joint mechanism reproduces the temporo-mandibular joint of the case in hand, and hence the movements of the mandible. A record of the path of the mandibular condyles during these movements or its equivalent is therefore necessary. In order that the casts may be mounted upon such an articulator in the same relationship to its joint mechanism as the jaws they represent bear to the temporo- mandibular joint, a record of this relationship must also be obtained. .335 336 JJATA TO BE USED IN CONSTRUCTING DENTURES. As a matter of convenience, it is usually customary at this time to select the shade of artificial teeth suitable for the patient under con- sideration in accordance with principles to be outlined in a succeeding chapter. To recapitulate, the data which are to be obtained are: 1. Relationship of the jaws in the position of occlusion. 2. Fulness of plates necessary to restore external contours. 3. Median line of mouth. 4. Amount of dentures displayed in movements of lips. (High lip line.) 5. Relationship of the jaws to the temporo-mandibular joint. 6. ^Movements of the mandible. 7. Shade of artificial teeth. These are usually included under the term "taking the bite," which by an extension of its old meaning has now come to commonly cover these several operations. It originally referred to the first of these pro- cedures, which, as first in point of time and probably of importance, is now to be discussed. SECURING THE RELATIONSHIP OF TI^E JAWS IN THE POSITION OF OCCLUSION. TAKING THE BITE. If it were required to make an artificial denture for an edentulous case immediately after the teeth were lost, and if it were possible to obtain a record of what the occlusal relationship of the jaws was before the loss of the teeth, this would of course serve for the setting up of the artificial teeth, and the dentures would then establish the mandible in a correct occlusal position. As no data are obtainable which will enable the dentist to tell what this was, and as the changes in the tissues following the loss of the teeth soon alter the requirement of an occlusal relationship of the jaws for artificial dentures, an occlusal position for the mandible must be determined by the considerations now to be discussed. ^Vhen the teeth have been lost, the position of the mandible is determined by the balance estabhshed between the muscles which actuate it within the restriction offered by the temporo-mandibular joint. One characteristic, however, of the occlusal position when the natural teeth remain is that the condyles occupy the most distal posi- tion in the glenoid fossae which they can assume. This distal position of the condyles is one of the requirements for the occlusal position of the mandible for an edentulous case. It is only in such a position that the muscles can be free from strain and the jaw be in a state of equilibrium. This position of the condyles is the ultimate one reached by them from the various movements incident to mastication, and the artificial teeth must be arranged so that w^hen the condyles reach this point the teeth Avill fit together in proper occlusal relationship. With the condyles of the edentulous mandible in their most distal position in the glenoid fossae, only the movement of its anterior end as it is raised or lowered bv the muscles has to be reckoned with. This BITE-PLA TES. 337 simply means that It has to be estahhshed a certaui (Hstance from the upper jaw, and this distance is determined wholly by considerations relating to the appearance of the tissues of the face. This latter is the only available guide by which the position of the mandible can be fixed, and it must be such that the contours and proportions of the face are harmoniously arranged. Were it possible to keep the condyles in the back part of their fosste at all times during the determination of the position of its anterior end, it would be simple enough to secure a satis- factory occlusal position for the mandible. It would then be only necessary to hold the jaws apart the amount dictated by a correct judgment of the case in hand when the occlusal position would have been found. But as will be seen later, it is very difficult to get and to keep the condyles back in their fossae during such manipulations, so it is necessary to solve the problem by a different method. This consists in providing a means of maintaining the jaws a certain distance apart (already ascertained as proper for the case in hand) and then to induce the condyles to slide back to their most distal position. This is the rationale of all proper bite-taking methods. In securing this relation it is necessary to make use of plates which fit the jaws, and, by aftording mutual bearing surfaces, provide a means of keeping them a definite distance apart. If these plates are fixed to- gether while the jaws are in the occlusal position, they thus make a record of it, so that the casts may be mounted in the articulator in accordance therewith. They serve also as a means of recording the other data mentioned above, and as a guide in mounting the teeth. They are called "bite-plates." BITE-PLATES. By reason of the temporary nature of its service it is desirable that the bite-plate should be made of a material which will admit of being molded into requisite form over the cast and without injury to it. Sufficient rigidity at the temperature of the mouth to insure the firm retention of the bite-plate, and sufficient hardness to prevent change in its shape while in use, are qualities which it must possess. Be- cause of the fact that its original form is wholly tentative, as the plate must be trimmed or enlarged to meet indications when the bite is taken, the material should also readily allow these changes. Wax and its various combinations, as pink paraffin and wax, white wax, etc., are the substances in most common use for this purpose, but they are open to the objection that they are not hard enough, nor are they sufii- ciently rigid at body temperature unless used in such amounts as to make the plates unwieldly. Gutta percha and wax,i ideal base plate,' and other materials of greater stiffness have been used. Of the molded materials, modelling compound seems to possess the largest number of qualities to recommend it, while objections to it may be overcome by attention to the details of its use, as vdW be seen later. Because of their rigidity, bite-plates swaged of ^-acuum-chambe^ metal^ and other soft 1 Evans. 2 Head. . ^ Essig. 22 >38 DATA TO BK USED L\ CONSTRUCTISG DENTURES. metals have been used, the occlusal portion of the plate beinij made of one of the heat-softened materials. The amount of time necessary to execute the various steps in the making of swaged bite-plates for den- tures to be made of one of the molded bases is so serious a drawback to the method that in most instances it is impracticable, but their advan- tages should not be forgotten in deaHng with difficult cases. For the Upper Jaw. — To construct a bite-plate of modelling com- pound for the upper jaw, the cast should be placed upon its base on the work bench with the distal portion toward the operator. (Fig. 309.) The plate outline should have been marked upon it, but the placing of the vacuum-chamber form, and any alteration of the face of the cast by additions of tin-foil for retention purposes, should be Fig. 309 Steps in t deferred until a subsequent time, because they would probably be dis- turbed in the forming of the bite-plate. The method of making the })ite- plate in one piece proposed by Dr. W. W. Evansi is to be recommended. Three-fourths of a cake of modelling compound is softened in warm water, kneaded in the hands until homogeneous, anrl rolled into an ellip- soid about two inches long. One side of this should be thinned out by pressure between the fingers, and the mass so i)laced upon the cast that the thinned portion ])rojects slightly beyond the posterior margin of the plate outline. By manipulation with the thumbs the remainder of the compound is gradually worked forward so that the vault of the cast is covered by it to the thickness of about ^\ of an inch. The thickness of this maV be readily- gauged, for the cast chills the material as it come? • International Dental Journal., vol. xx., p. 221. BITE-PLATES. 339 ill contact with it, thus liardcMiini;- it, wliile tlie overlyiiis^ soft portion may he pushed forward. When the top of the alveohir ridge has been reached, the compound should be carried over it and sUghtly be- yond the jJate outUne, along the labial and buccal surfaces, the most of the mass, however, remaining upon the ridge and being shaped to represent tlie occlusal portion of the bite-plate. The pr()l)al)le relation of this part of tlie artificial denture to the alveolar ridge and the prob- able fulness of the buccal and labial portions should be borne in mind and the compound disposed accordingly, since the bite-plate when completed should be a rough model for the denture in these particulars. It should be taken from the cast, chilled in cold water, and trimmed around its periphery to the plate outline. It ought then to be replaced upon the cast and its margin brought into close contact therewith, around the plate outline. This is to insure firm retention of the plate in the mouth, which is of the greatest importance, and should be secured, even if it be necessary to make at this time the changes in the sur- face of the cast which provide for the adhesion of the future denture. The form of the bite-plate at this time is largely tentative, as it is pur- posed to complete its modelling when the bite is taken, in accordance with the requirements which shall then be indicated. During the process of forming the plate, to prevent adhesion, the hands should be wet and the compound occasionally taken off the cast to break up its adhesion while it is soft, and then replaced, but under no circum- stances must the cast be wet, as this will injure it for subsequent use. Rubbing its surface with soapstone or talcum powder will effectually prevent the adhesion of the compound. It is possible to construct the plate in two portions, that in contact \\\t\\ the mucous membrane being made of one piece of modelling com- pound rolled into a thin sheet and adapted to the cast, that representing the occlusal portion being formed of a roll bent to the shape of the ah-eo- lar ridge, and made to adhere by dry heat. The occlusal portion is made of wax by some practitioners because of the greater ease with which it may be carved, but its softness and tendency to yield under pressure make it less safe than modelling compound in preserving a fixed distance between the jaws. The Lower Bite-plate. — The lower bite-plate is more easily made than the upper. With the cast face up on the work-bench, a piece of compound equal to about one-half of a sheet is softened and worked into a long uniform r.oll, bent to the shape of the alveolar process, and placed upon its summit. (Fig. 310.) With the thumbs and fingers it is worked down the lingual and labial sides to a point slightly be- yond the plate outline, that portion over the ridge being shaped to represent this part of the future lower plate and made to correspond in outline to the arch of the upper bite-plate. It is removed and trimmed to the plate outline like the upper, its occlusal surface being left rough. If the lower plate must be very thin, it may be strengthened by im- bedding in it a piece of iron or brass wire shaped to conform to the alveolar outline. S-tO DATA TO BE USED IN CONSTRUCTING DENTURES In taking the bite for a swaged or cast metal plate, for a continuous- gum denture, or for a vulcanite plate where the base-plate has been previously vulcanized, the base-plate itself is used for the bite-plate with an addition of modelling compound ovev the alveolar ridge to give it an occlusal surface. Technique of the Operation. — The securing of the relation between the jaws may be divided into two stages — the fitting and shaping of the Steps in the formation of a bite-plate for a full lower case bite-plates, and the use of them in securing the relation of the jaws in the position of occlusion. The patient should be seated in an erect position in the dental chair. (Figs. 311, 312.) The upper bite-plate is first tried in and the plate outline followed by it verified or corrected if necessary. It is essential to the success of the operation that the bite-plate should remain firmly in position and it should be trimmed where any movable tissues tend to displace it. Its adhesion may be improNed by scraping from the centre of the palatal side to relieve pressure at this point and by accentuating the contact of its periphery by bending. It should then be observed whether the buccal and labial portions of the plate restore the contours of the lip and cheek tissues which they support, in accordance mth prin- TECHNIQUE OF THE OPERATION. 341 ciples contained in Chapter XII., and the plate added to or trimmed as may be indicated. ^Yhile the modellinf>' in these regions cannot be completed at this stage, it is advisable to have the form answer the re- quirements of contour as fully as may be possible. The inclination of the occlusal portion of the plate to the ridge may be changed by reheat- ing in water and bending after replacing it upon the cast, but additions of compound to the plate should be made to adhere by means of dry heat applied to the two surfaces, by which means its adhesive property is best developed. Any trimming may be easily done while the mate- rial is slightly warm, as it may be cut with a knife without dragging at this stage, and as it becomes brittle and hard when cold. A most important point of reference which will be utilized in the con- struction of the denture and which determines the length of the occlusal Fig. 311 K 1^^^. 1^1 ^ M *. ^^^1hi^^» p^.,.^^ Fig. 312 Patient with edentulous jaws. Front view. Preparatory to taking the bite. Patient with edentulous jaws. Profile view. Preparatory to taking the bite. portion of the bite-plate is now noted. This is the lower margin of the upper lip when in repose. This should be marked upon the labial surface, and the occlusal portion of the plate trimmed to wdthin one-six- teenth of an inch of this, so that the plate will project that amount below the lip.^ (Fig. 313.) The anterior portion of the bite-plate should not be trimmed or added to in subsequent fitting together of the two plates, as the lower margin of this surface is to indicate the position of the incisive edges of the anterior teeth of the artificial denture. The distal portions of the occlusal surface should be trimmed to curve upward in accordance with the probable plane of occlusion of the future denture. The lower bite-plate is now- tried separately in the mouth, the plate outline verified, and then the two are put in together. The length of 1 Dr. A. D. Gritman. 342 DATA TO BE USED IX CONSTRUCTING DENTURES. the iii)por l)ite-j)late haAin^; been (Infinitely fixed, it is i)ur[)()se(l to trim the lower one so that when in oeclnsion with the ui)per, when the condyles are in the most distal part of the glenoid fossae, the jaws will be held apart the distance it has been decided is proi)er for them. If it were possible to ascertain what the occlnsal position of the man- dible was when the natural teeth were in ])lace, it is probable tiiat this would fulfil the requirements of occlusion for the artificial dentures in a majority of cases, although when the teeth have been out some time the tissues of the lips and cheeks ha\e contracted and accom- modated themselves to a lessened distance between the jaws. It will be necessary to determine this distance by a judgment based upon the external appearance of the face and particularly that of the mouth, and as this is affected by the fulness of the bite-plates as well as by their length, these two dimensions should l)e considered conjointly. Patient with full upper bite-plate trimmed to one-sixteenth inch below the upper lip. The tentative form of the upper bite-plate is sufficient guide in the matter of contour to begin with, so the lower plate is trimmed to what is judged to be its proper length, and the contours readjusted subse- quently if necessary, or if later alteration of the buccal or labial surfaces should show that the bite-plates are too short, they must receive addi- tions to meet the indications. In judging of the final contours, three places require especial attention. (1) The lips need to be supported at their point of contact, as the margins fall in when the teeth have been lost. This is provided for by the amount of projection of the bite- plate at the point corresponding to the edges of the upper incisors, and by the inclination of its labial surface. (2) As a great deal of resorption has taken place at the site of the canine tooth, this area should be held out by the bite-plate, and the corner of the mouth given proper fulness. The upper bite-plate must be fuller and higher here than anywhere else. TECHNIQUE OF THE OPERATION. 343 (3) The other place at which fuhiess is often required is at the site of the bicuspids and first mohirs, where the cheek may be sunken in. When the labial surfaces of the bite-plates have been built out to proper ])r()porti()ns, the distance between the nose and chin siiould be such that the lips just come in contact. (Fig. '314.) When the proper position has been obtained an equal amount of the mucous surface of both lips is displayed, the upper lip is inclined outward slightly from its base, and the lower lip is sufficiently everted to bring out the graceful curve between it and the chin. (Fig. 315.) If the bite is too long the lips will be strained in trying to cover the plates, or if too short, they will appear compressed or curled upon themselves. Where the teeth have been out a long time and wdiere the tissues have wrinkled in consequence, it is impossible to take out all the wrinkles or to establish the ideal con- tour and proportions above suggested; and this is also true, of course, Fig. 314 Patient with full upper and lower bite-plates in place. Front view. Patient with full upper and lower bite-plates in place. Profile view. where they were not originally possessed. It should be remembered that during the fitting and trimming of the plates the condyles of the lower jaw must be kept in their most distal position. When they have once been secured in this position, the lower bite-plate should have its labial surface marked and trimmed or built out until it is flush with that of the upper bite-plate. During subsequent manipulations the relationship of these labial surfaces is a guide as to whether or not the condyles are in their correct distal position. In some cases patients have contracted the habit of protruding one condyle, thus throwing the lower bite-plate to one side. In such instances it will be found advantageous to mark the median line of the face upon upper and low'er bite-plates at this junc- ture, being certain that both condyles are retruded at the time. Dur- ing subsequent procedures lack of correspondence in these two lines will at once indicate a protrusion of one condyle. 344 DATA TO BE USED IN CONSTRUCTING DENTURES. The lower bite-plate must be trimmed to occlude e\enly with the upper throughout its whole extent, and when the mouth is closed the contact between the two should be equal at all points. It is important that the bite-plates should be evenly trimmed , since there is danger that they may be in contact anteriorly and aj)pcar to be i:)()steriorly, when they are really too short, and have been forced oft' the ridge by leverage on their anterior portions, and it is wise to leave the lower bite-plate longer behind than in front and trim it to accord therewith. \Vhen the plates have been fitted by trimming as closely as possible, an absolute fit may be obtained by removing the lower bite-plate and very slightly softening its occlusal surface over an open flame, quickly returning it to the mouth, and having the patient close in the occlusal position, gently pressing the plates together. It is important tliat in Fig. 31G Fig. 317 Patient with bite-plates of correct fulnes.s, but too .short. Patient with bite-plates of correct length not full enough. this procedure the uj^per bite-plate be kept cool enough not to ha\e its form altered at all. The final fitting may be conveniently judged by placing the index-finger of each hand in the mouth in contact with the buccal surfaces of the two plates, which serves to hold them on the al- veolar ridges, and by pressing the ball of the finger slightly l)etween the occlusal surfaces of the plates as the patient is directed to close the mouth slowly, they are held apart at places where they do not fit prop- erly. These occlusal portions afford rigid and unyielding surfaces that maintain the jaws apart the distance which has been decided upon. The bite-plates so fitted should now be removed from the mouth. Their occlusal portions should be trimmed down from the inside until they are not more than three-eighths of an inch wide. This is to give room to the tongue and to make them as comfortal)le as possible for the patient. It may be stated that the more nearly they approx- TECHNIQUE OF THE OPERATION. 345 imate the future denture in form and size, the greater will be the hke- lihood of accuracy in taking the bite. They should have grooves cut upon their occlusal surfaces, as illustrated in Figs. 31S and .319, trans- verse ones at the site of the first molar and first bicuspid teeth, and a longitudinal one between. The grooves receive the wax to be used later in fixing the plates together. Having thus determined by means of the rigid occluding surfaces of the bite-])lates what the distance between the jaws shall be, the second stage consists in securing the mandible in such position that the plates are in contact and the condyles are back in the glenoid fossae, and the position of occlusion is obtained. If the bite-plates are then fixed together and taken from the mouth, the casts may be placed in position in them and mounted upon the articulator. The Fig. 318 Fig. 319 Full upper bite-plate, with yellow wax on occlusal Full lower bite-plate, showing grooves cut on surface. occlusal surface. difficulty which presents itself in this procedure is in getting the con- dyles back in the fosste. When the natural teeth are in the mouth consciousness of the occlusal relation is derived from sensations transmitted through the teeth, which are sensitive tactile organs and give instant knowledge of the position of the mandible w^henever they are in contact. Absence of muscular strain and absence of a strained feeling at the joint also contribute to a knowledge of this occlusal relation, but the muscular apparatus itself does not possess a sufficiently accurate muscle sense to convey this in- formation. If the mandible is not forM^ard of the incisive relation the inclined planes of the cusps of the teeth form guides, which serve to insure its return to the occlusal position when the elevators are con- tracted. At the posterior end of the mandible the fossae present in- clined surfaces which serve as guides to the condyles in their return to the distal position occupied by them during occlusion. With the teeth missing we have only the temporo-mandibular joints as guides in directing the mandible back to the occlusal position. A further consideration of this joint will serve to give a clearer understanding of the problem under discussion. The temporo-mandibular articulation is a condyl-arthrodial joint. Into the glenoid fossa, which at its posterior extremity presents a 346 DATA TO BE USED IN COySTRUCTINO DENTURES. marked concavity, fits the condyle of the lower jaw, with the interarticu- lar fibro-cartilaije interposed between them. The condyle is attached to the rim of the fossa 1)\' means of the capsular liuament, to tlie centre of which latter the cartilage is attached. The li,i;ament thus hinds the condyle in its fossa and imposes certain limitations upon its movement. The anterior portion of the fossa presents a flat articular surface over which the condyle and cartilaii;e ma\' glide in the various movements of the maiKhhle. During the protrusive mo\enients of the jaw this arrangement permits the condyle to slide forward upon the floor of the glenoid fossa, in which movement it is accompanied by the cartilage. At any time during this forward movement the mandible may rotate about a horizontal axis, passing through the condyles as they rest on the cartilages, and a combination of these two movements frecjuently takes place. These movements may be bilaterally symmetrical or there may be movements of one condyle forward and inward, the other remaining in the distal part of the fossa. When the mandible is depressed the condyles also slide forward in the glenoid fossa. This is due to several anatomical peculiarities of the joint as well as to the fact that external pterygoid muscles pull forward both the condyles and cartilages. The external and internal lateral ligaments are attached to the neck of the condyles and extend so far forward in their attachment to the rim of the fossae that they serve as falcrums to force the condyle forward during the depression of the mandible. In returning to the position of occlusion from any of these excursions it is by retraction and elevation or some combination of these movements according to the position from which the return is made. Normally this is accomplished by the contraction of the massetcr, temporal, and internal pterygoid muscles, sometimes in combination with the muscles attached to the anterior end of the mandible. By the contraction of the elevators the main l)()dy of the jaw is drawn upward and backward when the condyles slide up the inclines of the glenoid fossae into their most distal parts. The lower fibres of the temporal muscle, the inner lEibres of the internal pterygoid, and the posterior fibres of the masseter muscles serve to draw the jaw back, opposing the action of the external pterygoid, though not directly in line with it. F'or complete retraction to occur there must be total relaxation of the external pterygoid, as these muscles have the power of fixing the contlyles at any point in their return path. If the condyles are thus fixed at any point before their most distal position is reached, the elevation of the mandible may con- tinue by rotation about this now fixed axis. When the anterior end of the mandible is completely elevated it will then be forward of the occlusal relation, when, if the natural teeth are present, their contact serves to give information of the fact. But with the edentulous patient this source of knowledge is removed, and he is iniable to assist in the operation of securing the proper occlusal relationship. Even though there is a complete understanding on the part of the patient as to what is required, he no longer possesses the means of judging if this is ob- TECHNIQUE OF THE OPERATION. 347 tained, and his efforts are not only not hel[)fnl, but often greatly hinder the work of the dentist. After the loss of the teeth changes in the joint incident to this con- dition freciuently further complicate the problem at hand. The elevator group of muscles, being no longer accommodated to a fixed position of the mandible, usually shorten, as do the other tissues extending between the jaws. The ligaments of the joints become stretched and the articulation becomes loose and wandering. The anterior portion of the glenoid fossa becomes absorbed, resulting in a flattening of its floor, all of which factors tend to make the protrusive movements easy and those of retraction uncertain. The longer the teeth have been absent, the greater will be the tendency toward protrusive movements. Lack of precision in the movement of the jaw will also frequently be increased by the various manipulations incident to fitting the bite-plates. It becomes very difficult, therefore, to secure the jaw in its distal position. In this operation eflforts are directed to compel the patient to close the mouth and bring the bite-plates together, and to have the jaw slide back to its proper position. Measures which are utilized to fix the plates together at the same time that the mouth is closed are open to the criticism that they serve to increase the protrusive tendency if much force has to be exerted in bringing the jaws together, and great care must be taken to avoid this error. It may be stated as a truism that the smaller the amount of force necessary to close the jaws, the less likelihood will there be of protrusion. Goslee^ has called attention to the tendency to a continuation of the incising relation which may be caused by having too much wax upon the anterior part of the plates. This offers the same sort of resistance to the closure of the jaw as is met with in incising food, and the muscular movements which are useful in inci- sion are reflexly provoked thereby. Instructing the patient " to bite" usually conveys the idea "to incise," that being the common meaning of the word, and the movements of that operation are suggested. It is safest to use the term "close the mouth" in giving directions during the procedure, as that more nearly than any other suggests the desired movement. The operator must be able to tell when the jaw- is in its posterior position, in which he may be assisted by-.palpation of the external end of the condyle, and by inspection of the surface of the skin during its motions, and by resorting to the measures which induce its backward movement. While it is there, the bite-plates must be marked and so trimmed that their anterior surfaces are flush, to serve during sub- sequent procedures as an indication as to whether the jaw is protruded. The median line may also to advantage be marked upon both plates so that a protrusion of one condyle would be noted. The plates should fit firmly, be comfortable and stable in position, and the patient should be engaged in conversation, and, if possible, made to forget the operation. Then they should be taken out, wiped dry with a napkin, and replaced. 1 Transactions Odontological Society of Great Britain, 1900, p. 167. 2 Taking the Bite. The Dental Review, vol. xvii., p. 509. 348 DATA TO BE USED IN CONSTRUCTING DENTURES. and a small (|uaiitity of very soft yellow wax placed over the grooves on each side of the upper plate. The patient is then directed toswallow, and, as the lower jaw has to be fixed for this operation to give a base from which the elevators of the larynx may work, the condyles are usually forced back in the fossae. By this means the patient is en- gaged in the performance of an act which is naturally done when the condyles are back in their fossne and the tendency to protrusion is thereby considerably reduced. It must be observed that the bite-plates have been brought into the proper relation, and also it must })e assured that they are in position on the ridges. It may occasionally be necessary to hold them in position, especially in the case of a lower jaw in which there has been much absorption. If the head is thrown backward the tissues of the front of the neck attached to the lower jaw, particularly the platysma myoides, are put on the stretch, the tendency being also to carry the jaw backward. In- structing the patient during closure of the mouth to touch the palatal vault with the tip of the tongue as far posteriorly as possible also, has the advantage of tending to keep the jaw far back. Asking the patient to bite the back teeth together may assist in securing this position, be- cause of the fact that in the natural denture this is not possible when the jaw is protruded. Efforts to hold the jaw back })y clasping the fingers behind the ligamentum nuchae and pressing with the thumbs on the chin, or pushing it back forcibly by pressure on the chin during the closing, have been recommended and in some cases they may be suc- Pj^ 32Q cessful, but frequently they are so resisted by the patient as not to be effective. Dr. Molyneaux, in cases of extreme diffi- culty, recommends the use of Garretson's de- vice as follows: "A little apparatus, which was invented by Dr. Garretson, of Iowa, has been satisfactorily emploved in several cases, and is illustrated in Fig. 320. "It consists of two steel strips about 6 inches long, at one end of which are projections to enter the external ear, and a leather strap pass- Garretson's device for keep- iug OVCr tllC OCciput which prCVCUtS the Caf ing the condyles in the distal pjeces from slipping dowu. At the Other ends part of the fossae in taking the oi i •• i. i, i-i i bite. of the metal strips is a chin plate wliicli works on a ratchet, and which may be moved for- ward or backward as the case requires. After placing the ear pieces in position and tightening the straps, the chin jilate is to be moved up firmly against the chin. The patient should now open and close the mouth repeatedly, and as the lower jaw is drawn backward the chin plate is moved upward until it is certain that the condyles are at rest in the glenoid fossae. In this position the patient can open and close the mouth comfortably, but any attempt at protrusion will meet with resistance by the ear-lugs. "The base plates are then adjusted and the bite is taken as usual." THE HIGH LIP LINE^ 349 The wax iutorposod between the two phites should be small in amount and cjuite soft. It serves two purposes — to unite the bite-plates in the position of occlusion and to compensate for any failure of their occlusal surfaces to be in uniform contact. It is difficult in trimming an unyielding substance like modelling compound to get two surfaces to fit absolutely accurately, although, of course, this should be attempted. The wax at properly distributed points offers enough resistance to hold the plates in contact with the membrane, but not enough to interfere with the accuracy of closure. Care should be taken to see that the labial surfaces of the plates are flush, since by this means it may be seen that the jaw is in its most distal position. The bite plates may be fixed by fusing their edges together with a hot spatula, or by the use of staples of wire inserted on each side to bind them together, or in very difficult cases in which it is desired to interpose no resistance to the contact of the bite-plates, thin plaster of Paris may be used for the purpose as has been recommended by Ottolengui.^ The High Lip Line. — In the usual movements of the lips in speaking the teeth are more or less. exposed to view, and even greater display al of the denture is made in laughing and smiling. The relation between the lips and the teeth in laughing and smiling has much to do with the beauty of these acts. It is usual, therefore, to take into account the amount of this exposure in the construction of artificial dentures, and the highest point to which the patient can elevate the upper lip should be indicated by a line drawn upon the upper bite-plate at the lower margin of the upper lip. This lip is more mobile than the lower, because a larger number of muscles move it, and more of tlie upper teeth are displayed in smiling and laughing. In separating the jaws the lower teeth are depressed below the margin of the lower lip, and it is usual to disregard this record on the lower bite-plate for the addi- tional reason that the upper having been determined, the proportions of the lower are more or less harmonious therewith. It may, however, be marked upon the lower bite-plate and is then "the low lip line." The Median Line of the Mouth. — This should always be recorded, for in a normal natural denture the line between the central incisors prac- tically coincides with the median line of the mouth. The median line of the mouth is more or less difficult to determine for lack of an accu- rate guide in judging it. The frsenum of the upper lip is usually in the median line, but it is by no means a safe landmark, for in a good number of cases it is a little to one side or the other. The median line of the cast and the little tip on the front of an edentulous upper cast, repre- senting the incisive pad of the rugse, are also unreliable ; because the cast is frequently unsymmetrical from the fact that the teeth on one side may have been lost earlier and the resorption taken place to a greater extent than on the other side. The median line of the mouth must, therefore, be secured by data obtained from the face. The tip of the nose is so frequently out of the median line that it should not be used. The philtrum, when not obliterated, is a safe point of reference. The 1 The Dental Cosmos, vol. xliv., p. 446. 350 DATA TO BE USED IN CONSTRUCTING DENTURES base of the septum of the nose and the median Hne of the chin may also be used. The mouth itseU' is sometimes from hal)it or otherwise one sided, and then the Hne fixed upon as the niechan hne should occupy a position between the median line of the mouth and that of the face. After it has been determined upon, it should be marked upon both bite-plates perpendicular to their line of division while they are still Pig. 321 Fig. 322 L'pper and lower bite-plates fixed together Bite-plate for full upper denture in case in with wax and showing high lip line and median which the lower natural teeth remain. Ca,st line of the mouth. set in bite-plate preparatory to mounting in articulator. in the mouth. Fig. 321 shows the bite-plates removed from the mouth and exhibits the median and high lip lines. BITES FOR FULL UPPER OR LOWER PLATES. The bite for a full upper plate where the lower natural teeth remain in whole or in great part, involves the construction of a bite- plate in accordance with principles already outlined. It should be tried in the mouth, trimmed tentatively for contour and length to the lower margin of the upper lip, and the patient requested to close the lower teeth upon it. Special care should be taken to see that the lower teeth strike evenly upon its occlusal surface, and, when the jaws are* closed in the position of occlusion, the lips and cheeks should be lifted so that it may be seen whether this is the case, and an excavator should be used to try to pry the jaws apart at each tooth to prove the contact. The external contour and distance between the jaws should be estab- lished in accordance with the principles which underly their determin- ation in full upper and lower ca.ses. After the plate has been satisfac- torily shaped to fulfil these requirements, it is removed from the mouth, wiped dry with a napkin, and the occlusal surface covered with a thin layer of very soft yellow wax, which should be made to adhere closely, and only thick enough to receive indentations of the occlusal surfaces of the lower teeth. It is replaced and the mouth closed as in the pre- viously described bite-taking operations. The patient should bite BITES FOR FULL UPPER OR LOWER PLATES. 351 into the wax until the cusps of the teeth touch the bite-plate, which insures that the distance between the jaws is the same as was deter- mined by the bite-plate. It is important that only a small amount of wax be used because of the increased tendency to protrusion caused by too great a cjuantity. The bite-plate may be marked with a line to note the position between the lower central incisors to assist in bring- ing the lower jaw to a correct position, and the relation of the incisors to the edge of the bite-plate will also assist in this purpose. The high lij) line and median line of the mouth should be marked upon the bite-plate, since the line between the lower central incisors is too frequently out of centre to be depended upon for setting up the teeth, and the median line of the mouth is a safer guide. The bite- plate should be removed from the mouth, the wax chilled in water and laid aside, and an impression taken of the lower jaw, a cast made and fitted into the depressions in the wax of the bite-plate. (Fig. 322.) Bites for Full Lower Plates. — In taking the bite for a full lower den- ture, a bite-plate must be made for the lower jaw as described earlier in this chapter. It rarely occurs that the upper natural teeth remain in a mouth in which all the lower have been lost, and the common case requiring a full lower denture is that in which there is already a satis- factory upper. The general procedure is the same with either artificial or natural upper teeth. The lower bite-plate is formed and trimmed to occlude evenly with the upper teeth, and of such length that the dis- tance between the jaws as determined by the appearance of the patient is correct. The bite-plate is given a layer of wax over its occlusal surface and the bite taken in the usual way. Where there is an artificial denture for the upper jaw, it is well to remove this with the bite, to mount it upon the articulator, and to articulate the lower teeth to it rather than to a cast formed from the tooth depressions in the wax. \Yhere the denture has been made some time before the lower teeth were lost, and particularly where the teeth are set at irregular le^'els to occlude with the natural ones, it is advisable to reset them with the lower teeth, as a much more satisfactory occlusion may be obtained. Bite-plates for Temporary Dentures. — These dift'er in no wise from the plates already described, except that they do not extend over the labial surface of the cast, this being more especially true of those for the upper jaw. This arrangement is, of course, necessitated by consid- erations of contour. They are, in consequence, more difficult of re- tention, and this must be looked out for and pro^•ided for by means already discussed. Bite-plates for Partial Dentures. — Except in those cases in which only a few teeth have been lost, bite-plates must be constructed for tak- ing the bite for partial dentures. In general it is better to construct them of a thin sheet of modelling compound rolled out to about the thickness of Xo. 12 gauge, adapted to the cast, and trimmed to the plate outline. Vacancies between the teeth are built up with additions of modelling compound to the level of the adjacent teeth at such places as it is necessary that the plate should oppose a tooth in the opposite jaw 352 DATA TO BE USED IN CONSTRUCTING DENTURES. and stay the bite. (Figs. 323 and 324.) Where the natural teeth remaining occlude and stop the closure at that point, these spaces on the partial bite-plate may be built up with soft wax just before the closure for fixing the bite is made, the soft wax receiving an impression of the opposite teeth. Partial bite-plates may often be strengthened to advantage by im- bedding iron or brass wire of proper shape in them at such places as will be exposed to strain, anrl this is particularly true of partial lower j)latcs. Taking the Bite in Partial Cases. — (1) In the construction of den- tures for jjartial cases where the remaining natural teeth occlude prop- FiG. 324 Bite-plate for partial lower denture. Bite-plate for partial upper denture. erly and particularly if only a few have been lost, it is only necessary to be assured that the teeth of the casts occlude in the same way that the natural teeth do, and, if there are enough points of bearing, occasionally they may be mounted on the articulator without taking the bite. If, how- ever, a roll of soft yellow wax is placed between the patient's teeth and they are brought to the position of natural occlusion, the wax may be pressed up with the fingers, cooled with water, removed from the mouth, and the casts may be fitted into the wax with the assurance that their relation is correct. It is important to be certain that the teeth come into the po.sition of correct occlu.sion when the jaws are closed upon the wax, and it is expedient to mark oppo.site points with a lead pencil upon the external surface of two teeth that occlude, and to observe if these bear the same relation when the jaws are closed for the bite. This includes partial upper or partial lower dentures or both. (2). Where the teeth remaining do not occlude, as in the case of a par- tial upper and partial lower denture, it is, of course, necessary to have bite- plates to establish the distance between the jaws, as was done with the full upper and lower. These are trimmed tentatively to correspond in the length of their occluding portions with the adjacent teeth and are tried in the mouth. The same general principles which determine the length of the full upper and lower bite-plates must be borne in mindi A R T/CULA TORS. 353 but the lengtli of the remaining natural teeth will be the main guide in determining the trimming. The plates and teeth should occlude with the same evenness and firmness which is demanded of the full plates, each tooth striking upon the bite-plate. The plates are removed, wiped dry, and their occlusal surfaces covered with softened wax, and the bite taken as has been before described. With a full denture for one jaw and partial for the other, as in a case in which some of the lower nat- ural teeth remain, the obvious method is to combine the principles of bite taking for partial and for full dentures, being guided by the nat- ural teeth as to the length of the bite-plate, by the external appearance of the patient as to the contour and the distance between the jaws. (3). There are occasional cases requiring partial upper or lower dentures or both where the remaining natural teeth occlude, but where they are either much abraded or have been driven out of their places by the force of occlusion, and the distance between the jaw^s which they establish is insufficient. It is usually ad^^sable in such instances to "open the bite." The bite-plates are trimmed to a length in accordance with the amount it has been determined the jaws are to be separated, and the procedure is the same as where there were no points of oc- clusion. It must be borne in mind that the natural teeth separated by this measure should be built up by operative measures or by crowning, if possible, as they cannot otherwise participate in the masticatory functions of the denture. Instances demanding this proceedure are not of frequent occurrence, buta few cases are much simplified and the mouths much improved by thus establishing a new position of occlusion. The determination of the relation of the jaws to the temporo-maxil- lary articulations, and the determination of the path of the condyle in its forward and lateral excursions are questions so closely related to the subject of articulators that it will be necessary to consider them con- jointly. ARTICULATORS. As originally designed the instrument known as the articulator was simply intended as a means of maintaining the plaster casts in the posi- tion of occlusion during the operation of arranging and mounting the artificial teeth. -The bite having been taken, the casts were arranged in the occlusal position and attached to the metal frames of the instrument, and thus their occlusal relation was preserved while the teeth were being set. In its simplest form the instrument consists of two frames joined by a hinge, which permits the separation of the casts without detaching them from the articulator. The invention of the device is attributed to J. B. Gariot about 1805.^ Many forms of articulators constructed upon a similar principle have been used since that time, a common type in use at the present day being illustrated in Fig. 325. Except for convenience in handling while the teeth are being mounted, 1 Guerini: The Historical Development of Dental Art. The Dental Cosmos, vol. xliii., p. 8. 23 354 DATA TO BE USED IN CONSTRUCTISG DESTniES. the occlusal relations of the casts may be j)reser\'e- in the arc of a circle in its depres- sion and elevation (see Chapter IV.), but the centre about which this occurs is not in the condyle except at the beginning of the depression, and no simple hinge could imitate it. The casts mounted upon a simple hinge articulator, therefore, occupy the positions occupied by the iaws onlv when the casts are in the occlusal relation, and the joint of the • • • 1 articulator serves solely for convenience m separating the casts. This form of articulator has a distinct field of usefulness, but its boundaries are prescribed. In those cases in which a number of the natural teeth remain in the mouth, as in partial dentures, or full upper or lower, where most of the opposing series arc natural teeth, this in- strument may be used with satisfaction. In these several instances the forms of the morsal surfaces of the artificial teeth are determined by those of their opponents of the natural series, which latter serve as a A R TICULA TORS. 355 safe index of tlie forms best adapted for masticatory purposes for the eases in question. No type of this articuhxtor should be used which is not sufficiently accurately ma(ie to avoid lateral mo\'ement at the joint. This mechan- ical defect may alter the relation of the casts in the occlusal position and the dentures will l)e correspondingly inaccurate. It must also be noted that the frec^uent practice of attempting to correct defects in the vertical relations of the casts originating from an inaccurate bite, by raising or lowering the upper portion of the articulator, as the bite is found to be too short or too long, is to be decried because the relation thus established between the casts cannot be that e^•er existing between the jaws. In the use of this instrument the casts are placed in their proper positions in the bite-plates, having been trimmed so that they may be accommodated between the jaws of the articulator. x\s they are to be attached by means of plaster, and, as after this has set and dried out it becomes so hard that there is danger of fracture to the casts when they are removed, it is advisable to coat the bases of the casts with sandarac varnish, with the exception of a small area about the size of a twenty- five-cent piece in the centre of this surface. The casts are wet and a plaster batter used to attach them to the articulator, the union obtained from the small unvarnished area being sufficient to hold them in place, and yet they may be removed without difficulty after the teeth are set up. The first instrument in which an attempt was made to imitate the temporo-mandibular joint with the joint mechanism of the articulator and thus to permit a reproduction of the masticatory mo^'ements of the mandible was that of W. G. A. Bonwill. It was designed by him in 1858, and its well-known form is shown in Fig. 326. With this apparatus an effort was made to imitate the movements of the mandible occurring in its forward and lateral excursion, as well as in its depression. The joint mechanism consists of a ring sliding upon a bar, in imitation of the sliding movement of the condyle, the direction of the bar deter- mining the path pursued by the lower cast. The ring is attached, however, to the portion of the articulator carrying the upper cast, the bar being a part of that carrying the lower. So long as the upper and lower loops of the instrument are parallel, the path of the condyle as represented by the bar is parallel to them, and hence in a horizontal direction; but as the portion carrying the lower cast is depressed, rotat- ing about a horizontal axis passing through the rings, the bar, being a portion of this, is correspondingly elevated, its angle with the horizontal plane is changed, and the path of the condyle is represented as de- scending, the angle which it makes with the horizontal being determined by the amount of depression of the lower portion of the instrument. This is a faulty principle, and the instrument does not accurately repre- sent the movement of the jaw. It was shown in Chapter IV. that the path of the condyle is downward and forward, that it is rarely horizontal, and although the paths differ in different individuals, an 356 DATA TO BE USED IN CONSTRUCTING DENTURES. instrument representing the condyle path by a bar, the inclination of which is changed as the jaw is depressed, cannot correctly imitate the mandibular movement. The instrument represents the condyles as four inches from the centre of one to that of the other, and it is directed that the casts should be placed upon the articulator with their general alveolar planes parallel to the loops to which they are attached, the centre of the lower alveolar ridge in front being located four inches from each of the rings representing the condyles. This presupposes a uniform intercondylar distance of four inches, and a like distance between the centre of each condyle and the lower alveolar ridge, conditions which do not obtain in Fig. 326 Bonwill articulator. (Photograph of instrument in collection of the Department of Dentistry, University of Pennsylvania.) the natural jaw. Cryer^ has called attention to the difference in inter- condylar distance in two mandibles in his possession: furthermore, such uniformity in the dimensions of the jaws of different individuals is not in accord with actual anatomy. The instrument, besides being constructed upon a faulty principle, is not adjustable in any way to the individual case, and represents the ideal of the designer. The principles of tooth articulation, however, enunciated b>- Dr. Bon- will in connection with this articulator are extremely valuable, and will be discussed in a later chapter. Various efforts have been made to reproduce the movements of the lower jaw by means of an instrument. The articulator designed by > Internal Anatomy of the Face. ARTICULATORS. 357 W. E. Walker,! illustrated in Fig. 327, was an improvement upon the Bonwill articulator, in that the bar determining the path of the condyle is adjustable to any angle. By means of a "facial clinometer," to be Fia. 327 Walker's articulator. used in connection with the instrument, the angles made by the paths of the two condyles with the general alveolar plane are determined for each patient and the articulator is set to correspond therewith. Fig. 328 The Gritman articulator. It is a somewhat complicated apparatus and has never come into general use. The Gritman articulator'- (Fig. 328) represents the path of the con- 1 Facial line and angles in prosthetic dentistry. The Dental Cosmos, vol. xxxix., p. 789. ! Introduced in 1899. 358 nATA TO UK rsh'i) jy constuuctjm; dicstcres. dyle as a strai^dit line niakiiit; an an^^le with the ^^eneral alveolar plane which the designer obtained as an average hy the measurement of a large number of skulls. The superior mechanical construction and coinenience of this instrument have recommended its use to many, but it is not capable of adjustment to the exact requirements of the Fig. .'529 Christensen'a articulator. individual case. The articulator of Christensen^ (Fig. 329) is capable of adjustment in the matter of the direction of its condyle paths. The New Century articulator^ (Fig. 330), designed by George B. Snow, has the mechanical excellence of the Gritman articulator and, in addition, Fig. 330 New Centun,- articulator, designed by George B. Snow. has adjustable condyle paths. In common with all anatomical articula- tors so tar described, it is constructed on a plan of a constant inter- condylar distance of 4 inches and is not adjustable in this particular. 1 Ash's Quarterly Circular, December, 1901, p. 409. 2 Introduced in 1907. ARTICULATORS. 3o9 For li't'iitTal use in the lab()ratt)ry it will be found an instrument of sufficient precision for all practical purposes. While the fact that the path of the condyle is not a straight line has been proved by the work of various investigators, notabl}' J. B. Parfit,' George C. Campion,' Thos. E. Constant,^ it has been represented as such in all the instruments so far discussed. The articulator of J. B. Parfit represents the path as curved, and pieces of metal plate, having l)een cut out to correspond with the ascertained paths of the case in hand, are clamped to the frame of his articulator. Inasmuch as the paths of movement of the condyle while the teeth are in contact are the only ones with wdiich wt are concerned in the articulation of artificial teeth, and as this contact is maintained only in the movement of the condyle from its most distal position to a point somewhat short of the summit of the eminentia articularis, during which time the condyle Fig. 331 Articulator of Gysi. moves nearly in a straight line, for most practical purposes in tooth articulation it may be represented as such in the articulator. The articulator of Gysi (Fig. 331) is the most accurate device so far designed as a mechanical representation of the temporo-mandibular joint. The following is a description of the instrument:* "This articulator (see Fig. 331), like all others, consists of a mov- able upper part (Fig. 331, 0), and a fixed lower part, u. To the upper part the straight bow (Fig. 340, O), is attached and secured by screws, S. An extra bayonet-shaped bow (Fig. 331, o) can be inserted as in Fig. 331, o, or inverted to correspondingly suit the height of the plaster ' A new anatomical articulator. Transactions Odontological Society of Great Britain, 1903, p. 337. - Method of recording graphically the movements of the mandibular condyles in the li\dng suD- ject, The Dental Digest, 1903, p. 841. 5 The movements of the mandible: Brit. Jour. Dent. Science, 1901, p. 807. ■" The Dental Cosmos, vol. lii., p. 148. 