TCYT31 — — — ety Columbia (Sbtftmitp W$ in tfjeCftpofJtrttigork fetfjcol of Bentai mb 0val iburgerp LIBRARY OF Dr. CARL F. W. BODECKER 1846-1912 The gift of Dr. Henry and Dr. Charles Bodecker 1929 DENTAL METALLURGY A Manual for the Use of Dental Students. BY CHAS. J. ESSIG, M.D.,D.D.S., PROFESSOR OF MECHANICAL DENTISTRY AND METALLURGY IN THE DENTAL DEPARTMENT OF THE UNIVERSITY OF PENNSYLVANIA. THIRD EDITION, REVISED. PHILADELPHIA: The S. S. White Dental Mfg. Co 1893. Copyright. The S. S. White Dental Manufacturing Co. 1882. Copyright by Chas. J. Essig, 1887. Copyright by Chas. J. Essig, 1S93. ALL RIGHTS RESERVED. PRESS OK PATTERSON & WHITE, PHILADELPHIA. \ PREFACE TO THE THIRD EDITION. IN the revision of this work for a third edition, the author has adopted the system of spelling and pronunciation of chemical terms as recommended by the Chemical Section of the American Association for the Advancement of Science, among the promi- nent features of which is the dropping of the final e from all words terminating in ide, the pronunciation being id, as chlorid, iodid, etc. ; and in sulphur and its compounds the spelling is changed to sulfur, sulfid, sulfite, sulfate, sulfo, etc. The work has not been greatly enlarged, the author believing that for the present its original scope is sufficient for the needs of dental students. The chapters on Amalgams, Aluminum, and Iron and Steel have been somewhat amplified, and made to embody the recent improvements in the production of those materials. Charles J. Essig. Nov. 6, 1893. PREFACE TO THE SECOND EDITION. THE practical results of the publication of the first edition of the Manual of Metallurgy, then the first text-book of the kind adapted to the use of dental students, showed that it filled a want long felt, and demonstrated the necessity for preparing a second edition. I have accordingly carefully revised the work, and while it has not been greatly enlarged, the more recent improvements in the reduction of metals and the formation of alloys and amalgams used in dentistry have been incorporated. As in the first edition, my aim has been to avoid extraneous or merely hypothetical matter, as well as that belonging to the purely chemical study of the metals, his knowl- edge of which it is presumed the student should derive from other sources. The success of the first edition, in so far as it has been of service to students of dentistry, and the kindly reception accorded it by my collaborators, are subjects of peculiar gratification and grateful appreciation on my part. Charles J. Essig. PREFACE TO THE FIRST EDITION. THE object of the author in the preparation of this Manual was to place in the hands of students of dentistry an outline of the scientific principles in- volved in the reduction of the metals, their properties, the modifications resulting from alloying, and their application to dental uses. While the properties of many of the metals have been only incidentally or illustratively referred to, special consideration has been given to those most commonly used by dentists. In Chapter V a resume of the author's experiments in the formation of alloys for amalgams is given. These were made for the purpose of enabling him to present the subject systematically to the students who sat under his teachings ; and, while the chapter is far from being a complete treatise on the subject, it is hoped that it will prepare the dental student for a better comprehension of that branch of metallurgy, occupying, as it does, so conspicuous a place in the practice of dentistry. As the atomic weights, specific gravity, and fusing- points of the metals are somewhat differently stated by various authors, the figures given in this Manual 5 6 PREFACE TO THE FIRST EDITION. have been made to correspond to those in Fownes's Chemistry, — the text-book commonly used by dental students. The decimal system has been used to express pro- portions, in the belief that its comprehensiveness and simplicity would commend it. In expressing temperatures the Centigrade and Fahrenheit scales have been occasionally referred to separately. The rule for translating one into the other, and comparative tables, have been omitted because they are to be found in Fownes's and nearly all the other recent works on chemistry, with some one of which the student is supposed to be more or less familiar. C. J. E. CONTENTS. CHAPTER I. Metallurgy . II. The Metallic Elements . III. Properties of the Metals IV. Alloys V. Amalgams .... VI. Modes of Melting Metals VII. Combinations of Metals with tallic Elements . VIII. Gold IX. Silver X. Platinum XL Iridium . XII. Palladium XIII. Iron XIV. Mercury . XV. Copper . XVI. Zinc XVII. Cadmium . XVIII. Aluminum XIX. Lead XX. Tin XXI. Electro-Metallui iGY . Non-Me- PAGE 9 17 34 46 74 103 116 169 187 199 202 207 217 227 235 242 244 257 260 269 CHAPTER I. METALLURGY. THE art of separating metals from their ores, or from simple combinations with non-metallic ele- ments, and their application to useful purposes, may be regarded as a separate branch of chemical science. It is essential that the student, before com- mencing its study, should acquire a good preliminary knowledge of chemistry and mechanics. The empirical reduction of the ores of metals seems to have been practiced at a very remote period, its origin being attributed to Tubal Cain, seventh only in descent from Adam.* The remains of numerous mines have been discovered on the southern and eastern borders of the Ural Mountains, in which have been found hammers and chisels of copper, and other instruments of the same metal of which the uses at present are unknown, t That these implements were those of a wandering people would seem to be evi- denced by the absence of any traces of masonry in the neighborhood ; and the fact that no iron tools were found near them would indicate their great antiquity. *In the fourth chapter of Genesis Tubal Cain is spoken of as an " in- structor of every artificer in brass and iron." t Percey's Metallurgy. 2 9 IO DENTAL METALLURGY. Gmelin found in the eastern part of Siberia the remains of nearly one thousand smelting-furnaces, very primitive in character, surrounded by heaps of scoria, broken pottery, and other evidences that metallurgical operations of considerable magnitude had been at some distant period carried on in that locality.* The alchemists of the Middle Ages were the metal- lurgists par excellence of that period, and there is evidence that they were acquainted with chemical processes for the reduction of metals, which were brought to a great state of perfection, thus showing that the practical part of metallurgy was far in advance of the theoretical, t To these early experimenters, however, must be awarded the credit of great industry. They knew nothing of the metals as ultimate bodies, nor of the * Percey's Metallurgy. f The following experiments, from manuscripts discovered by M. Ferdinand Hoefer, will serve to convey an adequate idea of the status of metallurgy from the third and fourth centuries down to a comparatively recent date : " Experiment No. i. — A piece of red-hot iron is placed under a bell, which rests in a basin lull of water. The water diminishes in volume, and a candle, being introduced into the bell, sets fire at once to the gas inside. Conclusion — water changes into fire. " Second Experiment. — A piece of lead, or any other metal except gold or silver, is burned in contact with the air. It immediately loses its primi- tive properties, and is transformed into a powder or species of ashes or lime. The ashes, which are the product of the death of the metal, are again taken and heated in a crucible, together with some grains of wheat, and the metal is seen rising from its ashes and reassuming its original form and properties. Conclusion — metals are destroyed by fire and re- vivified by wheat and heat. " Third Experiment. — Argentiferous lead is burned in cupels composed of ashes of pulverized bones ; the lead disappears, and at the end of the operation there remains in the cupel a nugget of pure silver. Conclusion — lead is transformed into silver." (It is probable that upon this and analogous facts was founded the theory of the transmutation of metals.) METALLURGY. 