360 DATA TO BE USED IN CONSTRUCTING DENTURES. models. To the lower part a straifjht bow is attached and secured by screws, S. An extra bow can l)e attached ha^'ing a base that is H ccm. higher. With these four bows six diilerent combinations can be effected to correspond to the height of the plaster models, thus saving much time in their preparation for the articulator. The bows should be oiled and pushed as far into the articulator as they will go, so that should it l)e necessary for any reason to remove them, together with the models from the articulator, they can be easily and accurately placed in their former positions. "To the upper bow an adjustable supporting ])in, St, is attached, which rests on the narrow end of the small plate. T, of the lower bow. On this narrow end of the lower bow is attached an inclined plane, E, on which the su}3porting pin moves upward in the side movements. The supporting pin should always rest at the foot of this inclined plane. This inclined plane forms the incisor guide, and serves on the articulator as a substitute for the overbite. I'p to the present the artificial incisors, attached with wax to the trial plates, have taken the place of this incisor guide. This was a most uncertain guide, especially in warm weather. For practical reasons, which will not be further discussed, this inclined plane is attached to the lower part of the articulator instead of to the upper part, just as all articulators have a fixed lower and a movable upper part, entirely contrary to the natural relations, but, as is well known, answering the same purpose. "As the supporting pin acts as a guide to the height of the bite, it is placed in front of the incisors, because it is only here that a true and secure support can be obtained. If this supporting pin interferes with the setting up of the incisors, it can be removed until the latter are placed in position. "File-marks (Fig. 331, F) can be made on the supporting pin, so that there is at all times a guide to show that the height of the bite has not been changed in the setting up of the teeth. "The placing of this supporting pin in front of the incisors offers the further advantage that the articulator is quite unobstructed at the back, so that the lingual surfaces of the teeth arc clearly visible and can easily be reached with the fingers and wax sj)atula. This is important, in that it permits of the correct articulation of the lingual surfaces of the teeth. "The most important function of this supporting pin and the in- clined plane consists in the prevention of the wrong downward move- ment of the upper part of the articulator produced in all articulators up to this time when reproducing lateral movements. "The upper bow, O, is connected with the upper part of the articula- tor by a hinge joint, so that in the setting up of the artificial teeth open- ing and closing movements can be made that are independent of the true joint movement: consequently the two condyle parts of the triangle can be combined exactly and precisely with the incisive part of the triangle in such a way that all unnatural movements are impos- sible. In this manner the downward movement of the mandible com- ARTICULATORS. 361 bines with the forward movement in the direct forward and lateral mo^•ements. This combined movement, which has long been recog- nized, is for the first time accurately reproduced by the articulator. The importance of this fact is fully explained in another chapter. "The upper and lower parts of the articulator are connected through the real joint, G, which permits of the lateral movements. Two springs, F, automatically bring the two parts back to their normal positions. " The joint is formed by the fork (g) of the lower part of the articula- tor, the prongs of which pass through the E-shaped part of the upper half and receive the slotted plate, Sp. The fork (g) rotates, and with the slotted plate can be fixed bv the screw (5) at an angle of from 0° to 50°. "The two identical slots in the slotted plate which receive the prongs of the fork (g) correspond in form to the path which the condyles take in their movements during mastication. "To be quite exact, a number of different forms of slotted plates should be kept. From my long experience, however, I have found that the two average forms are quite sufficient, and if the artificial teeth are placed in the exact position necessary to secure the full value of the gradations of these tw^o condyle path forms, the result is most satis- factory. If a special form for every case is thought necessary, as advocated by Campion and Parfit, it can be easily and quickly sawed out of thin brass plate and placed on the articulator. "The pin, st, found on the slotted plate, serves to bring the latter into its proper place, and should always be placed in the hole provided for it in the middle of the E-shaped joint. "In order to change the slotted plate, the binding screw, 5, is first loosened, then the prongs of the fork g are turned to a vertical position, which will allow the plate to slide over the head of the binding screw. With the prongs in the same position another plate can be put on and fixed at the desired angle. This is accomplished by placing the index of the slotted plate at the required degree on the engraved scale, W, in the E-shaped joint. "When making full upper and lower dentures \\dthout measuring the individual condyle paths, the index of the slotted plate should be placed at the average angle of 30°. It is not absolutely necessary to measure the condyle path for partial dentures. In such work the slotted plate can also be set at the average angle of 30°. In this way, in extreme cases there can only be a difference of 20° above or below this average slant, which is not much when we consider that in using a Bonwill articulator there can be a difference of 45°. "The two small supporting pins, D D, at the back, with the large supporting pin, St, form a secure triangle which is not to be found in any other articulator. This, together with the solid cast pieces, pre- vents any looseness or any springiness, and thus insures the possibility of true and exact work. "The small supporting pins, D D, in the slotted upper part, 0, can be moved sideways. They rest on the transverse piece of the lower 362 DATA TO BE USED IS CVySTIiUCIIMJ DENTURES. part, on wliich is an eiitiravcd scale to iiKllcate the positions of the pins. As the positions of these sni)pt)rtin '^^^pn 1 I^PI/ ^l^^^^-^-^^^-^i^s:^ W ^-'/ "" m ^^ [ -. - ^^^^^^^^^^^^^^^^ i Patient with Snow face-bow in place. THE PATHS OF THE CONDYLES. 36'5 tury articulator or any other with adjustable condyle paths with which it may be employed, it is not necessary to take these precautions about havin^j the general alveolar plane of the casts parallel with the bows of the articulator for the reasons above mentioned, because the condyle paths are subsequently adjusted to the proper angle. However, as a matter of convenient positioning of the casts this plan will, under these circumstances, be found advantageous. AVhen the casts have been properly adjusted (Fig. 336), they are fixed in place by additions of plaster, and the casts are ready for the subsequent setting of the teeth. The Snow face-bow was designed for use with the Gritman articulator, but it may be used with the Walker or Christensen articulator, if these Fig. 336 Snow face-bow. are altered slightly by the addition of projections at their joint-mechan- ism to receive the projecting ends of the face-bow. Since the ad^-ent of the New Century articulator, with which it may be very conveniently employed, the results obtained in tooth articulation by the use of this combination of instruments have been most satisf^^ng. THE DETERMINATION OF THE PATHS OF THE CONDYLES It was shown in Chapter TV. that in the typical natural dentures there is a definite relation between the paths pursued by the condyles, in their forward and lateral excursions, and the forms and arrangement of the occlusal surfaces of the teeth, and that during the sliding contact of the teeth the path pursued by the mandible is determined by the teeth on its anterior extremity and the condyles and glenoid fossae posteriorly. In the articulation of artificial teeth, it is desirable that they should be arranged in such a way that the sliding contact may be possible during their use. In order to accomplish this, it is necessary, therefore, that they be set up to accord with the paths pursued by the condyles in the 366 DATA TO BE USED IN COySTHUCTIMJ DESTURES. Fig. 337 individual case re{|uiriii,ii: tlioni. Tiiis drmaiuls that tlie paths of the condyles be ascertained, if this l)e })ossihle, and that an articnUitor capable of imitating these movements be utilized in setting the teeth, and that it be set to imitate the condylar paths in the given case. No articulator has yet been constructed which is exactly- able to reproduce these movements, but of the several anatomical articulators already described, all possess sufficient accuracy to make them of some service, and while no one of them is perfect, the use of them greatly impro\es the results in the articulation of the artificial teeth. Given a satisfactory articulator, it then becomes necessary to have some means of ascertaining the paths i)ursued by the condyles of the edentulous jaw for which the denture is to be made. These must then be recorded by some means, and the record transferred to the articulator. This is a somewhat difficult matter, as in the living subject the condyles and fossa^ are beneath the skin, and this makes it im])ossible to directly observe the operation of the joint. Campion^ has devised a means of accomplishing this by the use of a bow carrying two points ; the bow is attached to the teeth of the lower jaw, and the points are placed over the exter- nal ends of the condyles pre- viously located. The points carry a pen or pencil, and the position of the condyle at any position of the lower jaw is recorded by a mark upon the skin of the face and the record may then be trans- ferred to a piece of paper. This method is chiefly for use in study- ing the mo\ement of the mandi- ble, and is not designed for use in the setting of artificial dentures. Walker has, however, designed a method for determining the condylar path on each side independently, which he applied in the construction of dentures. He used a com])li- cated apparatus which he called a "facial clinometer," by means of which the angle between the path of the condyle and the plane of the upper alveolar process was determined, and the articulator bearing his name set according to this for the case under consideration. The ap- paratus is so complicated that it has never come into general use. Christensen^ has proposed a method for harmonizing the articulation of the teeth with the mo^'ement of the condyles. His methc^d is based ' Method of Recording Graphipiilly the Movements of the Mandibular Condyles in the Living Subject: George C. Campion. The Dental Digest, 1003, p. 841. 2 A Rational Articulator. By Carl Christensen. Quarterly Circular of C. Ash & Sons, December, 1901, p. 409. I I Schematic drawing, showing relation of dentures in position of occlusion and in forward po.sition of mandible. (Christensen.) THE PATHS OF THE CONDYLES. 367 upon the following- facts, to which he calls attention. During the movement of the mandible in the individual with natural teeth, while these teeth preserving a sliding contact, the condyle can move only from its distal position downward and forward a distance of 4-5 mm., with 12 mm. as the probable maximum. It is during this sliding con- tact of the teeth that the movements of the condyle occur which are important from the standpoint of the function of the denture: we are only concerned \\\i\\ such movements of the condyle. Therefore, the path of the condyle would be a curve of large radius, or, for all practical purposes in tooth articulation, would be a straight line. If an articulator C(Hild be set so that the parts representing the paths of the condyles would be either identical or concentric with those actually pursued by the jaws, the teeth could be set up according to this and would articulate when placed in the mouth. Christensen's articulator is used upon this principle. The bite is taken in the ordinary w^ay and the casts are mounted upon the articulator with their median line corresponding to the centre Fig. 338 Casts mounted on Christensen articulator, in position of occlusion. (Christensen.) of its bows, and the line between the positions of the lower central inci- sors, 10 cm. from each joint, representing the temporo-mandibular joint. (Fig. 338.) If the bite-plates are then returned to the mouth, with a small lump of soft wax upon the occlusal portion of the upper bite-plate, and the patient is instructed to throw the lower jaw forward and then to bring the bite-plates together, a record is made of the jaws in this posi- tion. (Fig. 337.) If the casts already mounted upon the articuhitor can be placed in the bite-plates, the former will then assume the position occupied by the jaws, and if they were originally placed upon the articu- lator in the same relation to those portions of it representing the con- dyles, these latter must now^ be in the position of the condyles in the forward position of the mandible. The path of the condyles to this 368 DATA TO BE USED IN CONSTBUCTING DENTURES. point is approximately a straif^ht line, and as we have the distal' end of this line recorded on the instrument when the bite was taken, we now have its anterior end determined, because the bow of the articulator, like the mandible, moves as a whole. If the portion of the articulator, therefore, which represents the path of the condyle (the bar with the spring and sliding ring) is set in this position by screws, C and C (Fig. 339), the casts may then only be moved in relation to each other as the sliding of the rings on the bars will permit, and they thus imitate the movements of which the jaw is capable. It will be seen at once that this imitation of the jaw movement by the articulator cannot be mathematically precise, because of i)robable errors Fig. 339 Casts arranged in forward position of mandible by means of bite-plates; articulator adjusted thereto. (Christensen.) in the record, and because there is some play in the natural joint which no articulator can ever imitate. It is sufhciently accurate, however, to give results in tooth articulation which far excel those obtained without the use of an articulator capable of indi^•idualizing the move- ments of the condyles. Since the publication by Gysi of the results of his work along the line of greater accuracy in securing these various records, and since the publication of descriptions of his instruments, prosthetists have been provided with a valuable addition to their armamentarium. Especial attention is directed in the following quotation' to the means of obtain- ing the balancing points in the lateral excursion of the jaw, the first recorded effort to adjust the articulator to a varying intercondylar distance. 1 The Problem of Articulation. By Alfred Gysi. The Dental Cosmos, vol. lii, p. 1. (Being a translation of Professor Gysi's book, "Beitrag zum Articulationsproblem," published by Hirschwald, Berlin, in 190S, with some practical additions written since its publication. THE UY8I MEASURING INSTRUMENTS. 369 THE GYSI MEASURING INSTRUMENTS. "According to Bonwill, the two condyles form an equilateral tri- angle with the contact point of the lower central incisors. I have, there- fore, constructed two measuring instruments. With the one (Fig. .'540, G) I determine, from the forward and opening movements and the combina- tion of these two, the form and direction of the two condyle points of the triangle in a vertical plane; with the other instrument (Fig. 340, S) I determine, from the lateral movements, the path of the incisor point of the triangle in a horizontal plane. " From the registered paths of these three points of the triangle in their separate directions, I can direct the movement of the mandible in all the combinations of the masticatory movements. Pig. 340 A, Articulator; O, U, extra bows; G, condyle path register; H, horseshoe plate; Sch, extra pair of condyle path guides; W, angle measure for condyle path slant; T, type plate for molar groove; S, small register; St, holder for register G (to be used when plastering models to articulator). " The Large Registers for Determining the Slant of the Condyle Path. — This instrument (Fig. 340, G) serves to measure the path taken by the condyles in the movements of the mandible. The important part of this instrument consists of two lead pencils, which in every individual case can be placed in the region of the condyles (Fig. 341). " Its attachment to the mandible is accomplished through" the horseshoe plate (Fig. 340, H), which, with its points on the under side, is pressed into the lower wax model which has been used in taking the bite (Fig. 343). The whole is so well balanced and weighted that it will, without any further help, remain iirmly in its position in the mouth. "Parallel to the horseshoe plate (which we shall consider as being in place) on the wax model and on a line with the plane of occlusion, 24 370 DATA TO BE USED IN CONSTRUCTING DENTURES. parallel ])lates run backward on both sides (Fig. 341, P), carrying mov- able spring lead jjencils in the region of the joint (Fig. 'M\, B). For extreme cases these lead pencil holders may be changed from the left to the right side, or moved up and down on their vertical bars with a screw (Fig. 340), so that the pencil points form as nearly as possible a right angle to the writing surface of the recording card (Fig. 341). The parallel plates with the lead pencils may be adjusted for individual cases b^' moving the plates on the cross-bar which hokls them (Fig. 341, T)*. "This instrument also serves as a compass, like the American Snow face-bow, w^hich fixes the distance of the plaster models in correct rela- tion to the axis of the joint. Fig. 341 Shows the method of detenniaing the slant and form of the condj le path. (Wilson, after Gysi.) "In order to find out the movements of the mandible in different individuals, the wax bite-plates are prepared in the usual manner, except that the base plates must be either of modelling compound, 1.5 mm. in thickness, or of some other equal firm material reinforced in the usual way w^ith a piece of wire. After proper attention has been paid to all the details, such as securing the proper fulness of the wax models to insure proper lip contour, the right height of the plane of occlusion, the length of the incisors, the median line of the face, etc., the patient is THE ay SI MEASURING INSTRUMENTS. 371 Fig. 342 directed to open and close the mouth several times, during which the positions of the condyles are ascertained by feeling in the region of the ear where they move. Then the i)ositions of the condyles in the resting position (with the moiith closed) are found, and marked on the surface of the skin of the patient with a pencil or with chalk. Usually the con- dyles are found about 1 cm. in front of the tragus of the ear, in the direc- tion of the outside corner of the eye. (Fig. 342.) The horseshoe plate is then fastened by its points to the lower wax bite-plate and both are placed in the mouth. " If some natural teeth are left in the mandible, a horseshoe plate without points can be used. Some hot modelling compound is placed on the under side of this plate and pressed over the teeth, just as though an impression were to be taken. In most cases where there are nat- ural teeth, it is not absolutely necessary to measure the relations of the joints. " The condyle path register is now attached to the horseshoe plate, which in its position in the mouth is fixed to the wax bite-plate. The lead pencils are put at the marked places which indicate the positions of the condyles; by ad- justing them on their perpendicular bars and through the sliding arrangement on the cross- bar, they are brought close to the surface of the skin. "The form and slant of the condyle path will then be found by inserting a piece of card- board, as shown in Fig. 341, between the pencil points and the skin in the region of the joint, with the lower edge parallel to the parallel plates of the register. The latter plates are again moved on the cross-bar until the pencils are brought in such close contact to the writing surface of the card that the springs holding the lead pencils are under a pressure. The patient is directed to move the mandible up and down, and from side to side (chiefly the latter motion), until the condyle path is clearly drawn on the surface of the recording card. (Fig. 341.) "The lateral movements of the mandible show that the cur^-e first made by the opening and closing movements is followed in the lateral movements, and that while one condyle moves forward, the other moves more or less backward. (See explanation to Fig. 348.) The measuring of this moving path of the mandible occupies hardlj' three minutes' time. "After one side of the mandible has thus been measured, the same card is held on the other side, and the same procedure is again followed. After the measuring is finished, the lead pencils are moved outward by the sliding arrangement on the cross-bar, so that the recording card and the register may be taken away, without, however, removing the horseshoe plate. The lead pencils with their holders must not be moved or turned on their vertical bars, as their position is of importance 372 DATA TO BE USED IN COySTEUCTING DESTURES. ill attaching the models to the articiUator, as we shall see later. The wax models may then be taken from the mouth. "To find the angle of the registered path in relation to the occlusal plane, a slotted plate is selected which corresponds as nearly as possible to the form of the registered path. The plate chosen is placed directly over the lead pencil drawing, so that the drawn line can be seen ap- FiG. 343 Shows the condyle register with the horseshoe plate attached, to which the lower wax bite-plate is fastened: .ST, spring lead pencils; P, parallel plates; H, horseshoe plate; F, tinger rests; A, place for insertinff holder. inserting holder proximately parallel to the sides of one of the slots (Fig. 344). Press the point of the axis of the slotted plate through the card, and then mark the points of the long axis of the plate with a pencil. This will show the main direction of the path. The slotted plate is taken away, and both marked points are joined by a direct line (Fig. 344), which line Fig. 344 Shows how the main direction of the condyle path is found by means of the slotted plate. is continued to the lower boarder of the card, so that with the angle measure one can determine (see Fig. 345) the acute angle which this line forms with the lower border of the card. As this lower border of the card is held (in the registering of the path of the condyles) parallel with the parallel plates, which in turn are parallel to the horseshoe plate, and this latter lies in the plane of occlu.sion, the angle measured THE nysr measuiuxg instruments. 373 ill this way must corresjjoiul exactly with the shuit of the path of the condyle to the plane of occlusion. "The slotted plate chosen is then i)laced in position on the articulator and the known slant fixed by means of the engraved degree scale on the joint of the instrument (see Fig. 331). Then proceed in the same manner with the registered condyle path of the other side. Fig. 34.5 Shows the mode of measuring the angle formed bj* the condyle path and the lower border of the record- ing card. "Fig. 346 shows a recording card with the registered condyle paths of the right and left sides, which have been measured in the manner stated. On the right side the condyle moves in a slant, the angle of which is 44° to the plane of occlusion (Fig. 341, P); on the left the measured angle registers 34°. "In cases where the curve is more or less horizontal, and the length- ened line does not reach the lower border of the card, a line is drawn from Fig. 346 Shows the completed measurements of the condyle path. the side of the card tangent to the lengthened line and parallel to the lower border of the card (Fig. 347). "The objection may be raised to my method of measuring that the plane of occlusion represented by the wax bite-plate is not a fixed point, because in setting up the artificial teeth one is obliged to change this temporary plane of occlusion according to circumstances. 374 DATA TO BE USED IN CONSTRUCTING DENTURES. "This objection, however, is only apparently justified, because in reahty not only the angle of the condyle path to the tenii)()rary plane of occlusion is obtained, but also at the same time that to the alveolar ridge, or to the mandilile itself as a whole. "If the temporary ])lane of occlusion is changed, the degree of the angle of the condyle path is, of course, also changed to agree with the Fig. 347 new plane of occlusion, but the relative relations of the position of the mandible itself to the condyle path remain just the same. "I will now give an analysis in detail of the condyle path curve as illustrated in Figs. 344 and 346. "Fig. 348 shows an enlarged diagram of a left and right condyle Fig. 348 c R Analysis of a right and left condyle path as secured by method shown in Fig. 311: C, condyle path; L, left; R, right; Oc, plane of occlusion; 35-degree angle of middle part of path to plane of occlusion; r, resting position of condyle; /?', path of condyle in a right lateral movement; Z/>, the same in a left lateral movement; o, forward bite or wide opening and closing movement. path curve, each of which is divided into its chief parts, as indicated by the different lines. "From the description of this diagram it can be seen that during the forward and downward movements of the one condyle, the other runs more or less backward horizontally. "The extreme forward movement and the opening and closing move- ments may be divided into two parts: First, the path which the con- TIIK arSI MKISURINO JNSTJiUMh'NTS. 375 (lyle takes in a sli' cases in which through disease or surgical operations the form of the jaw is so altered that any sort of adhesion is impossible. There are also a few cases of extremely flat mouths or greatly contracted jaws in which the use of spiral springs is necessary. Care should be taken that their point of attachment is correctly located or they will have a tendency, when the mouth is open, to displace the dentures upon their base or even to protrude them through the lips. These points must be ascertained by trial, the usual point in the upper jaw being opposite the second bicuspid and in the lower jaw between the first molar and the second bicuspid. These points of attachment should be at least seven-eighths of an inch apart in a vertical direction. The spring should be two inches long. The detail of the construction of the THE RETENTION OF FULL LOWER DENTURES. 401 spiral springs and of their attachment is given in former editions of this book and many other older text-books. Their present infrequent use renders a description of their mode of construction unnecessary here. THE RETENTION OF FULL LOWER DENTURES. Full lower dentures are maintained in situ by their weight and by ad- hesion. The latter is not of great value in a majority of cases, but acts to a greater or less extent in all. The former is the most important of the active forces of retention, but, as in the upper jaw, the plate must be freed as far as possible from all displacing influences due to faulty articulation and incorrect plate outline. Lower plates are more subject to displacement through the movement of adjacent tissues than are upper plates, and especial care should be devoted to establishing cor- rect outlines for them. Full lower dentures are the most difficult of all types of dentures for the patient to learn to use, as they have the least mechanical aid to re- tention of all plates. They are chiefly held in place by the tongue, cheek, and lip muscles, and every measure tending to assist the patient in acquiring facility in using them should be carried out. Plate Outline for Full Lower Dentures. — In making the plate outline for a full lower denture the same general principle of avoidance of movable tissue, as obtained in the upper jaw, is to be followed. The muscular structures attached to the mandible around the periphery of the proposed plate are located at a level relatively higher than in the upper jaw. The tissues attached to the labial and buccal surfaces of the mandible are placed at a higher level than those attached to its lin- gual aspect. The plate outline should, therefore, be sufficiently above the point of reflection of the mucous membrane to avoid any possible movement transmitted through this tissue. The muscular structures to be avoided are the attachment of the levator labii inferioris, the de- pressor anguli oris, the buccinator, and. the muscles of the floor of the mouth. The frsenum of the tongue must be given full play, and it is desirable to avoid the plicse of mucous membrane around the external border of the plate, those most commonly found existing opposite the canine and bicuspid teeth. The outline for a typical case is shown in Fig. 370, A, while that for a case in which there has been great resorp- tion of the process is shown in Fig. 370, B. The plate outline for the full lower denture should be carefully tested when the bite is taken, by requesting the patient to touch the roof of the mouth with the tongue and noting the impingement of the plate upon the tissues on the lingual side of the jaw. k stretching of the lip and cheek tissues upward by means of the thumb and forefinger of one hand, the other hand holding the plate in situ to counteract any displacing force which this may develop, will enable the operator to judge of the im- pingement in these regions. It is impossible in some instances to en- tirely free the plate from the influences of moving tissue, but the eftort should be made to reduce this to the minimum. 26 402 THE RETENTION OF PLATE DENTURES. Gravity. — The force of gravity is of service in tlie retention of all lower plates, whether full or partial, being the chief physical force which maintains full lower dentures in place. A recognition of the part played by it in the maintenance of lower dentures has led to the construction of dentures characterized by considerable weight for very flat mouths in which retention is difficult. This object is efi'ected in the use of so-called "weighted rubber" for lower vulcanite dentures and the use of metallic dentures of all forms, fast metal dentures, cast metal base-plate with vulcanite attachment, swaged metal plates with soldered teeth and with vulcanite attachment, and continuous-gum dentures all possess the advantage of considerable weight. In the case of a very flat mouth, as is shown in Fig. 370, B, the use of a weighted lower denture is frequently advisable. It must be remem- A. Plate outline for full lower cast with well-defined ridge. B. Plate outline for full lower east with little ridge. bered, however, that the weighting of a denture should not be carried to the point that it is burdensome to the patient. The author has seen a case where extreme weight has so stimulated the absorption of the bone that the mandible had become very thin and dangerously liable to fracture. This contingency must, of course, be avoided. The use of weighted dentures is entirely confined to full cases, as partial lower cases should receive sufficient maintenance from the natural teeth. THE RETENTION OF PARTIAL DENTURES. Partial Upper Dentures. — These are maintained in place by adhesion, by atmospheric pressure, and by clasps. Adhesion is of service to a greater or less extent in all cases, but it cannot alone be depended upon: its chief utility is in conjunction with atmospheric pressure. It is manifest that in j)artial cases the easy access to the palatal surface of the plate which the spaces about the natural teeth afford to the air, makes adhesion a less eftecti^'e means of retention THE RETENTION OF PARTIAL DENTURES. 403 than in full cases. When the plate is held in place upon the mucous membrane by atmospheric pressure through a \-acuum-cavitY or by clasps, adhesion between the plate and the membrane supplements the other retentive means. In most partial upper cases, therefore, where advantage may be taken of the position of the posterior margin of the plate, the contact of this is to be accentuated by scraping the caist as in full upper dentures. This, of course, has to be done advisedly and only in those instances in which the margin rests upon comparatively soft tissue. Contact of the plate with the natural teeth just above their gingivae serves to stay it and maintain it in place under the stress of mastication. This contact also serves to prevent the pinching of the membrane be- tween the edges of the plate and the teeth, and should always be estab- lished, except those cases in which the insertion and removal of a plate thus made would be prevented because of the contours or inclination of some of the teeth; in such instances the plate must be cut away to permit its easy placement. In this latter class of cases provision must be made to prevent the edge of the plate from sinking into the mucous membrane about the teeth not touched by it, by the employ- ment of lugs attached to the clasps and resting upon the occlusal surface of the teeth as described in Chapter XIV. Advantage is taken of the bracing or staying effect of natural teeth by the use of half-clasps or stays attached to the plate and placed to grasp the tooth or teeth in such a way as to resist the displacing force. Atmospheric pressure is to be preferred in most cases as the means of retention of partial upper dentures. Exception to this occurs when the configuration of the vault and the consistence of the tissues is unfavor- able to this means, and in those cases in which its use would unduly extend a plate carrying only a few teeth. In some cases, as of those supplying the posterior teeth, it is the only available means of retention. Where it is to be employed, the vacuum-chamber is to be located and shaped in accordance with principles already outlined for full dentures. Clasps as a means of supporting partial upper dentures are discussed later in this chapter, while the details of their construction and mode of attachment are given in the chapter on Swaged Metal Plates. Plate Outline for Partial Upper Dentures. — In marking the outline for a partial upper case it is advisable to draw it so that the edge of the future plate is just in contact with the cervical margins of the teeth. Teeth which are to be clasped must, of course, have sufficient room about them for the placing of the clasp. Single isolated teeth in the distal part of the mouth are to be included in the plate outline, its buccal edge being made continuous, and the plate carried around them, unless, as the last molars in the series, they would cause the plate to extend too far back. Then the plate abuts upon them, as in the anterior part of the mouth. At the site of the artificial teeth when they are to rest upon the gum as in the anterior part of the mouth, in cases where little resorption has occurred, the plate is only to extend up to the tooth and not under it, except in partial metal plates where a tongue of the plate should underly 404 THE RETENTION OF PLATE DENTURES. one-half of the portion of the tooth in rehition with the process as de- scribed in Chapter XIV. In the distal part of the mouth where the teeth are not so visible in the movements of the lips, the plate should extend up as high on the outside of the process as the movable tis- sues will permit, the same principles obtaining as for full dentures. It is usual, when the teeth back of the canine are to be supplied, to per- mit the first bicuspid to rest upon the gum, the buccal margin of the plate ascending from the second bicuspid. The location of the posterior margin of the plate, which determines its size, is to be settled by the number of the teeth it is to carry, and by the locations of the remaining natural teeth. If the plate is to be re- tained by atmospheric pressure, the posterior margin must be laid down in comparatively soft tissue, and the plate must be large enough to admit of properly placing the vacuum-chamber. If clasps are the retentive device, the plate area may be markedly decreased, and the reader is referred to Chapter XIY. for a discussion of this feature. Retention of Partial Lower Dentures. — These are retained in place mainly by gravity and by contact with the remaining natural teeth, usually through the instrumentality of clasps. Lower dentures cover so small a plate area that adhesion plays little part in their retention. Because of their small plate area it is necessary that every measure be taken which will assist in maintaining the plate upon its proper resting- place. This fact, together with that of the known difficulty attending the use of full lower dentures, should indicate the importance of retaining all natural teeth in the lower jaw which might be utilized to stay the plate even for a short period. A partial lower denture can be advantageously equipped with clasps in nearly every case. These are intended chiefly to maintain it in place upon its supporting base, but occasionally they must be designed to transmit some of the force of mastication to the natural teeth. Thus, for example, a denture supplying all the posterior teeth often exhibits a tendency to move backward in use, and clasps attached to the natural teeth adjoining the vacant spaces on either side should be arranged to prevent this movement. In cases of this kind where a natural second or third molar remains in the mouth, the plate abuts upon this and the backward tendency of the plate is obviated. Far greater liberty is permissible in the way of contact of the plate and clasps with the natural teeth in the lower jaw than in the upper jaw. In the former case the fact that the teeth are bathed to a greater extent in the saliva, which by reason of gravity is more plentiful about them and thus the acid products of fermentation and bacterial activity are more quickly neutralized, renders this contact less harmful to the teeth. Plate Outline for Partial Lower Dentures. — For partial lower plates the margin of the denture is to be determined according to the following method: the plate should rest upon the remaining natural teeth for the maintenance and slight additional support which is thereby gained. In the front of the mouth the plate outline is to be marked half way up THE RETENTION OF PARTIAL DENTURES. 405 the lingual surfaces of the teeth, tlie remainder of the periphery of the plate being determined as for full dentures by the location of the movable tissue. A single isolated tooth perpendicular to the ridge is to be included in the plate outline, the buccal margins being continuous and an opening made in the plate to receive the tooth. The high attachment of the tissue of the cheeks upon the buccal sur- face of the mandible makes it usually necessary to have the buccal margin of the plate relieved at this point. Experience has demon- strated that this edge of a plate made upon an unaltered cast has a tendency to bury itself in the tissue. It is, therefore, advisable in many A Diagram illustrating addition to lower cast. full lower cases and in all partial lower cases to make an addition to the cast along this line as represented in Fig. 371. Where the summit of the alveolar ridge is represented as being sharp and thin, it is advisable to relieve the plate somewhat at this point by making an addition to the cast. This permits the plate to have its point of bearing on the sides of the ridge instead of upon its summit. Fig. 371 illustrates the manner of making this alteration. CLASPS. " Clasps^ are metallic bands partially encircling the crowns of natural teeth, and serving as a means for the retention of artificial dentures. Tiie employment of the device is prompted by necessity, and not by choice. In upper plates they are employed where the vacuum-chamber is found to be insufficient to retain a denture in position, where the configuration of the vault renders the chamber inapplicable, or where the positions of the replaced teeth render the covering of the vault with a large plate unwarrantable. They are attached to partial lower den- tures to prevent displacement by the movements of the tongue, cheeks, and lips, and by the forces to which these pieces are subjected during mastication. The great advantage of the employment of clasps is an increased stability of the piece, the disadvantage is, if worn long enough, they eventually cause the loss of the crown of the tooth clasped, through chemical solution, and mechanical abrasion. The food-deposits beneath and about the clasp are the seat of lactic fermentation, so that a gradual solution of the crown by lactic acid occurs, if the clasps are not kept in an aseptic condition. ' Burchard: American Text-Book of Prosthetic Dentistry, Second Edition. 40() Till-: RF.THSTloy OF PLATE DF.STrilF.S. Not infrecjueiitly tlie teetli are so mechanically strained I)}' the force of mastication transmitted through the clasps, that the rt-tentive appa- ratus of the teeth succumbs and the teeth are dislodged. The latter is a more serious consideration than the loss of a crown; an artificial substitute for the latter may be provided and serve for clasping, but loosened teeth are the heie noir of dentistry. This danger is lessened by accuracy of adaptation of the plate, and by having the clasps of sufficient elasticity to yield to stress and diminish the strain on the teeth. Clasps properly adapted serve but to stay a plate, not to support it. The support should be derived from uniform pressure upon the soft tissues ; the clasps are an adjunct preventing displacement. The violation of this principle is responsible for many of the ills attributed to the wearing of clasps. In selecting teeth to be clasped, where a selection is possible, they should be chosen with a regard to their form, the position and condition of the tissues of the teeth, and the surrounding parts." In the upper jaw the most satisfactory tooth to afl'ord attachment for a clasp is the first molar. Its location, strength, and shape are es- pecially favorable to its use for this purpose. Next in order come the second bicuspid, second molar, and first bicuspid. The disadvantageous position of the third molar, its shortness, and irregular shape render it seldom of use for clasping, though sometimes it may be the only depend- ency and must be used. Only in extreme cases should the upper canine be clasped, and even less frequently the central and lateral incisors. These teeth are all poorly shaped for clasping, besides being weaker than the distal teeth, and in addition the displayal of a gold clasp about them is objectionable. When it is necessary to clasp the canine, the clasp should be shaped and located to appear as a cervical gold-filling in the tooth. In the lower jaw the teeth most commonly used for clasp purposes are the bicusjjids, as these frequently adjoin the vacant spaces to be supplied by the plate and are most satisfactory for this purpose. The first molar has the advantage of strength and favorable shape, but is less frequently used because its presence in the mouth either indicates a case in which the molars back of it alone are missing — a case in fact in which it is doubtful if the advantage gained by supplying these few teeth com- l)ensates for the trouble of wearing it, experience proving such plates unsatisfactory — or it may mean a case with teeth anterior to the first molar missing in which bridgework would be greatly preferred. If possible two teeth on opposite sides of the mouth should always be selected for clasping, so located that a line drawn from one to the other would pass through the centre of the plate, if it be an upper denture, and if it be a lower plate, they should be located to be of the greatest mechanical support. In difficult lower cases even a single central in- cisor may be clasped to advantage, thus serving to hold the plate on the alveolar ridge, and hence greatly aiding the patient in acquiring the use of the denture. CHAPTER XII. THE SELECTION, ARRANGEMENT, AND ARTICULATION OF ARTIFICIAL TEETH. By Charles R. TurxVer, D.D.S.,M.D. In a general way it may be said that the objects of artificial dentures are the restoration of the impaired functions and the altered facial ap- pearance which have ensued from the loss of the natural teeth. Artificial dentures should restore any lost parts of the mechanism concerned in the preparation of food for subsequent stages in its digestion, or should establish a satisfactory means for the execution of this function. If the normal voice and articulate speech have been impaired by the loss of the teeth, the dentures should restore these by providing a substitute for the lost tissues concerned in the sound production, and they should impose no impediment to the proper exercise of these functions. They should restore the lost portions of the apparatus by w'hich the expressive movements of the face are produced, and not interfere with the normal operation of these activities; and finally, they should restore the facial contours and the fixed expression of the face, and provide such sub- stitutes for the missing tissues in those portions of the mouth which are exposed to view in laughter or speech, as shall be in harmony and accord with the other features of the face. In addition, they must be con- structed with the view of utilizing to the best advantage the materials at command, and in consonance with such sound mechanical principles as will insure the efficiency of the appliance. To design a denture which shall successfully meet these requirements demands an acquaintance with the normal operation of the impaired functions, a recognition of the alterations which have been produced by the loss of the teeth, and a knowledge of the materials and principles which may be best adapted for the restoration of these functions. Furthermore, the placing of an artificial denture is to be regarded as a rational therapeutic measure, in which the requirements of each in- dividual case are recognized, and the appliaince designed and constructed to meet the indications thereof. Here follows a tabulated list of the ends to be subserved by artificial dentures, together with the factors concerned with promoting these ends. r ( 1 . Physical means of retention. A. Stability of the plate. \ 2. Correct plate outline. / 3. Correct articulation of teeth. B. Functional efRciency j 1. Form, of the teeth. ' ( 2. Arrangement. I. Food preparation f 1. Strength of teeth. ■Mr -u ■ 1 ai ■ 12. Strength of base-plate. Mechanical efficiency j 3 ^^^^^^^ relation of teeth of plate and teeth. to base-plate to resist stress. 407 408 SELECTION, ARRANGEMENT, AND ARTICULATION. TT ,. . , , ( A. Form, position ;iii(l ainmi'ciiK'iit of teeth. II. \oice and speech, j j^ p^,^^^ ^\^j 1;,,^,,,.^, eontoursof plate. III. IV. Express! \e movements of face. Facial expression. A. Positions of the teeth. H. Form and labial contours of the plate. C. Relation estal)lished l>et\veen the jaws. r Facial con- tours. H. Teeth as fea- I tin-es of the [ face. 1. 1-ips. 2. Cheeks. .3. Relation !)e- "l tween jaws. ( 1. Color." 2. Form. 3. Relative positions. 4. Gums. Positions of teeth, liuccal and labial contours of the plate. These factors will be discussed in detail from the standpoint of the principles underlying their correct adaptation to the individual ca.se. They will, however, be considered under headings somewhat different from the above, .since many of these factors are concerned with both the utilitarian and cosmetic purposes of the plate, and besides avoid- ing unnecessary repetition, the method selected seems to afford the simplest means of elucidating the subject. Fig. 372 Casts arranged upon articnlatur, with liitc-philos removed. The design of artificial dentures to effect the various purposes above outlined, includes the selection, arrangement, and articulation of the teeth, and the arrangement of the plate form and contours. The technique of denture construction using the various bases will be de- scribed in succeeding chapters. Many of the data which are to be used in the process have been ob- tained from the patient at the time the bite is taken, and it is supposed that the casts have been mounted properly upon the articulator, and that the bite-plates have been laid aside for reference. (Fig. 372.) THE SKLECTION OF ARTIFfCIAL TEETH. ' 409 THE SELECTION OF ARTIFICIAL TEETH. Suitable artificial teeth for use in the vast majority of cases which the prosthetist is called upon to treat may be obtained by a judicious selection from the stock of the manufacturers, which it has been said in Chapter III., is sufficiently comprehensive to meet most of the needs of the present day. The teeth selected will, in most instances, require some modification on the part of the dentist to fit them for the case in hand. Details of these procedures will be discussed subsequently in this chapter, and it is sufficient to say here that the alterations in the teeth are with a view of adapting them to meet the peculiarities of the individual case. The considerations which determine the choice in the selection of artificial teeth for a given case are, first, anatomical: that they shall have the appearance required of substitutes for the natural teeth, and the form demanded by their proposed functional activities; and secondly, mechanical : that they may be adapted to the base upon which they are to be mounted, and satisfy such other mechanical demands as are incident to the shape and relation of the jaws in the individual case. The physical characters of the teeth relating to their appearance are determined in edentulous cases by the consideration of temperament. This requires a diagnosis of the predominating temperamental type or combination of types existent in the individual. A careful study of the physical attributes associated with the basal temperamental types and of their combinations in the excellent tables of Dr. A. H.Thomp- son, is urged upon the student, and a thorough acquaintance with the details of the physical characters of these types is necessary. In gen- eral it may be said that, in the matter of color of the artificial teeth, the complexion of the individual and the coloring of other pigmented tissues of the body, are the guides which may be most safely used, and that, as for the dimensional characteristics of the teeth, the general physique and, in particular, the physical characteristics of the head and face, are of best service for their determination. In addition to this, something more will be needed in the way of an appreciation of form and color, which will enable the dentist to produce a harmonious result. The existence of this harmony in nature, for the most part, is the basis upon which the restorations are made. This harmony is not, how- ever, universal, for there are occasional exceptions to the principles which have long been established. There are people who have teeth apparently too small and of a color too light, but they are always the exception, and for this reason, attract an attention, which it is desired the patient with the prosthetic appliance shall avoid. From a cosmetic standpoint the end to be aimed at first is the re- establishment of the personal appearance of the individual. This is not always possible, because the data which are at hand are at best not sufficiently complete. Nor is it in every case advisable, even if it were possible, for instances occur in which the exact reproduction of the conditions which existed before the loss of the teeth would require 410 SKLECTIOS, ARRAyClEMENT, AND ARTICULATION. tlic imitation of a disfiguring defect. In such cases it may be said that the reproduction of a condition known to exist before the teeth were lost, which condition amounted to a deformity, was in poor waste, or was greatly lacking in aesthetic effect, is only justifiable when there are strong reasons for perpetuating it as a means for preserving phys- ical identity. 1. The Size and Form of the Teeth. — In the selection of teeth for an edentulous case, it is desirable to know the size and shape of the lost natural teeth for which substitutes are required. This is seldom pos- sible, except in the few cases in which the prosthetist has extracted the natural organs, in which event they should be carefully preserved for guidance in the selection of the artificial teeth. In general other means must be used for determining the size and shape of the artifi- cial teeth. Fig. 373 Full upper cast, showing the trial of artificial teeth of proper size with second molar reaching center of the maxillary tuberosity. Because of the resorption of the aheolar process and the consequent diminution in the size of the maxilla, but twenty-eight teeth are used for full artificial dentures, the third molars being omitted. If the full complement of teeth were used, they would either be too small, or if of proper size, could not be correctly arranged in relation to the process. The data which indicate the combined width of the teeth must be ob- tained from the casts of the mouth. When placed in trial upon the upper cast in that relation to the alveolar ridge which it is proposed they shall occupy, with the centrals on either side of the median line, the distal side of the second molar should rest upon the centre of the maxillary tuberosity. (Fig. 37.3.) In cases of extreme resorption of the alveolar ridge, the cast alone does not furnish all the desired in- formation, but for the most part it is usual to judge of the \yi(lth of the teeth by their relation to the length of the upper alveolar ridge. THE SIZE AM) FORM OF THE TEETH. 411 As obtained in full sets from the manufacturers, the sizes of the in- dividual teeth of a series are intended to be correctly proportioned. The "fronts," or six anterior teeth, and the "backs" are matched to have the same general proportion and may be fairly trusted for this. Cases sometimes present, however, for which the size of the "backs" may be increased to advantage, because the canine of the series comes only to about the position of the canine eminence, while the backs do not reach the tuberosity. Minor differences in the proportion of the back teeth met with in the natural series need not be imitated. A'ariations in the proportionate size of the six anterior teeth may, however, be imi- tated to great advantage, and the selection of larger centrals or canines for use in appropriate cases will often be attended with happy results. This will be mentioned later. Having decided upon the proper width for the upper teeth, their length is next to be determined, and this is largely decided by the tem- peramental indications of the patient, together with a purely physical factor, namely, the position of the high lip line. This latter landmark, it will be remembered, indicates the highest point to which the lip is ele- vated in smiling and laughter, and it is desirable to have the portion of the artificial denture which is displayed by these acts represented by teeth, unless this would make them too long. However, the correct proportion between the length of the teeth and their already determined width must be preserved to accord with the predom- inating temperamental indication of the patient, and in no case should this consideration be sacrificed. The mobility of the upper lip varies in individuals and in some it may be elevated so high as to dis- play a good deal of the adjoining gum and process. Therefore, this space cannot always be filled in with the artificial teeth, and in cases of a very high lip line, the use of gum-section teeth is sometimes justifiable. This will also be discussed later. The distance between the high lip line and the lower edge of the bite- plate which corresponds to the proposed position of the cutting edges of the anterior teeth, should be the trial length of the artificial teeth, the correct proportioning of this length to the width in accordance with the temperamental in- dication of the patient should be the final deciding factor as to this dimension. The general form of the teeth is, of course, determined by their width and length, but their appearance is also affected by the character of their outline and their surface. These are matters which are wholly determined by temperamental indications for a tabulated list of which the reader is referred to pages 259-262. in Chapter V. Except in rare instances in which mechanical advantage is obtained by a differ- ent form of tooth, as of those of protruding jaws, the surface contours and outlines of the teeth should correspond with the temperamental indications of the patient. The rounded form of the sanguine temper- ament should be used for that temperament, while the flat faced angular tooth with constricted neck should be used for the nervous 412 SELECTIOX, ARRANGEMENT, AND ARTICULATION. (see Fig. 374). The niaiiuracturcrs siii)j)ly a sufficient variety of molds for satisfactory selection, but they also provide teetli made of the shade of one temperament and of the characteristic color of another. These are made largely for commercial reasons to supply a demand which arises without a just appreciation of the harmony desirable in such matters and their use should be avoided. 2. The Color of the Teeth. — The most difficult physical characteristic of the artificial teeth to determine is their color. In edentulous cases as has already been indicated, this must be selected upon a judgment based upon other })hysical characteristics of the patient. A few remarks upon the color of the natural teeth may not be out of place. Fig. 374 fffUl fffiM Four basal temperameutal types of artificial teeth : A, bilious ; B, nervous; C, sanguine; D, lymphatic. The color of teeth is, in general, due to two things — the intrinsic color of the enamel and dentine, and their proportion and distribution in a given tooth. The intrinsic color of the enamel and dentine is in accord with that of the other pigmented tissues of the body. It harmonizes with the color of the hair, the eyes, and in particular with the color of the skin. This latter is of the greatest importance in determining the color of teeth. Ivy^ has said that the complexion is of great impor- tance in deciding the color of artificial teeth. Joseph Head^ has stated his belief that the fundamental color of the skin over parts of the body protected from the sun, and that of the teeth is the same, and that if the pink element in the former due to the presence of the blood were removed by pressure upon the part, the color of the .skin thus observed should be the fundamental color of the artificial teeth. The author is not wholly prepared to accept this view, but has observed instances in which it was fairly accurate. The state of organization of the teeth has been supposed to partly account for the color. It is certain that the greater translucency of American System of Dentistry. \^ol. ii., p. 1034 In private conversation. THE COLOR OF THE TEETH. 413 the enamel in the nervous temperament is a result of its \\'\^\\ state of organization, and accounts in part for the blue cutting edge character- istic of these teeth, while the opaque enamel of the teeth of the lym- phatic temperament is commonl}- observed to be of poorer structure. The proportion between the enamel and dentine also is related to the color of the teeth. In the typical lymphatic teeth, for instance, the opacity of the enamel and the fact that it is generously backed up with dentine partly account for their color, while in the thin teeth of the nervous temperament, the enamel plates at the cutting edges of the incisors enclose little or no dentine, permit the light to be carried through, and appear blue at this point. Furthermore, the commonly observed yellow color at the neck of incisor teeth is due to the thin- ness of the enamel wdiich permits the yellow color characteristic of dentine to show through. As age advances there is a deepening in the shade of the tooth which is due to a molecular change in its tissues, and teeth are a shade or so darker in middle or late life than they were in early years. The fact has been brought out by E. A. Royce^ after a careful exami- nation of a large number of natural teeth that the individual members of any given natural set vary much in shade. He says, "In our study of this subject we must have some standard by which to measure the shade of each tooth, and for this standard the upper central incisors of the denture under consideration were always taken. Some means being necessary to convey to you the diflferent shades of the teeth in the mouth, I selected 0, or zero, to represent the shade of the central in- cisors, using numerals to express the other shades — the higher the numeral the darker being the shade." * * "The upper central incisors are the lightest, the laterals are darker, and the canines are darker still. The first bicuspids are generally lighter than the canines, and the sec- ond bicuspids lighter than the first. The first molars generally vary but a shade or so from the second bicuspids." The general color of the artificial teeth should be selected to accord with the temperamental indication of the patient according to the tables on pages 259-262, This should be modified in shade by the age of the patient, an individual past middle life requiring teeth usually as much as tw^o shades (of the manufacturer's shade-guide) darker than at an early period in life. For reasons of convenience the shade of the teeth should be selected at the time the bite is taken. The trial shade should be actually tested under the lips, because, from the shadow cast by the latter, the teeth will appear lighter in the mouth. The same shade-guide used in the patient's mouth should be used, if possible, at the dental depot, because there are many slight variations in teeth said to be of the same commercial shade which are due to details of molding and burning, and these differences exist in the sample teeth of the shade-guide. A happy effect may usually be produced by the selection of the an- terior teeth from different sets, thus breaking up the unnatural uniform- ly The Dental Review , Vol. xv., pp. 301 and 934 414 SELECTION, ARRANGEMENT, AND ARTICULATION. ity in color observed in the stock sets. Slightly darker canines usually give a very natural effect to the teeth, and frequently the use of slightly bluer lateral incisors, and bluer bicuspids will add to this. The differ- ent shades recommended by Royce may also be obtained by the use of oil stains, as described on page 429. We next come to discuss the selection of teeth in order that they may fulfill the mechanical recjuirements imposed by the base used, the re- lation of the jaws, and the amount of resorption of the process. 3. The Base. — Artificial teeth are constructed with a means of attach- ment and a general form suitable to the base upon which they are to be mounted. Vulcanite and celluloid teeth are similar and may be used interchangeably upon the two bases. Plate teeth are suitable for Fig. 375 A Diagram showing the ideal relation of an artificial incisor to the alveolar process. B Diagram showing the ideal relation of an artificial molar to the alveolar process. crown and bridge or metal plate work. In England and on the Conti- nent they are used to some extent for vulcanite work, for which purpose they are made wuth long pins which are bent into a hook for attachment to the base. For cast dentures, where the metal is cast directly about the teeth, vulcanite teeth are used and they are occasionally useful in continuous-gum dentures, although the teeth made especially for this work are the usual indication. 4. The Amount of Resorption of the Alveolar Process and the Distance Between Jaws and their Relation. — In the mounting of the artificial teeth, other things being equal, it is desirable to have their occlusal ends occupy the position with relation to the alveolar ridge which was char- acteristic of their natural predecessors. The incisors, canines, bicus- pids, and molars would be placed, therefore, over the alveolar ridge, in the upper jaw, and slightly to its buccal and labial sides. (Fig. 375.) In the lower jaw the incisors and canines would be slightly to the labial side and over the ridge, while the line of teeth then crosses the ridge, the last molars being somewhat to the lingual side of its summit. This ideal method of placing is by no means always pos- sible, because of the state of resorption of the process and the rela- tion of the jaws. The proper placing of the teeth will be discussed later in this chapter, but inasmuch as they may be selected of surh form THE AMOUNT OF RESORPTION. 415 as to greatly facilitate this, the mechanical demands upon them must be known at the time they are selected, and the choice made to fur- ther this end. There are also mechanical requirements incident to the strain upon the tooth in use, which may be furthered by judicious selection. The labial surface of the fronts and the buccal and occlusal surfaces of the backs are fashioned to meet anatomical demands; other portions of the teeth are designed, as was seen in Chapter III., to meet the mechan- ical demands above referred to. As plain rubber teeth are the ones in which this provision is best brought out, they will be used to illustrate the principles. The anterior teeth have the headed pin which affords attachment to the base-plate, a pin guard, which makes the division between "the bite" and the "shut," and also what is technically called "the ridge lap." Fig. 376 Diagrammatic drawing showing section through upper cast and lower bite-plate, with artifi- cial incisors properly filling in the space. The dotted Une indicates the contom- of the upper bite-plate. Other things being equal, it is desirable in full sets to have the pins near the alveolar ridge. The "ridge lap" is intended to be in relation with the alveolar ridge, and "the bite" makes up the other part of the length of the tooth. Long bite teeth, it must be remembered, ase not the strongest type of teeth because of the leverage upon them, the pin being the fulcrum, the length of "the bite" part of the tooth repre- senting the power arm. Long bite teeth are, therefore, not recommended for use in cases demanding great strength in W'hich other types will answer the requirement. In most full cases, however, they may be used with reasonable assurance of sufficient strength. The upper bite-plate having been removed, the space between the an- terior edge of the lower bite-plate and the alveolar ridge is to be filled in with "the bite" of the teeth selected, the remainder of the length of the tooth has to be made up in ridge lap. (Fig. 376.) Thus when there has been little resorption of the process and the jaws are some distance 416 SELECTION, ARRANGEMENT, AND ARTICULATION. apart, the teeth may rest almost upon tlie v\(\gii, and a tooth with a short ridge lap is selected. (Fig. 377.) Where the jaws are close to- gether, however, a tooth with long ridge lap must be used, and, of course, of short bite, since in teeth of a given length the ridge lap and bite are usually inversely related. (Fig. 378.) Where there has been much resorption of the process and the teeth are to be mounted almost Fig. 3' Fig. 378 Use of short ridge lap with small amount of resorption of process. Use of long ridge lap with a short distance between the jaws. upon the ridge, a short ridge lap is indicated. (Fig. 379.) To avoid the use of a long bite tooth in a given case, a tooth with a long shut may be used, but it must be remembered that the plate must not be made too thick at the portions about the pins, as it may interfere with speech. (Fig. 380.) Use of short ridge-lap tooth with much resorp- tion of process, mounting tooth directly over ridge. Use of long shut tooth, with short ridge lap, where there is much space between jaws and little resorption of the process. Where plain teeth are to be mounted directly upon the gum, as in all temporary dentures, a short ridge lap must be used. When the antero-posterior relation of the jaws is abnormal, as in pro- truding upper or lower jaw, this condition may be accommodated in the teeth selected. Thus, in a protruding upper jaw, a short ridge lap is indicated for the upper teeth, while in a protruding lower jaw, teeth with a long ridge lap may be used advantageously in most instances. THE AMOUNT OF RESORPTION. 417 (Fig. 381.) In a protruding upper jaw, too, the use of teeth with a flat face provides a means of making the protusion less apparent, while on the other hand, when the lower jaw is anteriorly placed, so-called "bow-faced" teeth will lessen the apparent protrusion of the lower jaw and often permit a better arrangement of the upper and lower incisors. (Fig. 382.) Fig. 381 Use of flat-faced tooth for protruding upper jaw. Use of bow-faced tooth for protruding lower jaw. In the bicuspid and molar region, the adjustment of these teeth to the alveolar ridge may be made with greater ease. The use of teeth, as long as the distance betw^een the ridges will permit of their proper placement, is indicated. This brings the tooth closely in relation with the ridge, thereby lessening the amount of base used, and, if the amount of the tooth above the pins is as great as possible, this is an added ad- FiG. 383 Fig. 384 Kitting of molar teeth : filling in space between the ridges. Use of saddle-back teeth in case with short distance between ridges. vantage, for in the articulation of these teeth it permits the making of the sulci deeper by grinding. (Fig. 383.) Cases will be met with, how- ever, where the distance between the two alveolar ridges is so short that recourse must be had to so-called "saddle back" teeth, although this is more especially necessary in partial dentures. (Fig. 384.) The use of countersunk pin teeth is only possible when the distance between the jaws is sufficient to permit them to be placed almost di- 27 418 SELECTION, ARRANGEMENT, AND ARTICULATION. rectly over the ri(l' well for the purpose, and a small selection, consisting of sepia, ivory black, iProm paper on "Artistic Staining of Artificial Teeth," in Ohio Denial Journal, by Dr. George H. Wilson. 430 SELECTION, ARRANGEMENT, AND ARTICULATION. rose pompadour (gum color), ivory yellow, brown yellow, celestial blue, and relief white will be sufficient with which to form almost any shade required in the imitations of the usual discolorations of the teeth as met with in the natural orijans. The implements recjuired for the mixin;niiist the vault of the base-plate and the test made as before, — when, if the base-plate sets firmly at every point and the peripheral line is correct, the operator may be assured the finished denture will adhere after it has been in the mouth a short time. A satisfactory base-plate having been obtained, wax should be formed over the alveolar ridge of the base-plate to restore the contour of the gum and the outline of the teeth, by repeatedly trying in the mouth and adding to or taking away the wax until the desired effect is produced. (Fig. 437.) The objects sought are: (1) that the hps and cheeks be Fig. 437 The base-plate with wax added used for the bite-plate. held in as near the normal condition as possible; (2) that the wax be built the full length of the lip, thus indicating the incisal line of the teeth; (3) that the occlusal edge of the wax be carved to receive a slight im- print of the cusps and incisive edges of all the lower teeth, and (4) that a line be drawn in the wax to indicate the median line of the face and also the high lip line at the highest point at which the patient can elevate the lip by muscular action. Sufficiently well-softened pure beeswax should be placed in a lower tray and a perfect impression of the occlusal and incisive surface and at least half the length of the lower teeth be taken. This impres- sion may be filled with regular dental plaster or better, Spence com- pound, because of its great hardness. Teeth of suitable size, form and color (see Chapter XII.), should now be selected and the patient dismissed. The base-plate with its wax occlusion model and cast of the lower teeth should be adjusted upon an anatomical articulator. (Fig. 438.) If there are heavy undercuts in the base-plate, they should be filled in with soft wax, but if the undercuts are not suflacient to interfere with its easy removal from the supporting cast, no wax will be needed. The cast of the lower teeth should be adjusted to the imprints of the teeth in the occlusal edge of the wax model and sufficient regular dental plas- ter mixed to fill the base-plate and attach all to the articulator. At- tention should be given to placing the case squarely upon the artic- ulator, the occlusal surface of the teeth parallel with the low^er bow, and 486 VULCAXlZh'D nriiHh-R AS A BASE. thv median line at the incisive edge of the teeth four inches from the condyles. The Snow face-bow in connection witli the Snow ar- ticulator furnished a good means for accurately adjusting the case. Fig. 4o!) siiows wax model removed. Arranging the Teeth. A section of the wax model extending from the median line and tiie iiigh lip line hackward to the first molar, upon Casts mounted upon artioilator : wax removeri for setting: np of teeth on one side. Fi... v:<.\ Base-plate on articulator with wax removed. one side only, should be removed. (Fig. 438.) Arrange the teeth in this space so as to restore the contour of the removed wax, and at the same time follow the instructions in Chapter XII. Wax should be melted about the lingual and cervical portion of the teeth to securely fasten them. The remaining segment of wax representing the molars should now be removed and the teeth secured in their position. The PROVING ANTAGONTZATION, CONTOUR, AND EXPRESSION. 487 opposite side is treated in like iiiauner. The contour of the gum is restored by aid of a wax spatula and the addition of wax where needed. The wax can be nicely smoothed by passing it through the Bunsen flame. The teeth may be cleaned of wax by using a cloth wet with chloroform. The case should now be taken from the articulator. If any wax has been placed upon the maxillary surface it must be removed. Proving the Antagonization, Contour, and Expression. — The case should be placed under the faucet to chill the wax and moisten the maxillary surface; it is then placed in the mouth. The occlusion must first be considered. Examine the molars and bicuspids w^ith a mouth mirror and use a thin spatula blade to determine if there is equal pressure upon both sides. The antagonization can be inspected by requesting the patient to carefullv close the teeth in various positions. The in- Teeth set up and strings in place. cisors are studied by taking a position in front of the patient, and re- questing the patient to raise the upper lip, thus observing the location of the median line and the inclination of each of the six anterior teeth. (See Fig. 457.) The contour is proven by studying the face. Standing at the side of the patient observe the profile and see that the upper lip is in harmonious relation to the lower, also that the lip has its normal fulness throughout its length. (See Figs. 455 and 456.) The operator should again take a position in front of the patient. Attention should be given to the incisive fossae and canine eminences, and especial atten- tion be given to the triangles formed by the wings of the nose and cheeks, which are influenced by the apices of the canines. If it is desirable, the cheeks can be contoured to a limited extent by the addition of wax contours (plumpers) over the molars. Having obtained the desired re- sults in a state of rest, the operator should engage the patient in conver- sation and observe the effect. The case is now cleansed of the secre- 488 VULCAyiZED RUBBER AS A BASE. tions of the month hv liohUii*; iiiKk-r the faucet and (h'viny; wltli a cloth. If any of the teetli are k>o,se, they must be securely fastened, and the wax smoothed witliout chaiiLriiiii; its form. The double vulcanization method is especially adapted for restoring the contour and expression, because this work is done in wax, when the wax form can l)e easily and ])erfectly reproduced in vulcanite. Preparing the Case for Flasking — Strings are used for outlining the festoons and periphery of the gum. The object of the festooning string at the cervical portion of the teetli is to give the proper thickness to the margin of the gum. The string used for this purpose is waxed dental floss, twisted very hard, doubled, and twisted again. In doub- ling, the loop will show the direction in which it should be twisted the second time. Wax the string well with softened wax and apply it by grasping the left heel of the plate between the fingers and thumb of the left hand, with the occlusal surfaces of the teeth upward; place one end of the string at the distal surface of the second molar, pressing it gently into the wax; outline the margin of the gum, using the wax spatula to carry the string well into the interproximal spaces. The peripheral string should be well-waxed wrapping twine, placed at the outer edge of the wax, and secured in place by melted wax made smooth with a hot spatula. (See Figs. 440, 449, and 450.) The next step is to cover the buccal and labial surfaces with a strip of No. 60 tin foil. Instructions are necessary in applying the tin over the strings. The No. 3 instrument of the Evans set of carvers (Fig. 40f)), is especially adapted for adjusting the tin foil. The strip of foil is placed over the wax and teeth and pressed as closely as possible into position with the fingers. Hold the work in the left hand, seize the ivory-pointed instrument by the hand grasp, rest the thumb upon the occlusal surface of the second molar and burnish the tin closely to the tooth and against the festoon string. Continue this operation with all the teeth. With a sharp chisel cut away the excess of tin upon the teeth to within one-sixteenth of an inch of the festoon string. (See Fig. 441.) After readjusting the tin about the teeth, the metal must be burnished over the string to give the desired thickness of the gum and the contour of the festoon. This is done hy holding the plate and burnisher in the same manner as before. The instrument must extend one-sixteenth of an inch behind the string and at the same time must rest upon the body of the tooth while pressing the tin down over the festoon string. By this means a proper thickness and contour is given the margin of the gum, without forming an unnatural beaded edge. After all the teeth have been thus treated, the position of the plate should be reversed in the left hand, so that the thumb of the right hand may rest upon the periphery of the plate while burnishing the tin from the festoons towards the edge of the plate. With a pair of sharp curved scissors trim the tin flush with the peripheral string, but do not permit it to overlap the vulcanite base-plate. (See Fig. 441 .) The case is now ready for tinning the lingual surface. Use No. 60 foil and if the vault is a high one, slit the tin from the middle of one side to the centre. Place the inner end of the slit over the middle of the vault, and one edge FLASKING. 489 of the slit along the median ridge to the incisor teeth; press the side of the foil gently against the wax and teeth ; press the other half of the tin in the same manner into position, permitting the slit portion to overlap the- first half. With sharp scissors trim the tin nearly down to the Fig. 441 Tin-foil applied to labial and buccal portions of denture. teeth. Remove the foil and place it upon a plaster or metal cast^ having well-defined rugae and burnish the rugse into the foil. Remove the foil, turn it over and fill the impressions of the rugse with wax, also smear the remainder of this surface with a thin layer of wax; now re- Fig. 442 ^^^pH^HKiVL^^^ ^'Nj i^^l ■■^^y''"iiT "" ^^^^^^H H^HjIil ^^^^JESMRySfc ■ % ' ^^^^B^wfc . ^^ ^IM| ^n^^_ ^H HBSHhi Tin-foil applied to lingual surface. Showing rugae and carving of the lingual surfaces of the teeth. place the waxed surface against the vault of the plate and nicely ad- just wath the fingers. The tin must be securely burnished against the teeth. The lingual contour of the teeth - is reproduced by burnishing their forms in the tin. (See Fig. 442.) 1 A suggestion of Dr. A. DeWitt Gritman of the University of Pennsylvania. 2 In the International Dental Journal for August, 1905, Dr. William M. Fine describes and beau- tifuUv illustrates his method of carving the lingual contour of the teeth. 490 VULCANIZED RVIIUER AS A BASE. Flasking. A flask with a narrow rim is imperative. The Star flask will do \(Ty well, hut one made by the Cleveland Dental ]\Ianufaetur- \\\)l Company, railed the Wilson flask, will better serve the purpose. Fig. 443 Cast and denture invested in Wilson-flask. This fla.sk is desiijned to l)e used with the Donham elamp. (See Figs. 4()8 and 4G4.) The maxillary surface of the case, having been cleansed of wax, is filled with Spence's compound, which forms a cast upon which the denture is vulcanized. This vulcanization cast should be not less than Fig. 444 Counter portion of Wilson lla.sk. one-fourth of an inch thick at the thinnest portion of the vault, and should not overlap the tin facing. In one hour this cast will be suflS- ciently hard to place in the first section of the flask with regular dental plaster. (See Fig. 443.) The peripheral string will be a great aid SKPARATfNd THE FLASK. 491 in outlining the denture in the investment plaster. After the plaster has set it is coated with a separating fluid, and then held under the I'aueet so as to moisten the tin foil and thus facilitate the flowing of plaster into the interproximal si)aces at the time the flasking is com- Invested case: flask opened and wax removed ready for packing. Fig. 446 Lower portion of flask, showing invested base-plate and outlet vents for the excess of vulcanite. pleted. The flask should stand about twenty-five minutes and then be placed over the stove in a stew-pan of cold water to be heated up as before described. Separating the Flask.— When the heat of the water indicates the 492 VULCANIZED RUBBER AS A BASE. time of opening, the flask is grasped with a cloth holder in the left hand and separated h\ the point of a knife blade or wax spatnla inserted at the heel of tlie flask. The instrument should be guarded by the thumb and finger of the right hand to avoid the possil)ihty of marring the ease. The strings and as mueii of the wax as possibk' are removed with the spatula, when the remainder is removed by pouring boiling water upon it; then with a cloth the vulcanite base-plate and tin lining are w'iped dry. (Figs. 445 and 446.) The excess space is cut with small gates (see Fig. 446), and the separated flask is placed over the sheet iron to warm as pre\'iously described. Packing. — Sufficient Gilbert Walker's granular gum or pink rubber is cut into strips to form a layer of one thickness over the tinnerl surface. First pack a narrow strip of red rubber about the pins (Fig. 447, C), and small square or triangular pieces of granular gum between the cervical portions of the teeth. The strips of granular gum are then placed over the labial and buccal surfaces of the matrix with the fingers and wax spatula so that no space remains through which the red rubber can escape. The strip of red rubber about the pins Invested case partially packed. A, half circle of granular gum; B, thin granular gum; C. strip of red rubber about the pins. should be pressed down with a wax spatula to form a symmetrical outline. Apiece forming half a circle of granular gum is then placed over the anterior portion of the lingual surface (Fig. 447, A), and with the wax spatula the circular edge is joined to the red rubber about the pins of the teeth. Apiece sufficiently large when stretched to half its thickness (Fig. 447, B),is then applied over the remaining portion of the lingual surface, and its edges are united to the contiguous rubber. Strips of red rubber are then placed over the teeth to nearly but not quite FINISHING. 493 fill the mold. A separating cloth of closely woven cotton, or the cloth removed from the rubber after the sizing has been washed out, is satu- rated with water and placed over the rubber in the mold — when the two sections of the flask are placed together. If the packing has been ex- peditiously done and the rubber is sufficiently warm, it is placed in the flask press and gentle pressure applied. The flowing of the rubber should be followed every ten seconds with a partial turning of the screw until the flask is closed. The flask is then removed from the press and separated. If there is not an excess of rubber, the cloth will easily separate from the rubber, but should there be strong adhesion, satura- tion of the cloth with water will facilitate its removal. If the plaster has become nearly cold while packing, the flask should be placed in boiling water for five minutes before it is pressed. Vulcanizing.^ — The case is vulcanized in the same manner as the base- plate. Preference is given to low temperature and long time, 300° F. for three hours from the time of applying the heat. It should not be taken from the flask until it is cold. Finishing. — After washing to remove the loose plaster, the tin can be easily stripped oft', and the excess vulcanite filed from the periphery of the denture. A sharp chisel should be used to trim about the labial and buccal surfaces of the teeth, but the lingual surface should be trimmed v.'ith a scraper. The file marks about the periphery of the plate should Fig. 448 Completed denture for case A, be removed with fine sandpaper. The labial, buccal and lingual sur- faces are buffed with felt wdieels and cones carrying pulverized pumice and water. The spaces between the teeth are best reached with, a stiff bristle brush wheel, using wet pumice. All the surfaces including the maxillary, are glossed by using a rapidly revolving soft brush wdieel and whiting wet wdth alcohol or water. Case A.— Figs. 438, 439, 440, 441, 442, 448. Constructed for a lady about fifty-five years of age, and of the bilious temperament modified by the nervous. Teeth, S. S. W. natural forms, mold num- ber 227, shade number 47. The illustration will show that nearly all the teeth were ground for the purpose of occlusion or to represent age. and the laterals to represent a personal peculiarity. When the finished 41)4 VULCAMZKD RUBBER AS A BASE. (k'liture was placed in the mouth it was observed that the centrals and canines were too much curved in the long axis of the teeth. The labial surface of these teeth was ground to give them the square angular Flo. 449 Full u|)|)er ami lower cases waxed up, shr>winK larRO contours and method of using waxed string. Fig. 450 Completed dentures for case B. Note large contoms on buccal surface of upper plate. form indicative of the bilious temperament, thus completing the har- mony in form. These ground surfaces were then given a dull polish as described in the paragraph "Grinding and Polishing" imder the ILLUSTRATIVE CASES. 495 heading — " General Technique Instruction." These photographs were made before the grinding of the labial surfaces. Attention is called to the canine eminences and the incisive and canine fossae. Fia. 451 Lingual surfat-e of upper plate shown in Fig. 450. Fig. 4.52 Occlusal surface of lower plate shown in Fig. 450. Fig. 4.53 Maxillary surface of plate showu in Fig. -150. Case B. — Patient about forty-five years of age, of a strong sanguine temperament, with a nervous temperament modification. The patient had W'Orn artificial dentures for fifteen years. Teeth used, S. S. W. 496 VULCANIZED RUBBER AS A BASE. natural forms, mold number 202, shade number 40. Attention is called to the extensive contours; also the general rounded outline indicative of a sanguine temperament. (Figs. 449 to 457 inclusive.) Case C. — Lady thirty to thirty-five years of age. Marked nervous temj)('rani(Mit with sanguine temperament modification. Teeth S. S. W. natural forms, mold number 215, shade mmiber 31. (Fig. 458.) Manilihuliir surfarc of i)liitL' as shown in Fig. 450. Fig. 455 Fni. 4.')0 Profile view of patient described as case B Before the insertion of the denture. ew of patient with denture in place. Case D. — Lady about fifty years of age. Sanguo-lymphatic tempera- ment. Teeth, S. S. W. natural forms, mold number 223, shade number 40. Gum restoration about the thickness of a sheet of base-plate wax. Onlv suitable for construction bv single vulcanization method. (Fig. 459.) ILLUSTRATIVE CASES. 497 Three-quarter view of patient with denture in place. Fig. 458 Completed denture for case C. Fig. 459 Denture for case D. 32 498 VULCANIZED RUBBER AS A BASE. FULL UPPER: SINGLE VULCANIZATION METHOD. A Spence compound cast is made from a plaster impression, upon which is formed a base-plate of pink paraffin and wax, modelling com- pound, or any of the hard gum base-plate materials upon the market. The wax is arranged in the same manner as upon the \ulcanite base- })late for obtaining the occlusion models. (Chapter X.) The cast and occlusion models are attached to the articulator. The teeth are arranged, the gum contoured, and the case tried in the mouth. It is obvious that it will be more difficult to prove the occlusion, con- tour and expression, because of the imperfectly fitting trial plate. (Fig. 440.) The denture is then cleansed of the secretions of the mouth, dried, and the tin foil adjusted upon the wax. With a hot spatula and a little wax the case is sealed to the cast. The cast is then well saturated with water, when it can be easily removed from the articu- lator by inserting the end of a knife blade between the cast and attach- ing plaster. It is then ready for flasking, as in the previous case. The labial and buccal surfaces are packed with granular gum cr pink rubber as in the other method, but the lingual surface is packed entirely with red rubber. Care should be exercised to properly distril)ute the rubber about the mold and to use no excess for the first closing of the flask. After removing the separating cloth, sufficient rubber is added to fill the deficiencies and give a slight excess when the flask is closed. It is then vulcanized and finished as in the former case. FULL UPPER: SINGLE VULCANIZATION SUB-METHOD. . The procedure is the same as the preceding method, up to the pre- paring of the case for flasking. The strings and tin foil are not applied, but the wax form is nicely smoothed in the Bunsen flame or by use of soft cloth or cotton and some solvent of wax, — chloroform is the least objectionable. The case is then flasked and the rubber packed against all portions of the plaster, except the cast, which should always be finished either with the thin tin and soap, or silex. Owing to the rough surface produced by contact with the plaster, it will be necessary to file, scrape and sandpaper the entire labial, buccal and lingual surfaces before they are ready for polishing, thus more than offsetting the time saved by not applying the strings and heavy foil. The surface pro- duced is not as hard and dense as that obtained from contact with the metal. FULL LOWER: DOUBLE VULCANIZATION METHOD. The only difference in method of constructing the lower and upper dentures is in the application of the peripheral string, which in the lower should completely encircle the base-plate; while in the upper the distal edge is not outlined from heel to heel. GENERAL TECHNIQUE INSTRUCTIONS. 499 FULL LOWER: SINGLE VULCANIZATION METHOD. The one detail in the construction of this lower denture, which differs from that of the upper one, is the necessity for addins; a 16-gauge soft ir(Mi wire stiftener over the wax base-plate, to give rigidity in securing the wax occlusion models, and later while proving the occlusion by try- ing in the mouth. The wire is removed from the flask with the wax. See Chapter X. for other methods of taking the bite. PARTIAL CASES. These are usually made by the single vulcanization or sub-method. They differ from a full case in the manner of flasking and packing, which will be explained under the heading "Flasking." GENERAL TECHNIQUE INSTRUCTIONS. Flasking. — Rule I. — All full cases and partials with gum restora- tions should be flasked so as to separate at the periphery of the wax. The "Wilson or Star flask is best adapted to this class of work. If the Star flask is used, the case must first be imbedded in the narrow rim portion of the flask. Rule II. — All partial dentures without gum restorations, and most repairs must be so imbedded in the first section of the flask, that the teeth are held securely in apposition with the cast, when the flask is opened. The Star flask, using the wide rim flrst, or any of the various forms of flasks of the Whitney type, are best for these cases. Under this rule the cast and denture are imbedded in the encasing plaster of the first section of the flask, the wax only being exposed. The plaster in the first section must be so shaped that the two portions of the flask can be easily separated. In comiection with this rule, attention is called to a class of repair cases that perplex the no^-ice; namely, those having an extensive frac- ture upon both the lingual and buccal or labial surfaces. This diffi- culty in flasking is overcome by attaching one or more shafts of wax, one-fourth inch in diameter, at suitable locations over the fracture upon the labial or buccal surface. The shaft of wax must be sufficiently long to extend through the encasing plaster. ^Yhen the plaster has hardened, a portion of the wax shaft and the surrounding plaster is cut away to form a cone-shaped depression, which will be filled in with the plaster in the top section of the flask. These wax shafts will form openings through which the rubber can be packed, and which are to be filled with rubber. After vulcanization the shaft of rubber may be re- moved with a mechanical saw. (See Figs. 515 and 516.) The salts NaCl and KjSOi should never be used to hasten the setting of the plaster in the flask, as they weaken it and make the warp- ing of the plate during the closure of the flask more liable. Packing. — Wlien the case is prepared for packing as described in the double vulcanization process, some operators prefer placing the cold 500 VULCANIZED RUBBER AS A BASE. rubber in the flask, because it is firmer and easier to handle, and will soon become warm and plialile from the heat of the flask; other oper- ators j)refer warmino- tlu> rubber as well as the flask, and it is better for all students to do so until they have gained dexterity and confidence in their ability to quickly pack the case before the flask becomes too cold. A convenient way to warm the rubber is to heat in hot water a piece of soap-stone (a small-sized foot warmer), and after the rubber is cut into suitable strips, it is placed upon the stone. By this means the stone does not become too hot, and holds the heat well. Another device is a porcelain dish upon which the rubber is placed and is kept warm by placing the dish upon a receptacle containing hot water. A specially constructed warming-oven as described by Drs. Essig and Evans in the former editions of this book, serves an excellent purpose. (See Fig. 47 1 .) It is very essential in packing a case that the rubber shall be evenly distributed through the mold so that in closing the flask there W'ill be no undue pressure at any point, thus avoiding a warped denture or elongated teeth. Vulcanizing. — For convenience, cleanliness and uniformity of results, dental vulcanization is accomplished in a specially constructed machine, called a vulcanizer (see Figs. 472-481). which is a steam-tight boiler. The operation can be accomplished in a sand, glycerine or oil bath, but this is not desirable for obvious reasons. The time of vulcanizing will be influenced by both temperature and the surrounding medium. Vul- canization begins at 248° F., but at that temperature it would require many hours; at 280° F., it will require 5^ hours; at 300° F., 2.} hours; and at 320° F., fifty minutes. The medium surrounding the flask in the vulcanizer must be either water, steam or air. The time re- quired to produce the same result will be in inverse ratio to the conducting power of the medium; w^ater being the best conductor of heat will require the least time, steam second, and air being the poorest conductor, wull require the longest time.^ It is always best to permit the vulcanizer to stand until it is cold before opening, but wdien it is necessary to expedite matters, the steam may be blown off, the vul- canizer opened, and the flask be set in a pan of cold water. This last procedure would be condemned by some, but ow ing to the poor con- ducting and radiating property of plaster, it is doubtful if this method of cooling ever accounts for fractured teeth. Under no condition how- ever should the flask be opened until the plaster in the centre of the flask is cooled to blood heat, which will be sometiine after the flask is cold if placed in water. Finishing. — The filing is done with a rather coarse file especially made for this work. See Fig. 483. A seven or eight inch half-round file having an oval and a flat side, a coarse and a fine end, is a very desirable instrument. The cutting is done with a push stroke. A' iThe wTiter's custom is to have the flask submerged in water and usesuflBcient heat to reach the required vulcanizing point, (300° F.), in about thirty minutes, and to maintain that temperature for two and one-half hours; but when time is an object, to take thirty minutes to attain the 320° F., and maintain that temperature for fifty minutes. INSTRUMENTS USED IN VULCANITE WORK, 50I round or rat-tail file is also very useful. For scraping and trimming the plate, short shank Kingsley scrapers numbers 1 and 5, and an Ivory scraper number 9, and a Wilson or a Spaulding chisel, make a very complete equipment. See Figs. 484-487. These instruments should be kept very sharp by grinding and honing. The sandpapering is done with a two inch square piece of number o- or 1, followed with number 00 paper. The paper is cut into squares by placing the sand side down over a crevice and cutting with a knife. Various substances are used for buffing wheels and cones, but felt has proven the most satisfactory. The wheels have either a square or a knife-edge. The knife-edge soon becomes so blunt that it fails to do the work for which it was designed. This difficulty can be overcome by using a worn-out half-round file ground to a sharp edge, held by an end grasped in each hand, when the felt wheel can be quickly and safely turned to shape. The felt carries the abradant, of which pumice is the best, and must be kept well moistened; otherwise the vulcanite will be heated and possibly warped. The wheel should be run at a moderate speed, and the hands holding the work firmly against the felt carrier must be kept in constant motion, so that symmetrically curved surfaces may be formed, and not ridges and grooves, as the result of too long contin- ued contact at any one place. A large and small size of each of the forms of the buffers used should be provided. A stiff converging bristle brush-wheel, in which the bristles have been worn or cut away one-half, will be more effective for buffing between the teeth. Striking the work against the moderately revolving brush will cause the bristles to reach the innermost spaces. As before stated, the glossing is done by using a rapidly revolving soft bristle brush-wheel and whiting wet with alcohol or water. Very good results can be obtained by rubbing the surface of the vulcanite in the hand with dry plaster of Paris. Grinding and Polishing the Teeth — It will often be necessary to grind the exposed surfaces of the teeth, which should again be polished. The abraded surface should be well sandpapered. The paper is held against the ball of the thumb which will conform the paper to the sur- face of the teeth and greatly facilitate the removal of the facets formed by the grind-stone. It is then buffed with the felt wheel and pumice; or a specially prepared rubber wheel can be obtained for this work. Notching the Cervical End of the Teeth. — In some cases where there is gum restoration, it is desirable not to have the rubber overlap the cervical end of a tooth, and in these cases cruciform notches should be cut in the cervical end with a small knife-edged stone, into which the rubber will flow and harden and aid in supporting the tooth. INSTRUMENTS AND APPLIANCES USED IN VULCANITE WORK. Wax Spatulas. — The instruments shown in Figs. 460 and 461, are some very useful forms. The Evans 'numbers 2 and 3 and one of the others, will meet every requirement. 502 VULCAMZED RUBBER AS A BASE. Fig.. 461 b '-J Fig. 400 ^y Evans' carving tools. Wax spatulas: Dorance, Burchard, Gritman. FLASKS. 503 Flasks. — There are a great many varieties of vulcanite flasks upon the market. They are made of iron and brass. Iron has the greater affinity for oxygen and sulphur in vulcanizing and so is not as desir- able as brass for this purpose. The brass flasks are the more easily Fig. 462 Star flask. cleaned. Each time they are used, they should be thoroughly cleaned with a stiff brush and sapolio. The Star flask is one of the oldest forms, and being reversible, is prob- ably adapted to more cases than any other. (Fig. 462.) The Wilson flask is characterized by a very narrow rim upon the lower section, with 504 VULCANIZED RUBBER AS A BASE. a correspondingly wide rim in the npper section. It is designed to l)e used for full cases only and with the Donham spring clamp. (Fig. 463.) The Donham flask is shown in the Donham spring clamp. (Fig, 464.) Fig. 463 Wilson flask. Fig. 464 Fig. 465 Donham clamp and flask. Whitney flask. The Whitney flask is very much used. There are two sizes, the lar- ger being five-sixteenths of an inch deeper than the smaller. Fig. 405 shows the regular size with springs upon the bolts to aid in closino- the flask. ^ FLASKS. 505 The box flask is designed for interdental splints and any extra large pieces of vulcanite. It is made in two sizes, one as large as can be used in a two-flask ^•ulcanizer and the other for the three-flask vul- canizer. (Fig. 466.) Fig. 466 Box flask. Fig. 468 Edson flask S. S. W. No. 1 press. The Edson flask is especially designed for the Edson julcanizer, and may be used for both vulcanite and celluloid. (Fig. 46 / .) 506 VULCANIZED RUBBER AS A BASE. Flask Presses. — The flask press is an indispensable appliance in a well-ecjiiipped laboratory (P'igs. 468, 469, 470), and yet probal)ly its improper use has caused more misfit vulcanite dentures than all other causes. When the principles involved in the flask press and its use are understood, there should be no trou})l(' in handling it. All Fig. 470 Buffalo No. 2 press. Dr. E. Wilson's press. plasters expand and are compressible, some excessively so. French's regular dental plaster is the best and most commonly used by the pro- fes.sion, so these statements are in connection with this plaster. A molar tooth one-half inch in diameter under a thousand pounds pressure would be driven into well set plaster one-twentieth of an inch. Rubber when cold is very tenacious and will resist a very heavy pressure for a short time, but will gradually yield. Plaster compresses to its full extent in a very few seconds. It is ea.sy to comprehend that if an excess of rubber is placed over the teeth upon one side, and heavy pressure is applied, that the teeth will be driven into the plaster encasement and consequently the teeth upon that side of the denture will be too long. It can also be comprehended that if the cast is formed of regular plas- VULCANIZERS. 507 ter, and excessive rubber and pressure be applied to the vault of the cast, that it will be pressed upward and the plate warped. We shall now consider the power of the press. The screw is a combi- nation of the lever and wedge, and its power is calculated by multiplying the circumference described by the lever by the pitch of the screw. The Buffalo Dental Manufacturing Co. No. 2 press, Fig. 409, has a handle eight inches long, hence describes a circumference of twenty-five plus inches. There are ten threads to the inch, hence a pitch of one-tenth of an inch. An allowance must be made for friction in the screw, but one-fifth will be very liberal, when we shall have for every pound of force applied at the end of the handle, two hundred pounds pressure under the screw or a ton for every ten pounds of force. If the force is applied at the middle of the handle it will produce one-half as much pressure, or a ton for every twenty pounds of force. It is now easily understood why plates are warped, and these heavy malleable iron presses are sometimes broken. The student should now return to the Double Vulcanization Process, and read again how to use the press. Fig. 471 t^ ^^^^o ^^^^ ^ B c A -•"l''^^ \ 11 L.- Warming oven. Flask and Rubber Warming Oven. — Fig. 471 shows a convenient arrangement for the purpose. It consists of a double box, 12 inches wide outside and 9 inches inside measurement, by 9 inches outside and 6 inches inside in heigth, the depth of the oven, (A), may be 12 inches. Thespace, (B),an inch and a half wide, is partly filled wdth water, (C). The strips of rubber to be used in packing may be laid on the top to be softened while the flasks are heated in the oven, which, as will be seen by the illustration, is provided with a door. The screw-cap, (D), is for convenience in filHng the box wdth water, and is provided with a small hole for the escape of steam. The box should be made of copper and may be heated by a small Bunsen burner. Vulcanizers. — There are in use at the present time many forms of vulcanizers. It would be unnecessary to enumerate them all. These descriptions will, therefore, be confined to some of the best and sim- plest examples of the somewhat extensive list. 5U8 VULCAMZED RUBBER AS A BASE. Cam-lock.— Tliis is one of the new cross-bar vulcanizers. It is very simple in construction and is expeditious to liandle. It is fitted for either gas or oil. (Fig. 472.) Fig. 472 C'aiii-iofk vulcanizer. Edson. — This is an older form of screw-cap vulcanizer. It has n screw press within the boiler, and is suitable for both rubber and cel- luloid. It is not as easy to handle as the cross-bar variety. (Fig. 473.) The Lewis Cross-bar Vulcanizer (Fig. 474) is entirely new in its essential parts,^ and embodies many valuable improvements, and is probably one of the strongest, safest, and most convenient vulcanizers of the cross-bar pattern in use. The boiler is hand-made from copper, rolled expressly for this form SEA BURY VULCANIZER. 509 Fig. 473 of vulcanize!', and is of unusual thickness. The cap is ribbed on the under side to resist any strain which may be put upon it. This cap has but two holes drilled in it, one for the mercury bath, to which the thei'- mometer is attached; the other for the "manifold," which carries the safety-valve, blow-off, gas-regulator, or steam-gauge (Fig. 475.) The ring surrounding the boiler is of cast steel, and is therefore of ample strength. Beside the lugs for taking the strain off the cross-bar and bolt, it has a dovetailed projection for the insertion of a lifting handle (Fig. 476.) It will be observed that when the cross-bar and cap are re- moved, there are no swinging bolts or attachments to the pot. The cross-bar is of an im- proved form, and is made of cast steel. One end is at right angles to the main bar, and ter- minates in projections which catch under the lugs on the ring. Over the projections is a small rib which prevents the bar from dropping out of posi- tion. The other end of the cross-bar has an enlarged por- tion for the reception of the bolt, and is terminated by a handle. The vulcanizer is closed by one bolt suspended in a slot on the hand-end of the cross-bar. The bolt is squared to prevent rotation, and is surrounded by a spring for the purpose of dis- engaging it from the lugs when the nut is loosened, and for al- ways retaining the bolt perpen- dicularly and forcing it in place automatically. The vulcanizer is opened by loosening the nut on the bolt by means of the wrench furnished for the purpose (Fig. 477.) The bolt will be forced downward through the action of the spring. The handle of the cross-bar is then seized, and with the thumb against the nut it is pressed until the bottom of the bolt is disengaged from the lugs, when the bar may be lifted (Fig. 478.) The Seabury Vulcanizer. — This apparatus is so arranged that the vulcanizing is accomplished with dry steam. It has a dry chamber or oven for vulcanizing, which is distinct from the steam-generating chamber or boiler, the two being connected by a valve cut-off. The vul- canizing chamber has a capacity of three flasks. In the illustration the Edson vulcanizer 510 VULCAMZi:i) RUBBER AH A BASE. Fig. 474 Lewis cross-bar vuJcanizer, \\ith gas heating apparatus. Fig. 475 Manifold with safety valve and blow-off. Fig. 476 -piG. 477 Pot-lifter. Cross-bar wrench. jacket i.s cut away to show the relative positions of the two chambers, and their connection. It is claimed for this machine that plates made INSTRUCTIONS FOR USE OF TIME REGULATORS. 511 Fig. 478 Removing Lewis cross-bar from pot. Fig. 479 Seabury vulcanizer. 512 VULCAXIZED RUBBER AS A BASE. in it are as stroiiti; wlicii only lialf as tliick as those vnleanized in the onhnary way. (Fig. 479.) Hy eutting oft" the steam from tlie generating ehamber, eases can be removed and others inserted witliout loss of time, and, as the plaster is bnt slightly injured l)y the dry steam, warjiing of plates by the yielding of the investment is not likely to occur. INSTRUCTIONS FOR THE USE OF THE TIME REGULATOR. "The gas regulator (Fig. 480) is secured to the cap by means of the short iron pipe or coil. This is screwed into a hole drilled through the Fig. 480 Gas regulator. cap of the vulcanizer, and tapped with a 'one-eighth gas-pipe tap.' If the vulcanizer has a 'Lewis manifold' attached to the cap of the vulcan- izer, remove the screw between the blow-off and safety-valve and screw the coil-pipe in its place. After the gas regulator has been properly fitted, place the vulcanizer in the jacket and in the position in which it is to be used. Connections between the time regulator, gas regulator, and gas burner are made by means of rubber tubing. The engraving (Fig. 481) illustrates the correct method of connecting gas and time reg- ulators to vnlcanizers. Cut a piece of tubing of sufficient length to reach from the gas-supply tap to the titne regulator, and connect them; cut off another piece to reach from the time regulator to the c/as regulator. INSTRUCTIONS FOR USE OF TIME REGULATORS. 51^. and attacli to the gas regulator by the upright or straight nipple on top of the No. 4 Lewis gas regulator; then conneet the downward curved tube of the gas regulator to the gas burner under the vulcan- izer with another piece of rubber tubing. "The time regulator is more convenient when placed on a bracket near the gas-supply pipe. It is then out of the way, and not likely to be broken from contact with tools, and can also be used as a timepiece. To Set the Time Regulator. — When the valve lever on top of the time regulator (Fig, 481) is engaged with the screw upon the minute ar- Fig. 481 No. 4 graduated gas regulator, mounted on a Lewis cross-bar vulcanizer. bor on the back of the clock, the valve is held open for a length of time depending upon whether the lever is engaged with the first, second, or third thread of the screw; and the lever will be cast off, and the valve closed when the minute-hand reaches the figure XII. When the min- ute-hand is at IX the lever will be cast off at the end of fifteen minutes, if it is engaged with the first thread of the screw from the end; an hour and a quarter, if engaged with the second thread, and so on. A trial 33 )14 VULCANIZED RUBBER AS A BASE. should be made, and the time ascertained which is necessary for heat- ing the vulcanizer to the vulcanizing point, and tiiis time should be added to the proposed time for vulcanizing. We have, therefore, the following : — "Rule. — Turn the minute-hand to as many minutes before the hour as the number of odd minutes desired; then put the end of the lever in the threads of the screw upon the minute arbor at the back of the clock. The jirsi thread from the end gives the odd minutes to which the clock is set; the next and each succeeding ^hread gives a full hour. For ex- ample: For an hour and twenty minutes, set the minute-hand at the figure VIII, and engage the lever in the second thread from the end of the screw. At the end of that time the lever will disengage and automatically shut off the gas from the vulcanizer. If this w^ere to l)e an hour longer — l. e., two hours and twenty minutes — the lever should be placed on the third thread of the screw. For three hours, set the minute hand at XII and the hour in the third thread of the screw lever. "Those who use vulcanizers should be thoroughly informed as to the nature and properties of steam. The fact should be borne in mind that a vulcanizer is subject to the same laws and conditions as a steam boiler, which it is in fact, and, although it is comparatively safe and easily operated, it may, by carelessness or ignorance in its management, become exceedingly dangerous. "The following table of steam-pressure will be found convenient for reference, as it has been corrected so that it shows the true temperature for any pressure indicated by the steam-gauge. Fractions are omitted, and the nearest whole number is used instead. The French table gener- ally used shows 14.7 pounds pressure at 212°, whereas the steam-gauge at that temperature will indicate 0, unless by the expansion of heated air confined in the vulcanizer. The guage is therefore just one atmos- phere lower than French table: Table of the Elastic Force of Steam} (corrected to correspond with the steam-gauge) • Degrees of temperature, Elastic force in lbs. Degrees of temperature, Elastic force in lbs Fahrenheit. per square inch. Fahrenheit. per square inch. 212 ... . . . . . 390 ... 205 220 ... . . . . . 2 400 . . . 234 230 ... . . . . . 6 410 ... 264 240 .. . . . . . 10 420 . . 296 250 .... . . . . 15 430 .... . 440 335 260 ... . . ... 21 . . .375 270 ... . . ... 27 i50 . . .415 280 ... . . ... 34 460 ... 455 290 ... . . ... 43 470 . . .515 300 ... . . ... 52 . ... 63 480 . . 565 310. .. . 490 ... 603 320 ... . . ... 75 500 . . .663 330 ... . . ... 89 510 ... 721 340 ... . . ... 104 520 ... 793 350 ... . . ... 120 530 . . . 864 360 ... . . ... 140 540 . . .937 370 ... . . . . . IGO 550 . . 1015 380. .. . . ... 180 *. General instructions for operating dental \TiIcanizers, Buffalo Dental Manf'g Co., July, 1898 INSTRUCTIONS FOR USE OF TIME REGULATORS. 515 "It will be noticed that as the temperature rises the pressure of steam increases in a constantly increasing ratio for equal increments of heat, the pressure being nearly doubled by the addition of fifty degrees to the temperature. This fact will show the necessity of care and watch- fulness while vulcanizing. "The bulb of the thermometer is set in a mercury bath. This is the small cup, forming a part of the vulcanizer cap, to which the thermom- eter case is screwed. This cup should contain sufficient mercury to ensure its touching the bulb of the tube when the thermometer case is screwed down properly. This makes a metallic connection between the thermometer bulb and the vulcanizer cap, and is absolutely necessary for the proper indication of heat by the thermometer. "Should the mercury column separate, it can usually be reunited by removing the tube from the thermometer case, holding it perpendicu- larly, and striking the bulb with some force upon the palm of the hand, or by holding the tube by the bulb and giving it a sudden flirt. If the vulcanizer is used with the thermometer in this condition, it should be remembered that it is the whole column that denotes the heat, and allow- ance should be made for the broken part; i. e., if there is enough mer- cury separated to fill the space of ten degrees, the remainder of the col- umn should only rise to ten degrees less than the temperature desired. "Directions for inserting a new tube in the thermometer case will generally be found on the package containing the tube and scale. "Thermometers are accurately marked, by test instruments, at the 212° and 320° points, and the scales are especially graduated for each tube, as the positions ofthe points above named vary in different tubes. Each tube must, therefore, be used with its own scale, and in fitting it to the case, care should be taken that the black mark on the tube indicating the 320° point is brought exactly opposite to the 320° point on the scale. "The thermometer does not always give a correct indication of the heat of the vulcanizer. It only gives the temperature of the vulcanizer top, which may not be that of the flask. In fact, the indications of the thermometers employed on vulcanizers are almost invariably too low, owing to imperfect conduction of heat, radiation, etc.; and the vulcan- zation temperature, instead of being 320°, as indicated, is more usually 330° to 340°." The plan of providing a mercury bath for the reception of the bulb is a great improvement over the old way, and prevents the fracture of the bulb by the great pressure of the steam, which was of such frequent occurrence w^hen the thermometer was in direct contact with the latter. Damage to the glass bulb of the thermometer is manifested by a rise in the mercury, which cannot be brought down to the usual vulcanizing point by turning off the flame of the burner; consequently the ther- mometer ceases to correctly indicate the degree of heat, and imperfect vulcanization is the result. Leakage of steam around the packing of the vulcanizer should also be guarded against, as in such cases all of the water may escape from the apparatus before the vulcanizing is complete. Loss of all of the water in the vulcanizer may be detected by a per- Fig. ls3 Flask-tongs, Vulcanite files. TRIMMERS 517 Fig. 484 ? Fig. 485 \L Wilson .set. Spaulding trimmer, sistent fall of the mercury, even when the gas flame is greatly increased, and when this phenomenon is observed, the gas should be turned ott, the vulcanizer allow to cool, and new packmg adjusted. 518 VULCAXIZED RUBBER AS A BASE. Failure to strictly observe this rule has undoubtedly resulted in many serious accidents. An example of this kind occurred some years since in the laboratory of the Dental Department of the University of Pennsyl- vania. A student was endeavoring to vulcanize with an apparatus whicli leaked at the packing: noticing that the mercury persisted in falling, he continued to increase the gas fiame until the lower part of the vulcanizer was probably red hot. While he stood before it, holding a lighted match to the tube to enable him to see the column of mercury, the vulcanizer exploded with terrific force, sending the top through the ceiling and pieces of the boiler in every direction. It is quite likely that in this particular case the steam was partly decomposed by con- tact with the hot metal, producing a highly explosive com})ination of oxygen and hydrogen: no other theory would seem to account for the great force of the explosion. Fig. 486 Ivory Scrapers. Vulcanizing. — The flask or flasks are placed in the vulcanizer and filled about three-quarters with clean water. The packing should be without a break in its continuity, otherwise the steam will escape; the joint between the pot and cover must be protected from adhesion by slightly coating it with black lead or soap-stone. The cover is then put on, but the valve is not closed until the heated air which precedes the generation of steam has escaped; the valve is then closed. A close watch must be kept on the thermometer or gauge until the vul- canizing point is reached unless a time regulator is used. Flask-tongs, — Figs, 482 shows a useful form of flask-tongs for lifting flasks from the vulcanizer. They are made of sufficient length to reach the bottom of a three-case vulcanizer, and will securely grip the flask OCCASIONAL AiYD SPECIAL METHODS. 519 Files. — Fig. 483 illustrates some excellent forms. Scrapers. — There are a great variety of scrapers and chisels from which each operator may select such as seem best adapted to his hand. The writer's preferences are the ones here illustrated. Fig. 487 fejl Kingsley scrapers. OCCASIONAL AND SPECIAL METHODS. Gum Section Teeth. — There are two varieties of teeth used for the vulcanite base. They are known as plain and gum section teeth. The plain teeth are single and have no porce- lain gum, but may have a reproduction of the neck portion of the root of the tooth. The gum sections may be single teeth with a porcelain gum, but usually they are in the form of blocks, each containing two or three teeth. There are various combinations of the teeth in the blocks designed to meet special cases. The plain teeth are used only for the double and single vulcanization methods as previously described, but either plainer section teeth may be used for the single vulcanization sub-method. The gum section teeth require much grinding to adapt them to the wax or hard gum base-plate. The joints between the blocks must be ground to fit squarely against each other and not with a V- shaped space. The case should be flasked at the periphery of the wax. Much care should be used in packing and closing the flask. Do not use too great pressure, as there is danger of checking the porcelain gum and of opening the joints. Advantages and Disadvantages of Section Teeth. — Where the gum portion must be restored and there is extreme mobility of the Hp, the porcelain gum will appear to better advantage than the vulcanite gum 020 VULCANIZED RUBBER AS A BASE. restoration. In partial cases, blocks of one to four teeth can sometimes be used to very great advantage. The disadvantages are: (1) the forms of the sections preclude the possibility of the individualizing of the teeth by the dentist, they always have a stiff, unnatural appearance; and (2) there is a space between the porcelain and vulcanite for the accumulation of filth. This last objection does not apply to most of the oartial cases requiring gum section teeth, because in these cases the gum portion is not backed up with vulcanite. Some years ago the gum section teeth were in al- most universal use, but to-day there are sections of the country in which sets of fourteen gum section teeth are rarely used.^ Plain Teeth Without Gum Restoration. — Many cases of recent ex- traction antl in a few cases where tlie gum is sufficiently full after re- sorption has taken place, no restoration of the gum is required. In these cases the plaster of the cast is scraped away one thirty-second of an inch for the insertion of the cervical end of the teeth. Without this precaution the teeth in the finished denture would not set closely nor appear to grow from the gum. Black Rubber Single Vulcanization Method. — Some people fear the effect of the coloring matter in red rubber, so that it may be desir- able to construct the denture of black rubber, when it becomes very necessary that the black rubber should not show upon the labial and buccal surfaces. The two rubbers can be packed in the usual way or by this very ingenious method." Dr. Slabaugh's Method. — "After separating the flask and boihng out the wax, I coat the cast w'ith silex and then lay on my black or red rubber loosely over the cast. I then take a piece of the cloth that comes on the sheets of rubber and lay it over the rubber that is on the cast, and place the two parts of the flask together and set them in boiling water for a few minutes, after which I place the flask in a press and force it together, care being taken to have the flask entirely together. The flask is entirely filled with the rubber. -I now separate the flask and the rubber will adhere to the cast. Remove the cloth that has been put on and you will have perfect imprints of the teeth and pins in the rubber that have l)een made through the cloth. I then place a narrow strip of pink rubber on the black or red over the imprints of the teeth and up to about one thirty-second of an inch of the imprint of the pins. Place the two parts of the flask together and force them down and it is ready for the vulcanizer. Cut waste gates only in the rear so that when the flask is closed the second time with the little piece of pink rubber added to it, the slight displacement of black or red rubber will be en- tirely toward the rear." Waxable Rubber — Recently this material has been placed before the profession by John Hood & Co., Boston. Its physical properties are very much like those of sheet gutta-percha. It is not a patented 'The writer has put up but one set of gum section teetli in nineteen years. 2 Dr. Frank W. Slabaugh of Omaha, Xeb. REMOVING THE TEETH FROM A VULCANITE PLATE. 521 article. The claims for the material and the method of construction are given as follows: "An entirely new process with a material far sup- erior to rubber,— much lighter and one that will not become slimy in the mouth. To use, hold a sheet over the burner the same as a sheet of wax and make a trial plate; build out with rubber to the desired fulness and put the teeth in place with a spatula, by keeping the rubber soft and heating the tooth. Use the rubber just as you do wax, and smooth it be- fore adding pink rubber. After it is cool, add the pink rubber by first removing the teeth without heating, leaving the impression of the teeth in the rubber. Cut a strip of pink rubber wide enough to reach the ridge and cover the indentations of the teeth, just above the pin holes, and to reach from -heel to heel. Soften the rubber slightly and heat the tooth and replace it, carrying the pink rubber under the neck of the tooth. In this manner you can make a natural festoon as full as is de- sired. Care should be taken not to make any finger-nail marks or to mar the pink rubber. After the teeth have been tried in, before placing on the cast, paint the cast with the solution and heat the plate, pressing it in place with the fingers. Smooth the red rubber by the use of chlo- roform on a cloth. Moisten the cast and drive the air from it. Fill the flask with plaster and press in cast, shaking the flask well to prevent bubbles. Vulcanize, taking twenty-five minutes to run up, and then hold it one hour at 315 degrees." Thick Vulcanite. — Thick masses of rubber may be vulcanized by gradually increasing the heat from 280° F. to 300° F. (See "History of Vulcanite" in the early part of this chapter.) Another method is to fill the mold with vulcanite, having the rubber packed about it; or the mold may be largely filled with pink rubber. REMOVING THE TEETH FROM A VULCANITE PLATE. Sheet Iron Method. — Place the denture, teeth downward, upon the sheet iron over a gas stove, and when the vulcanite is thoroughly softened, the teeth may be pushed off one at a time by inserting the wax spatula between the tooth and the vulcanite upon the lingual side. The plate is held by a pair of pliers and the dislodged teeth are permitted to fall upon wood, or better, upon cloth, but not upon cold iron or stone. Any portion of the vulcanite remaining about the pins should be re- moved. If necessary, these small portions of vulcanite may be softened by grasping the tooth in a pair of solder tweezers and holding in the Bunsen flame. Glycerine Method — The denture is placed in a vessel of glycerine and heated to the boiling point of the liquid, when the teeth can be re- moved as in the other method. The glycerine is soluble in water and easily removed. The fumes of the heated glycerine are more ob- jectionable to some than those of the overheated rubber. Flame Method — The denture is grasped with a pair of pliers and the outer surface of the teeth heated by passing repeatedly through the 522 VULCAXIZEI) RUBBER AS A BASE. flame until the vulcanite is softened about the pins, when they are re- moved as before described. It is not advisable to remove the teeth b} heat from a denture which is to be used again, because of the liability of warping the plate. The vulcanite should be cut from about the pins of the tooth with a bur in the engine or with a chisel. Partial Cases. — It frequently occurs in constructing partial artificial dentures for the replacement of single incisors or canines that the ordi- nary rubber teeth are too thick to admit of their being arranged to con- form to the line of the natural teeth without interfering with the normal Fig. 488 Partial vulcanite plate arranged for case with marked overbite. occlusion (Fig. 488). In such cases a plate tooth may be used, and is attached by means of gold backings, bent at an angle with the base of the tooth, of sufficient length to allow of the projecting portion to be imbedded in the rubber plate, as shown in Figs. 488 and 489. The ex- tension of the gold backing shown in Fig. 489 has two or more holes punched and countersunk in it, so as to be held firmly by the vulcanized rubber. Fig. 492 Fig. 489 Fig. 490 Fig. 491 Tooth and clasps prepared for attachment to vulcanite plate. Gold Clasps. — Gold clasps, when used in combination with rubber, are attached in the same way. The clasp, after being accurately fitted to the plaster tooth, is provided with a piece of gold plate soldered at a point next to the rubber plate (Figs. 490-492). This attachment should be slightly raised from the cast, so that it will be entirely en- veloped by the rubber, as shown in Fig. 490. PARTIAL LOWER DENTURES. 523 There is some danger of these chxsps being forced sHghtly from their correct position by the pressure of the rubber in packing. This difficulty may be entirely overcome by soldering a temporary support of scrap gold to the clasp and bending it over the plaster tooth, as shown by Fig. 492. Usually this device will be found effective in retaining the clasp in contact with the tooth. After vulcanizing, the supporting piece of gold may be sawed off with a jeweler's saw. In packing a case arranged with gold clasps, a thin sheet of rubber should be worked un- der the gold attachment to further protect the latter from displace- ment. It will, of course, be understood that the clasps are to remain in position during the packing; therefore, in flasking such cases the plas- ter should be made to cover the portion of the clasp not actually in con- tact with the rubber; this affords additional support to the clasp during the pressure accompanying the closing of the flask in packing, and will keep it in correct relation to the plaster tooth. Partial Lower Vulcanite Dentures. — Gold is used in combination with that class of partial lower dentures designed to replace the bicus- FiG. 493 Fig. 494 Strengthener of clasp gold to be used in connection Strengthener and clasps for lower plate, with vulcanite. pids and molars and when the natural incisors and canines remain. For the purpose of strengthening the piece and to lessen its bulk in front a plate of gold is sometimes swaged to fit the cast back of the front teeth, and where the ridge is not well defined and not favorable to the reten- tion of the piece without some form of attachments, gold clasps are soldered. The gold plate is allowed to extend somewhat beyond the canine teeth; the ends are perforated by the punching forceps, as shown by Fig. 493, to ensure strong union with the rubber. This plate is then put upon the cast and secured in place by means of wax; the teeth are arranged in position, waxed up, and vulcanized in the usual way. The denture when finished presents to view a plate with the anterior part of gold, while the two parts holding the teeth and resting upon the ridge on each side are of vulcanite. The purpose of such a combination is to save labor and material, but a denture so constructed, while better in point of durability and because of the absence of bulkiness where it 524 VULCAXIZKl) RUBBER AS A BASE passes around back of the incisors and canines than vulcanite alone, is still far inferior to one constructed entirely of gold, for while such a den- ture is doubtless strono;or than one of vulcanite alone, it is not so durable as one made exclusively of gold, on account of the liability of the piece to break at the points where the gold is imbedded in the vulcanite. Dentures of the class above referred to should always be made entirely of metal, and the expenditure of money and labor is but little greater than in the combination plan, while the general result is in every way more satisfactory. Dr. P. T. Dashwood uses in place of the swaged plate above sug- gested for the anterior portion of lower partial dentures, an iridio- platinum wire, gauge 14, which is bent with round nosed pliers to conform to the lingual alveolar surface. The ends are flattened upon an anvil and cross pieces of platinous gold are soldered to them for the attachment of the vulcanite. He makes the just claims that the wire is "stronger than vulcanite, is nuich more comfortable to the patient, and is more hvgienic. Fig. 4'.>r, Completed plate. Natural Teeth upon Vulcanite Base. — "Where the anterior natural teeth have become so loosened by the ravages of pyorrhoea alveolaris, by excessive resorption of the gums and sockets or of the roots of the teeth, so that their complete loss is a matter of a very short period of time, a plaster impression may be taken of the mouth before the removal of the loose teeth. In constructing partial dentures for cases where the natural organs are prematurely lost, it is much the better practice to reset the natural teeth, provided, as is often the case, they are of dense structure and have not previously been attacked by caries. This is done by making a plate in the usual way, and in the spaces to be occupied by the natural teeth vulcanizing a strong platinous gold wire, being careful to place the gold pin in the centre of the space. The wire must have an at- tachment soldered to it, so that its connection with the rubber will be secure. The wire may be arranged with a simple piece of scrap gold NATURAL TEETH UPOX VULCANITE BASE. 525 soldered to the end to be imbedded in tlie rubber, as shown in Fig. 496, or it may be provided with a perforatefl extension, as shown in Fig. 497, by whicli union with the rubber may be secured and great bulki- ness avoided. The rubber portion of the denture finished, it only re- mains to remove the infirm natural organs and attach them to the plate made ready for their reception. This is done by sawing oft" the roots (Fig, 496), enlarging the pulp-canal witli a suital)le engine drill, fitting Fig. 496 Cast with plate in position natural tooth to be added. the neck of the tooth to the plate, and into the socket, as shown in same figures, and then attaching the tooth to the pin (Fig. 497) and plate by means of zinc-phosphate cement, being careful to dry the parts thor- oughly before the cement is applied. This method of resetting natural teeth is more conveniently done on gold plates than on those of rubber, but it is applicable to both. It possesses the following advantages: Fig. 497 Natural tooth mounted upon plate. First. The teeth are the patient's natural teeth, and this fact very greatly lessens the repugnance which many individuals of exalted sen- sibilities feel to artificial teeth. Second. It saves the individual from being seen without teeth — a matter of the greatest importance to many patients. Third. Artificial appearance is avoided, for they are the natural teeth of the patient, and nothing more need be said on the score of natural effect. The question is often asked. Do teeth reset in this manner suffer from dental caries ? It has been observed that such teeth 526 VULCAXIZED RUBHEIi AS A BASE. are not more liable to decay after their attachment to a plate than they were before removal from their sockets. If the infirm natural teeth are of poor (juality and have large fillings in them, it is better to use porcelain teeth, and the dentures can be en- tirely finished ready for insertion before the natural teeth need be ex- tracted. Care should be observed to allow the necks of the artificial teeth to extend well into the sockets of the extracted organs, to anticipate resorption of the parts which to some extent is sure to occur at such points. Combination Dentures. — Under this heading are included metal plates with vulcanite attachments, vulcanite plates with metal linings, vidcanite dentures strengthened with perforated metal plates, etc. Ex- cellent results may be obtained by attaching the teeth to metallic plates by means of vulcanized rubber. A denture so constructed will be found to possess greater strength than one of vulcanite alone, while it will have the additional advantage of being free from interstices, which favor the lodgment of decomposable debris. In other words, the com- bination of metal plate with vulcanite attachment thoroughly meets the objections raised against either method alone. Either gold, silver, platinum, aluminum, or any of their alloys usually employed in prosthetic dentistry, may be used in the construction of one of these combination dentures; preference, however, should be given to gold as a base. Platinum unalloyed is not well adapted for the pur- pose, on account of its great ductility and weight, but when alloyed with a small percentage of iridium its rigidity is so much increased that a plate of No. 29 thickness will be found to be cjuite as strong as a much thicker plate of 18-carat gold. Either ordinary silver plate of standard ^ fineness may be used with rubber attachment, or silver alloyed with platinum, the latter having greater tensile strength than the former. It must be remembered, how- ever, that silver has a powerful affinity for sulphur, the indurating agent in vulcanite, and that the presence of platinum as an alloy does not en- tirely protect the silver from the action of the sulphur. It is, therefore, necessary, where a silver plate is used, to interpose a layer of No. 60 tin-foil between the rubber and the plate; this precaution, however, is not necessary where celluloid is used. In silver dentures with vulcanite attachments the anchorages must invariably be made of platinum or gold w^ire. After the plaster wall is made and the wax removed from around the teeth, the exact positions of the anchorages are marked upon the plate with a sharp steel point to the number of eight or ten. The plate is then laid on a charcoal support and pieces of silver solder are fused at the points indicated. The wire is then cut into proper lengths, screwed in a vise, and one end of each flattened by means of a rivetting hammer into the form of a head : each pin is then taken up separately, the headed end dipped in borax, and placed on the plate at a point where a piece of solder has been fused. The borax will assist in retaining the piece of wire until the flame of the ^Coin. VULCAXITE WITH PLATE OF FUSIBLE ALLOY. Tyll blowpipe is directed upon it to remelt the solder and unite the pin to the plate. The wire anchorages are not to be bent into hook form, as shown in Fig. 500, until after the tin-foil protection has been adjusted. The pins are forced through the tin-foil and pressed with a rubber point and burnished closely to the plate. The holes made by the passage of the pins through the tin-foil, if care is used, will not be large enough to allow the rubber to reach the silver to any great extent. After the tin is in place the pins may be bent with pliers, as shown in Fig. 500. Another method is by directly tinning the surface to be covered by the rubber. The silver is cleansed and covered with a saturated solu- tion of zinc chloride. The tin-foil is pressed carefully against the silver and the plate is held above a Bunsen flame until the tin fuses. Its flowing is to be directed by means of a camel's-hair pencil which has been dipped in the zinc solution. Vulcanite in Combination with Plates of Fusible Alloy. — For the modus operandi of the preparation of plates of fusible alloys the reader is referred to Chapter XY. The Reese or AYeston fusible alloys can be cast very thin, and yet are sufficiently rigid to withstand the force of mastication. These alloys retain their color and make an admirable combination plate. Having finished the plates as shown above, the edges and raised rims are trimmed to the desired dimensions. A roll of softened modelling compound or wax is pressed around the gums over the alveolar ridges, and trimmed with a knife to the supposed height of the teeth. The plates are then tried in the mouth, and the wax trimmed from all sides until perfect occlusion and contour are obtained. The median line is marked on the modelling compound or wax, as the case may be, and the cutting edges marked in several places to serve as guides in restoring the upper and lower waxes to their correct relation with each other should they become separated. The articulating models are prepared in the usual way — pouring plaster into the lower plate, first allowing it to extend back sufficiently to receive the upper half, which is to be poured next. The modelling compound or wax is then to be removed and the teeth arranged and waxed up and vulcanized. The attachment of the vulcanite to the plate may be secured by freely nicking the ridge to which the teeth are to be fastened by means of a sharp-pointed graver, but without this the undercut of the rims and buttons will be ample to hold the vulcanite securely to the metal. Aluminum, though not affected by sulphur, is not as well suited for vulcanite attachments as the other metals named, on account of the want of reliable aluminum solder "^'ith which to fasten the loops or pins thoroughly; but by special treatment, which will be described in connection with the manner of preparing aluminum plates, a compar- atively durable denture can be made of that metal with vulcanite. Recently casting methods have been so improved as to make the use of cast aluminum with a vulcanite attachment a thorouglily satisfactory procedure. In constructing a denture of gold with vulcanite attachments the plate should be of the thickness of No. 27 of the standard gauge, and 528 VULCANIZED RUBBER AS A BASE. made in accordaiicc with the directions for tlie makiiifr of gold and silver plates in Chapter XI\'. It should he provided with a rim extending entirely around the labial and buccal edges and upon the palatal portion of the plate slightly pos- terior to the alveolar ridge, as shown in A and B in Fig. 498. This rim may be formed of Xo. 27 plate (Fig. 498) or round wire of Xo. 17 gauge (Fig. 499). A rim formed of round or triangular wire requires Fig. 498 Cross section of gold plate showing soldered rim. much less labor and time in its adjustment and soldering than if formed of a strip of plate, and when flattened with the file on the labial side, and the corundum wheel and graver on the lingual side, it has the same effect as if it w'as formed of plate. Fig 499 Wire rim. The rim may be dispensed with entirely, but, as it gives a more fin- ished appearance to the denture and adds greatly to its strength, it should, therefore, always be preferred. In attaching a flat rim to a gold or silver plate a strip of plate long enough to extend entirely around the rubber attachment, should be cut. VULCANITE WITH PLATES OF FUSIBLE ALLOY. 529 The rim should be annealed, and bent with the pliers to fit the la})ial and buccal edges on the plate. It is then placed on a charcoal support, and the rim held in contact with the plate by means of small nails or tacks : it is then united to the plate by a small piece of solder immediately in front at the frfenum and at one or two other points along the buccal edo-es. The plate is then cooled, placed upon the plaster cast, and with a small hammer and pliers the rim is brought in close enough con- tact with the plate to admit of complete soldering. The lingual por- FiG. 500 Location of wire attachments. tion of the rim shauld not be soldered to the plate until after the correct position of the teeth has been ascertained. This is accomplished by arranging the teeth according to the bite and other reciuirements of the case, and then making a wall of plaster around them, separated at the centre line. This enables the operator to mark upon the plate with a sharp instrument the correct point at which to solder the rim, so that it will leave an unbroken surface for the tongue, as shown by B in Fig. 498 and to mark the proper position for the loops or bent-pin attachments, as shown by C in Fig. 500. It is very important that the exact location of these fastenings should be ascertained, but this cannot be determined until after the teeth have been adjusted. Any attempt to solder the Fig. 501 Fig. 502 Incorrect location of rim. Incorrect location of rim and attachment. rim or fastenings previous to the fitting and arrangement of the teeth will be but guesswork, and nearly always result in either of the con- ditions shown in Figs. 501 and 502. 34 i30 VULCAXIZED RUBBER AS A BASE. Fig. oOS The wire rim is soldered to its place by simply clamping the wire to the plate, and then attaching it at single points in front and at the buccal edges, and, after the correct position of the teeth has been ascertained, bringing it entirely around at the Ungual portion, as shown by Cin Fig. 500 By simple pressure with an instru- ment or gently tapping with a riveting hammer, it may be brought into close con- tact with the plate and completely sol- dered. It need not be flattened and fin- ished until after the case is vulcanized. Owing to the difficulty in soldering aluminum, it is necessary to secure at- tachment for the vulcanite to the plate by means of perforations or countersunk holes along the top of the ridge. For this purpose ingenious perforating punch- es have been devised by Drs. Rich- mond, Peck, and others; those of Fig. 504 Perforating forceps No. 9. Fig. 505 Loop punch. Pei-forated plate. VULCANITE PLATES WITH GOLD-FOIL, ETC. 531 the two former are shown by Figs. 503 and 504, the latter throwing up a sharp square burr, the former a loop. The punch points entering from the under side of the plate, produce the desired result without in the least bending or affecting the fit of the plate. A rolled aluminum plate, constructed in the manner shown by Fig. 505, and roughened by means of the punches (Figs. 503 or 504), and with the teeth attached by means of vulcanite, will afford a light, strong, and comparatively durable denture. , Vulcanite is of great value in refitting gold plates which have ceased to fit the mouth in consequence of changes by absorption following the extraction of the teeth. These changes may continue in some cases for several years after the removal of the natural organs, to such an extent finally that the denture will no longer be of service. The resorption usually occurs along the alveolar ridge, and it is a matter requiring but little time or labor to adjust the denture to a new plaster cast, fill the spaces caused by resorption with wax, invest, pack, and vulcanize the piece. Care must be observed to make countersunk perforations through the plate at points where the vulcanite is to be attached, so as to secure firm union with the gold plate. Vulcanite Plates Lined with Gold-foil, Electro-deposits, etc — Various experiments have been made with this class of work in the last Fig. 506 The vulcan gold lining. twenty-five years, with a view to developing some process by which a durable metallic coating can be given to that portion of the vulcanite denture which is in contact with the alveolar process and maxillary por- tion of the mouth. There are two methods. One consists in coating the surface of the plaster cast vnth gold by electro-deposition, by first rendering it impervious to warm water, so that it will not take up and destroy the gold bath. The surface to be electro-plated must be hard and smooth and free from all greasy substances. It must be thoroughly 532 VULCANIZED RUBBER AS A BASE. coated with plumbago and painted with a solution of chloride of gold to facilitate rapid deposition over the whole surface. The next and simplest form is to coat sheets of No. 8 or 10 gold-foil with a non-conductor on one side, or by putting two sheets together with a non-conductor — as wax, for instance — between them, and seal- ing the edges with w'ax to prevent the gold solution from penetrating between or through the sheets. A rough granular coating of gold or copper can be deposited on the exposed sides, which will ensure com- paratively good adhesion with the plate after vulcanzing. Another method is w^hat is known as the "Vulcan gold lining." It is a pure gold sheet covered on one side with a thin coating of silver (Fig. 506.) The gold is applied in one piece to the surface to be cov- ered, and no extra care is required in packing the flask. The lining is of chemically pure gold on one side with a thin covering of pure silver on the other. The union betw^een the rubber plate and the gold lining is mechanical : the sulphur in the rubber acting upon the surface of the silver produces a condition of surface which favors adhesion. This foil is of the thickness of No. 40. In applying it, the case should be packed first; the flask is then separated, and any imperfections in the casts are to be repaired wuth thin plaster or oxyphosphate cement. The cast is then to be painted with a thin solution of equal parts of shellac and sandarac dissolved in alcohol. When dry, coat the surface with dextrin, gum tragacanth, or damar varnish, and while still moist and sticky, press small pieces of the gold lining on to the cast, bright side down. The gold lining is first cut into convenient strips of the form of rectangles, sc|uares, and triangles, to avoid wrinkling. The edges should slightly overlap, and the lining be kept free from varnish or any substance that would be likely to interfere with adhesion. Pres- sure on the granular side of the foil with a steel instrument should also be avoided. The rubber end of a lead pencil or the finger is the best means of pressing the gold into all the irregularities of the cast. The flask should then be carefully closed and the piece vulcanized. Dr. John A. Daly, of Washington, D. C, has described a method of lining new and old rubber plates as follows: "The vulcanite denture is constructed and finished in the usual way; the surface to be covered with the gold lining is washed with soap and water until perfectly clean. A sharp-pointed instrument — excavator, knife-blade, or needle-point — is then employed to scratch or serrate the entire surface to be covered. Care must be observed to keep it clean, and the roughened part should not be touched with the fingers. A solution of ordinarv dental rubber in naptha is then to be evenly painted over the prepared surface and allowed to dry to the point of stickiness. The lining of gold, which is the form of foil of about No. 60,hasone side roughened by the electro- deposition of gold. A strip of the foil is cut of sufficient size with which to form the rim, which, when in position, will enable the operator to handle the plate without soiling the rubber surface (no harm will re- sult from handling the gold). Where there are depressions, the lining should be pressed with a suitably shaped piece of rubber eraser to the VULCANITE PLATES WITH GOLD-FOIL, ETC. 533 lowest point in the plate. It should be cut in pieces of such size as will avoid wrinkling, and applied so that each piece will slightly overlap the other. All creases are to be removed by gentle pressure with an egg- shaped burnisher. The plate is then flashed in the usual way and vul- canized for twenty-five minutes at from 320° to 330° F. It will re- quire no finishing when removed from the flask, except where the edges of the lining overlap. The laps must be removed carefully with a blunt instrument, such as a dull knife-blade, burnisher, or the finger- nail, bending them back and forward until they break. If the plate has been worn for some length of time, it should be placed for fifteen minutes in a concentrated solution of lye or soda, and thoroughly cleansed before the surface is roughened and coated with the rubber solution. The gold lining of Dr. Daly may be applied during the construction of a new denture by first varnishing the plaster cast, after flashing, with sandarac varnish, followed by a coating of damar varnish; and while the latter is sticky the cast is covered with the gold lining cut in pieces of about half an inch in width. The brown side of the gold, which is the roughened surface, must be up, so as to engage the rubber in packing, the rest of the operation being in every respect similar to the ordinary procedure of filling and closing the flask. Fig. 507 Surface-cohesion forms for artificial dentures. The smooth side of the gold lining is like ordinary gold foil, and is the surface intended to be in contact with the mucous membrane. The roughened surface, which has a brownish color, is prepared so as to in- sure strong adhesion with the rubber, and the union is said to be so firm that it cannot be stripped from the vulcanized plate. The gold lining, extra thin rubbers, and all other materials used in this process can be obtained from dental depots. Dr. Joseph Speyer has introduced a method of lining vulcanite and celluloid dentures consisting of a thin metallic plate of the thickness of No. 120 foil, the surface of which is covered with minute papilliform prominences (Fig. 507), which are claimed to effect very strong surface 534 VULCANIZED RUBBER AS A BASE. adhesion, while they cause no irritation and leave no marked inden- tations on the tissues. Fig. 508 shows the prominences magnified four diameters. New Rubber Facings — Two comparatively new kinds of rubber have been introduced within a few years that commend themselves for use in the combination plate described above. One is the "gran- ular-gum" rubber facing by Dr. Gilbert Walker, in the use of which the following directions are given: "In waxing up a case, carefully model the gum portions to the exact contour desired, and make the festoons smooth at the necks of the teeth. After flasking, face with a layer of granular gum cut to lie close around the labial and buccal necks of the teeth, and pack against the outer wall of the plaster investment, so that the facing shall not extend above the edges of the plaster. I^ap the pieces of granular gum carefully, so that the red rubber will not be squeezed between them, and show on the facing after vulcanizing. In Fig. 509 Granular-gum faced \ulcanite denturos. packing the red rubber care must be taken not to have an excess, else the overflow may carry with it the granular gum and elongate its colored particles, thus interfering with the mosaic appearance on which the imitation of the gum depends. The lingual part of the plate may hkewise be faced, with care in lap- ping the pieces of granular gum and avoiding an overplus of red rubber. With this form of rubber, exposure to sunlight for the purpose of devel- oping its color is unnecessary; when well polished the moisture of the mouth will improve the tint. Granular gum vulcanizes with any of the ordinary rubbers; better results are, however, obtained by vulcanizing it at a low temperature. In finishing, care should be exercised to avoid cutting through the thin facing." Gear's shaded pink rubber is somewhat similar to the granular gum described above. It may be used in the same manner as the latter, and adds greatly to the beauty and natural appearance of the gum portion of the denture if the preliminary modelling has been done with taste and skill. Beaded or Grooved Vulcanite Dentures — For the more complete ex- clusion of air and moisture between the artificial denture and the mucous WEIGHTED rULCAXITE DENTURES. 535 membrane upon which it rests, a groove is cut in the plaster cast as shown in Fig. 510/ so that the vulcanized denture should have an in- tegral half-round smooth bead formed on its maxillary surface. The groove must be carried continuously across the palatal portion of the plaster cast and along the buccal and labial lines of muscle attach- ments, to form a bead-enclosure which should produce a supplemental chamber-like function of the entire inner surface of the denture. (Fig. 510.) The groove may be conveniently scraped on the plaster cast by one of the larger-sized Palmer's excavators, which, being rounded at its cut- ting edge, will afford a half-round bead in the vulcanized piece. Fig. 510 Grooved plaster cast. Weighted Vulcanite Dentures and Dentures with Contours — As a rule, lower dentures formed of vulcanite have not sufficient weight to overcome the resistance of the muscles of the cheeks and the sublingual integuments, and when the bite is unusually short they are also deficient in strength, so that breakage of lower dentures is a common occurrence. Both of these defects may be remedied by constructing a platinum or gold plate of two thicknesses of No. 29, soldering suitable anchorages near the top of the ridge in a position which will not interfere with the teeth, and vulcanize as described under the heading of Combination Dentures. A less expensive method of adding weight to a vulcanite denture con- sists in using rubber, which is prepared for the purpose with tin filings incorporated with it. By this means the requirements as to weight are very nearly fulfilled, but no additional strength is acquired, the only means of overcoming that difficulty being the use of a metallic plate. ^^^len the bite is unusually long it may be waxed and flasked in the usual manner and after the flask has been separated preparatory to packing, a cylindrical rod of wax may be laid upon the under side *■ Dental Cosbios of July, 1895, p. 55. 536 VULCANIZED RUBBER AS A BASE. of the blocks or single teeth, as the case may be, of sufficient length to extend from one second molar to the other. The wax rod is then carefully lifted from its place and invested in plaster to forma mold which should be in two equal halves, the line of division being exactly in the centre of the diameter of the wax rod. This mold should have a gate bored through the top for convenience in pouring the melted tin, while at tiie other extremity it should be provided w^ith a vent to allow the escape of air at the instant of pouring the melted tin. The tin may be melted in a small iron ladle with a suitable handle, and the melting may easily be accomplished over a gas-jet or alcohol "flame. When the casting is complete and the tin sufficiently cool, the mold may be opened and the tin facsimile of the wax rod placed in position in the flask, resting upon the teeth, as previously indi- cated in the description of the preparation of the wax pat- tern rod. The tin rod should be so arranged that all parts of it will be covered by the vulcanite. Fig. 511 shows the arrangement as de- scribed, A indicating the tin, B the vulcanite. This method possesses the additional advantage of preventing porosity of the vulcanite — an accitlent which is very liable to occur in bulky lower dentures. Fig, 511 Molded tin weighted vulcanite. It is sometimes necessary to amplify the denture at points where un- natural depression occurs in consequence of great resorption following the loss of canines or molars. If the amount of projection required to restore natural expression is not extraordinary, slight additions to the rim and the usual vulcanizing may be relied upon to accomplish the de- sired result; but if the case require a large mass, exceeding a quarter of an inch in thickness, the vulcanizing must be done at a lower tem- perature, of, say, 300° F., and three hours' exposure in the vulcanizer, in order to avoid porosity. Equally good results may also be attained by forming a core of some light material, enveloping it in rubber, and filling with it the recess in the flask representing the "contour." For this purpose cores of thin metal hermetically sealed, approximating the form of the contour and one-eighth of an inch smaller than the latter may be used. The preparation of metallic forms is, however, a matter requiring considerable labor and time. A much simpler and equally effective method is to form a core either of vulcanized rubber, sponge, or cotton wool tightly rolled and wrapped with thread. In packing the VULCANITE DENTURES WITH CONTOURS. 537 core is not to be placed in position until the case has been packed and the flask completely brought together, when it may be opened, the recesses representing the contours freed from rubber, and the cores, pre- viously wrapped with strips of soft rubber to the thickness of an eighth of an inch, put in its place. The object of first packing and closing the flask is to prevent the flow of rubber from displacing the cores and to Fig. 512 Cast of mouth with opening into nasal cavity. ensure their complete envelopment. In finishing such a case, care must be exercised to avoid cutting through the rubber or exposing the sponge or cotton when those materials are used. Probably of the materials named a piece of hard vulcanite affords the best results and is less likely to lead to failure through displacement, which is always liable to occur. Fig. 513 Denture constructed for case shown in Fig The same course as outlined above in the preparation of ordinary contours may be pursued in making plates to restore contours when large portions of the maxillary bones have been lost by disease or accident, such as gunshot wounds, etc. Fig. 512 shows a cast of the mouth in which the whole anterior portion of the alveolar ridge had been removed, leaving a large opening into the nasal cavity, by which speech was seriously affected. After obtaining the cast a thin plate 538 VULCANIZED RUBBER AS A BASE. of wax was pre])are(l to cover tht- j)alatal j)ortion extenciing around the teeth in the form of half clasps, and through the opening even with the floor of the nasal cavity. Upon the wax plate thus prepared the arti- ficial teeth were arranged, and the waxing and flasking done in the usual way. To prevent porosity of that part of the vulcaiu'te ap])liance wiiich extended into the opening, two or tliree drops of water were introduced, so as to keep the bulb hollow and in a state of expan- sion during the vulcanizing process. This water may be removed after vulcanizing by drilling into the bulb and then securely plugging the holes thus made with platinum wire tightly screwed in. Fig 513 affords a sectional view of the appliance. Spiral Springs. — A'ulcanite dentures are occasionally retained in situ by means of spiral springs. This method of retention, is, however, but seldom resorted to, except in cases of extreme flatness of the mouth or else in the correction of oral deformities. (For a description of the preparation and adjustment of spiral springs the reader is referred to Chapter XIV.) Vulcanite Plates and Flexible Rubber Rims. — The use of flexible rubbers in connection with artificial dentures is of doubtful value, on account of the inevitable loss of flexibility of all semi-vulcanizable rub- bers when worn in the mouth. REPAIRING VULCANITE PLATES. The breaking of vulcanite dentures is usually due to over-vulcanizing, by which elasticity and toughness are destroyed; to improper arrange- ment of the molars, by which the strain of mastication is thrown on the outside instead of on top of the ridge; or to a warped plate. The firstevidenceof the giving away of apiece is usually a fine crack appear- ing between the two central incisors, and sometimes, in partial dentures, in the border surrounding a natural tooth. Wax Method —A method particularly applicable to plates which are broken entirely in two, consists in adjusting the two parts of the plate together, and fastening them in correct relation to each other temporarily by adhesive wax dropped on the lingual surface until plas- ter can be run into the maxillary portion of the denture. As soon as the plaster hardens, the plate is removed from the cast, the line of divis- ion is enlarged with a file, and dovetails cut opposite each other with a jeweler's saw, as shown by Fig. 514. The dovetailed space is then filled with wax, invested in the usual way in a flask, packed, and vul- canized. This method is open to one serious objection; — it necessi- tates another vulcanizing and the consequent loss of elasticity and toughness. A plate so treated will never be as strong as it was before. By another method the edges may be adjusted as before described, and the piece be placed immediately in the lower half of the flask. After the plaster has set, the adhesive wax is to be removed from the lingual side of the plate and a line cut with a round engine bur along the REPAIRING VULCANITE PLATES. 539 full extent of the crack, or break, halfway through the plate and a quarter of an inch wide, with smooth, regular edges, without dovetails. The case is then waxed up and the other half of the flask poured — Fig. 514 Fractured vulcanite plate dovetailed for repair. when the case is packed and vulcanized. If the parts have been kept perfectly clean the union will be quite strong. Another modification \ which gives the best results is this. After Fig. 515 l~ M^l^HiM ^M ^Kr m} ^^ B^l ^^^M i^H P^i ^i ^^^^ ^^jfilll pi ^H^|l,^Fm%'" •T • WSg-'.' lo ■n mm ^ ''-'U^bJH^^^^B Fractured vulcanite plate prepared for repairing with beveletl surface and wax shafts. the cast is made, the portions of the plate are removed from the cast and with file and scraper a long bevel is cut, forming a thin feathery edge along the fractured edge and sloping away from this for an ^ The method preferred by the writer. 540 VULCANITE RUBBER AS A BASE. eighth to one-half inch as the case will permit. (V'l^. 515.) The pieces are then filed to give a slight bevel upon the maxillary surface. The portions of the plate which have been cut away are replaced with wax, and if necessary the plate may be thickened over the portion hav- ing the freshly cut surface. It is unnecessary to coat the vulcanite sur- face with a solution of rubber as the heat and i)ressure will make the union. Fig. 510 shows method of Hasking. For description of wax shafts, see page 499. Fig. 516 Fractured vulcanite plate shown in vested. Fusible Metal Method. — To avoid loss of strength by the second vul- canizing it is recommended that fusible metal, melting at 150° or] 60° F., be used to fill the dovetailed space. This can be .done by pouring the melted alloy into the space and packing it w'ith a hot spatula, which is readily admissible owing to the low fusing-point of the metal. AVhile the method has the advantage of not requiring a second vulcan- izing, the union of the metal at the point of fracture is not as close as when rubber is used, and it cannot be said to be reliable as a means of repairing broken vulcanite plates. A single tooth may be fastened to the vulcanite by filing the dove- tailed space as for repairing with rubber, the fusible metal to be put in place with a hot spatula; or the dovetail can be filled with amalgam. Replacing Vulcanite Method. — ]Much the better way is to fasten the parts together, run a ])laster cast into the denture, then make a bite of plaster to serve as a guide for the replacement of the teeth, remove the latter from the broken plate, reset them to the cast, w'ax up the piece, flask, and vulcanize. This affords practically a new case, and the time consumed is not much greater than is required in repairing the old one. INTERDENTAL SPLINTS. 541 Additions to Old Plates. — Additions of teeth to old plates are accom- plished after practically the same methods. Fig. 517 shows a case where six teeth have been extracted, and the old plate is prepared for the ad- tlition of as many porcelain teeth, so that the denture could be worn until the resorption of the alveoli and gums would admit of the con- struction of a permanent plate. The illustration shows the plate bev- elled oflf to a smooth edge, and several holes drilled into the filed portion. The correct occlusion of the new teeth is obtained by placing the plate Fig. 517 Plate prepared for the addition of several teeth. in the mouth after the bleeding ceases, placing two pieces of softened wax along the alveolar ridge and plate, and directing the patient to bite into the wax, and then gently pressing the wax while the teeth are in contact. This gives the correct relation of the lower to the upper teeth, and the impression of that portion of the alveolar ridge to be covered by the addition to the plate. The preparation of the plaster cast and bite is done in the usual way, plain teeth being ground to the gums to allow for the rapid resorption which always follows the extrac- tion of teeth. The waxing and flashing are done in the usual wav. Ironing-in Method. — This method is suitable for replacing a tooth or two, or filling a short crack or a hole. The vulcanite is cut with a file to give a dovetailed form to the space into which a tooth is to be added; and a crack or hole should be prepared for the new rubber with a scraper. The new rubber is ironed into place by using a hot wax spatula and firm pressure. Waxable rubber is better for this work than ordinarv rubber. INTERDENTAL SPLINTS. Interdental sphnts in conjunction with submental compresses and occipito-mental bandages have been used by surgeons in the treatment of fractured jaws since 1780. Drs. F. B. Gunning of New York and J. B. Bean of Atlanta, Georgia, 542 VULCANIZED RUBBER AS A BASE. were the first to describe methods of constructiii;e(l side by side so that they may be cut from it with the least waste. While the patterns should be made accurate, some little allowance must he made when cuttiufij the metal by them for any slight change in position the metal may take during Fig. 531 PaUern for a partial lower plate supporting anterior and posterior teeth, like Fig. a'ZX I'l*" metal form is cut as shown by the dotted lines. swaging. With all plates an allowance of a full sixteenth of an inch should be made on all sides, as shown by the dotted lines in Fig. 532. The indentations to accomodate remaining natural teeth should not be too closely followed. An allowance should be made in the front portion of forms for plates like Fig. 528, as shown by dotted lines in Fig. 530, to allow for a portion of metal to be bent or hooked over the die and so pre- FiG. 532 The shaded portion shows the pattern for a partial vacuum plate where the natural canine teeUi alone remain. The inner doited line tlie paltern tor the vacuum-chamber cover as ordinarily made, and as It should be when extended to the tuberosities to give rigidity to wide, flat plate. vent the plate changing its position during swaging. The writer pre- fers to make these allowances when cutting the metal form rather than when making the pattern; if the pattern shows the size absolutely needed, by fitting one pattern with another or by reducing the allowance at immaterial points, the plate can be cut more economically. x\t places PREPARING THE METAL FORM. 557 representing the necks of the teeth tlie phite shoukl always be cut with a rounded, never with an anguhir outHne. The phate is unavoidably subjected to considerable strain during the swaging process; this is apt to start a break or tear at any point of its periphery weakened by a slight cut or angular indentation. A greater surplus is allowed with partial lower plates to compensate for the greater danger of displacement in swaging; as they are very apt to slip to one side or the other, especially at that portion covering the distal portion of the ridge, it is, therefore, wise to make the form a little wider at this point than the pattern calls for. \Yhile it is well to remember that gold plate depreciates very much in value when reduced to scraps, it is not wise to cut so closely to the pat- tern as to risk losing valuable time in accurately adjusting the form within the plate lines, or perhaps spoiling the piece by its slipping to one side beyond recall. Having laid the pattern upon the metal of which the plate is to be made, selecting a position where it can be cut with the least waste, trace Fig. 533 Pattern for a pE.rtial vacuum plate such as shown by Fig. 526. The metal form is cut as indicated by the dotted line. its outlines with a sharp point or tracer, and cut to this line with straight or curved shears as may serve best, as closely as may be deemed prudent. In cutting the form for a partial upper plate it is best not to follow the outlines of the pattern where it is cut out to fit around the natural teeth, but to leave these portions to be removed after the plate is partly fitted to the die. If the form is accurately cut to the pattern, it must be accurately adjusted to the die — in practice a very difficult task. Fig. 533 shows the outlines of the pattern — the dotted line the size and shape to which the metal should be cut for a plate like Fig. 534. In cases requiring an extra rigid plate, or in cases especially difficult to swage, whether to make a heavy single plate or two thin ones and solder them together, is a matter upon which there is a difference of opinion. A plate made of two laminae thoroughly soldered together, is undoubtedly much more rigid, and is usually heavier than can readily be swaged as a single plate. Apart from this, the slight unevenness in the thickness of the solder, owing to the two plates not being in abso- lute contact at all points, tends to give it increased rigidity. Partial lower plates must be thus stiffened where they pass behind remaining 558 SWAGED METALLIC PLATES. natural teeth. Practice differs as to whether a narrow full lower plate, or a narrow upper clasped plate, horse-shoe in shape, should be so made. In the writer's judgment a full lower plate made of a proper thickness with its edges all around bound with half-round wire, will usually be sufficiently rigid for all practical purposes. A narrow upper plate requires to be doubled over the greater part of its area. The writer prefers to make the plate heavy, and to make the strengthening piece from one-eighth to one-fourth of an inch narrower than the plate, so as to leave its free edges of a single thickness. This does not impair its strength or rigidity, while it permits a ready bending up or down of these edges to accommodate those changes taking place in the mouth after the plate has been worn, which require at times that the plate be made to fit closer, and at other times that undue pressure upon the soft tissues be relieved. There are certain points in all partial plates to be in- FiG. 5:14 Partial vacuum plate to support several isolated teeth. dicated later, which are especially liable to fracture, and which re- quire additional strength beyond that necessary to make the plate suflBciently rigid to retain its shape when subject to the wear and tear of actual use. The resistance of the dies limits the thickness of plate liiat may be used. If to make a plate thoroughly strong, calls for a greater thickness than can be readily and accurately swaged, a double plate is required. MAKING THE PLATE. The sheet metal form is annealed by being heated all over to a dull red heat, being careful, however, especially when working with silver, that it is not made so hot as to be "burned" or "sweated;" that is, fused on the surface. When this has been done the plate is not only rough- ened, but its texture is destroyed, and it becomes in a measure brittle. It is a matter of no moment whether the plate is cooled quickly or slowly MAKIXG THE PLATE. 559 after annealing; it is usually plunged into cold water to save time. It should be annealed every time it is swaged: before doing so it should be placed for a few minutes in a bath of equal parts of sulphuric acid and water, or a boiling solution of alum, technically termed "pickle," to re- move any particles of base metal that may have adhered to or become attached to it. If these are allowed to remain, when the plate is heated they become alloyed with it, and either produce a roughness, or a hole, according to the amount present. There is but little difference between the manipulation of gold and silver in making a plate, except that gold, being stiffer and becoming hard sooner, requires a little more manipulation and more frequent annealing than silver. Making a Plate for a Full Upper Denture. — The poorer die is cleansed by brushing off any particles of sand or metal ; any inaccuracies are cor- FiG. 535 Horn or wooden mallet, used to adapt the metal form to the die preparatory to swaging. rected, and any roughness not shown on the model made smooth. Place it upon a portion of the work-bench, where it will rest solidly and firmly, or, better still, hold it in the jaws of a bench vise, the back part toward the workman. Have the plaster model near at hand so that the posi- tion and extent of the plate may be seen at a glance. Now place the annealed form on the die, having jQrst smoothed or rounded any sharp points left by the shears that might cut the hands, and note the position in which it will best cover the outlines of the plate. Holding it firmly in position with the left hand, with a horn or wooden mallet (Fig. 535) held in the right, bend down the outer edge with light, rapid blows, being careful that the position of the plate is such that it will ex- tend over the ridge far enough to meet the lines drawn on the cast, and also allow for the sides to be drawn in when the plate is swaged 500 SWAGED METALLIC PLATES. into the palatal arch. It is not desired at this stage to make the edge fit closely to the die, but simply to give it a downward tendency, so that when the mallet is used to drive the centre down, the tendency of the plate to slide backward will be checked, while the edges being thus bent, will at the same time naturally be drawn closer to the die. If the centre of the plate should be fitted first, the edges will naturally flare up; the effort to bring them into position would not only tend to again straighten the centre of the plate, but also to stretch and throw the outer edge into folds or creases. "When once this movement has started, it is very difficult to control; the pounding necessary to work out these folds will, by stretching the plate, increases the difficulty. It is best, especially in deep mouths, to work the plate well down to the centre of the die with the mallet before using the counter-die. The skilful use of the mallet "draws" the metal into place; it is free to slide over the ridges as it is bent dow^n into the die by the mallet; as soon as the counter-die is used, that portion of the plate over the ridge becomes "set" and does not so readily "draw in" to the concavity of the die. The metal is stretched, and in very deep mouths the plate may be split if the counter-die is used too soon. The mallet should be held lightly between the thumb and fingers, not grasped in the hand, and the blows struck should l)e light, rapid, and springy; otherwise the plate becomes battered and bruised. Some workmen prefer to fit the plate into the palatal portion of the die by the use of partial counter-dies, that is, counter-dies made to fit into the palatal portion of the die only, and not extending over the ridge. A series of these are recjuired, be- ginning with one taking in the middle portion of the palate only, each succeeding one extending further toward the ridge. During this pro- cess the plate will require frequent annealing, always preceded by a l)ath in the pickle. After the plate has been fitted as well as it can be with the mallet, place it on the die, first covering the face of the die with several thicknesses of thin paper, cloth, such as comes with vulcanized rubber, or a rubber dam, and placing a like covering of paper over the plate, accurately adjust them to the counter-die. At this early stage of the swaging there is more rulibing between the metal and the dies than later, and this paper not only protects the plate from the base metals of the dies, but saves it from being bruised and roughened. Place the die and counter-die, with the plate between them on the anvil, with the counter-die down, and while grasping the die with the left hand, holding it firmly in place, strike it a light blow with the large hammer. Now examine and make sure that the plate has not moved, as it is apt to do, then holding it as before, strike several moderately heavy blows. It is desired at this stage to simply fix the plate in position. Remove it from the dies, pickle and anneal it. In using the swaging hammer grasp the handle near the head of the hammer, and strike deliberately, firndy holding the hammer down so as to prevent any rebound. If the hammer is allowed to rebound, as it tends to do, the die follows, and in setding down again, is apt to slightly change its position. It requires very few of such blows to produce a plate no one can ^fAR'^XG THE PLATE. 561 make fit the model accurately. The character of the force used in swag- ing is a matter of great importance, not only in making a plate to fit, but also in making a plate that will not change its fit in subsec[uent opera- tions. The nearer the hammer blows resemble those of a drop- press, or a pile driver, the more effective they will be. If it were practicable to make dies and counter-dies suitable for it, a screw or hydraulic press would be ideal for swaging dental plates. Now, examine first if the plate is in a position to well cover all the Hues; next note if there are any wrinkles or creases on the palatal portion of the plate. With some forms of palatal vault they are apt to occur at the posterior edge of the plate, and should be promptly straightened with the mallet. Then see that the plate is well down to the die at the central portion of the palate. At times this first swaging tends to draw" the plate up, especially so if the counter-die extends too far over the die. If the plate does not fully cover all the lines, its position can be changed at this stage by bending with the mallet. It is important, before pro- ceeding further, to make the palatal portion of the plate fit closely to the die, using the mallet to drive it down if necessary. The portion extending over the ridge next requires attention. First straighten out any T\Tinkles that may have formed, and make the plate fit as closely as possible the outer portion of the alveolar ridge. To facilitate this, cut off with the shears any surplus portions of the plate that extend to a marked degree over the fines, remembering, however, when so doing, that the tendency of further swaging may be to draw these edges up. In some plates it is necessary to remove a V-shaped section at the frsenum be- fore they will lie close along the outer portion of the ridge; it should not be done, however, at this stage. During these manipulations w^henever the plate becomes rigid and unyielding, it should be annealed. When the plate has been made to fit well as it can be with the mallet, it is annealed and again swaged between the dies, using paper or cloth on each side as before. After a few light blows, separate the dies sufficiently to see that the plate has not changed in position; if found satisfactory in this re- spect, it should now receive a thorough swaging, moving the dies around with the left hand and striking hea\y blows over successive portions of the die. Attention is now given to the vacuum-ca\ity. If it is of the Gilbert pattern, swaged with the plate, two special tools are needed to give it a sharp outline, and to make it fit at its edges close to the roof of the mouth. First is a chaser made of bone. This may be made of a tooth- brush handle, one end of which is filed to a moderately sharp, smooth chisel-like edge, rounded at its corners, and about one-fourth of an inch wide. This edge ^\*ill require frequent rene■u^ng, as it is soon battered by use. It is used by placing the edge of the chaser in the imprint of the chamber while the plate is on the zinc die, and holding it at a slight angle as one would a chisel in cutting a groove, striking light rapid blow^s on the end with a mallet or bench hammer, so as to drive it down and forward. Pass it round the chamber in this way repeatedly, so as to coax the metal into place. 36 5()2 SWAGED METALLIC PLATES. Jf the blows are struck too hard, the chaser will indent the plate, and the indentations once made, are very difficult to remove. In some cases this bone chaser will complete the vacuum-chamber. If greater sharpness of outline is desired, it is followed by a steel chaser with a thinner, but well rounded smooth edge, polished like a burnisher. This is used in the same manner, but with caution. If used to excess it may cut a groove that must be filled with solder to make a neat finish. This chasing of the vacuum-chamber is only partly finished on the first die; it is completed after the plate has been lightly swaged on the second or finishing die. During the process, the plate must be frequently an- nealed. If a soldered vacuum-chamber istlesired, and the form for the chamber has been cast on the die, the proceedure is somewhat different. An opening is cut in the plate corresponding to the chamber, but not to its full size at this stage. Until the plate has been thoroughly swaged on the second or finishing die there is a possibility of its slightly chang- ing its position, for this reason accurate fitting to plate lines is deferred until that time. Roughly cutting off the surplus plate with the shears, chasing the chamber, or cutting the opening for a soldered chamber, no matter how carefully done, distort the plate to some extent; there- fore, all this should be done before commencing to make the plate fit the model accurately. The final fitting to the plate line is readily done with the file without risk of bending the plate, and at the time this is done, the opening for the chamber is enlarged until it exactly coincides with the base of the wax chamber on the cast. A covering piece is then swaged up as sharply as can be done with the dies; it is not necessary to use the chasers. This is then trimmed all round to leave a margin of about one-sixteenth of an inch; if it is desired that this margin should be well defined on the finished plate, the edge is filed square; if on the contrary this is not desired, it is filed from the under side to a feather edge. In some cases the back edge of this covering piece is ex- tended to the plate line, not alone for additional strength, but to increase the thickness at this point to provide for filing from the underside to re- lieve pressure upon the hard palatal ridge. Again, in a very broad mouth, the covering piece may be extended toward the tuberosities to give stiffness and rigidity to the plate. Instead of making a wax model of the chamber on the plaster model and reproducing it on the die, a better result is obtained by making the model of the chamber in copper or brass as thick as the chamber is in- tended to be deep. A piece of copper or brass of proj)er thickness is fitted by means of the bench hammer into the first die in the position of the desired chamber, held in place by a little adhesive wax, and swaged between the die and counter-die. As this usually changes its position it should be sufficiently large to well cover the lines of the chamber notwithstanding this, so that when it is shaped it will accurately fit the die in the position marked for it on the cast. This is done with the first die and counter-die. It is then made the desired shape and size, and the edges are smoothly finished with a slight bevel. An expert work- man has no diflSculty in attaching this to the finishing die with adhesive MAKING THE PLATES. 563 wax and swaging it into the counter-die so as to produce in the counter- die a depression into which the covering piece is swaged. There is, however, a marked tendency in the chamber model to shde backward or sidewise, to prevent which the writer is accustomed to tack it to the plate, whether of gold or of silver, with the least possible mite of silver solder. It can be readily held in position on the plate with a wire clamp while this is being done. By this expedient all risk of change in position is obviated, and after it has been swaged so as to make an impression in the counter-die, by inserting a point under its edge, and heating the plate, the chamber model is released. The impression made in the counter-die requires deepening by means of flat gravers before swaging the covering piece. This method has the advantage that making the sand mold is not complicated by the pres- ence of the chamber model; it also avoids bending up the edges of the chamber when swaging the plate. The plate, the chamber model, and the covering piece fully finished and ready to solder in place, can be swaged between the die and counter-die at the same time, making a close and easily soldered joint. The opening in the plate is roughly cut when the finishing counter-die is ready for swaging the covering piece, and is finally enlarged and shaped just before the cover is sol- dered in place. After the plate has been well swaged to the first die, and roughly shaped to the plate lines, especial attention is given to that portion ex- tending over the alveolar ridge. As a rule, the extreme edge of the plate has a greater length than the plate outline marked upon the cast; it is evident that to adapt one to the other, at least a portion of the sur- plus length must be cut out, or the plate must be so manipulated as to contract it along this line, or else the amount of surplus length will be represented in the plate by wrinkles. By removing the wrinkles by light blows with the horn mallet as they are formed, the plate can be so adapted that this surplus length may be accomodated by a slit, or a V- shaped piece removed from the plate at the site of the frsenum of the upper lip. By careful manipulation with the horn mallet, and by swaging with blows along the sides of the counter-die, driving the plate toward the die along the alveolar border, this slit may be avoided in favorable cases. While this can seldom be done, it is best to reduce the wrinkling as much as possible in this way; then, when the plate is fairly well fitted to the die, to simply cut a slit with the shears at the centre of the plate, extending it to nearly the top of the ridge, but not over it. If it is cut too far, the edges are apt to separate at the apex of the ridge, leaving a hole to be either filled with solder or covered with a scrap of plate, either of which is objectionable. After making the cut, bend the cut edges apart, and bevel the opposing sides so that when they lap over they will make a smooth joint. If the overlap is excessive, it should be reduced, but a liberal overlap is not objectionable. This slit is not to be soldered 564 SWAGED METALLIC PLATES. until the plate is fitted to the model and accurately trimmed to the plate lines; if it is closed too soon, it may be impossible to make the edges of the plate fit the model as closely as they should. The plate should be finished as nearly as possible upon the first die. The first bending of the plate into shape unavoidably subjects the first die and counter-die to an uneven stress; those i)ortions which are fir.st brought into forcible contact with the plate, together with the sharper prominences, become battered. It is on this account that the second die is required. One careful swaging on the finishing die should be all that is needed to make the plate fit the model accurately. If the plate has a soldered chamber, the covering i)iece, fully finished and readv to be soldered in place, should be in i)()sition when this is done. After this swaging, make the outer edge of the plate closely fit the cast at the plate line, and then solder the front slit. In many cases after this has been done, a little manipulation with the bench hammer to bring the plate in close contact with the model at its edges is all that will be needed to make the plate fit satisfactorily. Testing the Adaptation.— To test the adaptation of the plate, place it on the plaster model, and note first that it does not unduly bind on the alveolar ridge. If the outer edges spring in to the cast the plate will be held firmly on the model, although it may not fit. While the edges must fit closely, they must not bind, and may require to be slightly bent outward to avoid this. Now see that it rests solidly upon the model, that it does not tend to spring up when pressed down, or to rock when pressure is made along the top of the alveolar ridge alternately at the aides, or when pressure is made at the back edge of the plate and on the top of the ridge at the sides or in front. If the plate shows no movement under tliese tests, if it has a swaged chamber, all that remains to be done is to smooth the edges, clean and polish it preparatory to testing it in the mouth. If, however, it should spring or rock, the cause of this must be sought and the defect corrected. The spring may be due to the outer edge pressing unduly at some point, and holding the plate from contact with the model. When this point is located, a slight bending with the fingers will usually correct it. It not unfrequently happens that the plate will bind upon the outer side of the tuberosity, prevent- ing its back edge from fitting solidly to the model. When this is the case, a few blows with the bench hammer stuck along the back edge of the plate on each side just inside of the tuberosities, while it is firmly held upon the cast, will usually prove effective. There is quite a "knack" in using the bench hammer in giving the finishing touches to the fit of a plate while it is on the plaster model, so as to effect the object without bruising the model. It is something that cannot be described, it must be accpiired by observation and practice. That the final fitting needs to be iiit to he horiK" in mind. KiH"|)in- a molar tooth. A supplemental clasp on the canine tooth ensures greater firmness, and relieves the molar of a strain it would otherwise sustain. A supplemental clasp may be adjusted to any tooth deemed best, or most available. from which the teeth are removed by means of a cold chisel or a hack- saw, supplemented by a coar.se flat file until nothing more than an out- MAKING THE PLATE. 573 An unsatisfactory form of clasped plate supporting an anterior tooth and sustained by a ciasp on a molar tooth. Owing to the leverage upon the clasp, not only is the plate liable to displace- ment, but in addition the strain upon the sustaining tooth is excessive. In such cases it is more satisfactory to make the plate as shown by the dotted line, clasping a bicuspid tooth on each side. Fig. 543 line remains. Sufficient should be removed that the plate may be swaged over them without being torn, and yet enough left to plainly mark their posi- tion on the plate, as a guide when fitting it to the cast. To prevent the plate sliding backward it is occasionally desirable to let the tooth immediately posterior to the plate line remain, and to make in it a saw- cut to receive the back edge of the plate. This sliding backward of the plate during the swaging process must be guarded against at all stages of making a partial up- per plate, especially those embracing only the anterior portion of the cast. They do not, as a rule extend over the alveolar ridge, nor yet are they supported by the flat por- tion of the palatal vault. Resting as they do upon a sharply inclined surface, their natu- ral tendency is to slide backward as the die is forced into the counter-die. In making a plate such as Fig. 540, it may be treated, in all essential particulars as though it was a full upper plate, the only difference, in- , L°wer piate-bending pliers, used to , T • 1 i> n 1 1 • 1 bend a lower plate into a gutter-like deed, between it and a tuU plate bemg the form preparatory to swaging. They extent of surface it covers. In making a ^re also used to give a concave form smaller plate, such as Figs. 541 or 542, the r^Xi, ^Zr tS".:'"'' '° " r,Tl ^\vA(;i':u metallkj i'i.ates. cliicf (liffifulty encountered is usually to hold it in j)lace until suffic- iently swaged as to become fairly well fixed in position upon the die. This may be facilitated by sharply beiuKno; the edge with a pair of flat pliers so that it rests upon the portion of the die representing the ridge, or in some cases, first fitting it into the counter die. Now and again it may l)e more securely held by raising u})on the counter the with a graver one or more strong burs, just at the edge of the plate, bending them over the plate so as to hold it in position during swaging. A small narrow plate is less manageable with the mallet when fitting it to the die prelimiiuiry to swaging, and is liable to displacement during the early stages of this operation. When the plate is well swaged, on the first die, it should be roughly shaped to the plate lines, and then finally swaged upon the second die. Unless it is by its form securely held in position, this must be carefully done to avoid swaging it to a new position and thus spoiling the plate. In all cases until the final swaging is complete the plate should be allowed to extend slightly over the anterior line, as with all possible care there is a slight movement from it. The suggestions as to frequent annealing, and the precaut- ions to be ol)served to avoid contamination with base metals apply with equal force to all swaging operations. Small plates, and partial plates simple in form, may be constructed upon one die; there is, in such cases, less for the die to do. After the swaging is complete and the plate fits the model satisfacto- rily it is then carefully shaped to the plate lines, especially those along the lingual aspect of the remaining natural teeth. As a rule, the plate should fit their margins accurately. In exceptional cases the anterior plate line may be located w^ell within the line of the teeth, as for instance to avoid interference with opposing, teeth, etc.; some operators prefer that it should do this in all cases, holding that cleanliness and comfort are promoted thereby. The plate is now ready for soldering in the chamber, or to be fitted with clasps, if these are to be added, after which the edges are smoothed, its surface well cleansed and prepared for adjustment to the mouth. Full Lower Plate. — Before commencing to form a low^er plate upon the dies, when the ridge is sharp and prominent, it is an advantage to give it a gutter-like form by use of the lower-plate, bending pliers Fig. 54,3, or in default of these it may be readily done with the blade of the bench hammer, the plate being held over a V-shaped groove formed in the end of a piece of, hard wood held in the bench vise. The plate thus prepared can be fitted to the dies more rapidly and with less injury to the metal of which it is made than if the work was entirely done upon the dies. Where the ridge is wnde, however, it is better to form it over the dies, beginning at the mesial line by first slightly bending the outer edge down with the mallet, and then fitting the inner portion, and so progressively on each side toward the distal end of the ridge. When the plate has thus been made to conform to the die suflHciently well to retain its position it is lightly swaged into the counter-die. As in making an upper plate, this must be done cau- MAKIXCr THE PLATE 575 tioii-sly and any mal-position promptly corrected. If the sheet metal form is correct in shape but little difficulty will be experienced. If, however, this is faulty, the tendency to bend over inside the ridge if the form has been of too sharp a curve, or to the outside if it has not been sufficiently curved, will be pronounced and difficult to overcome. If the error is not excessive, patience, and careful manipulation with the fingers and the mallet may correct it. If the plate is made of a single thickness of metal, the swaging, fitting, and testing the fit, etc., differ so httle from that of an upper plate, that further description is unnecessary. It is usual, after a lower plate made of a single thickness of metal is complete and has been tested in the mouth and found to fit satisfactorily, to solder a half-round wire along its edges, inside and outside if the denture is not to be rimmed, and inside and around the distal ends only if a rim is desired. The object of this is not only to afford additional strength, but to make the edge thick and rounded and less liable to cut into the soft tissues. The wire should be quite light, say, one-sixteenth of an inch wide on the flat side. It is fitted and sol- dered to the plate in the following manner : take a piece of half-round wire sufficiently long for the work in hand, and beginning at a point about an inch from the left distal end of the plate, lay the flat side of the wire against the inside edge of the plate, bending it with the pliers so that it will fit accurately along the edge for about an inch. Do not begin at the end of the wire, but leave enough to well go round the distal end and hold this in place with two bands of binding wire (annealed iron wire of about 24 guage). The binding wire is passed over the plate and its free ends twisted together. Make the edge of the half-round wire lie exactly on the edge of the plate. There is a tendency in the binding wire to draw the half-round wire too far in; this is corrected after it has been made tight, by placing the edge of one blade of a pair of pliers ou the binding wire, just beyond the inner edge of the half- round wire, and the other blade at the edge of the plate; a slight pres- sure will bend in the binding "W'ire and hold the half-round wire securely in place. The binding wire should not be twisted too tight, or it will distort the plate. By twisting it just enough to hold, and then pressing that portion which passes under the plate into the gutter of the plate with a blunt instrument, it will be drawn sufficient tight without risk of bending the plate. This portion of half-round wire is now soldered; in doing this, the plate must be carefully supported at all points to prevent its changing shape during the operation, and the blowpipe flame carefully directed so as not to melt the free portion of half-round wire. A skilful workman will generallv at this stag-e fit the T^'ire all around and complete the operation in one additional soldering; this is not necessary, how^ever; if found more convenient to fit and solder an inch or two at a time, there is but little risk of warping the plate by repeated soldering if the plate is well supported. In applying heat do not direct the flame directly on the wire or it will spring away from the plate. The soldering should be done continuously from the point at which it is commenced. If it is tacked at intervals it is apt to 576 SWAGED METALLIC PLATES. spring from the plate when the intervening spaces are soklered. Do not use too much solder, if the wire is neatly fitted to the plate and held in position with binding wire at short intervals, very little will be needed. Sufficient heat should be applied to make it flow freely. If the case is to be rimmed omit the wire from that portion the rim will occupy. "Wiring" a lower plate, as this operation is called, is a decided improvement, and, while adding very much to its appearance and strength, makes it more comfortable for the patient. While it is not impossible to res wage a lower plate after it has been wired, the oper- ation is complicated by the presence of the wire, especially if the alve- olar ridge is thin and sharp. If due care is used in properly supporting the plate, and avoiding excessive heat, it is seldom, indeed, that there is any serious change in the fit, no more than can readily be overcome without recourse to the dies. In constructing a double plate, make each plate separately until both are swaged to fit the cast accurately, but do not trim them quite Fig. 544 Two thin lower plates held together with iron binding wire preparatory to soldering. The underneath plate, as shown, extends a little beyond its fellow so as to form a ledge convenient for placing the solder; this ledge is removed when the plate is finished. to the plate line, letting the one that is to be underneath extend a little beyond the other so as to form a ledge upon which to place the solder. After annealing, thoroughly cleanse the surfaces that are to be in contact and coat them with borax ground to the consistency of cream, very smooth, and free from any gritty particles. Then place them between the dies in the relation to each other they are to occupy when soldered, placing a thickness of paper on each side so that they will not come in contact with the dies, and swage the two together. The paper is used to avoid the necessity of "pickling" them before solder- ing. Usually when swaged together they are brought so closely in contact that, if carefully handled, they hold together with sufficient MAKING THE PLATE. 577 firmness to be laid upon the solder support and soldered without separating. It is desirable that they should do so; if they do not, hold them together with binding wire at, say, three or four points as shown by Fig. 544. In using binding wire for this and similar purposes, it is well to remelnber that twisting the ends together practically has the tightening effect of a screw; without apparently using much force it is quite possible to draw the edges of the plate together and seri- ously impair the fit. To avoid this, twist the ends of the wire barely tight enough, and further tighten the wire so as to hold the plates firmly together by slightly bending the wire that passes across the edges, either by pressing it in with a blunt instrument, or slightly by twisting it sideways with a pair of pliers. It is by these means made sufficiently tight without being so rigid as to bend the plate. Al- ways make it a rule to twist the ends of the binding wire in one direction : while it is in all cases a matter of choice, the habit once formed of doing it in a certain way will avoid the annoyance of disturbing the wire, by twisting it in the wrong direction when making the final adjustment before soldering. Usually one wire in front, and one on each side near the distal ends, are all that are required; if, however, there is a tendency in the plates to spring apart at any point, an extra wire or two may be used to restrain them. After adding a little fresh borax at the edges, the plates are adjusted to place on the soldering support, carefully supported at all points so that they shall not bend with their own weight when heated, and a few narrow pieces of solder of the same fineness as the plate, placed along the lingual border of the ledge formed by the lower plate projecting beyond the other. No solder is placed upon the labial and buccal portions of this ledge. The solder is to be drawn through frorn the lingual side, so as to give assurance of a perfect union of the plates throughout. The blowpipe flame is passed above it, not on it, until efflorescence of the borax ceases, when a broad flame is applied to the plate until it is heated to a uniform red, when the fine flame is directed against the solder pieces, fusing them one by one. Other pieces are added until there is a uniform line of solder along the lingual edge. Unless one is expert with the blowpipe, it is better not to place too much solder in position at the beginning, as should the plate not be evenly heated, it would tend to flow to one point, and perhaps over instead of between the plates. By adding a little at a time, it is much more manageable. A larger flame is now thrown upon the labial and buccal aspects of the plate until these portions are at a higher temperature than the lingual edge, the heat is cautiously carried forward until the entire mass of solder is seen to flow like water and appear at the labial and buccal por- tions of the joint, uniting the plates perfectly. This thorough union of the plates is very important, as if a small portion remains unsoldered the plate will be apt to rise up at that point in subsequent solderings, forming what is technically known as a "blister." When cold the bind- ing wires are removed and the plate boiled in the acid solution. The 37 578 SWAGED METALLIC PLATES. ledge of the lower plate is trimmed away, using for this purpose the plate nippers, or the points of a very sharp pair of curved shears, com- pleting the trimming with files until the plate outline corresponds with the i)late lines on the model; its edges are then rounded, and smoothed with fine sand- or emery-paper. The plate may be reswaged after soldering, if necessary, but if made to fit accurately before, antl carefully handled during that operation, it is seldom that there is any material change in the fit. It is a serious mistake to depend upon subsequent reswaging, and to solder the plates Fig. A partial lower plate supporting the posterior teeth, a natural molar tooth on each side remain- ing. It it desirable in some cases, in order to hold the plate more firmly in position, or to relieve the sums of the pressure of mastication, to extend the plate over the molar tooth of one or both sides, as shown. The reinforcing piece extends backward to the dotted line. together before they fit the model accurately. After the plates are soldered together they become quite rigid, and while a slight warping may be readily corrected by the dies, it is very difficult to effect any material change in their shape. A carefully swaged double plate should require comparatively little solder to thoroughly unite its .sep- arate portions. If by any mischance they should separate during the operation, and an excessive amount of solder be required, it will be apt to make trouble when soldering the teeth; sometimes by running out at the joint, and at other times by melting its way through the plate. The pos.sibility of this enforces the importance of care and exactness at each stage of the work. A partial lower plate is not as a rule cut away to conform to the gum line of the teeth as is an upper plate, but is allowed to pass up over MAKIXa Till': PLATE. 579 them, and is made to fit closely their lingual surfaces. This is done partly to give greater strength to the plate by increasing its width at points where it otherwise would be quite narrow, and partly to allow it to rest on or against the teeth, and thus assist the narrow ridge in bearing the pressure of mastication. It also makes a more comfortable plate, as the edges are not so liable to press into the gum, nor yet are they so perceptible to the tongue. In preparing the first zinc die, the teeth are cut off a little above the plate line, with a sharp downward 'bevel on the outside, so that the plate will hook over and be less liable to be driven down during swaging. Teeth standing alone, and the end Teeth of a series should be rounded on their approximal sides so as not to tear or split the plate. Teeth which are to be fitted with clasps, or when for any reason the plate is not required to extend over them, may Fig. 546 Showing the first used metallic die for a plate like Fig. 445, prepared for swaging. The molar teeth maj" be cut off as shown on the right, or a saw-cut may be made as shown on the left, to receive and hold the plate in position during the first stage of swaging. The reinforcing piece extends back- ward on both sides to the dotted line. be cut down to the gum line, leaving sufE.cient only to mark their out- line upon the plate. When the teeth to be supplied are in the front, and all together, as in Fig. 539, the manipulation of making such a plate differs but little from that of making a full lower plate. The backings of the teeth and the solder used to secure them to the plate, together with a heavy piece of plate extending from the clasp on either side to the backing of the nearest artificial tooth, added when the teeth are soldered, gives it all the stiffness needed. \^'Tien the front teeth are in and the back teeth of either side are to be supplied, as shown in Figs. 528 and 545, the task is much more difficult. The front teeth are filed from the first die nearly but not quite to the plate line as in Fig. 546, and sloped so as to leave a sharp edge, over which the plate is bent so as to hold it in place during swaging. The molar teeth in a case like Fig. 545 are cut off, so as to leave only enough 580 SWAGED METALLIC PLATES. of tlie teeth to make a slight indentation in the plate to serve as a guide in tiling it to fit around tlieni. In some cases instead of cutting off the molar teeth it is better to make saw cuts as close to the gum as possible, and let the ends of the plate pass into them when fitting it to the die, so as to assist in holding it in place. (Fig. 546.) There is usually some little difhculty in commencing a plate of this kind. It is apt to slip back, or when fitted to the front teeth it does not well cover the ridge on each side. This difficulty is very much increased if the pattern by which the piece of plate is cut is faulty or has not been accurately copied. After htting it to the die with the mallet sufficiently to permit it being placed in the counter-die, it may be bent and securely held in place by means of burs raised from the lead, and lightly swaged so as to fix it in position. The first effort should be to get the front part of the plate in proper position; this accomplished by bending and holding the sides in their proper place in the counter-die, the difficulty can be overcome by light swaging and frequent annealing. After the plate has been swaged to fully conform to the die, it is very difficult to make any change in its position. In swaging partial lower plates such as these there are three points that recjuire special attention: first, the tendency in the plate to slip down in front, which is continued, unless there is a marked offset at the lingual gum line, from the first to the last swaging of the plate. It is checked by making the plate hook over the front part of the die; and this portion should not be removed until the plate is so far advanced as to be ready to swage upon the second die. The plate should not be filed accurately to the upper front plate line until it is fully fitted to the cast, and is completely finished so far as swaging is concerned. If the front teeth have an outward lean, and there is but little offset at the junction of the teeth and gums, the plate will work down in spite of all care; to provide for this, allow it to extend a full sixteenth of an inch beyond the plate line at this point. Second, a tendency to fold over or form a crease, generally about the position of the canine teeth. This must be closely watched and the fold hammered out as soon as seen and before it becomes fixed. Third, the plate is liable to split at about the same points, and also, in some cases, about the middle of that portion of the plate covering the outside of the ridge on either side. Frequent annealing, skilful use of the mallet, and cutting away the surplus metal at these points as soon as it can be safely done, will usually prevent it. If the split is noticed in time, by cutting it off, if there is sufficient margin, or where this cannot be done, soldering a piece of plate over it, will usually arrest its progress. It is desirable that those portions of the plate covering the lingual surfaces of the natural teeth fit closely to them and into the interspaces; it is, therefore, important that the dies be accurate at these points. Usu- ally the portions representing the interspaces will need a little carving; this is not so necessary for the first die, but the second or finishing die should be made as accurate as possible. To secure close adaptation at this portion of the plate, proceed as follows: after the plate has been swaged as much as is considered necessary on the first die, and has been MAKIXG THE PLATE. 581 well annealed, place it on the second die and holding- it firmly in place, with a luininierorniallet antl a hone eliaser sucli as is used inniakinga swaged vaeuuni-eavity, drive the plate into the interspaces and the fes- tooned outlines of the gum of each tooth. If the interspaces are deep and sharp, strike light blows going over it a number of times, and if need be, annealing the plate during the operation so as to "coax" it into place. The edge of the chaser will need resharpening, as it breaks down rapitUy. Be careful not to cut through the plate, an accident that will occasionally happen, and while not a serious mishap, one that should be avoided. This operation usually bends the plate very much out of shape, and sometimes changes the position of that portion extend- ing toward the distal end of the plate. On this account the plate should be only approximately trimmed to the lines until it is completed and the plate well swaged on the second die. After the plate has been thus fitted to the lingual surfaces of the teeth, it is fitted on the second die and lightly swaged. First, see that it has not materially changed its position, that the front portion has not been driven down, and that the posterior portion reaches the plate line on both sides of the ridge. Any mal -position is readily remedied at this stage, by read- justing the plate to position in the counter-die and holding it in its proper place by burs raised along its edge while it is again swaged. Next, examine the plate on the model to see that it fits the model and die alike. It may be that at some points the die may have "dragged" slightly, which will be shown by a space under the plate at that point when the plate is placed upon the model. If this is the case, carve the die so that the plate fits both alike; this can be done very accurately by carving the die, placing the plate on it, and with a hammer making the plate fit the die at the point carved, and testing it on the model, repeating this until they are both alike. The points needing special attention are around the teeth next the spaces to be supplied, the interspaces, and the edges of the plate. When this has been done, again go over the front portion of the plate to make it fit against the lingual surfaces of the teeth accurately, using a steel chaser if necessary, holding the tool so that it will carry the plate into place, and not simply make an indenta- tion into it or cut it through. The plate is now thoroughly swaged on the finishing die, and made to fit the model accurately. It is then ready for the reinforcing piece. In some cases that portion of the plate extending inside the teeth is very narrow and so nearly flat that three thicknesses of plate are needed to impart the required stiffness. In other cases it is wide, or so corrugated that two thick- nesses of comparatively thin plate are quite sufficient. If three thicknesses are deemed necessary, the first reinforcing piece is made narrower than the plate lines, and to extend only a short distance beyond the last tooth of the series on each side. After being swaged it is filed to shape, and its edges all round filed to a feather edge. It is then adjusted to place on the plate, (both it and the plate having been made clean and prepared for soldering), held by a clamp or TN^ith bind- ing -vsire as may be most convenient, and a trifle of solder fused at some 682 SWAGED METALLIC PLATES. point where the two are in contact, i)referal)ly at a central point, with the object of holding them together so that they can be swaged together. After this, the soldering is completed. The second reinforcing piece is now made. This should be larger, fully the width of the plate lines as far as the natural teeth extend, tapering and extending beyond the last tooth of each side to fully half an inch, as shown by the dotted line. Figs. 545 and 540. No special care is taken to make this fit into the inter- spaces, but as soon as it is sufficiently swaged to j)crmit its being prop- erly shaped, it is tacked to the plate with a small portion of solder and swaged with it. The denture is more easily kept clean, is more com- fortable to the tongue, and has a more artistic finish, if this portion of its lingual surface is smooth and even; it is not desirable, therefore, that the front portion of the reinforcing piece should be swaged into the interspace with the same accuracy as was the plate. It is worthy of note that the reinforcing pieces are not made to fit the model ; they are fitted to the plate, and as the surface of the plate against which they fit difl'ers very much from that which fits against the model, the plate should be in place on the die, and be considered as part of the die, when the re- inforcing pieces are swaged. Before attaching the reinforcing piece to the plate, the upper edge of the tapering ends should be well bevelled. The plate should extend slightly beyond the plate line as far as the re- inforcing piece extends, so as to provide a ledge on which to place the solder; otherwise, the solder is liable to flow under the plate instead of between it and the added piece. If the two have been tacked together it is seldom that a clamp is needed when the final soldering is done. As in soldering a double plate, every care and effort should be taken to make the solder flow thoroughly, and to unite every portion of the two surfaces in contact with the least possible amount of solder. As to accomplish this the heat must be quite high, it is important to thoroughly support the plate to prevent its warping, especially if it is of silver. With pro- per care, it is seldom that the fit is materially altered. After the re- inforcing piece is soldered, and the plate has been cleansed from the borax, etc., it is ready for final adjustment to plate lines, and the portion beyond the reinforcing piece is bound with half-round wire. This done, the plate is fitted to the model, the edges neatly rounded and made smooth and its surfaces thoroughly cleansed, when it is ready for ad- (justment to the mouth. CLASPS. The metal of which clasps are made should possess stiffness and elasticitv in a marked degree; it should also be tough and free from brittleness. An allov of gold with about two grains of platmum to the pennvweight answers the purpose exceedingly well; if carefully made of pure metals it is tough, stiff, and, even when well annealed, is as elastic as a piece of spring-tempered steel. In practicai use clasps are CLASPS. 583 Fig. 547 Fig. 548 A properly shaped clasp. subjected to considerable strain; in addition to this, from their posi- tion, they are constantly liable to accidents while the denture is being handled and cleansed l)y the patient. To withstand this the metal must possess tenacity. To test its tenacity, grasp a corner of the well annealed metal firmly with a pair of flat pliers and quickly bend it to a sharp right-angle. If this causes the least sign of a break, the metal should be rejected. Other alloys than that of platinum and gold have been suggested and used for silver dentures on the score of economy. While they have answered the purpose, none possess any practical ad- vantage over the more generally used alloy of gold and platinum, while the slightly smaller cost is overbalanced by the difh- culty of profitably utilizing the scraps and filings. Laboratory practice is simplified by making an eighteen carat gold and platinum alloy the standard for all clasps used on soldered dentures, whether of gold or silver. The alloy is used for clasps in the form of plate of about No. 25 gauge — seldom heavier than No. 22, and seldom lio'hter than No. 27 — and in the form of CI half-round wire. Half-round wire is used when the teeth are short, so as to obtain strength with a narrow clasp, and when the clasp passes around the tooth in an irregular line to avoid pressing upon the gum, or to obtain a firmer hold upon the tooth. The peculiar shape of half-round wire permits its being bent in any direction, and on this account it can be more readily fitted to short molar teeth, and a clasp made of it will often grasp them far more firmly than would one made of plate. The thickness either plate or wire should possess depends entirely upon the work required of them and the position they occupy. It should never be made so hea\7 as to be practically Vigid : the free or unsoldered porton of a clasp should be elastic, not only to firmly grasp the teeth, but also to slightly A-ield under severe strain, and thus ease the strain upon the tooth to which it is adjusted. The strongest portion of all clasps should be where they are united to the plate, and from this point they should taper to the free ends. It is a common and a serious fault in making clasps to file from the under edges where they pass around the appromixal surfaces of the teeth, so as to keep thena from pressing unduly upon the gums, without at the same time so shaping the upper edge that the clasp shall be oi uniform strength. The effects of this is to make a weak spot at a point where the clasp should be the strongest; all the bending takes place at this weak point, not only impairing its value as a clasp, but invariably causing it to break. ' If after properly shaping the lower edge the clasp is reduced in width toward the free end so that the hollowed out A clasp improperly shaped. Having its plate edge filed away to fit the gum makes this por- tion the weakest point , impairing its usefulness b y d e- stroying its elasti- city, andmaking it liable to fracture. 584 SWA a ED METALLIC PLATES. portion ceases to be the narrowest part of the clasp, its strenfi:th, useful- ness, and (hiral)ility will he very nuich increased. A properly shai)ed clasp, opened out, would approximately resemble Fig. 547; one with the defect referred to is represented in Fig. 548. In each case the attachment to the plate extends from a to b. If it is necessary that the clasp be narrower near the plate than at the free end, as is sometimes the case, the narrow part should be nuide correspontlingly thicker — not by soldering an extra piece upon it, but by making it of thicker plate, and filing the wider portion thinner, so that in proportion to the strain upon it, it will be of uniform strength throughout, and as free to bend at one part as another. Before begin- ning to make a clasp carefully examine the shape of the tooth, its posi- tion in the mouth, and its relation to the plate, so as to form an idea of the direction and amount of strain it will be required to resist, and from this regulate its size, shape, position, and strength. In making clasps, fit them to the teeth before soldering only so far as is necessary to adjust them to their proper position — usually only the palatal and approximal surfaces, anterior and posterior — leaving that portion which is intended to pass around the buccal or labial surfaces straight. The object of this is, first, that they may not bind upon the teeth so tightly as to prevent their being readily removed from the model without change when cemented to the plate prior to investing for sol- dering; and second, for the reason that in many cases they are more securely held by the investment when so made. The remaining por- tion of the clasp may be fitted approximately either to the model, or when the plate is adjusted to the mouth, but only so far as to lightly retain it in place, the final fitting being left until the case is entirely finished. If prior to this they are fitted so as to hold the plate firmly, it gives a great deal of extra trouble in adjusting the teeth, without any corresponding advantage. In making clasps the following tools are needed: a pair of strong, but not too massive round nose-pliers and a pair of narrow-beaked flat pliers, with the inner edges of both blades rounded so as to remove the sharp square edge. These are essential. A tool known as a clasp bender is frequently useful, as is also the lower plate-bender, (Fig 543), the former is used to concave the clasp along its length to fit a markedly rounded surface, such as is occasionally found on molar and bicuspid teeth. If this tool is not at hand, the same effect may be produced with the riveting blade of the bench hammer. In making clasps for very difficult cases it is occasionally necessary after fitting them with the pliers, to swage them between dies: this is required when the surfaces to be fitted are rounded or the shape of the tooth very irregular. It is not usually necessary to make a pattern by which to the cut the clasp material to shape. It is better to prepare a strip of plate a little wider than the finished clasp is intended to be, so as to allow for filing the edge to fit the gum at the neck of the tooth, and long enough for several clasps; in fact a little longer, as it is quite diflficult in practice, without CLASPS. 585 unduly wasting time, to make the first bend precisely in the right place; it is always best, therefore, to have a little margin for possible errors. Having the strip long makes it very much easier to handle. In case the lingual surface of the teeth are markedly bulging, as is frequently the case with upper bicuspids and molars, a more accur- ately fitting and more effective clasp may be by first curving the strip making the side that fits against the tooth concave, so that it will fit over the bulging portion of the tooth. This may be done with the blade ' of the bench hammer, resting the strip upon a lead counter-die or on the anvil, but more conveniently by the lower plate-bender represented in Fig. 543. In fitting a clasp to the molar tooth, such as that seen in Fig. 547, lay the strip flat against the anterior approximal surface of the tooth, allow- ing one end to project beyond the buccal surface as far as it is intended to pass around that corner of the tooth, and mark with a pencil or point or note with the eye the position of the first bend to be made in fit- ting the clasp to the palatal surface, and with the round or narrow- beaked pliers or clasp-benders, as may be preferred, bend it, but at first not quite as far as necessary, so as to be able to correct any error made in the first bending without unduly straining the metal by bend- ing it back again. The clasp is now placed as nearly in position on the cast as its shape will permit. Note where it should be bent, so as to pass around the posterior approximal surface, and cautiously bend it into the required shape. Usually it will be necessary to file the lower edge of the clasp at certain points to allow it to fit well down to the gum at the neck of the tooth, especially at the palatal side; it is desirable to do this before the clasp is accurately fitted to the tooth, as it may somewhat change its position. This may now^ be done, at least so far as to allow the clasp to fit well down to the neck of the tooth, as it should do in the case we are now considering. In many cases the neck fit of the clasp is most important; fitting well down to the gum-line and close to the tooth, enables it to sustain a plate firmly and comfortably, wdthout that strain upon the tooth inevitable when the security of the plate depends upon the clasp firmly grasping the tooth. The clasp fitting under the bulging portion of the tooth holdsby interlocking with the body of the tooth, w^hile it is free to yield to slight movements of the plate inci- dent to its proper use. ^Yhen a tooth stands alone, as represented in Fig. 539, it is a matter of but little moment on w" hich side of the tooth we first make the clasp fit accurately; but when it passes between the teeth, after the clasp is roughly fitted, make the portion betw'eenthe teeth as accurate as possi- ble, and proceed to readjust the clasp from this point until it fits the tooth satisfactorily, as far round as it is necessary that it should before soldering it to the plate. Invariably begin to make the clasp at its shortest end; when this end terminates between two teeth, but does not pass between them, make it 586 SWAGED METALLIC PLATES. pass into the interspace as far as possible; it gives the clasp a much firmer hold iipcm the tooth. As a rule, clasps for lower teeth are not fitted close to the gum line : the teeth are usually longer and more wedge-shaped than are the upper teeth. To hold firmly the clasp must embrace the tooth nearer the cut- ting edge: this is made necessary by the fact that the neck is so much smaller than the crown. It is frequently necessary to make little hooks or lugs to catch over the grinding surface of lower bicuspids and molars in connection w^ith the clasps to prevent the plates pressing too hard upon the gums. These may either be made with the file — which, when it can be done, is the best plan — or they may be soldered on after the clasp has been soldered to the plate and fitted to the mouth. It is always desirable to avoid using solder for any purpose on that part of a clasp which is Intended to act as a spring, as it impairs its elasticity by making it rigid at that point. When clasps are applied to partial low^er plates carrving posterior teeth only, especial care is needed in their con- struction to conteract the natural tendency in such plates to slide back- ward. In clasping upper teeth that are markedly wedge-shaped, as for instance, the canines, and all teeth where the gum has receded and exposed the neck, the clasp should be made to fit the tooth at its largest part; the importance of this is self-evident. In some cases it is desir- able to have the clasps as far above the line of the plate that it is neces- sary to connect them by a narrow strip of plate, forming what is known as a "standard clasp." Occasionally, especialh for lower plates, when the axes of the clasped teeth are at such an angle as to interfere with the adjustment of the plate and clasps, the difficulty may be overcome by making the standards elastic, by making them of half-round platinous gold wire- Standard clasps are not as a rule desirable. They are not as cleanly as is a clasp soldered directly to the plate, and are more liable to accident. Stay or collar clasps, are useful appliances to assist in supporting partial vacuum-chamber plates, to assist in supporting clasped plates, or where a clasp is needed and no space exists through which a clasp may be conveniently passed. They are frequently employed upon the palatal surfaces of bicuspids, not unfrequently in pairs, each one, however, be- ing separately soldered to the plate with each end free. In fitting these partial clasps l)ear in mind the purpose for which they are used, so as to take advantage of every little point that may increase their usefulness. In fitting them to the bicuspid teeth, to make them fit ver\' accurately at the neck, the model may be slightl\- scraped at this point before the clasps are adjusted, so that they will spring over the bulging portion of the teeth when placed in the mouth. Let the strij) of plate from which they are made be amply wide, not too wide for proper adjustment, but quite as wide as the tooth is long, unless it is of unusual length. It may be that when the plate is fitted to the mouth the clasp must be quite nar- row, but if so made before it is soldered, being so lightl\- held by the in- vestment, it will probably move during that operation sufficiently to make it useless. If made wide, fitting well at the neck of the teeth, not CLASPS. 587 only is a change of position less likely to occur, but, if when fitting the plate in the mouth, it does not press as firmly against the teeth as it should, it may be grasped by the pliers and borne outward initil it does. The free edge is then shaped, being cut away so as to not extend beyond the bulge of the tooth at its palatal surface, but embracing its approx- imal surfaces nearly to the cutting edge. It not unfrequently happens that partial clasps upon bicuspid teeth, although fitting accurately, spring the plate down until the portion extending beyond the bulge is removed, they then take hold under the bulge, and hold wdth great firmness. When they are used in pairs, the ends which pass into the interspace must be filed thin, so that both clasps will pass w^ell in: it is a mistake, and a common one, to make one clasp short at this point. When soldering these partial clasps to the plate, solder sufficient only to make the union secure, both ends must be left free. Now and again stay clasps upon the bicuspid teeth on both sides of the mouth may be used to advantage to wholly sustain a plate. Favorable cases are where the gums have receded moderately and the teeth are quite firm. The clasps in these cases pass over the bulging surfaces of the teeth, and impinging lightly upon the narrow^er portion of the teeth, will hold a denture satisfactorily without exerting any marked outward pressure. When the gums are normal, and the root portion of the teeth not exposed, this construction is seldom admissable; to secure firmness the clasps must press against the teeth; the denture then acts like a regulating appliance, and in a little while the teeth are pressed out- ward and no longer sustain the denture. Placing and fitting clasps requires accurate judgment; while the gen- eral construction is the same in all cases, the endless variety of condi- tions met with require corresponding variations. First consider the object the clasp is designed to accomplish, and with this in view, pro- ceed with its construction. When all the clasps required by the denture are fitted to the teeth, they are adjusted to the plate in the following manner. Place the clasps on the model, one at a time, and file away the plate where it impinges upon them until it fits in place on the model with all the clasps in position. The next step is to cement the plate and clasp together so that they can be removed from the model to invest for soldering. In ordinary cases hard resin-and-wax cement will be sufiicient; for difiicult cases shellac, being stronger and more rigid, is to be preferred. In using either it is best to first heat the plate and clasps quite hot and run a little of the cement upon them, otherwise the cement may not hold firmly. While the plate and clasps are accurately in place on the model unite them with the cement; allow a few minutes for it to chill, then carefully remove them from the model, being very watchful that their relative positions are not changed. The cement may be chilled with cold water, and is then made more rigid, but with the disadvantage that if it should frac- ture in removing the plate from the model, the plate and clasps must be thoroughly dried before the cement is reapplied. In some cases it is best not to attempt to cement all the clasps at the same time, but to take 588 SWAGED METALLIC PLATES. those first wliicli may readily l)e removed with the jilate from the model, solder them, and then adjust the others. Those elasps in whieh the neek-fit is im|)ortaiit, especiallN' partial clasps upon bicuspid teeth, may l)e settled in |)()sition upon the model hy a few sharp, hut not severe, blows with the ix-nch hammer just prior to ccmentin<;- them to the plate. Cases are met with occasional l.\ where the clasped teeth occupy such a j)osition that the plate and clasp cannot be removed from the model without the cement breaking. In such cases use as much shellac as possible, let it get quite cold and rigid so that it will break without bending, and readjust the plate and clasp after their removal from the model, for which the fractured surfaces furnish a fairly trustworthy guide. It has })een suggested that if the case camiot be removed from the model it cannot be placed in the mouth. While this is true of ex- treme cases, it is not generally so; the teeth in the mouth are not so rigidly fixed as are their counterparts on the plaster model. After the plate and clasps are removed from the model they are in- vested for soldering in a batter of about four parts plaster to five of white sand. In placing the investment especial care should be taken to so imbed the clasps that they will not be drawn in toward the plate during soldering; and also that the relative positions of the clasps and plate are not changed by the pressure of pressing them into the invest- ment. When the investment is thoroughly hard, the cement is chipped away, and the investment so carved that the blowpipe flame can readily reach all points which are to be soldered. Fill with plaster, or whit- ing mixed w^ith water to the consistence of cream, as much of the joint between the clasp and plate as it is intended shall remain unsol- dered. It is not desirable at this stage to make a very strong union be- tween the clasps and plate, but simply to unite them at some point at which they are known to fit the teeth and which will be included in the final soldering. It is desirable that they be left as free as possible so as to permit an accurate adjustment when the plate is fitted to the mouth. Now scrape the surfaces over which the solder is to flow, making them clean and bright, coat them w^ith borax, lay over the joint a small piece of plate and over this a small piece of solder. While clasp soldering can be done entirely with the blow^ pipe without previous heating of the investment, it is a decided advantage to first make the investment nearly "red hot". If this is not done, and the blowpipe flame directed immediately tow^ard the part to be soldered, the uneven heating of the metal, or burning away of the investment is apt to change the position of the clasps, or by displacing the plaster placed in the joint will permit the solder to flow further than is intended. If the mass of the investment is quite hot the solder flow^s quickly, and without an excessive application of the blowpipe flame. First use a broad flame, and endeavor to heat the clasp and plate equally. As the heat approaches that required to fuse the solder, use a more pointed flame and concentrate it upon the parts to be soldered, but TRYING THE PLATE. 589 not directly upon the solder. If the solder is heated more than the plate, it melts into a ball, and it is then difficult to make it flow. When in this condition more heat is required, and sometimes the parts are made so hot that when it finally flows some of the surrounding plate is also fused with it. This is especially apt to occur if there is a large amount of solder in position. This is one reason for advising a small piece at first. After the solder has once bridged the joint there is no difficulty in adding more to make the joint strong. Sometimes, when the solder has "balled-up," as this condition is technically termed, it is best to add another small piece and endeavor to make this flow over or into the joint. If this fails, do not continue to add solder, but, with a pointed steel or iron rod, say, about one-eighth of an inch in diameter, and ten or twelve inches long, provided with a suitable wooden handle, flatten the ball of solder and direct it into place, at the same time by skilful application of the blowpipe flame make the plate hot at the point over which the solder is desired to flow. The sol- der always tends to flow to the hottest point: advantage is taken of this in all soldering operations and its flow directed by the skilful ap- plication of the blowpipe flame. The iron or steel solder director is a useful tool. Made of iron or steel, it is not apt to alloy with the solder although the solder tends to unite with it slightly. Before beginning to solder, see that this tool is at hand and in proper order; it should be filed to a moderately sharp point, and all adhering solder filed off clean so as not to contaminate the new solder. Now and asain moving it over the point where the solder should flow with a rub- bing motion, or with a wiping motion moving the molten solder toward the joint very much facilitates a successful soldering. When the soldering is completed there is no objection to quickly cooling the plate in water to save time. It is then "pickled" in acid and cleaned either by rubbing with white sand and water with the fin- gers, or at the polishing lathe with a brush-wheel and pumice-stone. The outer ends of the clasps are now made to fit the teeth, and their upper edges filed to the size and shape desired, leaving, however, a little margin for any changes that may be necessary when adjusting the- plate to the mouth. It is well to remember that it is far easier to take ofT a little more than to add. After rounding and smoothing the edges with sand-paper, if the plate and clasps fits the cast accurately, it is ready for adjustment in the mouth. TRYING THE PLATE. After the plate is finished on the model, the next step is to adjust it in the mouth, not only to test the accuracy of the impression, but also to make any changes that may be required to secure a satisfactory fit. This is more important with the plate for a soldered metallic denture than with one for a denture that is either cast or molded, as in this case 590 SWAGED METALLIC PLATES. the plate is not a mere model, but is designed to become a part of the denture, and should, therefore, be made to fit as accurately and as comfortably as possible. In an upper or vacuum-chamber plate, after relieving any points where it presses unduly into the soft tissues, or if it is a partial plate where it impinges upon any of the remaining teeth, test the fit in much the same way it has been tested on the model; this should be done before testing the eti"ecti\eness of the vacuum-chamber. This is important. In many cases a moderately strong suction will hold the plate so firmly that a serious misfit may pass unnoticed, and the mortifying failure that results when the denture is finished and its stability tested in the act of mastication will probably be incorrectly assigned to "warping during soldering." Examine closely the back edge of the plate. In some mouths the centre is quite hard and rigid, while at either side the tissues are quite soft and yielding. Changes may have been made to provide for this when preparing the model for making the dies. If it has been over- looked, or the changes made prove not sufficient for comfort and stabil- ity, any needed corrections may now be made. Allowance must be made for the changes the pressure of usage will make: a plate that fits quite loosely at this point may press uncomfortably hard after a few days u.se. Any needed changes in this respect are readily made at this time by bending the edge of the plate with the pliers, to make it press harder, or to relieve undue pressure. If there is much difference between the fit of the plate on the model and in the mouth, it is probably due to a faulty impression, and the only remedy for this is to obtain a better one. When the central hard ridge is well marked and the plate presses upon it unduly, it is sometimes necessary to reswage the plate with a thickness of paper between it and the die at this point, or an additional piece of plate may be soldered back of the chamber and extending t(j the edge of the original plate, to allow for filing at this point if found necessar\' after the denture is finished; or a suflBcient relief may be obtained by placing a few thicknesses of paper under the plate on the plaster model and gently striking each side alternately with the bench- hammer, at the same time holding the plate firmly to the model. If the undue pres.sure is at the extreme edge of the plate, it may be relieved with the pliers, but it is more frequently .so far in.side of the edge that they are not available. In a few cases the mouth is intolerant of a plate, its presence causing severe nausea even though it does not en- croach upon the soft palate. Persistent use will often overcome this, but not always. Usually the central portion of the mouth is more sen.sitive than is the alveolar ridge, and advantage may be taken of this to secure a more comfortable plate by extending it far back on either side and leaving the roof of the mouth free, or, in other words, making it horseshoe-shaped. Occasionally the edges of the vacuum-chamber press too hard; this is easilv relieved, and its correction had better be left until the denture TRYING THE PLATE. 59I is finished; the smoothing of these edges in the final poHshing may be all that is needed. Examine the edges of the plate and see that they are in close contact with the gums, and that there is sufficient room during the various movements of the mouth for the fra?num in front, and the "strings" on either side just back of the canine teeth. In partial cases see that the edges fit snugly against the remaining teeth. In partial cases, when a vacuum is produced, see that the plate fits closely to the gum where Jt extends through the interdental spaces; it not infrequently happens that a vacuum-cavity plate is slightly displaced, when drawn firmly to place; usually it is brought forward. This is more likely to happen with a small plate in which the vacuum-chamber is well in front; in partial cases this movement is occasionally so marked that it is necessary to take an impression \\'ith the plate in place in order to obtain an accurate guide for arranging the teeth. In full lower plates, first see that they fit solidly, and are without rock or spring, that the edges are in contact with the gum at all points, and yet do not press unduly. The lingual edge immediately back of the incisor teeth, and the lingual aspect of the distal ends, are points that frequently require bending inward. These points are often markedly undercut, and are points where the plate is apt to be inaccurate owing to inaccuracy of the model, inaccuracy of the die, or failure to thoroughly swage the piate into the undercut. In cases where the undercut is quite marked the plate cannot be swaged accurately by the counter-die alone. A blunt pointed chisel or chaser made of bone (a tooth-brush handle answers admirably), and at times a pair of pliers, will be required to make the plate fit accurately at these points. Next make sure that there is ample room for the franum of the tongue when the tongue is raised as in swallowing, and that the back part of the plate does not encroach upon the cheek, or interfere with the movements of the tongue. I^ower plates are frequently made wide at the distal ends, under the idea that the increased width adds to their stability and comfort. This is seldom the case ; but few mouths will tolerate a lower plate wider at the ends than the face of the alveolar ridge. In partial lower plates see that the plate fits closely to the teeth upon which it extends. Partial lower plates should be tried in the mouth and accurately adjusted before the reinforcing pieces are added, so that any changes found necessary can be readily made; after the plate is "doubled,' ' "wired," or "reinforced," it is very difficult on account of the stiffness imparted by the additional thickness and by the solder, to make any material change in its shape. The clasps on all plates retained wholly or in part by clasps should be filed into shape and bent around the teeth as far as they are intended to go at this stage, but should not be fitted closely to the teeth, except so far as they are intended to be soldered. To permit the ready removal of the plate from the mouth it is best to leave the clasps rather loose until the denture is finished, especially if it is desired to adjust the teeth in the mouth before they are soldered to the plate. 592 SWAGED METALLIC PLATES. In adjusting]; the clasps especial attention is needed to so shape the ends that they will hold firmly, and yet as far as possible be out of the way and out of si- seem necessary, so that the artificial teeth or gums will properly blend with the natural ones. Any marked peculiarities to be considered in arrang- ing the teeth should be noted and marked upon the model, so that later they may not be overlooked. The shade may now be selected, the kind and character of teeth de- cided upon, and the bite-impression taken. TAKING THE BITE. Apart from the fact that in full soldered dentures the operation tech- nically known as "taking the bite," is conducted with the plates that are ti) be used in constructing the denture, and not on temporarv trial plates, as is the case with molded or cast dentures, the procedure is pre- SELECTING THE TEETH. 593 ciscly the same for soldered, cast, and molded dentures. In partial cases it ma>- be done when the impression is taken or immediately after the plate is fitted to the mouth, and while it is still in position, as may be most convenient. In a few exceptional cases where the bite is very close, or where the occluding teeth interfere, a more satisfactory bite may sometimes be obtained if taken with the plate in position; mainly because there is less risk of misplacing it upon the model prior to making the articulating model. The method of taking the bite, the precautions ,to be observed to secure accuracy, and the construction of the articu- lating model, are fully considered in Chapter X. SELECTING THE TEETH. As the general subject of selecting teeth is elsewhere in this work con- sidered at length, it is unnecessary in this place to do more than briefly consider those purely mechanical matters especially concerned with selecting teeth for soldered dentures. Inasmuch as the teeth of a sol- dered denture must rest upon and fit closely to the plate, more care is needed in their selection than in selecting teeth for a molded or cast denture. In. the latter case, but little is required beyond satisfying the artistic requirements of the case; as the plate is made to fit the teeth after they are finally arranged in position. In a soldered denture, on the contrary, the teeth are accurately fitted to the plate by means of various forms of grindstones, and in addition to satisfying the equally exacting artistic requirements, the purely mechanical problems this in- volves must be considered when the teeth are selected. When plain teeth are used this involves but little more than sufficient extra length to allow the teeth to fit the plate solidly throughout the area of con- tact between the tooth and the plate, The selection of gum-teeth is more complicated. There must be sufficient body back of the gum to permit the porcelain to accurately fit the plate over the whole area of contact when the tooth is in its proper position. Considerations of cleanliness as w^ell as of strength calls for this accurate adaptation of the porcelain to the metallic plate. The usual form of teeth made especially for soldered dentures requires a metallic backing reaching nearly to the cutting edge. In a close bite this must be considered, especially when selecting anterior teeth for an upper denture, and teeth selected sufficiently thin to permit this without making the tooth un- duly prominent. The position of the pins must also be noted. Teeth for plate-work usually have two, arranged either transversely or perpendicularly, and are technically known respectively as cross and straight pins. Very large or very long teeth have three or four, usually vertical. The cross pin teeth are designed for cases of close bite, and now and again serve a useful purpose. They should be avoided wherever possible, as they are inherently weak from the following reasons: first, the position of the pins weakens the tooth at the point where it is most liable to break 594 SWAGED METALLIC PLATES. from stress of use or strain clurint; soldering; second, when arranged in place both pins are brought near the plate, making the strain upon the pins and the porcelain greater on account of the increased leverage between the pins and the cutting edges, and furthermore, this strain, in time stretches the metal of which the pins are made and permits a slight rocking movement of the tooth which causes the pins to break between the backing and the tooth. They are least objectionable in short molar and bicuspifl teeth, and in anterior teeth very broad in proportion to their length. It is a common error to always select cross pin teeth for a close bite. In all such cases the backings should be extended to near the cutting edges, so that they will relieve the teeth of part of the strain, AYhen this is done there is no advantage whatever in cross pins. The straight pin teeth are more reliable, mainly from the fact that the upper pin is closer to the point at which the force is applied, and the leverage upon it is, therefore, less. The position of the pins is also varied. Some teeth are made with the pins nearer the cutting edges, than are others, and in some, the pins are placed nearer together. Advantage should be taken of this, and teetli selected with the pins in the best position to resist the strain in- cident to constructing the denture and the strain of constant use. Pins near together are much more liable to cause fracture of the tooth during soldering, and do not so well resist the strain of constant use; straight pins so placed that they come near to the plate when the tooth is in position, while there is considerable space between the upper pin and the occlusal edge, provided that the backing can be extended so as to protect the tooth, are to be avoided; in all such cases teeth with pins better placed can quite as readily be used. While very thin teeth should be avoided on account of their weakness, where they are not especially required, very thick teeth are objectionable on account of their clumsi- ness. Porcelain teeth mounted upon a molded or cast base are, as a rule, better supported than are those upon soldered metallic plates; they are less strained during the process of construction; their initial strength and their ability to resist strain is, therefore, less important. Close attention to detail, a careful study of the varied strains porce- lain teeth are subjected to during the process of constructing a denture and thereafter, should be brought to bear in selecting teeth for each individual case. The artistic and the mechanical requirements are equally important, and it is important to know the resources of good workmanship in securing the best result when either one must be sacrificed. The large number of porcelain tooth makers, the individ- ual peculiarities and the variety of their products, are decided helps in obtaining that which is best for each case. This suggests the import- ance of keeping in touch with that which they have to offer in the im- portant matter of selecting teeth. ARRAXGIXG AXD FITTING THE TEETH. 595 THE ARRANGEMENT AND FITTING OF THE TEETH. The arrangement and fitting of teeth to soldered metalhc dentures requires much more care than does the same operation in the con- struction of cast or mokled dentures! Accurate fitting of the tootli to the metal phite is demanded on the score of strength and of cleanhness. If the tooth does not rest sohdly upon the plate the whole of the stress ■due to usage is borne by the platinum pins; at times this results in the pins being broken, or they may be stretched and so reduced in calibre that they draw out of the porcelain. "When this occurs the fault is usu- ally ascribed to careless heading of the pins by the manufacturer; the fact, however, that the pins in these cases fit the holes in the porce- lain loosely, and that if seen before they have separated from the teeth are found so reduced in diameter that they freely move while still held by the head imbedded in the tooth is conclusive that the tooth maker is not in fault. \Miile it is impossible to secure absolute contact between the plate and the tooth, every effort should be- made to secure accurate adaptation, leaving as little space for the accumulation of offensive mat- ter between the tooth and the plate as possible. The artistic problems of tooth arrangement are very much the same with all kinds of dentures; the mechanical procedure, however, by which this is attained, and the special mechanical problems involved are varied. It is to the latter, the special requirements of tooth ar- rangement for soldered dentures, that attention is now directed. Entire Dentures. — To properly adapt, arrange to the best advantage, and to correctly finish a full denture of gum-plate teeth is "one of the most difficult operations in prosthetic dentistry. Each tooth must be separately adjusted and fitted in position so as to secure the proper position and spacing of the tooth-part and at the same time properly contour the gum portion without breaks or offsets; to do this and to pro- \ade against accidents in soldering, and to accurately fit the teeth to the plate, tests to the utmost the w^orkman's skill. To describe how this is to be accomplished, beyond indicating the proper use of the mechanical means employed, is equally a difficult task. It must be borne in mind that in plate work, teeth are fitted by grinding from their substance; nothing can be added to replace that which has been needlessly remo ved. This will suggest the importance of first fixing, definitely, the position of each tooth as nearly as can be done, by roughly arranging a series of teeth approximately in position; then to proceed carefully, making such corrections as may be needed in slant, length, fulness, and spac- ing; finally finish the joints between the teeth and make the gum por- tion form an even surface. There are three points to be first determined when making this preliminary arrangement; first, and first in im- portance, the median line. This should be accurately marked when taking the articulation while the plates are in the mouth, and should coincide with the median line of the face. To correct any error in its position, however slight, may involve changing the position of every 596 SWAGED METALLIC PLATES. tooth upon the denture, and if this is done after the teeth have been accurately arran^^cd, it will, in many cases, imi)air beyond remedy the adaptation of the teeth to the plate. A tooth accurately fitted in one position can seldom be as accurately fitted to another. If there is any doubt about the median line being accurately marked upon the model, hold the case so that a line drawn through the centre antero-posteriorly is exactly perpendicular: if the central teeth are in their proper positions, the bicuspids will be directly opposite to each other. There are oc- casional exceptions to this rule, but it is so generally reliable that it is well to regard with suspicion any centre mark that does not conform to it, and to ascertain beyond doubt its correctness before so far grind- ing the teeth, that their position cannot be changed without injury. The two other points are the junctions of the canines and bicuspids of each side. This point marks, first, the change in contour line of the denture from the anterior arch to its abutments, the posterior teeth, which are from this point arranged in a straight line; and second, a change in the slant of the teeth, from a marked leaning toward the median line of the anterior, teeth to the nearly perpendicular position of the bicuspids and molars. It also determines accurately, the space to be occupied by the six anterior teeth. This joint, that between the canine and bicuspid teeth, is, in gum plate teeth, by far the most dif- ficult to make of any in the denture, the artistic blending of the gums of the anterior and posterior teeth depends upon the workman's skill, whereas in gum block sections it is provided for by the tooth maker. The first step in arranging a set of plate teeth is to clean thoroughly the platinum pins, especially to remove from them a slight coating of porcelain acquired when the teeth are made, being careful that this is done close up to the body of the tooth, where a little cone of porcelain is fre- quently found encircling the base of the pin. See, also, that the pins are straight, at right angles to the tooth, and parallel to each other. This can be done at this stage better than later, as it not unfrequently hap- pens that a tooth may be ground away so close to a pin that while its hold in the porcelain is not materially weakened, a slight strain upon it might break it out and ruin the tooth. If the wax or modelling composition used in taking the articulation has been carefully carved to represent the desired length and fulness of the upper and lower denture, that on the lower plate may be used as a guide in the preliminary arrangement of the upper teeth, the wax, or composition, for the time being taking the place of the lower denture. The upper teeth are usually arranged first, as they are more important in giving expression. Now, arrange upon the upper plate a layer of adhe- sive wax, not too massive, and yet sufficient in amount to hold the por- celain teeth securely when their pins are pressed into it. It is not desir- able that this adhesive wax should be at all brittle, and yet it should be sufficiently hard to hold its form well, and to hold the teeth in position sufficiently firm when trying in the mouth, when with the wax spatula, it is fused^ around their pins. This layer of adhesive wax should ex- tend nearly to, but not to touch the wax on the lower plate, and should ARRANGING AND FITTING THE TEETH. 597 be molded to the general contour desired in the teeth, allowance being made for their thickness. On this layer of wax arrange the eight anterior teeth, first placing in position the central incisors, grinding them sufficiently only to get them approximately in position. These are followed by the lateral incisors, the canines, and the first bicuspids; by arranging the teeth in pairs when placing them roughly in position, and also when adjusting them finally, one has a better idea of the general effect. During this part of the work, accuracy in making the articulating model will be appre- ciated. If an articulator is used, it is important that the centre line of the articulator and the centre line of the denture coincide ; and also that the occlusal line and the top and bottom of the articulator are all parallel. In order to see that the teeth have a proper slant, and that the occlusal line gives to the teeth of each side the right length, the workman holds the articulating model before him on a level with the eye, and the eye is instinctively guided in judging of these matters by the upper and lower fines of the articulator. If the denture has been misplaced in the articulator, or the articulating model is defective in these respects, it is best to correct the fault as far as it can be corrected before proceeding. With these eight teeth roughly in position, and the place the canine tooth should occupy noted, the amount of joint-grinding necessary to bring the artificial teeth to their correct anatomical position is readily determined. This amount is to be divided among the five joints. In this preliminary arrangement let the teeth be a trifle too long to allow for a slight shortening when they are accurately fitted to the plate. Note care- fully the surface of that portion of the plate upon which the teeth and artificial gums rest. If this is markedly uneven, but little grinding should be done until the spacing is well advanced. If a tooth is hollowed out to fit over a prominence, and is afterward moved forward, it is quite likely that the hollowed out portion will remain a vacant space. Note also that nearly all single gum plate teeth have the sides of the gum portion slightly rounded; the object of this is to provide a means for making the gums even when the teeth are fitted to an irreg- ular surface, or when the tooth portion is arranged slightly unevenly. When it is desired that a tooth should set in more than its neighbor, this can be accomplished without making a noticeable offset by grinding more from the side of its gum when spacing. Advantage is taken of this in making the joint between the canine and bicuspid; quite frequently it is necessary to grind no more from the canine gum than to make the edge smooth, removing all that is necessary to close properly the joint from the bicuspid gum in order to place the bicuspid in its normal position, that is, slightly less prominent than the canine. And again, to bring the tooth portion of these two teeth into proper relation, it is not unfrequently necessary in order to make the gums blend, to make the joint at a slight angle, instead of in line with the axis of the teeth. When these eight teeth are roughly fitted into position, the wax guide b removed from the lower plate, and the anterior eight lower teeth are 51)8 SWAGED METALLIC PLATES. in like manner roughly fitted in place. If the upper and lower teeth are of proper widtli, and properly spaced, the points of the upper canines will rest between the points of the lower canines and first bicuspids. They should be made to take this position during this rough fitting while spacing the teeth. Up to this stage the teeth will usually retain their position sufficiently well by simply pressing tlieir pins into the wax; some little care is required, however, to prevent the central incisors from being pushed from the median line, and to avoid displacing the upper teeth outward while arranging the lower teeth. When the eight anterior teeth on the upper and lower plate are of exactly the right length; the overbite as it is intended to be; the curve of the arch, the fulness, so far as the tooth portion is concerned ; and the general expression quite satisfactory, they may be, for the time being, considered finished. The gum portion, especially of the upper denture, may be a little too prominent, and the spaces a trifle too wide; it is well that they should be, to permit a final and more accurate adapta- tion of the gum portion to the plate, and a consequent closing of the spaces. The six anterior teeth of each denture are now firmly "waxed" in position so that they will not be disturbed while the posterior teeth are being arranged. This is done by passing a heated wax spatula on each side of the pins over each tooth, beginning with the centrals. This spatula should be sufficiently hot to melt thoroughly the wax and heat slightly the pins and the surface of the teeth; the teeth being held in place while this is being done by curving the fore finger of the left hand around them. The spatula is first passed between the centrals, and held until the wax is thoroughly melted, when it is withdrawn, the denture in the meantime being so held that the wax will not flow from its position. When this wax has chilled, the same procedure is repeated between the centrals and laterals, and between the laterals and canines; in each case the wax must be allowed to chill before proceeding, other- wise the teeth would become displaced. The lower posterior teeth of each side are now roughly fitted, and then the upper bicuspids and molars, with especial care to make them normally articulate by proper spacing. The usual form of the gum por- tion of plate bicuspids and molars tends to make them assume an .in- ward curve owing to the lingual side being made narrower so as to save grinding in making the joints. This must be guarded against. When the posterior teeth of each side are satisfactorily arranged, they are to be "waxed" firmly in place as were the front teeth, and it is well at this stage to adjust them in the mouth. Little changes may be needed that can now readily be made without detriment that if attempted after the final adjustment might result in imperfect joints. Prior to this, the teeth should be securely waxed; more wax being added where needed and the whole made neat and smooth. A little wax run in along the ends of the gums at the outer edge of the plate assists very much in holding them securely. There is always a risk that the patient may, in- advertently, close a little too firmly; this will displace them if the cement- ing with wax is not thoroughly done. ARRANGING AND FITTTNG THE TEETH. 599 After making any corrections suggested while trying the teeth in the mouth, they should be carefully gone over and the final fitting of the teeth to the plate, eN'ening of the gums, and accurate adjustment of the joints receive needed attention. The arrangement of the teeth is now complete. Arrangement and Fitting of Teeth to an Upper or Lower Denture. — In arranging the teeth to an upper or lower denture, the same general plan is followed as in arranging a full denture. If the antagonizing teeth are markedly irregular, some little ingenuity may. be required when fitting gum teeth to make the occlusal edges of the anterior artificial and natural teeth fit sufficiently close to prevent lisping, or to prevent a hissing sound when pronouncing certain letters, and at the same time preserve a desired alignment of the artificial teeth and gums. This is more readily accomplished when plain teeth are used as it is not complicated by the artificial gums. Nevertheless, much care is required in these cases to make plain teeth fit solidly upon the plate. The same precautions are required; the teeth must first be approximately adjusted to position, the grinding cautiously done, and the final accurate fitting deferred until the position of the teeth has been definitely determined. The anterior lower teeth,' on account of their narrowness are more readily fitted; they are less concerned in expression, and the artistic requirements are less exacting. Gum and Plain Teeth. — Whether to use gum or plain teeth in con- structing soldered plate work is decided by the same general rules ap- plicable to all dentures. Gum teeth are required to restore a normal fulness, or to hide the plate. In some cases where gum teeth are re- quired^ and where it is desired to have as little fulness as possible, the gum teeth may be set directly upon the natural gum, the plate being cut away to allow them to do so. It is seldom necessary to cut it away further back than the canine teeth. The plate should be allowed to extend as far under the teeth as possible, the edge of the plate being beveled quite thin so that after the teeth are soldered it can be burnished up to them and a close joint made. It is a common fault to cut the plate away too much, not only depriving the teeth of the support it should give, but impairing the adhesion and bringing the edge so directly under the solder that it is too rigid to burnish up to the teeth. To avoid this, if the teeth have been recently extracted and the alveolar border is quite uneven, it is allowable to trim down the more prominent portions; even in cases where resorption is well advanced, the portion of the model against which the artificial teeth fit may be somewhat freely carved or rubbed down M^ith fine sand-paper. It is desirable that in the mouth the artificial gum should fit closely the natural gum; inasmuch as the wax does not usually hold the teeth quite close to the model, and since there is tendency for the teeth to move out slightly during soldering, teeth fitted to a model thus carved will fit in the mouth more satisfactorily than if it were not done. The teeth are now fitted to the model without the plate sufficiently to determine accurately the position they should occupy. This done, cut away the plate nearly, but not quite as much as will be 600 SWAGED METALLIC PLATES. re(iuirc(l, miuI iiftcr hcNoliiii;- the e(l<,^(', make the ed^e fit \ery close to the model, e\en sinkiiit;- it slij^ditly into the ])laster, and proceed to fit the teeth in position with the j)late in place. As the work proceeds more mav l)e remoxed from the plate as it is seen to be necessary. \\\ thns proceed- ing, and by carefully fitting the teeth to the plate, nsing for this ])nrj)ose very small grindstones, the teeth will he well supported by the plate, and the joint between the plate and the artificial gum, after the edge of the plate has been burnislunl close to the porcelain will be cleanly, smooth, and uon-irritating to the tissues against which it will rest when the finished denture is in use. Cases are met with where an artificial gum is recjnired on account of resorption of the alveolar l)order, and where the upper lip is so short that the edge of the plate is lia})le to show above the gums of the teeth. In these cases the plate is cut away so that the gums of the artificial teeth w'ill just cover it, and the teeth are so fitted that the edges of their gums rest upon the natural gums. In such cases the edge of the plate is not beveled, })ut made sciuare. The plate is cut aAvay only sufficiently to allow the artificial gum to extend from one-sixteenth to one-eighth of an inch beyond, a groove is then cut in the model just beyond the plate-line a full thirty-second of an inch deep, as far as the artificial gum is to rest upon the natural gum. This facilitates an accurate fitting of the artificial gum to the edge of the plate, and en- sures a close fit of the artificial gum to the natural gum. If too much is removed from the model, and in consequence when the denture is finished the artificial gum presses too hard upon the natural gum, this is readily corrected by the careful use of a small grind-stone, whereas if it does not fit clorely enough the fault is without remedy. Plain Teeth. — Plain teeth are more readily arranged than gum teeth. Where conditions favor them, they make a stronger and more cleanly denture. It is exceptional, however, when they can be used on a den- ture where the plate extends over the alveolar border. Cases are now and again, met with, however, where the lips are long and the tips of the teeth are alone seen, and where the teeth themselves will impart sufficient ful- ness; in these plain teeth may be used with decided advantage. The differences in the technique of their arrangement from that of gum teeth in full, and in full upper and lower dentures is self-suggestive. It is quite as important, and the same care is required, although less labor is involved, to make them fit accurately to the plate on acount of cleanli- ness and strength. Plain teeth are especially called for when a denture is made soon after natural teeth have been extracted, or where the alveolar border is broad and the teeth are set upon or into the natural gum. In some cases the four, six, or eight, anterior teeth are so set, the teeth further back being set upon the plate; in some cases, indeed, on account of resorption of the posterior portion of the alveolar border, plain teeth may be used in front and gum teeth at the back. The decision to use plain teeth set upon the natural gum for the front portion of the denture is usually reached when marking out the plate line, and the plate made accord- ARRANGING AND FITTING THE TEETH. 601 ingly. It should, however, be made to extend over the alveolar border a little more than seems necessary, as, until the teeth are partly arran,o;ed it is impossible to know exactly where it should end. It is very desir- able that the ])latc should extend under the teeth as far as it ma.\', and yet be well co^•ered. In order to ascertain how to trim the plate, first arrange the teeth to the model without the plate; while so doing carve the model so that the teeth will press hard into the gum. How freely this should be done depends very much upon the condition of the gums themseh'es. If the teeth ha\'e been recently extracted, or if the gums are quite soft and yielding, the model may be carved quite freely; if on the contrary they are hard, it must be done more cautiously. This however, may be remembered; the gums usually yield under pressure, even when the pressure is very slight, there is usually a shrinkage of the gum tissue shortly after a denture is inserted. When first inserted a tooth may fit upon the gum quite neatly; a few weeks later, on account of this shrinkage, it may not touch the gum at all. If the pressure is too great the fault is quickly remedied by a touch of the grind-stone. The beauty of plain teeth adjusted to the natural gum consists in so ar- ranging them that they appear to be growing out of the gum. Having determined to what extent the teeth should press into the natural gum and the position of the two central incisors, form a groove of the de- sired depth, its lingual side sloping and its labial side quite perpendicular, into which the teeth are to be fitted. This groove is to be extended as the laterals, canines, etc., are fitted into place. A groove is preferable to bodily cutting out a seat for the teeth, as it readily permits a sidewise change of position, and shows at all times how far the teeth extend be- neath the surface of the model. When as many of the teeth as are to set upon the gum are in place and cemented upon the model, with a fine needle-point make a traced line upon the model around their necks, thus marking the exact position of each tooth. Remove the teeth, and cut the plate so that the edge shall be about one-sixteenth of an inch within the line, not following the festoons, however, but making the edge straight and well beveled. The triangular portion of plate between the teeth can be readily and neatly cut away with a fine file when finishing the case. Then replace the teeth, grinding them to fit over the plate. Usually the last tooth of the series resting upon the natural gum will be the first or second bicuspid, depending partly upon the condition of the al- veolar border as regards resorption, and partly upon how far the plate is visible at the angles of the mcuth. As a rule this last tooth rests partly upon the gum and partly upon the plate; it should be so set as to hide as much as possible of the plate beyond it. In grinding plain teeth a much smaller stone is required than for grinding gum teeth. Plain teeth are more Hable to be displaced when trying them in the mouth than are gum teeth; on that account they should be more care- fully cemented to the plate. The technique of fitting teeth of partial soldered dentures differs but slightly from that of fitting teeth of any form of partial dentures beyond the fact that in one case the plate is fitted to the teeth after the teeth have 602 SWAGED METALLIC PLATES. Fig. 549 Fig. 550 Lingual aspect of a f aul t y gum- joint, in contact on the labial side only. Lingual aspect of a properly mad e g u m - joint between two upper central teeth, in contact throughout. been fitted into place, antl in the other they are themselves fitted to the plate. Further than this the mechanical and artistic requirements are verv much the same, and are elsewhere in this work given in detail. General Suggestions, — Corundum, or carborundum wheels of coarse grit, three-sixteenths of an inch thick by one and one-fourth inch in diam- eter, having a round edge, are used for the rough grinding. For the final grinding smaller wheels of a finer grit are re- quired. For joining, some prefer a ratherlarge wheel, and use the side instead of the edge ; oth- ers a wheel about one and one-half inch in di- ameter, and about one-fourth of an inch thick, of fine grit, and with a square edge, kept especi- ally for this purpose, and use it by holding the tooth between the thumbs and index fingers of both hands, pass the joint rapidly across the'edge of the wheel backward and forward. It is best to be provided with wheels suitably for side or edge use, and not to confine one's self to either method, but to practice both. It is very important to make the joints in such a manner that the joint surfaces are in contact throughout, as shown in Fig. 550. Great care must be exercised that there be no ^'-shaped space at the back of the joint (Fig. 549) : not only is such a faulty joint uncleanly, but with close contact of the pink borders anteriorly the gum will almost inevitably flake when the teeth expand by heat during the process of soldering. The bases of all the teeth should rest solidly upon the plate (Fig. 551, A), thus bringing the strain upon the bodies of the teeth, and not upon the pins (Fig. 551, B), as occurs when the teeth are imperfectly fitted. When fitting teeth to the natural gum it is well to remem})er that this portion of the model, especially the labial surface of the gum and that portion immediately adjoining the remaining natural teeth, is apt to be reproduced a trifle larger on the model than it is in the mouth, even though the model is from an otherwise faultless impression. This may be due to "creeping," when wax, or the so-called "impression com- pounds" are used, that is, a slight change of form after the impression has been pressed into place and before the material has been chilled sufficiently to make its form permanent. AYhen plaster is used, it may be due to the sluggish manner in which the plaster flows through the interspaces, or to neglect in pressing the plaster to the gum, or the plaster may be drawn away from the gums by the lips before it has set, or in removing the impression this portion may break in such a manner as to prevent accurate replacement. The defect frequently occurs, and while it is generally slight, it is sufficient to make a decided misfit be- tween the natural and artificial gum. This may be corrected by carv- ing this portion of the model before fitting the teeth. Accurate adjustment of the teeth to the plate may in many cases be facilitated, especially in partial cases and with teeth that stand alone by flattening the surface of the plate upon which the teeth rest with a TRYING THE TEETH IN THE MOUTH. 603 flat file; the amount removed may be trifling, and yet it may make a surface much easier to fit. Pigments of various kinds have been suggested to indicate the point of contact between a tooth, and the plate or cast, which must be ground away in adjusting a tooth m position. Thin carbon paper cut so as to lay evenly against the plate, has been recom- mended; the tooth pressed against this will be marked at the point of contact. Rouge, made into a paste with oil; a black or blue crayon, or a blue pencil, may be used to color the sur- face against which the tooth is to fit. They may be helpful to some; a skilled workman, how- ever, seldom makes use of them. A cultivated sense of touch enables one to know with remarkable accuracy the exact point to which the grind-stone should be applied. A slight rubbing motion makes a white spot upon the tooth where it touches the plaster, and a dis- tinctly discernible mark where it impinges upon either gold or silver, provided the metal is not coated with wax. These marks will continue to be made. Pigments must be constantly re- newed or they are deceptive, and most of them are unpleasant to use, they smear and soil the work and the workman's hands. A. A gum tooth, accurately- fitted to, and resting upon the plate B. A gum tooth imperfectly fitted to the plate. TRYING THE TEETH IN THE MOUTH. In full upper or lower or entire dentures the points to be carefully considered when trying the teeth in the mouth are — first, the accuracy of the articulating model ; if that is correct the teeth should articulate in the mouth very nearly as they do upon it; second, whether the joint between tne central incisors is in an imaginary line bisecting the face; third, whether they are of a proper length, and in the case of an entire denture, whether the relative length of the upper and lower teeth is as it should be; fourth, whether they have the proper slant and relative length, one with the other; the upper teeth should slant toward the mesial line, and in a young person the centrals should be a trifle longer than the laterals, for persons advanced in age, the length of all the in- cisors should be the same ; the lower incisor teeth should be of the same length, and slant but little if at all; fifth, the fulness, that is, whether the normal lip outline is restored; and sixth, the expression and the changes in expression and in articulation due to the movements of the mouth and adjacent parts, and their general harmony. Inasmuch as nearly all changes in soldered plate work must be made with the grind stone, and that any portion of a tooth once removed cannot be restored, it is a wise precaution when constructing full, or 604 SWAGED METALLIC PLATES. nearly full (U'liturcs, to make a j)reliiniiiary trial of the teeth in the mouth when they are partly arranged. It is a satisfaction to know that the models are correct, and to have a i;eneral idea of how the teeth should he when accurately adjustinjj; them to jjosition. Trifiinj;- changes in the articulation noted us needed when trying the teeth in the mouth, chanf^es that it is seen can l)e readily made with the i-rindstone, may be left until the denture is finished; some changes that may seem neces- sary at this time may not be needed when the patient can close the mouth firmly without danger of disi)lacing the teeth from the plate. If the articulating model is correct, required changes in length, slant, or fulness may be indicated l)y moving the teeth concerned into new positions until the desired effect is produced, making no attempt, how- ever, to refit them until a satisfactory rearrangement has been effected. If these changes are at all extensive, it is best to try the denture again in the mouth after they have been made. If the articulating model should prove defective, or the teeth be found too long or too short, it is better to place a little soft wax between the teeth on each side of an entire denture or all around an upper or lower denture, and proceed to take a new articulation, allowing the teeth to remain upon the plate if they do not interfere, as their presence aids in the correction of the defects of the old articulation. ]\Iake also a new articulating model: this course is far more satisfactory than to attempt to alter the old one, and saves time in the end. Trying partial cases in the mouth is frecjuently a difficult task, es- pecially if the teeth are scattered. If a hard cement is used, it is nec- essarily brittle, and the teeth are broken from the plate with the slight- est strain; if the cement is soft, they will move, and unless carefully watched and their position in the mouth compared with their position u})on the model, the tr\ing in will amount to but little. Quite fre- cjuently the teeth fit so tightly between the remaining natural teeth, or are placed at such an angle, that the plate cannot be removed from the mouth without displacing them. In these cases endeavor to judge what changes are necessary and adjust the teeth to the model after the plate is removed from the mouth. It is not always desirable to arrange the teeth so that they can be readily placed in the mouth while fastened on the plate with cement; it may be a decided advantage to make them fit too tightly for this. Or, again, the natural teeth may so lean that, although it is quite difficult to pass over them the plate with the teeth attached with cement, the finished case, with the teeth and plate firmly united with solder, will readily spring into place, and be held more securely and comfortably than if made to go into place easily. A well fitting partial denture may have to be manneuvered into place, one side being inserted first, or be twisted and turned to accomodate leaning or mis- placed teeth, or it may require a little force to press it into position. This can seldom be done with the teeth secured to the plate with ad- hesive wax. In these cases the main purpose of trying in the teeth is to discover and correct inaccuracies of the model, to see that the teeth in size, shade, and character harmonize with the natural teeth, and RIMMING. 605 that the artificial teeth are in a position where they can be made to fill properly the spaces when they are firmly soldered to the plate. Cases now and again occur where, when the teeth are in their proper position, it is impossible to remove them and the plate from the model, and it becomes necessary to break off one or more of the plaster teeth in order to remove the case from the model without displacing them. It has been suggested that a case which cannot be removed from the model without mutilating it cannot be inserted in the mouth; it must be remembered that the plaster teeth of the model are very much more rigid than are the natural teeth in the mouth, and that they are not al- ways accurate in shape. Such a case may require more or less "fitting," when it is finally inserted after finishing, but will, as a rule, prove more satisfactory fitted to the mouth than if it had been fitted to the model. ^Vlien satisfied with the arrangement of the teeth, before investing, see that they hold in place sufficiently firm that they will not be displaced when pressed into the investment. Teeth standing alone may require for additional security, a drop of wax underneath, at the point where the plate and porcelain meet; a little soft wax flowed inside long and w^eak gums may ward off an accident when the case is removed from the in- vestment after soldering. RIMMING. The addition of a rim to dentures of single gum or block teeth — that is, a band of gold or silver upon the border or edge of either an upper or lower plate to receive the gum extremities of the teeth — adds very much to the strength and to the artistic appearace of the denture. It can be added only when the plate extends beyond the gums of the teeth, and when the lip is sufficiently long to prevent its being seen whei: the mouth is widely opened and the lips retracted, as in laughing, etc. Rims are not confined to full dentures; they may be applied to either upper or lower partial cases where there are a number of single gum or block teeth together. If the work is well done, rimming makes a neat finish to the denture ; the thick, smooth, rounded edge is more comfort- able to the patient, and the rim, covering the joints between the plate and the teeth with its free edge burnished closely to the gum extremities of the teeth prevents the lodgement of food, etc., and thus adds very much to its cleanliness. Rimming also materially strengthens the den- ture, not only by the amount of gold or silver added to a weak part of the plate, but also by forming a groove or socket in which the ends of the teeth rest, protecting them from injury and assisting the platinum pins in resisting the strain of mastication, especially those of the molars and bicuspids. Rimming a plate before the teeth are fitted to it is a very simple matter. It is done immediately after the plate has been adjusted to the mouth, the articulation secured, and the articulating cast made. The wax used in taking the articulation, giving also the intended fulness of the gum, indicates the width of the rim. In locat- 606 SWAGED METALLIC PLATES. ing the position of the rim two things are to be considered : first, the probabiHty of the border of the plate extending too far over the alve- olar ridge; and second, the probability of the rim being seen when the mouth is widely opened. Provision must be made for reducing the size of the plate without injury to the rim should its border press too firmly upon the soft tissues. To this end the rim is located a little inside the outer border of the plate, but not so far as to be exposed in laughing, etc. ; and in soldering it to the plate the solder is allowed to run into and fill the apex of the angle between the rim and the plate. In construct- ing such a rim take a strip of plate sufficiently long, about No. 26 Amer- ican Standard wire gauge (.01594 of an inch thick,) — ^the wadth of the intended rim. After cleaning the plate, adjust the strip at about the middle of its length so that one edge touches the plate at its median line and other stands out from it at an acute angle, forming a groove or socket into which the teeth are to be fitted. This may be Fig. 5.52 Method of tightening wire in operation of "tying down." held in place for soldering, with binding wire, with a wire clamp, or it may be held with adhesive wax and invested for soldering. Begin soldering by "tacking it" — that is, flowing a little solder so as to unite the rim and the plate a portion of the distance they have been fitted : the object is to unite them firmly at one point, so as to facilitate fitting the remaining portion of the rim. If the portion of rim soldered does not form a desirable angle \f'ith the plate, it may be bent with the pliers, in or out, until it does. Now proceed to fit the strip to the plate as far as it can be conveniently done, securing it in position with binding wire, and .solder. Repeat this until the rim is fitted and .soldered nearly as far as it is intended to go. As the exact po.sition of the last molar cannot be determined at this stage, an ample length of the strip is left unsoldered, enough to continue the rim around the posterior of the last molar. This is to be fitted and soldered after the teeth are arranged in place. The rim is now neatly finished by being reduced with a file to a proper width, and the free edge slightly beveled. This RIMMING. 607 method of forming a rim may be used on either an upper or lower plate. Of the many methods which have been from time to time suggested for rimming dentures of single gum teeth, after the teeth have been fitted, the following are selected as being simple and practicable. For many years the writer invariably rimmed these cases after the teeth were soldered to the plate: in doing so there is a slight risk of shattering the gums during soldering, and of warping the plate, but with care in investing, in applying the borax, and in soldering, acci- dents seldom occur. It is done quickly, and with much less labor than by any other method; a rim so made is neat, strong, and effective. When this method is followed, after the case is soldered and cleaned by acid, and before commencing to finish the backings, carefully wash the denture to remove all trace of acid, and then grind the gum ends of the teeth until they conform to an even line from heel to heel of the plate, removing the rounded edge, and forming an even bevel of a somewhat obtuse angle to receive the rim. It is important that the finished rim should fit with absolute closeness to the teeth at its free edge; and, as it is more difficult to burnish it to a rounded surface than to a flat one, this should be borne in mind in preparing a denture for rimming. It is important also to have sufficient room between the ends of the teeth and the edge of the plate, not only to solder the rim, but also to give the rim sufficient width to avoid making the edge too abrupt. If the rim is narrow and of the same width from end to end, half- round wire, letting the flat side rest against the teeth, is usually to be preferred; if wide, or wide at some points and narrow at others, it is best to cut a paper pattern and make the rim of plate about No. 24 gauge (.0201 of an inch thick). Usually the rim of a low^er denture is made in one piece, that for an upper denture is sometimes more conven- iently made in two pieces, each extending from the mesial line to the tuberosities of either side, overlapping at the mesial line to make a neat joint. When preparing to solder a case which is intended to be rimmed in this manner, a piece of plate should be fitted around the back part of each end molar, extending from the backing as far around the molar gum as it can be soldered, and soldered in place at the same time as the teeth are soldered to the plate. This is intended to form part of the rim, and is fitted and soldered at this time on account of the difficulty of reaching that part when the case is invested for soldering the other portions of the rim. The method of fitting the rim is the same for either plate or half-rouud wire. Begin at the median line, and carefully bend the plate or half- round wire with either the fingers or pliers until it fits accurately in place, bevehng each end so as to make a neat joint with the piece ex- tending from the backings around the last molar at one end, or if made in two parts, with the other half of the rim at the median line. It is hardly probable that it can be fitted so accurately when invested in (306 SWAGED METALLIC PLATES. })lace as not to sprin<^ somewhat, hut it sliould so lu'nrly fit that the iron hiiKhng wire used to hokl it in position (hiring sohlering simply hohls it in place, and is not depended upon to draw it close to the teeth. If the fitting is imperfectly done and the rim is drawn close to the teeth by the wire, it may seem to fit accurately when invested; but when the case is heated for soldering, the binding wire, becoming hot before the rim, will expand and allow the rim to spring from the teeth, so that after it is soldered it will be a difficult, if not an impossible task to bend or l)urnish it to fit as closely to the teeth as it should. fVfter the rim is fitted as accurately as possible, it may be held in place for investing with adhesive wax, or preferably with iron binding wire. For this purpose annealed iron wire No. 30 or 32 may be used; strength is of small importance; there should be but little strain upon it. Before placing the rim in position, paint the porcelain against which it is to rest with a thin coating of whiting and water or plaster and water, let- ting it run in between the gums of the teeth and the plate, but carefully keeping it from any part upon which the solder is intended to flow. This is mainly to prevent the borax from coming in contact with the porce- lain. Adjust the rim in place, and pass a wire around the case in such a position as to impinge upon the rim on both sides — say between the bicuspids or the bicuspid and molar — passing it between the teeth or over the occlusal surface as may be most convenient; twisting the ends together so as barely to draw it tight enough to hold the rim in place; then, taking hold of the wire with the flat pliers, draw it sufficiently tight by bending it upon itself. If the tightening is done entirely by twisting the ends, there is a great risk of changing the shape of the plate either while placing the wires in position or during soldering; by bending the wires as suggested sufficient strain is exerted to hold the rim securely, without the slighest risk of bend- ing the plate; the wire, so bent, is not a rigid band around the case, but exerts an elastic pressure. Place as many wires around the case, in any direction most convenient, as may be necessary to hold the rim securely. If half-round wire is used for the rim, it should be wide enough on the flat side to reach from the plate sufficiently far over the gums of the teeth to cover the bevel made with the grindstone, but not to extend beyond this. Where it passes over slight depressions in the plate, do not bend the wire to fit them, but depend upon fitting a piece of plate to fill the vacancy. Wlien the rim is accurately adjusted and held in place with adhesive waxor with wire, proceed to invest the case for soldering, using for this purpose a mixture of plaster and sand precisely the same as is used for investing to solder the teeth to the plate. First fill the lingual surface of the denture with the investing material to a level with the tops of the teeth then invert it over a massof investment previously placed upon a piece of glass, pressing it down so as to leave about one-quarter of an inch under the cutting edges of the teeth; then build it up outside of the teeth about one-half of an inch thick, high enough to protect thoroughly the gums and RIMMING 609 to extend a trifle over the free edge of the rim: build the batter up quite wide at the top, so that there will be no danger of its cracking, so as to expose the gums of the teeth. Build it up at the back part so that the posterior edge of the plate is well covered, and extend it over the palatal surface to a little beyond the margin of the vacuum-cavity, so shaping it, however, so as to permit soldering the rim far enough around the last molar of each side to meet the piece extending from the backings. When the investment has set proceed to solder the rim. If binding wire has been used, do not disturb it. Apply borax carefully and not too freely, add sufficient solder to "tack" the rim at a number of points, and proceed to heat the case preparatory to soldering. When using the blowpipe to flow the solder, avoid directing the pointed flame against that part of the plate immediately back of the gums of the teeth. En- deavor to heat the case evenly, and after fixing the rim in position by fus- ing or flowing the solder first placed, cut or break those strands of bind- ing wire not fused or burned in two, and carefully bend their ends out of the way, taking especial care not to disturb the investment. Any por- tions of binding wire held by the solder or borax, and not readily removable, may be allowed to rest until completing the soldering when, if in the way, it may be pushed aside with the soldering-point. Then begin at one end of the rim and reflow the solder, adding enough more to fill the sharp angle between the rim and the plate and to permit a smooth, neat finish. This method of rimming is more rapid than any other, and has the decided advantage that after the teeth have been adjusted to the mouth and finally fitted they are not again removed from the plate. It re- quires careful attention to detail to avoid fracturing the teeth or warp- ing the plate, but in careful hands it is by no means as hazardous as might be supposed. The description refers to a full upper denture, but the method may be used equally well for rimming partial or lower dentures, the changes needed when it is a applied to these will readily suggest themselves. To rim a denture after the teeth have been finally fitted to the plate and before they are soldered in position, proceed as follows : — Place the denture fully prepared for investing upon the plaster model, having previously made one or more cone-shaped holes, or a vertical groove, in the back part to serve as guides to readjust to the model the model now to be made; and after varnishing the surface of the model so that plaster now to be added will not unite with it, proceed to build plaster upon the model precisely as though making an articulating model, allowing it to extend above the teeth and just to the outer edge of their occlusal surface and over the back part of the model, so as to have a solid bearing. When this is hard, smooth the edge immediately over the teeth, making around it a number of cone-shaped pits, and after properly pre- paring the surface of the plaster to prevent union, add plaster so as to ex- tend over the surface of the teeth as far as the gums, and to extend over the plaster model previously made sufiiciently far to give it a soHd bear- 39 610 SWAGED METALLIC PLATES. iiifj- (Fig. 553.) The object is to imbed the teeth thoroughly in plaster, so that in fitting the rim they can be removed from the plate, and readjusted as though they were one piece. This guide is made in two sections, so that if desired when the rim is finished the last-made section may be re- moved without disturbing the relation of the teeth, to permit of the den- ture being invested in the manner presently to be described. This plaster, v.hen set, will hold the teeth securely and the denture may be removed from the cast and the gum ends of the teeth ground even and RiG. 5oa Guide to hold single gum teeth together whUe adjusting a rim, a portion broken away to ghow its construction. This guide is made in two parts, the first rests upon the cast and fills the inner portioa of the denture, extending over the teeth and just to the outer edge of their occlusal surface. The second part fits onto the first part, and extends over the teeth to near the extreme ends of their gums. The two parts hold the teeth together so that they can be removed from the plate and handled during the fitting of the rim as one body; they are not separated until the rim is quite finished and the denture ready to be invested for solderin?. to a bevel precisely as described for rimming after soldering the teeth to the plate. The rim may be fitted to the teeth by swaging, making the die for each half of the rim by molding in sand directly from the denture; or by taking a plaster impression of the parts to which the rim is to be fitted, and from this making a plaster model to be reproduced in zinc by the usual method; or by taking this impression in a mixture of molding sand and plaster or other impression material into which zinc can be cast. After the zinc die is made a lead counter-die is obtained in the usual manner. Then, cut a paper pattern by which to cut the metal strip of which the rim is to be made. Swaged rims should be made to extend to the border of the plate; thus made, they are much easier to swage and to solder and make a neater finish. There is usually a little diflBculty in RIMMING. 611 holding the strip in place upon the die when commencing to swage, and a constant liabiUty of its moving out of position during that operation unless great care is used. After the rim is swaged and fitted to its position, the two halves being slightly beveled and overiapping at the median line, so as to make a neat joint, and that part extending around the last molar of such side adapted to make a continuous band to and uniting with the backings, it may be held in place for investing by shellac, or by a little plaster - placed at several points, or by drilling several rivet holes and riveting it in place. This latter method is by far the most secure and is readily done if there is room between the gums and the edge of the plate for the rivets. The rivets are to be placed in the holes and tightened before the teeth are removed from the plate, so that there will be no risk of the rim changing its position during investment and soldering. Then slightly warm the plate by holding it over a Bunsen burner or a spirit lamp and remove the plate, leaving the teeth imbedded in the plaster molds. The plate and rim are removed from the teeth for investment in the same way if the rim is held by shellac or plaster : it must be done, however, very carefully. The plate with the rim is now invested in a mixture of plaster and sand such as is used for investing teeth. If the rim is very narrow and does not extend much over the plate above the teeth, it should be, before investing, painted on the inside with whiting or plaster to keep the solder from filling it up; in other cases, it is better that the solder should run in slightly, so that there will be no danger of filing through if it should be found necessary to reduce the size of the plate: a little care in soldering will prevent the solder filling the socket so as to interfere with the teeth. By a second method, place the denture fully prepared for the re- ception of the rim on the model and proceed to make a plaster impression of the parts to which the rim is to be fitted, allowing the impression to extend around the last molar as far as the rim is to go, and to rest upon the front part of the model sufiiciently to permit of its removal and accu- rate replacement, the model having been previously prepared to prevent the new plaster from uniting with the old. When the impression has set, remove it by inserting a thin knife-blade between the new and old plaster; it is seldom that its shape wdll permit its removal in one piece, but with a little care it can generally be broken away in but few pieces. Then remove the denture, and after warming the plate remove it, leav- ing the teeth imbedded in the guides. Clean the adherent wax from the plate, and after replacing it upon the model replace the impres- sion, securing it in position with adhesive wax; oil, or otherwise pre- pare the impression so that new plaster will not unite with it, and pro- ceed to run plaster mixed with pumice stone or marble dust so as to be non-shrinking when heated into it, in order to secure a fac-simile of that part of the teeth to which the rim is to be fitted, allowing the plaster to extend over the lingual surface of the plate so as to give the cast sufficient strength. Any of the prepared investment materials 612 Sn'AUJW METALLIC PLATES. may l)e used in place of plaster. When this has set, remove the im- j)ressi()n. Now proceed to adjust a rim to this fac-simile of the teeth in precisely the same manner as though it was the teeth soldered to the plate, holding it in place with binding wire precisely as directed when describing that method, taking especial care, however, not to draw the wire so tightly as to crush the plaster. The rim is soldered without in- vesting. Success by this method depends upon careful mani])ulation, and upon securing an accurate fac-simile of the teeth in a material making a smooth, sharp model, and one not readily abraded, and that does not shrink when heated to the fusing point of the solder used. By the third method the rim is fitted to the teeth themselves, as is a swaged rim, and held in place with shellac or plaster while the teeth imbedded in the mold are removed from the plate. The plate and rim are then invested, the iuA'estment being molded to extend slightly over the free edge of the rim to hold it in place. It is seldom necessary to place whiting or plaster inside the rim to keep the solder from entering; the investment usually prevents its encroaching upon the space required for the teeth. The writer prefers this latter method. It is simple, less labor, and as the rim is fitted to the teeth, errors of adaptation owing to a model being abraded or broken are avoifled. When time is no object, and the work- man not accustomed to fitting rims, or for special cases, swaging gives good results. Theoretically, it should give more accurate results than any other method, but practically there are so many difficulties en- countered in constructing the dies, in keeping the strips of metal be- tween them, in so arranging that the impact of the blow will swage the rim evenly, that it is seldom practised by practical workmen. It is a mistake to make a rim of very thin metal. Gold should not be less than No. 26, while silver should be a little heavier. The thicker metal holds its position better during soldering; its free edge is readily thinned with a file as may be found necessary when burnishing it close to the tooth gums, and is not so fiable to stretch under the burnisher. Where it can be used, half-round wire has decided advantages. It is readily bent in any direction, and has thickness where thickness is most desirable; it requires, however, owing to its rounded edge, the free use of solder or the addition of narrow strips of plate to avoid an objectionable groove at its junction with the plate. The investment has but a slight hold on the rim, this will suggest care in preparing for soldering, and care during that operation to pre- vent its displacement. It is usually best to heat up invested rims slowly and to heat them as hot as one would for soldering teeth; otherwise when the hot blowpipe-flame strikes the narrow rim it is suddenly ex- panded and curves up; making a displacement difficult to contend with when finishing the denture. Care shoidd also be exercised to avoid fusing the rim, an accident very difficult to repair; or allowing the solder to flow over it, especially near its free edge. Small pieces of solder should be placed at intervals, and these should be first fused tacking the rim to the plate quickly. This accomplished, begin at one end and flow RIMMING. 613 the solder in sufficient amount to make a joint that will finish up neatly; where much filling in is needed, narrow strips of i)late may be added. After the rim is soldered and the j^late cleansed with acid, place it on the model to see that it fits as accurately as it did before — it is seldom that there is any material change if the work has been carefully done; then proceed to adjust the rim and to finish it as far as is desirable to do before the case is invested. As it is difficult to get a smooth, even edge to a rim after the teeth are soldered to the plate, the rim edge should now be finished, using for the purpose a dead-smooth file and very fine emery or sand-paper. Do not bevel the edge too much; it should be rounded rather than beveled from the labial side, and left thick; it will be somewhat further reduced when it is burnished close to the teeth in finally finishing the case, and if made too thin it is apt to tear and break during that operation. The next step is to readjust and secure the teeth to the plate for in- vestment; the procedure for doing this is the same, no matter by what method the rim has been constructed. Naturally, one would think that this could be accomplished by simply heating the plate so that it would soften the adhesive wax, placing it upon the model, and then press the guides, in which the teeth are imbedded, firmly in place. If the rim has been swaged and riveted to the plate before the teeth were removed, this will often give accurate results; in other cases, however, it is very uncertain. There is a tendency in the rim to creep bodily toward the alveolar ridge during soldering, so that when the teeth are placed in it after soldering they are a trifle too long, not enough to affect seriously the articulation, but enough to prevent their fitting solidly to the plate. Wax, particles of plaster, slight changes in the form of the plate, etc., may prevent the guides fitting exactly as they originally did. In order to ensure accuracy in this respect, before removing the guides from the model make a slight cone-shaped depression in the guide and another immediately below it in the model at about the median line; into these adjust the point of a pair of dividers, and accurately measure and note, or mark their distance apart on some portion of the model, so that the dividers can be again set in the same position. It is only necessary to apply the dividers again to these points to determine when the guide is in its original position. In many cases it is necessary to remove a portion of the wax and to cut the plaster away from the guide so as to free the plate entirely before it will fit in its original position, not disturbing, however, that portion which rests upon the back part of the model. Occasionally it is necessary to grind the ends of the gums; this should be done care- fully, and not until it is found absolutely necessary; usually a little taken from the plate side of their extreme ends will be sufficient. This can gen- erally be done without removing the teeth from the guide. If the teeth are kept together and not removed from the guide, one may depend upon accurately replacing them; if they are removed, it is very difl&cult to replace them without some slight changes in their position. After the teeth are accurately fitted to the rim, so that the guide with the teeth imbedded in it will go on the model in its original position. 614 SWAGED METALLIC PLATES. secure the plate to the teeth "vvith adhesive wax and remove the plate and teeth thus held together from the model, and add a coating of invest- ing material to cover the rim and exposed ends of the teeth, letting it extend under tlie plate. When this has set, remove the outer portion of the guides, that portion which covers the face of the teeth and which was made so as to be removable; cut from the remaining portion so as to expose one-third of the grinding surfacesof the molars and bicuspids, and add a fresh portion of investment, allowing it to cover all exposed portions of the teeth and to overlap the portion first added, wetting this with water so that the two will unite. When this has set, carefully remove the remaining portion of the guide, heating the plate to soften the wax and permit its removal. If any teeth are disturbed during this operation, they should be replaced and made secure. After wrapping this pre- liminary investment with binding wire as a precaution against the in- vestment cracking, the investment of the denture is completed in the Fig. o54 Upper denture rimmed. usual manner, [f this procedure has been carefully carried out, there will be no material change in the position of the teeth, and much less time will have been occupied than if the teeth had been separately adjusted to position, with the further advantage that the even gum line made as the first step toward the construction of the rim will have been preserved throughout. The changes in technique for rimming a full upper or lower or a partial upper or lower denture are so slight that further de- scription is unnecessary. After the teeth are soldered, the first step toward finishing the rim is to make its free edge fit closely to the gums of the teeth; this is accomplished by the use of the burnisher and the file — the burnisher to bend the edge, and the file to thin the edge when it is too thick for the burnisher to bend it. This accomplished, by the use of suitable tools the rim is to be given smooth and even surface. The finished riii\ is shown in Fig. 554. The operation of burnishing, when the object is to change the shape of a piece of metal, is analogous to "metal-spinning," and is very different from burnishing to produce a hardened pohshed surface; in the first the burnisher is used with considerable pres.sure, its motion is slow and de- liberate, so as to effect the desired change before the metal has lost the softening effect of annealing and become hard and elastic. In the second WIRING PLATES. 615 the pressure is light and the motion rapid, an effort is made to condense and harden the surface so that it will receive and retain a high polish. WIRING PLATES. ^Mien plain teeth have been adapted to a plate, and it is designed to place over them an artificial gum of one of the vegetable bases, it is advisable to attach to the labial and buccal edge of the plate a contin- uous wire, to round the edge, increase the means of retention of the arti- ficial gum, and to lend additional beauty of finish to the piece. After the teeth have been fitted and tried in the mouth, the palatal line of the base of the last molar of each side is marked by a scratch on the plate. Be- ginning at this point at one side, a wire is curved over the ridge and to follow the edge of the plate until it terminates at the base of the opposite terminal molar: the stays of these teeth are to abut with the ends of the wire. A piece of triangular wire No. 18 gauge of the proper length is pro- cured. Tliis is annealed, boiled in the acid solution, and one face of it scraped to exhibit a fresh surface. The middle of the wire is usually attached first. A clamp holds that point of the wire against the plate at the depression for the frsenum, the edge of the wire level with the edge of the plate, which is then set on a block of charcoal, the alveolar ridge portion resting upon the surface of the latter. The point of junction of the wire and plate is covered with borax and a small piece of solder placed over it. A fine blowpipe-flame is directed against the plate beneath the wire carefully avoiding contact of the flame with the loose ends of the wire, as these latter fuse very readily. When the solder flows, attaching the %vire at this one point, the clamp is removed and the plate plunged in the sulphuric acid solution. The plate is placed upon the model or die and the wire bent, following the line of the plate. For about half an inch on both sides of the soldered point the wire is brought into close apposition with the plate, the edges of wire and plate in a fine; the junc- tion is boraxed, and held in place with a clamp or with binding wire, and joined to the plate by small pieces of solder. The remainder of the wire is, little by little, fitted and soldered until the extremities are attached and are soldered fast to the point at which the disto-palatal corner of the sec- ond molar touches the plate. As a final measure the entire length of the joint between the plate and wire is covered by borax, one or two small pieces of solder placed at points where the solder may be deficient in amount, and then the blowpipe flame is passed along the joint, filling the latter completely with the solder. Should the case be one retained by clasps the wire is to be attached before the clasps are fastened to the plate. Before rimming or wiring it should be determined whether the plate edge is of the proper height or depth, for, should subsequent trimming of this portion of the plate be required, it is possible that a portion of the wire might have to be filed away, and thus mar the finish of the denture. Cases which have the third molars remaining should have the wire 616 SWAGED METALLIC PLATES. carried around the plate at tlie ])ase.s of tliese teeth (FiV. 555), Tliis device may he used on upper, lower, full or ])artial dentures. If the teeth have been fitted to the j)late before the \virin<>; i.s done they may be held together by plaster guides, and invested, nuieh in the same way that gum teeth are in rimming. It is usualh , however, better to wire the plate first, l(>aving the ends extending around the molars to be soldered when the backings are soldered. Fig. •■)-')5 A metallic upper plate intended for a, plain tooth denture, wired along its upper edge. If the teeth are to be attached to the plate l)y means of vulcanite the wire is continued across the palatal aspect of the palate, following a line which marks the base of the wax. A pair of special, long clamps will be required to hold the wire in contact with the plate during the solder- ing operation. PREPARING THE CASE FOR INVESTING. All dentures of plain teeth, and partial dentures of gum teeth where they stand alone, or where not more than two or three are together, usually need no preparation for investing beyond seeing that the teeth are held so securely by the adhesive w^ax that there is no danger of their displacement when the denture is imbedded in the investment; but in cases where the gums are very long and are not supported by the plate, or have been ground very thin, it is a wise precaution to place a drop of yellow wax inside the weak gums, so as to prevent the plaster from coming into actual contact with them, and thus lessen the risk of their being broken when the case is removed from the investment after solder- ing. In cases where there are a number of gum teeth together with the gums closely jointed, some provision must be made to prevent accident from their expansion when heated for soldering. If this is not done, there is serious risk of chipy)ing at the edges where they are jointed to- gether, or the teeth may be so broken as to require replacement. In INVESTING. 617 addition to this tlienMS also a risk of the plate warping tlnring soldering: where in lower dentures this oeeurs the plate usually eui'ves inward, the distal ends being brouglit nearer together; in ujjper dentui'es the ehange in shape makes the plate press upon the hard palate, producing what is known as' a "side rock." To prevent these aceitlents take a strip of thin soft paper, such for instance, as the margin of a newspaper, and, beginning at the centrals, remove, one at a time, each alternate tooth, and place in the joint be- tween the gums a thickness of paper, slightly wetting it so that it will lay flat, removing from the tooth with a grindstone a sufficient amount to allow for it. During soldering this paper is burned out before the heat has expanded the porcelain to any great degree, leaving between the teeth a slight space; if carefully done, however, the space made is so slight as to be unnoticed in the finished denture, while it effectually prevents the accidents referred to. To remove a tooth without dis- placing the adjoining teeth, pass a hot wax spatula by the side of the pins to soften the wax around them, and then push it off. If this is done carefully the wax is left so that the tooth can be accurately replaced, an application of the wax spatula securing it in place again. INVESTING. An investment is a device designed to hold the teeth and plate in their relative positions during the soldering operation. It is made of a material having a low degree of heat conductivity and sufficient coher- ence to ensure that it shall maintain its form when raised to a very high heat. By its relatively low conductivity it prevents too rapid heating of the porcelain teeth it encloses, and also the too rapid cooling when the source of heat is removed; either of which is a prominent factor in. casing fractures of porcelain. The same physical property lends to the investing material the feature of maintaining the teeth at a constant temperature during the soldering operation. Plaster of Paris is the basis of the investment; to it are added beach sand or asbestos, so as to overcome its tendency to contract and to fracture when heated to a sol- dering temperature. About five parts of sand, or of a mixture of sand and asbestos, to four of plaster, makes a satisfactory investment. Water is first placed in a plaster bowl, and to it is added a sufficient quantity of short-fibre asbestos, or woolly asbestos, or beach sand and asbestos, until the materials are just covered by a film of water, and the mixture is well stirred to distribute the asbestos or sand evenly. Plaster of Paris is next sifted in and stirred until a soft, plastic mass is made. About a spoonful of this is placed upon a glass slab, making a layer about half an inch thick. The denture is wet so that the investing material will flow freely into the spaces between the teeth. A portion of the material is taken upon the point of a spatula and worked into the deepest portions of the palatal surface of the plate : little by little more investment is quickly added until the plate is full, when the material is 618 SWAGED METALLIC PLATES. tlien packed between the teeth, filling the spaces perfectly. It is then inverted upon the bed of the material upon the slal), and the investment built up about the teeth until they are covered by a layer half an inch thick. The lingual surface of the plate is covered to within about half an inch of the bases of the teeth. To ensure against fracture of the in- vestment, a piece of round-wire, or several strands of binding-wire formed into a ring is imbeded in the investment. This is so bent that its arch shall be about a quarter of an inch larger than that of the teeth. When binding wire is used (Fig. 556), several strands formed into a circle are placed over the teeth, the front portion a little below the cutting edges of the teeth, and the back portion is bent downward to about the level of the plate. In either case this reinforcement is set in position when the investment is half complete and thoroughly imbedded in it. Fic. 556 Several strands of iron binding-wire formed into a ring to reinforce the in^■estment of an upper denture, showing the relative position of the wire and the plate. The investing material is harder, but more friable and quicker setting than plaster alone; these manipulations must be quickly done. (Fig. 557.) When investing delicate partial cases it is better to have the material rather thin, placing a mass of it upon the slab and then carefully laying the case upon it, work it down with a gentle rocking motion until there is rather less than half an inch under the deepest part of the plate: after this has become somewhat set, add more of the investment, building it up to and around the teeth. The investment of lower dentures differs so little from that of upper dentures as not to re(iuire a separate description. (Fig. 558.) ^ When investing a denture, three points are to be borne in mind: First, be sure that the material is in contact with the under surface of the plate at all points, especially that it fills the bands or clasps of partial cases. If any vacancies exist there is danger of the plate being overheated and burned at that point, or of sinking in and thus im- INVESTING. 619 pairing- tlie fit Second see that the teeth arc well covered, not only to hold them in place secnrelj, bnt also to protect them from direct contact witii the blowpipe-flame. Third, see that the edges of the plate are Fig. r,57 An upper denture invested, and prepared for making the backings and for soldering. A por- tion fs broken away to show the terminal of the reinforcing wire imbedded in the investment. well covered: if this is neglected they are apt to curve up. It is de- sirable, however, that the surface of the plate be well exposed; for soldering, otherwise there may be difficulty in getting it hot enough (Figs. 557 and 558.) Fig. 558 A lower denture invested, and prepared for soldering. A portion is broken away to show the terminal of the reinforcing wire imbedded in the investment. Preparing the Invested Denture for Making the Backings, and for Solder- ing. — First, trim off such portions of the investment as encroach too far over the plate and the edges of the teeth ; being careful, however, that while removing enough to give free access to the teeth for making the 620 SWAGED METALLIC PLATES. li.-ickiiigs, aiul ('X])().siiio; all portions over wliicli solder is to be flowed so that they can he readily reached hy the hlowpipe-llanie, that enoutrh is left to hold the teeth securely and to protect all parts liable to be injured by unnecessary contact with it. Then, remove all i-edundant portions and trim to a neat easily handled shape. Next, remove the adhesive wax, which to this point has held the teeth in place. This should be done mechanically; the use of heat, wet or dry, is inadmissible. A fine pointed instrument, or the narrow blade of a pen-knife, is an effective tool for this purpose. Care is needed in this operation to avoid fracturing the teeth by getting the instrument between their pins, or displacing the teeth. By picking and scraping every particle of wax under, between, and around the teeth must be re- moved, and the plate surface over which solder is to flow made clean and bright, especially if the plate is of silver. The pins should next re- ceive attention, and be made clean and free from porcelain or other foreign matter adhering to them. Partial dentures require during this operation very gentle handling. Teeth standing alone are easily dis- placed, and the gums of gum teeth are readily broken. If the backings have been previously made, the denture is now ready for soldering; if this has not been done, it may now- be proceeded with. MAKING THE BACKINGS. As a general rule for all full dentures, and for all but the smaller partial dentures, it is more satisfactory to fit the backings after the teeth are invested. When the denture has been finally tried in the mouth with the teeth cemented in place, it is a serious risk to remove them again from the plate; notwithstanding the careful use of guides, slight changes of position are inevitable, and the artistic and mechanical adjustment thereby endangered. Before beginning to make the backings for a full denture, have the fol- lowing tools close at hand upon the workbench: the plate shears; punch for rivet holes (select for this purpose a punch with short jaws; many are made with the jaws so long as to require a strong force to per- forate the thick metal used in making backings); a strong pair of flat pliers (not too large), with beaks that will hold firmly; a five inch half- round file; a point that can be used as a reamer; a tool for countersink- ing the rivet holes — a large, obtuse-pointed spear drill, or a triangular scraper answers for this purpose; a small hammer; the bench anvil; a pair of broad tweezers or solder tongs; and a chisel for splitting the pins. The thickness of metal to be used for backings will depend upon the length, position, and character of the teeth. Plain teeth, and all teeth that stand alone require stronger backings than do gum teeth, where the backings may be united a portion of their length. Long teeth and very narrow teeth refpiire stronger backings than do short or wide teeth. Gold backings should be about No. 22 gauge, silver about two numbers heavier. The addition of a little platinum to the gold used for backings gives them greater strength and stiffness. MAKIXG THE BACKIXGS 621 The hackings should be made nearly as wide as the teetli. The length is sometimes a matter of taste; at other times it will depend upon the requirements of the case. If the anterior teeth are likely to be sub- jected to much strain in mastication, their backings should extend quite to the cutting edges, so that the opposing teeth will strike against them rather than against the teeth. In other cases they should terminate about half way between the upper pin and the cutting edge; this leaves the cutting edge more translucent. The backings should be uniform in height and shape, have the corners rounded and their surfaces neatly curved to a convex form. (Fig. 559.) ^Yhen backings are to be made for a full denture, it is time-saving to cut several strips of metal, about two or three inches long, and of suit- able width, bevelling for their entire length both edges upon that side Fig 559 Upper denture ready for tho mouth. vrhich is to be the face of the backing: the strip is more quickly bev- eled than would be the separate backings. An upper denture requires the same width for the centrals and molars, another width for the later- als and bicuspids and canines, so that if each strip is cut slightly wider at one end than at the other, two strips will make all the backings. A lower denture usually requires three strips differing in width; one for the six anterior teeth, one for the bicuspids and one for the molars. The author prefers to begin ^-ith the centrals in making and finishing the backings, making them in pairs, as by so doing they can be made more uniform. Ha^'ing prepared the strips, place one before the tooth to be fitted, and, holding it in the position the backing will occupy, pro- ceed to make the end resting on the plate fit the plate accurately by cut- ting with the shears or by filing. "Wlien this is done, mark the position of the pin-holes by placing it in position and while pressing it firmly against the pins impart to it a slight movement so that it will rub against at least one pin. This will accurately mark where the pinhole is to be punched. Punch a hole at the pin-mark most plainly to be seen, and after countersinking this hole proceed in like manner to mark the posi- 622 SWAGED METALLIC PLATES. tion of tlie remaining pin. This method of determinig the position of the pin-holes has been satisfactory to the author for more than two score of years; it is cleanly, requires no additional tools or appliances, is quick- ly done and is accurate. If the holes when punched are too far apart, or to close, so that force is required to press the backing in place, there is danjections to the method, will be discussed at the appropriate place in this chapter. As a minor objection may be mentioned the liability of fracture of the plate. Proper admonition to the patient as to the care of the denture should always be given, and by the exercise of due caution in cleansing the piece, any annoyance on this score may be avoided. Fracture in the mouth is of rare occurence. The tissues of the mouth react more kindly to the presence of a platinum base-plate than they do to any of the vegetable bases. All the materials of which the denture is composed being good conductors, the thermal variations are readily communicated to all the underl\'ing parts. From a hygienic standpoint, the platino-porcelain plate stands alone. There are no interstices to catch food debris, and as the denture is absolutely uninfluenced by the secretions of the mouth, or the pro- ducts of bacterial action, it may be made as clean and fresh as new by ordinary cleansing methods which may be instituted by the patient. One of the chief points of excellence in favor of the continuous-gum method is the facility and accuracy with which the natural organs are imitated. Any loss of gum or palatal contour may be so faithfully reproduced in form and color as to defy detection. JMPBESSIONS AND CASTS. 637 Continuous-gum dentures are applicable with most satisfactory results in full cases, upper and lower, but it is quite possible to employ them with good effect in many cases requiring partial dentures. IMPRESSIONS AND CASTS. The first requisite for success in constructing continuous-gum den- tures is a good impression, and for this purpose plaster of Paris is the material par excellence. The manner of taking the impression and its subsequent treatment are described in detail in another section of this work. The cast is obtained and dies and counter-dies made as for Fig. 570 Showing form of arch where chamber may not be necessary. other varieties of metal dentures. The indications which govern the use of vacuum-chambers are the same as for other metal plates. If in the judgment of the operator a vacuum-chamber is indicated, it may be carved in the impression thus forming an integral part of the cast or it may be placed upon the cast after the latter has been removed from the impression in any of the ways described in Chapter XI. In Fig. 571 Cast of the upijer jaw with ledge for turning the rim. cases presenting well defined alveolar ridges and moderately deep vaults the vacuum-chamber is unnecessary. The firmness of the ad- hesion of the plate to the underlying tissues depends upon the accuracy of its adaptation to them. If this is perfect it v^^ill adhere with sufficient 63S CONTINUOUS-GUM DENTURES. tenacity, but if the plate is poorly adapted or warped in any degree, the stability of its retention is thereby lessened proportionately. If it is intended to swage a rim to the plate, especial care must be exercised in marking the outline of the latter on the cast. It should follow such a line as will prevent any possibility of its impingement upon the muscles of the lips and checks or other movable tissues. If after the piece is finished such impingement does exist, the beauty and finish of the plate would be seriously marred by trimming. A layer of wax slightly thicker than the proposed rim is adapted to the cast, following accurately the plate outline. It should form with the wall of the cast an angle, the de- gree of which should not be so acute as to interfere with accurate mold- ing. The wax is then trimmed away to blend gradually with the sides of the cast. The cast thus prepared is now to have the necessary varnish applied and the die is obtained in the usual way. Results obtained with dies made of nickel Bal)bitt metal, or Babbitt metal of Haskell's formula, will be found satisfactory in this work. The latter alloy contracts less than zinc, and plates made over dies composed of it fit the plaster cast as they do the die. FORMING THE FULL UPPER PLATE. The pattern of the plate in heavy tin foil is reproduced in pure platinum No. 32 to 36 gauge for upper plates, the thickness of the platinum depending upon the form and size of the plate. The writer has used all gauges and finds plate of these numbers, properly rein- forced to meet the individual requirements, not only sufficiently rigid for all purposes, but contributing much to the lightness of the completed denture. The face of the die is covered with a thin piece of wet muslin, and the platinum after being well annealed by placing the platinum in the electric furnace, bringing it to a white heat, and allowing it to cool slowly, is a.dapted as closely as possible to the palatal \'ault by pressure applied with the thumbs, and by the use of the horn mallet. Great care must be observed while annealing the platinum that no particles of zinc or lead are attached to it, for they would become alloyed with it, thus producing a hole in subsequent heatings, or causing the plate to crack or break in swaging, or possibly subsequently discoloring the porcelain paste. Using muslin over the face of the die, and pickling the piece each time before it is annealed to remove any possible trace of base metal will effectually prevent this. The process of swaging is continued, reannealing the platinum whenever its returning obduracy necessitates, until a fairly close approximation to the ^•ault is obtained. A partial counter-die made of good modelling composition may now be used. The composition is softened and molded to the vault of the plate, completely filling the latter. It is then hardened by dipping into cold water. The composition is now placed in position on the plate which is in place on the die, and the whole is clamped securely to the bench with FORMING THE FULL UPPER PLATE. 639 an ordinary bench clamp, one arm of the clamp engaging the under sur- face of the bench, the upper arm engaging the composition. In this manner the plate is held securely in position on the die while its alveolar borders are adapted with the swaging mallet. The soft platinum exhibits a tendency to wrinkle and tear under manipulation, so that all undue haste and force are to be avoided. If, however, the metal should be torn, small breaks in its continuity may be repaired by soldering a piece of thin platinum , gauge 36, over the hole with platinum solder. When the adaptation to the die is as accurate as can be obtained by pressure and the mallet, the plate is trimmed approximately to the line marked on the cast. It is now ready for the counter-die, and extreme care should be observed to prevent contact of the platinum with the base metal of the counter-die, particularly if the latter be made of lead or an alloy containing that metal. As is well-known, lead and platinum combine with remarkable ease, and should small particles of lead become attached to the platinum when the latter is heated, the base metal forms an alloy with that part of the platinum with which it is in contact. This alloy is very fusible, and if the plate is not at once per- forated, the part so contaminated will always be liable to this accident upon subsequent heatings. To avoid any such contingency it is well to interpose betweeen the surfaces of the counter-die and plate, wet muslin or thin rubber dam. This will also assist in disengaging the plate from the counter-die if, as frequently happens, it becomes wedged therein from the force of the swaging. Should the platinum become contaminated with the base metals, it must be pickled in hot nitric acid before further annealing. The plate is at first lightly swaged between die and counter-die, then removed and examined carefully for wrinkling or breaks. Should wrinkles develop they should be corrected with pliers and the horn mal- let. The swaging should alternate with pickling and annealing until the adaptation is perfect. The plate is then closely trimmed to the plate outline, leaving an excess sufficient for the rim if this is to be turned, and it is ready to be tried in the mouth. The rim is to be com- pletely turned after the first coat of body has been baked. The operator may prefer to solder a wire along the alveolar border of the plate instead of turning the rim, in which event the plate is to be trimmed to its true dimensions which are determined by trial in the mouth. The wire rim permits a trimming of the plate margins after the body has been applied and even after the plate has been worn. The vacuum-chamber in a continuous-gum denture is preferably formed in the plate itself, although it may be cut and soldered using for the chamber cap a piece of plate of the same thickness and soldering it in with 25 per cent, platinum solder. This is the solder em- ployed to best advantage in continuous-gum work. The relative position of the pieces united by it will not change in the intense heat of the furnace. Pure gold may be used if the pieces are in absolute contact and a minimum amount of solder used, but an excess of gold should be avoided, as it is thoroughly melted and widely diffused by 640 COyTINUO US-G UM DENTURES. the hifh temperature at which porcelain fuses. Solders containing base metals siiould never be used. Strengthening Pieces. — The object of these is to add strength to the parts of tlie plate which are liable to undergo alteration in form, either when it is subjected to the great heat of the furnace, or under the stress of mastication. Those places in which experience has demonstrated the necessity of additional strength are the posterior Iwr- der of the plate, usually i-eferred to as the heel, and that portion of the plate extending posteriorly for a small distance from the alveolar bor- der along the median line. The form these strengthening pieces should take is governed by the judgment of the operator. The method of forming and adjusting them employed by the writer is as follows: patterns of heavy tin-foil are pre- pared; the pattern for the anterior piece is cut roughly to the form of a triangle, the base slightly overlapping the ridge anteriorly; the apex ex- tending l)ack toward the vault of the plate and lying on the median line, the size to be governed by the size of the plate. (Fig. 572.) This pattern is to be reprofluced in iridio-platinum plate, No. 24 to 26 gauge, according to the case. The piece is well annealed, adapted to the die with the mallet and then swaged between the die and counter-die. It is now placed in posi- tion on the plate and both are swaged together. The posterior pattern is cut in the form of a strip three-sixteenths of an inch wide, extending across the entire posterior border of the plate. This pat- tern is also reproduced in iridio-plati- num No. 24 or 26 gauge. It should be observed that the object of this piece is two-fold; first, to give additional strength to the heel of the plate; sec- ond, to engage the porcelain and give finish to this part of the plate. In or- der to serve best this double purpose, the strip should be so formed that when finally adjusted its anterior edge is distinctly raised, while the posterior two-thirds of the body of the strip lie in close apposition to the plate. To accomplish this result a strip of brass is swaged to con- form to the portion of })late immediately in front of and covered by the anterior Iwrder of the iridio-platinum piece when the latter is in jM)si- tion. The posterior border of the brass strip should be filed to a feather edge. The strengthening piece, having been adjusted to the die and swaged in the usual manner, is now placed in position on the plate with the brass strip ])roperly interposed, and the pieces are swaged together Reinforcing pieces for full upper denture. ARTICULATION. 643 the !)akiii<>- of the body reduces the contraction, and maintains the con- tours in these regions. When the rerjuirements of the case cull for very short teeth, those employed in vulcanite or celluloid work may he used Any artistic skill which the operator may possess has wide latitude for its display in the arrangement of the teeth. The varying facial ex- pressions of tiie patient are studied closely and the endeavor is made to fix in the mind the natural expression both in repose and when the face is animated as in the acts of talking and laughing. Not only- must an accurate knowledge of all the elements entering into and con- 'stituting what is known as the natural expression be borne in mind, but what is as equally important from a less aesthetic standpoint, a thorough knowledge of the mechanical principles involved in the reten- tion of the denture. A just consideration of these observations leads to the further statement that in the construction of an artificial denture, the process is often attended by a series of compromises by which the Fig. 574 Set of continuous-gum teeth. various aesthetic features are reconciled to the practical requirements. Therefore, a comprehensive knowledge of all these factors will best place one in a position to so adjust the conflicting interests that the fin- ished product will not suffer. The central incisors are usually arranged first. Each tooth is ground to rest directly on the plate, and the pin should come in perfect contact with the wire on the top of the alveolar ridge presently to be described, or with the plate, in order to hold the tooth in position during the pro- cess of fusing the porcelain. Between each tooth and the platinum base there should always be two points of contact, viz : the platinum pin and the porcelain root; the teeth will then be less liable to change their position by the shrinkage of the porcelain. If the length of the roots is not sufficient to enable them to touch the plate, it will be necessary to support them in position by interposing little pieces of broken porcelain teeth. Slight irregularities of alignment commonly found in the natural teeth 644 coxTTXT^n rs-o um dentures. are easily reprorliiced and materially aid in giving a natural appear- ance to the case. There are often conditions of the remaining natural teeth resulting from decay and organic discoloration, which make it impossible to find in the stock of the manufacturers a desirable match for a partial den- ture. Such needs of the operator can only be met by selecting a porcelain tooth, the body of which is the same general color as the natural teeth to be matched and treating it with mineral colors in imitation of the natural organs. After the arrangement is completed in conformity with the ideas of the operator, the teeth are attached to the plate with adhesive wax. Fio. 57.3 Iia.-('-iilat<> with Teeth arraiijie'l uiion it. Wax is also loosely molded over the l)uccal and labial walls of the plate to approximate the natural contours. The piece is now transferred to the mouth, the articulation noted and any slight corrections or alterations made. I'he amount of wax necessary to restore the contours is also to be noted; this serves as a guide in the application of the body. The next step is the adjustment of the iridio-platinum wire around the crest of the ridge, which serves for the attachment of the pins and also gives additional strength to the plate. The writer advocates the use of Xo. 10 iridio-platinum round wire instead of the platinum strips Fig. 570 Clamp for holding reinforcing pieces on plate during soldering. which are sometimes employed. The porcelain adapts itself more uniformly to the wire and fuses with less tendency to crack. In determining the proper position for the wire, the plate is placed on the cast, and a wall of soft modelling composition is adapted to cover the anterior and lateral walls of the cast and all exposed portions of the teeth, and extending from the la.st molar on one side to the same tooth on the other. The composition is hardened and the wax removed thus freeing the pins which are bent slightly upward. The wire is adjusted INVESTING. 645 under the pins and waxed to the plate. The teeth should then be re- moved en masse with the composition. The wire is now held in posi- tion on the plate with clamps like that shown in Fig. 576 and soldered. The modelling composition wall containing the teeth is then returned to the plate in position on the cast, the pins bent down in contact with the wire, and securely attached with adhesive wax. The composition wall may then be softened by dry heat and removed , taking care that the wax about the pins is not melted and that the relation of the teeth to the plate is undisturbed. A plaster wall may be used instead of the one described. After the teeth are attached to the plate, the case is ready for the investment. INVESTING. The investment best adapted for continuous-gum work is composed of fine asbestos, one-sixth; coarse calcined silex, one-half; plaster of Paris, one-third. The investment best adapted for continuous-gum work is composed ciently plastic to flow freely between and around the teeth. All surfaces of the teeth to be embraced in the investment should receive a coating of thick shellac varnish. This is to prevent the fusing of the investment with the porcelain teeth during the soldering, an accident which is likely to occur at the high temperature required to fuse platinum solder. The shellac burns out when the plate is heated up and leaves a small space about the teeth. To make the investment small, uniform in thickness, and compact, a matrix into which it may be cast, is formed by placing the plate with teeth attached upon a flat surface like the work bench, the occlusal surface of the teeth being uppermost. Then a strip of base-plate wax is bent into the form of a ring, w^hich shall surround the plate, follow its general form, and be one-quarter of an inch larger in each direction. The ring should be deep enough to extend one-quarter of an inch above the cutting edges of the teeth. The ring thus prepared is now set on a glass slab and attached thereto by passing a hot spatula around its edge. This forms a cup-hke matrix into which the invest- ment is cast. The investment is now mixed as described above and first applied thoroughly to the palatal surface of the plate, allowing it to flow between and over all exposed portions of the teeth. The matrix is filled three- quarters full and the plate inserted, palatal surface downward, and pressed down until it is within one-sixteenth of an inch of the glass slab. The excess of material is forced up and around the teeth, cover- ing the incisal and occlusal surfaces to a depth of one-sixteenth of an inch and filling in the lingual side of the vault. The investment should be fully set before it is removed from the matrix. After it is thoroughly dry and hard, the wax may be detached from the teeth by subjecting the case to dry heat until the platinum be- comes warm, when the wax may be readily removed. Hot water should 646 CONTINUO US-G UM DKyTURES. never be used as it destroys the integrity of the investient material, thereby frequently eausing it to fracture under the subsecjuent applica- tion of heat. All surfaces from which the wax has been removed are to be thoroughly cleansed with chloroform. SOLDERING. The pins of the teeth are bent so they are in perfect contact with the wire and plate. Small sfpiares of platinum solder are placed at these points of contact and retained in position by a coating of borax. The borax is not intended as a flux, as the noble metals do not oxidize and a flux is not required, but it keeps the solder in place. There are numerous methods of heating up the case preparatory to soldering. Quite a satisfactory one which the author has frequently used is to set the invested case in a bed of broken pieces of burnt fire- clav over a Bunsen burner. This fire-clay is prepared by incorporating with it starch or sawdust, breaking it up into pieces of suitable size, and Fig. 577 S^^ W^k (b' *^ dBfeki- ^ \ 3K'* i I ^ m t .1 ^^>^: r Full upper base-plate, showing wire soldered upon top of ridge and teeth soldered on. firing them. The combustible constituents burn out, leaving the pieces porous. When thus prepared the mass may be heated thoroughly in a very short time, and it also possesses the advantage of cooling slowly. With the aid of an ordinary gas blowpipe the case is gradually heated to a dull orange color (2000° F). The flame from the oxy-hydrogen blowpipe is then directed against the parts to be soldered until the sol- der flows. Each pin should be carefully examined to see that it is securely at- tached to the wire or plate. When we are assured of this the case is allowed to cool gradually; this is conveniently done by placing a cover over the receptacle containing it. When quite cool the investment is removed and the denture boiled in dilute sulphuric acid, and Wiished in water until every trace of foreign substance is removed. It is now readv for its final trial in the mouth. The adhesion and articulation SOLDERING. 647 are noted; any slight corrections in the arrangement of the teeth may be made at this time. After it is removed from the mouth the plate is placed on the cast, and all surfaces of the platinum to which the porce- lain is to be applied are roughened by gentle scratching with a sharp excavator. The plate is made scrupulously clean by washing with chloroform or alcohol and is ready for the first application of body. The utmost de- gree of cleanliness must be observed in all the subsequent manipula- tions. Any dust or particles of foreign matter coming in contact with the porcelain body will seriously affect the beauty and integrity of the finished product. The Porcelain Body and its Manipulation. — The degree of success to be achieved now depends upon the skill and judgment manifested in dealing with the purely ceramic features of the case. The manipula- tion of the porcelain paste is considered in three stages; upon the ap- plication and carving of the first coat of body largely depends the final artistic effect. For the application of the paste and its aesthetic carving the set of instruments illustrated in Fig. 578 will be found very useful. Fig. 678 Instruments for carving the porcelain paste. The porcelain body is prepared on a clean glass slab. It is mixed into a paste with distilled w^ater to the consistence of thick cream. It is applied first to the lingual concavity of the palatal portion of the plate, betw^een and about the roots of the teeth, jarring the plate at in- tervals to insure compactness of the body. The moisture rising to the surface is to be absorbed with clean blotting paper cut into strips for the purpose. If slight pressure is exerted with the blotting paper upon the surface of the body, it will be condensed and less shrinkage will en- sue. When this portion is covered uniformly to the proper thicknesS; which is that of No. 24 gauge, the rugae and other features found in the roof of the mouth are modelled in the body. On the labial and buccal surfaces the paste is applied in the same manner. The contours of these regions are roughly brought out in the manipulation of the body, but no attempt is made at accurate carving. No instructions may be given as to the correct car^dng of these features. This must be learned by a close observation of nature and by abundant practice. The crowns of the teeth are kept well defined and thoroughly freed of any adhering porcelain paste. To prevent the teeth from mo\ang and the plate from warping by the contraction of the body in fusing, it is necessary to take into account the shrinkage which will take place in this process. Shrinkage occurs (j48 nJMlSUOUS-GUM J)Ju\TURh\S. hi three stages of the baking :-first, as the water dries out of the mass and the sohd particles more closely approximate each other; second, when the particles begin to combine b) fusing; third, as the mass becomes vitrified. The principal cause of shrinkage may be said to be the agglomeration of the particles which were previously mechan- ically separated by the water expelled in the drying out process. This is followed by the fusing of the component parts of the body, and fur- ther contraction is due to this. When the mass of porcelain is attached to a platinum base, it will either separate at its weakest place or the plate will warp; consequently, to prevent either of the above contingencies, shrinkage must be pro- vided for by dividing it into small masses so disposed that their contrac- tion will neither disarrange the teeth nor warp the plate. A fine ribbon saw is passed between each tooth dividing the body entirely to the base. The cuts are continued on the lingual aspect of the plate. It is noticed in firing the low fusing bodies, that the contraction is largely vertical, because of their tendency to assume a gloV)ular form when fused. The high fusing bodies stand as carved, contracting sym- metrically to the mass when uninfluenced by any extraneous factors. The case now ready for the first baking, is placed in the furnace resting upon the palatal portion of the original investment, or better, upon a support of iridio-platinum wire bent into a "V"-shape with up- turned ends. FURNACES. The great length of time necessary to heat up the old style porcelain furnaces was a strong objection to this kind of work; consequently, to meet the requirements of the present day a furnace intended for con- tinuous-gum work must be available, which can be used with the ex- penditure of less time than formerly. It must be so constructed that there is no possibility of a contamination of the porcelain denture by the products of combustion. Wherever it is possible to use it the operator will make no mistake in selecting one of the approved electric furnaces which the manufactur- ers are now offering. Among the many advantages possessed by this type of instrument, may })e mentioned the facility with which it may be handled, and the small space which it occupies. With it there is no danger of "gasing" the work. Its cleanliness, the complete absence of odor and noise, and its beauty of finish as now constructed, recommend it highly in the furnishing of a well ec|uipped laboratory. If lack of space in the laboratory prevents, it may even find a place in the operating room. The chief advantages, however, of the electric furnace are the ease and accuracy with which the requisite heat may be applied, con- trolled, and maintained. The construction of the furnace is such that the case may be heated uniformly and evenly throughout, the degree of heat is under perfect control, and it is claimed that porcelain fused by this method possesses unusual clearness and density. furnaces: 640 In those localities where the electric current is not available, the operator may find his demands satisfactorily met by the use of some form of the gasoline furnace. Very excellent results may be obtained by this method of fusing porcelain and the country practitioner need not hesitate to undertake the work with a furnace of this character. The electric, gas and gasoline furnaces are described in the Chapter on the Laboratory. A thorough acquaintance with the management of the furnace is indispensable, as probably the most prolific source of failure in continuous-gum work is lack of knowledge in this regard. Each indi- vidual furnace should be accurately tested by the operator and its working capabilities carefully observed. The temperatures at which the different porcelain bodies and enamels fuse should be noted; also the effects produced upon them by variations above and below their fusing points. Where a case necessitates several bakings with the same body the temperature scale must be marked for the proper limit of fusing for each baking. Because of the fact that the formulae of porce- lain bodies and enamels on the market vary in composition, fusibility and other physical properties, no general rule can be given for the regulation of temperatures. If the composition of all the bodies was the same and their working qualities were constant, the fact would still remain, that each furnace is a law unto itself. This applies not only to furnaces of different manufacturers ^^^'- ^"^^ but to those of the same make. Until recently, no positive test for the correct fusing point was known. Fairly good results may be obtain- ed by placing a pellet of pure gold in the muffle of the furnace near the plate, and after this melts at 2016° F,, by maintaining the heat for a definite length of time. More accurate results are secured by using the pyro- meter (Fig. 579), designed by the author. This device " by the author. is exceedingly simple in construction. A small cube of soap-stone is excavated to resemble an hour glass; into the upper ex- panded cavity is placed a small spherical mass of an alloy composed of platinum and gold. The relative proportion of the two metals governs the fusing point of the alloy ; so by varying the proportions, the tempera- ture may be graduated to any extent within the limits desired. The pro- per temperature of fusing foi a given body is matched with a pellet of alloy. The pyrometer is set in the muffle, and when the temperature at which the porcelain fuses has been reached, the little metal ball melts and runs down into the lower chamber. Below are tabulated the results of experiments conducted by the writer with the pyrometer to determine the fusing points of some of the more prominent porcelain bodies found on the market. Allen's body 2340° F, Close's body 2290° White's inlay 2260° Brewster's foundation 2220° Consolidated continuous-gum 2200° 650 coyTiNUous-auM benturks. Consolidated inlay 2140° Whiteley's 21-10° Brewster's enamel 2090^ Ash high fusing 20 10'^ Jenkins prosthetic 1830° Jenitins inlay 15800 Gum enamel (Close and Whiteley's combined) 2140° In the further prosecution of these experiments, the writer is led to the conckision that the maximum strength obtainable in porcelain is devel- oped by the proper regulation of the heat. If underbaked it will be brittle, flaked and incapable of withstanding the stress of mastication; while overbaking causes porosity and diminution in power to resist crushing stress. First Baking. — The piece is placed in the muffle of the electric fur- nace. It is desired to heat it up slowly at first, so that it may dry out, and so that it will not flake by the sudden formation of steam in the body from the water used in mixing it. The heat is turned on and gradually raised, placing the lever of the rheostat on the first button. It is allowed to remain on the first button for five minutes and then turned to the next button; on this and each of the following buttons it remains two or three minutes, until the last one is reached, w here it rests. During this time the case is watched carefully to determine the proper degree of baking. Less heat is required for this than for subsequent firings. The baking- is carried to what is known as the granular stage, at which point all shrinkage has taken place. The surface is not cjuite glazed, but presents rather a frosty appearance. When the process of fusion has reached the desired stage as indi- cated by the pyrometerj the rheostat is thrown back and the current turned off and the case allowed to remain in the muffle until cooled. It is then placed upon the die and the rim turned with flat-nose pliers and riveting hammer, holding the plate firmly in place. The rim is turned to the desired angle, which depends upon the thickness of the gum, the purpose being to make the external surface of the rim and that of the porcelain of the gum continuous in the finished piece. The case then presents the appearance illustrated in Figs. 580 and 581. The plate is now ready for the second application of porcelain. The paste is mixed and applied as in the first instance, filling in the fis- sures and restoring more fully the contours. When this is done, all traces of the paste are removed from the surfaces of the crowns of the teeth and platinum base which is not to be covered with porcelain. This may be done with a camel's hair brush, the bristles of which have been cut oft' close to make them stiff. The plate is now placed in the muffle as before and subjected to the second firing. The temperature for this operation is carried a few degrees higher than in the first baking in order to produce more complete fusion of the porcelain body. The porcelain body is brought just to the verge of a glaze: its surface should show a sparkling granulated appearance. Carrying the heat beyond the stage necessary to produce this effect lessens the strength of the APPLYING THE GUM ENAMEL. 651 porcelain, aiul makes it impossible to obtain the translucent appearance of the gum when the gum enamel is applied. (Fig. 582.) After beino- allowed to cool as before, the plate is tried in the mouth to note if any alterations in its form have taken place. The extent of the buccal and labial contours of the plate is also noted. If too full at any place they may be reduced by grinding; if lacking at others, more porcelain may be added, and the piece rebaked. Fig. 580 Labial and buccal view of denture after first baKing. Applying the Gum Enamel . — The gum enamel is now prepared by mixing it with distilled water and it is applied in the same manner as the body. The denture should be wet to facilitate the placing of the enamel. This layer should be thin and so applied as to preserve the distinctness of the gum outlines. By varying its thickness those differences in shade observed in the natural gum over the roots of the teeth may be secured Fig. 5S1 ^^m ■€ -^^^K H ^t\ Cm ■ ^1 Ftg- 582 Lingual view of full upper denture after first baking. Lingual view of partial upper denture after second baking. as the yellowish white of the body is allowed to show through more or less clearly at these points. The outlines of the necks of the teeth, and rugae are clearly defined. All superfluous portions of the paste are care- fully removed, using small camel's hair brushes and instruments de- signed for the purpose. The piece is again subjected to the heat of the furnace. 652 CONTJNUO US-G I'M DESTURKS. The exact degree of heat necessary to develoj) the full beauty and strength of the case can only be deterniin(Hl by ex])erience. ^Vhen the heat approaches the fusing point, it is well to turn off the current and make an inspection. When finished, it should present a smooth, glossy appearance. If the heat is carried a few degrees too far, the color begins to fade and its brilliancy is lost. Frequently little pieces become detached and minute cracks present themselves. These defects may be remedied by applying more enamel, and repeating the baking. The natural gum effect cannot always be secured with the enamels as found on the market. A combination of several of them often produces the shades desired. The writer obtains desirable results in many cases by combining the Close and Whiteley gum enamels and adding asmall quantity of White's inlay body. Doubtless other combinations pro- (iuce equally good results. With the exercise of patience the operator can obtain almost anv shade desired. Fig. oSo Ftg. 584 Lingual view of upper denture after liaking of gum enamel. Lingual \iew of upi^er denture after baking of gum enamel. The finishing process consists simply in smoothing and polishing the metallic surfaces and washing the plate. The metal portions must have any marks due to files or pliers removed, and the polishing i^ done on the lathe in the same manner as for any other metal plate. Mineral Paints. — As previously mentioned, there are often condi- tions of the natural organs associated with decay and discoloration from other causes, which may be imitated in the ])orcelain teeth by the judicious application of mineral paini's. The method of applying and fusing these paints may be readily learned, and a little study in the mixing and blending of colors will enable the operator to produce very correct imitations of these defects in natural teeth. (See Chapter XII.) These stains are applied on or around the cervical margins, the cut- ting edges, occlusal surfaces, or any other portion of the tooth, imitating those discolorations seen in the recession of the gums, mechanical ab- rasion, devitalized or decayed teeth. The effect is still more pleasing FUJJ. LOWER DENTURE. 653 If previous to staining, the cusps or edges of the tectli are ground off to imitate the wear incident to age. Pitting or other defects of structure which obtained in the natural teeth, may be well imitated by this pro- cedure. There is practically no limit to the possibilities offered by this process in the hands of a skilled operator. FULL LOWER DENTURE. The general considerations set forth under the discussion of the full upper continuous-gum denture, apply equally to other forms of den- tures embraced under this title. Hence in discussing the full lower or the partial upper and lower dentures, it will only be necessary to men- FiG. 585 Buccal and labial view of denture after baking of gum enamel showing imitation of recession of gums, and staining of necks of the teeth. tion those departures from the general line of procedure which are made necessary by the requirements of the particular case. The first step in the construction of the full lower case is that of form- ing the pattern. Heavy tin foil is accurately adapted to the die and trimmed shghtly larger than the plate outline to provide for any pos- FiG. 586 n Buccal and labial view of denture after baking of gum enamel showing omission of one tooth and irregular alignment. sible displacement of the plate in swaging. Great care must be ob- served in preparing patterns for lower and partial cases to prevent change of form when the tin foil is flattened out, for should this occur, a plate cut to such an inaccurate pattern would be useless. Because of the inherent weakness in the shape of a lower plate, heavier gauges a,re used than for upper cases, and furthermore, these are specially reinforced. 654 CONTIN UO US-G UM DEyTURES. The quality of li<^htness is not so mucli a desideratum as in the upper phite, and .strength may he gained in this manner without atteeting the sueces.s of the denture. Tlie pattern is re[)ro(hieed in No. 2S to No. 30 gauge, phitinum phite, and this is reinforeed around the anterior Hngual curve with a piece of iridio-platinum plate No. 26 gauge. The supj)lementary piece should extend from the rim on the lingual surface, U]) and well over the ridge as in Fig. 587. The rim may be turned as for an upper plate if it is desired, but a platinum wire No. 18 gauge, soldered along the borders of the plate is to be preferred. This will not only provide additional strength, but it permits the trimming of the plate edges which is so often necessary Fir.. 587 Reinforcement of full and partial lower plates. with lower dentures. An iridio-platinum wire No. 14 to 10 gauge, is to be adjusted on the ridge of the plate under the pins of the teeth, as in the upper plate. This serves for the attachment of the pins. The subsequent procedures are the same as those outlined for a full upper case. PARTIAL DENTURES. Continuous-gum is employed to best advantage for partial restora- tions in those ca.ses in which the teeth to be replaced are in an unbroken column. More patience and skill are required in the construction of partial dentures than in either a full upper or lower. Each individual case presents features peculiar to itself, and each will suggest the proper procedure. The chief difficulties encountered are in preventing the plate from folding or splitting during the swaging. Frequent anneal- ing, the skillful u.se of the pliers and mallet, and the cutting away of the surplus metal at difficult points about the natural teeth, will aid in avoiding the.se dangers. Partial Upper Dentures. — When a partial upper case presents, the first thing to be determined is the mode of retention. There are two general methods by which the stability of the plate in the mouth may be secured. In the one case, the retention is obtained l)v the combined PARTIAL DKNTURKS. 655 forces of atmospheric pressure and adhesion, the plate covering the en- lire surface of the hanl pahitc. In the other case the retention depends upon attachment to the natural teeth remaining in the arch, the plate being in the form of a horseshoe, and covering only the alveolar ridge. To illustrate the application of the first method, we may take a form of denture carrying the posterior teeth. Obviously, retention by clasps in this case would be unsatisfactory, although it is possible even here to obtain fixation in or to the natural organs by special means. Two pat- terns are prepared: that for the primary plate extending over all sur- faces intended to be covered; viz — the vault of the mouth, alveolar ridges and buccal surfaces. This is reproduced in No. 34, platinum plate. The second pattern is reproduced in No. 30, iridio-platinum plate. It conforms to the shape of the first except that it does not ex- tend over the ridges. Fig. 588 Full upper denture with lingual surface of teeth carved in porcelain; denture carrying large contours. (Wilson.) Each lamina is swaged separately and then the two are swaged to- gether. They are then united with platinum solder, the lateral margin of the supplementary piece being left free to form the lingual boundary of the porcelain. In this case the palatine surface is not covered with (he body and no rim across the posterior border is required. The buc- cal rim may be formed by turning or wiring. The patterns for the denture to be retained by the second method are adapted to the cast, and conform to the horseshoe shape, leaving the height of the vault free. The denture is preferably constructed of two laminae, the primary of No. 30 gauge platinum, the second of No. 26 to No. 28 gauge, iridio-platinum plate. They may be cut to the same pat- fern, though the borders of the first should be allowed to project shghtly to provide support for the solder. The rim and lingual boundary of the porcelain is formed by attaching No. 18 platinum wire. This style of denture may be used to replace the anterior teeth with some of the bicuspids or molars remaining to afford attachment for the clasps. These latter are adjusted as for other swaged plates, but iridio-platinum 656 coy TIN ijo us-a um i>ks tur ks. must DC used instead of tlie elasp metal, as the latfcn-will not stand the high temperatures of poreehiin hakiug. Coml)ination ehisps of phitinum and elasp metal may be made and attached in the following manner: the clasp metal is cut and adapted to conform properly to the tooth; around this is adapted thin iridio- Fir.. o8ody and enamel are made as previously described in the usual pro- cedure. REPAIRING CONTINUOUS-GUM DENTURES. An objection formerly urged against the employment of continuous- gum dentures was the difficulty of repairing them in case of fracture. The validity of this objection could not be questioned when the old methods were in vogue, but now with a more precise knowledge of the materials at our disposal, and with the valuable acquisition to our arm- amentarium of the electric furnace, we are enabled to undertake oper- tions for the repair of these cases with the assurance of complete success. When a case is presented for repair, before any other steps are taken, all foreign or extraneous material must be completely removed, not only from the free surfaces of the piece, but from the cracks and fissures as well. Any such material allowed to remain will exercise a deleterious influence upon the porcelain. The plate should first be scrubbed in water containing a little ammonia. To ensure effectually the further removal of the accumulations, the plate is encased in an investment of asbestos and plaster, placed over a gas stove, and gradually heated to redness. During this process the foreign matter is carbonized. After cooling slowly the plate is removed from the investment, washed with soap and water, and further cleansed with alcohol. It is then placed in the furnace and again heated to redness. A detached fragment of a tooth, provided the line of fracture be dis- tinct, may be readily replaced by the aid of liquid silex. The silex is applied to the broken surfaces, the fragment pressed to place, and the case heated in the furnace to an orange-red color. This gives a ready and quick method for this kind of repair wdiich may be particularly useful in emergencies. No attempt should be made to replace a broken tooth or teeth until the remaining portions are ground away. On the lingual surface the porcelain is removed until the platinum base or wire is exposed. The grinding on the buccal or labial surface should be sufficient to permit the proper replacement of the lost tooth. Teeth are selected of a mold and color to correspond with those remaining on the denture. They are ground to fit the spaces prepared for them, and at the same time are given a correct articulation. They are waxed in position and, if the length of the column of teeth to be replaced demands it, they are given a coating of shellac varnish and the piece invested to hold them in position. COMBINATIONS OF CONTINUOUS-GUM. 659 This wax is removed, and the pins are attached to the plate as pre- viously described, this time however, using pure gold as the connecting medium and the furnace as the means to fuse it. The unequal heating involved in soldering with the blowpipe would endanger the integrity of the porcelain. The case is allowed to cool, removed from the in- vestment and cleansed. The body is applied and baked to the granular stage after which the enameling is done as before. It is feasible to replace one or two teeth without attaching the pins to the plate with solder, the porcelain material alone maintaining them in position. In simple repairs of this kind it is only necessary to grind away enough porcelain to permit the tooth to be set in place. The teeth may also be retained in proper position by adapting a platinum wire No. 30 around the plate to rest upon the labial and buccal surfaces of the teeth. The ends of the wire are engaged between or around the posterior teeth remaining on the plate. If there is a break in the continuity of the platinum base and it is not extensive, it may be repaired by grinding away the porcelain im- mediately about it, adapting a thin piece of platinum plate to the break, and soldering it with pure gold in the furnace. The porce- lain body and enamel are then applied as already described. Cases sometimes present which after years of service cease to be use- ful because of the changes in the mouth incident to resorption of the process. In such a case it will be necessary to take a new impression, obtain dies and counter-dies and reswage the plate after removing the teeth and porcelain. This may be done by subjecting the denture to a red heat under the blowpipe or in the furnace and then plunging it into cold water. The porcelain will fracture and may be easily detached. All the porcelain portions of the denture may be effectually removed by immersing the plate in hydrofluoric acid for a few hours. After reswaging the process is the same as for constructing the original plate. COMBINATIONS OF CONTINUOUS-GUM AND VULCANITE. The ingenuity of the operator may find a fruitful field of employment in the various combinations of continuous-gum with vulcanite. The lightness, cheapness, and accuracy of adaptation of vulcanized rubber may at times be joined advantageously with the aesthetic qualities of porcelain. Cases presenting extremely long bites in which the increased weight of the continuous-gum might prove objectionable, may with advantage be restored by such a combination. A cast of the jaw is obtained, covered with a base-plate of gutta-percha, and from the model so pre- pared, dies and counter-dies are made. A platinum plate No. 34, is swaged to cover the alveolar ridge, making no provision for the rim. The plate is perforated at various points, and through these perforations after the porcelain body has been applied, but before bak- ing, retaining pits are made in which to engage the vulcanite. (560 CONTINUOUS-GUM DENTURES. The plate is warmed and set over the gutta-percha; the bite taken and the articulation of the teeth secured in the usual way. It is then separated from +he gutta-percha, invested, and the teeth soldered to the plate or stays. The body and enamel are applied and baked. When this part of the denture is complete, it is set over a wax base-plate, the ' articulation adjusted, and the waxing process completed. The case is then flashed and the succeeding steps are analogous to those described in the Chapter on Vulcanite. Continuous-gum sections or full pieces may readily be constructed and mounted in the vulcanite without the platinum base. Upon a base plate of wax, continuous-gum teeth are arranged and properly articu- lated. The hihial and buccal contours are restored in wax; a coating of shellac is applied to the teeth and the Case is removed and invested with the teeth downward in a horse-shoe shaped bed of investment material, half an inch thick, placed on a glass slab. The investment is built about the wax gum covering it to a depth of half an inch and ex- tending well over its edge. The wax is thoroughly removed after the investment becomes hard. An iridio-platinum wire. No. 18, is adapted against the necks of the teeth under the pins bending the latter securely around the wire to maintain the teeth in place. That part of the in- vestment previously covered l)y the wax is now oiled, and the porcelain paste packed in, applying it between and in front of the roots flush with the lingual surface of the teeth to take the place of the wax. The case enclosed in the investment is now introduced into the muffle with small pieces of pure gold applied to the junction of each pin. The gold sol- der unites the pins and wire and at the same time the porcelain body is baked. When cool the case is removed from the investment and cleansed; given its enamel baking, and then set on the cast, waxed in proper posi- tion and the plate is finished in vulcanite. CHAPTER XV I. ARTIFICIAL CROWNS. By H. H. Burchard, M.D., D.D.S., and F. A. Peeso, D.D.S. When the crowns of teeth have suffered such extensive loss of sub- stance that restoration by means of filhng material is inadvisable, the restoration is an operation of prosthetic dentistry. The term "artificial crown," as technically appHed, includes only such devices as are made in the dental laboratory and subsequently set as a single piece upon a prepared root or remnant of tooth. The pieces knctwn as "partial crowns" are also included in this category. The first example of crown substitution, mounted according to the principles governing contemporary crown operations, was the setting of a crown, of a natural tooth upon a prepared root, the support being afforded by a post extending from the enlarged pulp-chamber of such a crown into the enlarged canal of the root. Fig. 591 Porcelain crown with wood post. The mechanical principle involved in this mode of support has had constant application. The next variety of crown employed was that of porcelain, the post support being, as in the preceding form, a hickory post (Fig. 591.) Subsequently metallic posts were substituted for those of wood, and this variety is the typal form of one of the two great classes of crowns in present use. Metallic crowns resembling those of the present day were employed early in the present century. CLASSES OF CROWNS. All of the varieties of artificial crowns may be divided into two great classes, according to their means and modes of support. The first class includes all of those crowns which depend for fixation upon a post anchored in an enlarged pulp-canal. (Fig. 592.) The second class in- cludes those which have their retention secured by means of a con- tinuous band encircling the neck of the root. (Fig. 593.) 661 662 ARTIFICFAL CROWNS. Class 1 is subdivided into two orders; first, those in which the post is an integral part of the artificial crown, being baked in it or soldered to it (Fig. 592, A and B) ; second, those in which the post is firmly an- chored in the pulp-canal, as a primary measure, and upon this support the crown itself is fixed as a second operation. (Figs. 592 (\ 595, 596.) A sub-order includes the collar and post crown (Fig. 592 D), the band Fig. 593 Class 1, Order 1, encircling the root acting as a subsidiary support to the root, protecting it against fracture, the post being the retentive de\dce proper. All of the artificial crowns in present u.se will be found to be a variety or some modification of one of these classes. Fig. 595 Fig. 596 Class 1, Order 2. Each variety is designed and fitted to meet definite indications, and the application and choice of variety are determined by the anatomi- cal, physiological, and pathological condition of the root to be crowned and, it may be, of the surrounding parts. ANATOMICAL RELATIONS. The first consideration is the position of the root to be crowned; and the second, its form. Its position includes the class of tooth, whether it be an incisor, canine, bicuspid, or molar; next, its relative position to its neighbors and to its antagonists, and what will be the relations of the artificial crown in the.se particulars. Each cla.ss of tooth has a definite office to perform, and there is in- volved in the performance of its function an amount and variety of stress governed by the position of the tooth — i. e. the class to which it THE FORMS OF THE TEETH. 663 belongs. This demands in the supporting structures of the crown and root sufficient resistance to secure tlie integrity of tlie crown and roct in tlie performance of their normal functions. Incisors by their positions and forms are designed to receive and re- sist stress in one direction, that tending to force them outward. Canines in their normal relations receive stress in two directions: two forces act at an angle upon the axis of the tooth, and the resultant of these forces is a line outward. Bicuspids are subjected to three stresses — vertical, outward, and in- ward; the relative amounts of stress are in the order given. The amount of the outward and inward stress is governed primarily by the lengths of the buccal and palatal or lingual cusps ; the vertical stress, by the area of the occlusal surface. Upon molars the vertical stress is greatest, and in the direct ratio of the extent of masticating surfaces; the lateral stress is governed by the lengths of the cusps. Artificial crowns should be made of varieties to meet and resist the several directions of stress. The line of greatest mechanical resistance in any root is in its ver- tical axis, and is the only line of stress which does not tend to displace the tooth mechanically. As to the vital resistance of a root, this rule is but partially true, for roots appear to rebel against stress in any other direction than that due to their normal anatomical positions. In normal occlusion the stress upon any individual tooth is lessened or modified by the occlusion of the other teeth of a denture, so that the conditions of any tooth as part of a denture are not those of the same tooth standing alone. For example, the incisors normally receive a stress which ceases as soon as the molars and bicuspids are in perfect contact; in the absence of these latter teeth the entire force of occlusion falls upon the incisors, and they are unduly strained. With the molars, the greatest stress being vertical, support is de- manded from the entire root area underlying the crown. The latter represents primarily a block resting firmly upon a broad base. The lateral stress is guarded against by having one or two posts in the axes of the roots or by the periphery of the crown grasping that of the root- walls. It is evident that the crown best adapted to meet these stresses is that having a barrel form, grasping firmly the periphery of the root; the retaining cement becomes mechanically part of the tooth, so that these crowns rest uniformly upon the entire area of the root-face. Pin crowns of the variety placed upon bicuspid roots are occasionally employed upon molar stumps, but, as a rule, their intrinsic resistance is not as great as that offered by barrel crowns. THE FORMS OF THE TEETH. The great consideration as a governing factor in the placing of arti- ficial crowms is the forms of the teeth. This includes the shapes and 664 ARTIFICIAL CROWNS. sizes of the roots to be crowned as factors deterniiiiiii- under the gum. (Fig. ()43.) The baitd is then made and fitted to the stump and cut out on the labial side flush with the tooth. (Fig. 644.) The palatal side of the band is then pressed in close to the stump and the mesial and distal sides of the band spread out nearly or quite to the width of the facing which is to be usetl, and a floor of coin gold sweated or soldered to it. (Fig. 645.) A thin facing is then ground so as to leave a little space between it and Fig. 642 Fig. 643 Fig. 644 A the cap, touching the latter only at the tip. (Fig. 646.) The facing is now backed with thin platinum or crown metal, letting it extend about one-sixteenth of an inch over the incisal edge. It is then waxed to the cap, invested and soldered, flowing the solder between the facing and the cap and if necessary over the palatal portion of the band. (Fig. 647.) Another way of making this crown is to carry the band to the full height of the tooth, to contour it, and cut it out on the face to the depth of the facing. The facing is then ground to fit the edge of the cap so formed and a backing of coin gold fitted carefully to it. This is then adjusted to the cap, waxed, the facing removed and the backing sol- dered to the cap with 22 carat solder. The facing is then put in place, Fig. 645 Fig. 646 Fig. 647 the pins waxed on the inside of the cap, and the crown invested. The crown is invested face down and covered but lightly, leaving the opening fully exposed as in Fig. 648. It is thoroughly dried out and flux placed on and around the pins. A piece of 18 or 20 carat solder is then placed over the pins, the whole brought to a bright-red heat, and with the fine blue point of the blowpipe-flame thrown on the inside the solder is melted, uniting the facing to the cap. In the making of a porcelain faced crown for a bicuspid having a vital pulp, the tooth is prepared in the same way as in the anterior teeth, cutting it well out on the buccal side and grinding away the inner cusp. (Fig. 649.) The band is then made as for a full gold crown, and cut even with the cusp at the top and enough on the buccal side to JACKET CROWNS. 697 allow for the facing. (Fig. 650.) The facing is then ground to fit the edges of the cap (Fig. 651) and a backing of coin gold fitted to it. (Fig. 652.) The backing being fitted to the facing, it is placed in position on the cap and waxed and soldered with 22 carat solder. The backing is then cut off e\-en with the rest of the band and filed flat. (Fig. 653.) The tip of the facing is then ground on a bevel with an angle of about forty-fi\-e degrees, the lower edge of the bevel being on a line with the top of the cap. (Fig. 654.) A cusp is then selected, the under surface filed flat and the buccal side beveled to meet the bevel of the facing. (Fig. 655.) The cusp is then wired to the cap and soldered with 22 carat solder, the facing having first been removed. The cap is then Fig. 649 Fig. 650 Fig. 651 Fig. 652 cleansed in acid and the facing replaced in position and waxed. It is then invested and soldered from the inside as already described. (Fig. 648.) Another method of attaching porcelain facings to the gold barrel crown is by first making the entire crown of gold, the barrel and articu- lating surface being completed. The external wall of the crown has the segment made visible by the movements of the lips sawed out, and the cut edges of the metal beveled. A porcelain facing is selected of a size to fit the space with the minimum grinding. It is to be ground in until all of its edges fit those of the barrel. A stay of No. 34 pure gold is burnished over the back of the porcelain tooth. The edge of Fig. 653 Fig. 654 Fig. 655 the stay should be accurately adapted to the barrel. The crown ana facing are cemented together with adhesive wax, covered by a thin investment, and soldered by means of a blowpipe-flame directed against the portion of the investment covering the facing. The buccal and articulating faces of molars and bicuspids may be made of porcelain, the attachment of the crown to the root being secured by means of a gold barrel. The barrel is made as for an all-gold crown. A wax-bite and impression are taken, and an articulation mounted. Before cutting away the buccal wall of the barrel for the reception of the porcelain, measure by means of a ^dre and dentimeter the circumference of the upper portion of the barrel. The loop made is taken to the depot, and a saddle-back or a plain rubber tooth is Gds ARTll'ICIAL CROWNS. Fig. 656 selected, the eircumference of which agrees with that of the barrel (the wire loop). The tooth should have but little thickness of porcelain above the pins (Fig. ()5()); the S. S. \V. cusp crowns are designed for this special use. A scratch is made along the buccal portion of the barrel, marking it slightly above the gum line and between the adjoin- inii; natural teeth along the line of exposure. A fine saw is used to cut away the buccal walls to these lines. The palatal wall of the barrel is cut down if necessary to admit the face, so that it will articulate with the antagonizing teeth. Should there be any lack of correspondence be- tween the outlines of the barrel top and the cusp crown or tooth, the gold is bent to fit the latter accurately. Ey means of fine-grit corun- dum wheels the edges of the porcelain are closely adapted to the cut eclges of the gold at the cervical and approximal borders, and articu- lated perfectly with the antagonizing teeth. The tooth and barrel may now be set with cement: it is preferable, however, to solder the porce- lain to the barrel. A piece of 24-carat gold No. 33 is fitted as a stay to the under surface of the porcelain and burnished into accurate contact. The tooth and stay are set in the barrel, and the latter is cut away at points interfering with its correct placement. It is boiled in the acid solution, and invested so that the interior of the barrel and the stay ex- posed form a concavity. Borax is painted around the line of junction and Fic. 657 - — 4--^' Cusp crowns. over the pins, a small piece of solder placed over each pin, and three or four pieces around the joint, and the piece is graduall} raised to a high heat; a fine flame directed into the concavity fuses the solder, uniting the piece;^ perfectly. In finishing the crown the gold should be dressed down to the porce- lain, making a perfectly smooth joint. No projection of the gold be- yond the surface of the porcelain should remain. Fused porcelain may be used in lieu of solder to attach the crown to the barrel, as described by Dr. Robert Huey: "The barrel is fitted and cut out as descrilied. One of Ash & Sons' diatoric teeth is selected and fitted to the barrel. Openings are drilled through the mesial and distal walls of the barrel, which shall exactly uncover the openings of the tube in the tooth. A piece of platinum wire is thrust through holes and tube, holding the porcelain to the gold. The platinum wire is now PORCELAIN JACKET CROWNS. 699 either riveted or soldered to the barrel The hue of jiinetioii between o-old and porcelain is painted with a paste of dental glass, which is then fused in a Downie furnace." Dr. W. A. Capon claims excellent results for a platinum and porce- lain jacket crown, the details of the construction of which are as follows: "This crown is made by fitting a platinum band (gauge No. 31) to the root of prepared tooth (Fig. 658) in the same way as with gold cap work, except that the joint must have overlapping instead of abutting edges. The lingual and labial outlines of the adjacent teeth are marked on the tube (Fig. 659), as a guide to grinding those portions away to gain shape instead of cutting with scissors. The lingual side is shaped with a wheel on the lathe and a piece of the same gauge platinum soldered to fit it with very small amount of pure gold. (Fig. 660 and 661.) After Fig. 658 Fig. 659 Fig. 660 Fig. 661 Fig. 662 Fig. 663 trimming and fitting to the root, the labial surface is ground thin enough to burnish and fit over the tooth (Fig. 662), after which a thin porcelain veneer is fitted and held in position by the porcelain paste. It is care- fully dried and baked in the same way as other porcelain crowns. The crown is now fitted to the root and its requirements noted, such as pro- per size, shape and thickness. If the surface of the veneer requires grind- ing, it should be done at this stage, so that it will be glazed again by the last heat, which should be strong and of uniform degree. After final baking the platinum portion is polished and the crown is ready for set- ting, using thin cement and very gentle pressure (Fig. 663.) The crown should fit easily, as there is danger of breaking the thin porcelain on the sides of the crown, or of even checking the veneer itself. The joints are lapped and made as close as possible, so that great and frequent heating will not entirely destroy the union ; any excess of sol- der will flow over the surface of the platinum, and destroy the porcelain adhesion, which may not be noticed at the time of the operation, but will be more forcibly noted later on. The lingual surface is ground thin to give shape, so that there may be two flat surfaces to hold porcelain. When finished it gives the proper tooth contour." 7QQ ARTIFICIAL CROWXS. BAND AND PIN CROWNS. While a pin and plate crown or any of the manufactured crowns are especially adapted for temporary work, where permanent oj)erations are desired, the root should always be banded so as to reduce to a min- imum the possibility of a fracture. It is always best to be on the safe side in the beginning and to treat a root so that there is no possibility, or at least a very remote one, of trouble of this kind. Of all the crowns placed in the anterior part of the mouth in which the roots are banded, the Richmond crown is the one which is most frequently used and is made as follows: The root having been properly prepared as already described and the band fitted to it, the latter is marked around its inside and cut off flush with the top of the stump, which should be about one thirty-second of an inch under the o-um on the labial side and about one-sixteenth of an Fig. 664 Fig. 665 Fig. 666 Fig. 667 inch above it on the palatal. A floor of No. 30 coin gold is then sweated or soldered to the band. It is now replaced on the root and a hole made in the floor over the enlarged root-canal for the reception of the pin, which latter is preferably made of platinized gold. This is placed in position and fastened with adhesive wax (Fig. 664), removed, invested and soldered. It is then put back on the root and the impres- sion and articulation taken, after which the cast is prepared in a manner already-described. After separating, the cap is removed from the cast with a pair of heated pliers and the floor on the labial side of the pin is ground or filed perfectly flat. It is then replaced on the cast, and^a suitable facing selected and ground to fit the floor of the cap. (Fig. 665.) A backing of thin platinum or crown metal is then made, extending from the floor of the cap to about one sixteenth of an inch above the tip (Fig. 666.) The facing is then waxed in place with adhesive wax, and when this is nearly hard, it is pressed tightly against the backing to bring them into close contact. The crown is now removed from the cast, in- vested and solderefl. The investment should be made to cover the band partly and take in the backing which expends beyond the cutting edge, thus holding it down and preventing it from drawing away from the facing in soldering. (Fig. 667.) A favorite and easy way of setting in the facing, is to grind it away so that only the tip of it touches the floor of the cap at its outer edge, and then to let' the backing extend all the way to the end of the facing, and BAND AXD Piy CROWNS. 701 fill it in with solder. (Fig, 668.) The principal objection to this method, is that in many cases the gold can be seen from the front or side of the mouth, or there is a dark shadow between the teeth toward the gingival portion of the crown, which renders it unsightly and un- natural in appearance. In the making of the so-called Downie porcelain crown, where a facing is used, the band and cap are made in the same way as for a Richmond, with the exception that iridio-platinum plate No. 32 gauge, Fig. 668 Fig. 669 is used instead of gold and the band is cut lower on the palatal side. For these crowns it is well to have only the tip of the facings touch the outer edge of the cap, as this permits the porcelain body to be worked under the facing better than when it is close to the floor. (Fig. 669.) The Half-cap Crown.— While the full Richmond crown in point of strength and impermeability, may justly be considered as the ideal crown, the one serious objection to it is the difficulty of conceahng the labial portion of the collar. No method of crown construction can be Fig. 670 Fig. 671 said to be perfect which allows any of the metalhc portion to be visible after it is permanently fixed. For this reason a large number of crowns which are set on upper anterior roots are constructed on the half-cap plan. When the half-cap is accurately fitted at the lingual aspect, it is nearly as strong as would be a full cap or collar. An additional advan- tage will also be found in the fact that the use of the half-cap ob\dates the necessity of forcing the collar under the gingival margin at the labial portion to such an ex-tent, in orderto get it out of sight, that perios- teal disturbances may result. As shown by Fig. 670, in the absence of 702 ARTIFICIAL CROWNS. liiiijual ivinf()rccment,the post bocomos a lever, and its foree is exerted from the centre of the root and falls upon its labial or bueeal half, re- sultino- eventually in splitting of the root. The half-cap, as shown by Fi'i-. 671, places the reinforcement where it is most needed, while it Fig. 672 admits of so nice an adjustment of the porcelain facing that its neck may be made to pass far enough under the gum-margin to simulate closely the appearance of the adjoining natural teeth. Much of the Fig. 673 preliminary work in the construction of such a crown may be done on a good ]:>hister cast. The root should first be prepared, including the enlargement of the canal for the reception of the post; the latter is then placed in position and a plaster impression obtained. The plaster READY-MA DE CRO WNS. 7u:i cast re(iuircs no modification, except that the phister should be cut away at the gum-margin of the hngual aspect, as shown by Figs. 672 and 673, to enable the operator to carry the half-collar slightly under the gum (Fig. 674.) The half-collar should be accurately fitted to the convexity of the remaining portion of the crown and as much of the root as it embraces; the floor of the cap is then made and soldered to the pin. The collar portion of the cap is then to be tacked to the floor by a minute particle of solder, as shown by Fig. 675; the cap should then be tried upon the root to ascertain whether the adjustment is perfect, and to im- prove it, if necessary, by pressing the edge of the collar to complete con- tact with the root at the gingival margin with a burnisher. After which Fig. 674 Fig. 675 Fig. 676 the collar and flat piece with the post are united by the smallest amount of solder practicable. The selecting and fitting of the porcelain tooth to the cap and the subsequent soldering and finishing are the same as in the full Richmond crown. Fig. 676 shows the completed crown. READY-MADE CROWNS. Of the ready-made porcelain crowns there are two varieties — first, those designed for fixation upon a post which is previously fastened in the root; second, those having a pin baked in them. To the first class belong the Bonwill, the Davis, the S. S. White detachable pin crown, the Fellowship and various others on the market : in the second class are included the Logan, the Brewster and several others. Crowns which are formed and adapted by means of sets of ready-made appli- ances, such as the Hollingsworth and the mandrel systems, belong to the class of built-up crowns. Of the ready made porcelain crowns, the Davis, and S. S. White de- tachable post crown are those having the widest range of usefulness. The methods of fitting and mounting them are similar, and as a de- scri])tion of the technique of these operations would applv equally well to all, the following is quoted from Dr. G. W. Schwartz concerning the use of the Davis crown. 704 ARTIFICIAL CROWNS. Rapid Method of Setting Davis Crown. — "We shall first consider the easiest and most rai)i