1 1 particular force governing their union with the non- metallic elements, and finding that an earthy matter, such as an ore of iron, became converted by fire into a metal, they naturally believed the change of earth into metals to be possible, and in the search for gold, the philosopher's stone, etc., they really, by mere accident, discovered many valuable chemical agents. In this way sulfuric, nitric, and hydrochloric acids were produced, and these, made to act upon the metals, in turn yielded the metalline salts. Thus it will be seen that from the gradual aggre- gation of facts resulting from the pursuits of the alchemists ultimately sprang an exact scienoe, and toward the latter part of the sixteenth century appeared a set of investigators of a very different order, who, instead of wasting their time in the pur- suit of such fanciful theories as that of transmutation, etc., devoted themselves to the unraveling of the principles that govern the composition and forma- " Fourth Experiment. — A strong acid is poured on copper; the metal is acted upon, and in process of time disappears; or, rather, is trans- formed into a green, transparent liquid. Then a thin plate of iron is plunged into the liquid, and the copper is seen to reappear in its ordi- nary aspect, while the iron in its turn is dissolved. Conclusion — iron is transformed into copper." Practically the fourth experiment, quoted from Jules Andrieu's paper on "Alchemy," written for the Encyclopaedia Britannica, is electro- lysis, the principle by which a compound of a metal with a non-metal is decomposed by galvanic electricity ; but the transmutation theory was generally accepted as accounting for the phenomena noticed in this experiment, and it would seem that at least some of the savants of the period were sufficiently shrewd and unscrupulous to turn the process to profitable account, since we find that " St. Thomas Aquinas, in his theological writings, forbids the sale of ' alchemist's gold,' and in a special treatise on the subject unmasks an imposture of the char- latans of the day, who pretend to make silver by projecting a sublimate of white arsenic on copper. " 12 DENTAL METALLURGY. tion of bodies already known. Thus, Paracelsus* was the first to distinguish the true character of some of the well-known salts, such as alum and cop- peras, showing that they contained metals, — a matter of great importance at that day, inasmuch as it eventually led to the discovery that many of the well-known crystalline salts were compounds of dis- similar elements, as a metal with a non-metallic body, and to a knowledge of the particular force governing their union ; and finally the investigations of Beecher and Stahl of Cronstadt, Klaproth, Wollaston, Ber- zelius, Wohler and Deville, and others, have dispelled many illusions and rendered accurate the present literature of the subject. During the latter half of the 'eighteenth century the list of metals was augmented by new discoveries, and the application of the voltaic current to the decomposition of the alkalies by Sir Humphry Davy in 1807-8 added a dozen or more. The employment of the spectroscope by Kirchhoff and Bunsen, in i860, brought to light so many new metals that the total number now exceeds fifty. * Paracelsus, though the author of many fanciful doctrines, seems to have been the first to offer a true chemical explanation of the action of mercury, lead, etc., upon the human system. CHAPTER II. THE METALLIC ELEMENTS. MODERN CHEMISTRY assumes that the metals are elementary bodies, yet there have been other theories presented regarding their ultimate character, and it is thought by some that "when man shall have mastered that great power of nature, electricity, many of the so-called elements will be found probably to be compound bodies." Others have entertained the theory of but one ultimate ele- ment,* while nearly all agree that as we advance in knowledge new elements will be brought to light. The old philosophers applied the term element to imaginary principles of matter, such as fire, water, and air ; while the elements of the alchemists were salt, sulfur, and mercury. The term is now used as syn- onymous with simple body, or one of the undecompos- able constituents of any kind of matter, or that which cannot be divided by chemical analysis. The elements known at present number sixty-seven, divided into the metallic and non-metallic. Of the former there are fifty-two, as follows : * Prof. Graham's Researches with Hydrogen, in 1869. 13 14 DENTAL METALLURGY. • COMBINING NAMES. SYMBOLS. WEIGHTS. Aluminum. Al. 27.4 Antimony. Sb. [Stibium) 122 Arsenic. As. 75 Barium. Ba. 137 Beryllium. Be. (or Glucinum.) 9.4 Bismuth. Bi. 210 Cadmium. Cd. 112 Caesium. Cs. 133 Calcium. Ca. 40 Cerium. Ce. 92 Chromium. Cr. 52.2 Cobalt. Co. 58.8 Copper. Cu. (Cuprum) 63.4 Davyum. Da. (?) Didymium. D. 95 Erbium E. 168.9 Gallium. Ga. 68 Gold. Au. (Atirum) 197 Indium. In. H3-4 Iridium. Ir. 198 Iron. Fe. (Ferrum) 56 Lanthanum . La. 93-6 Lead. Pb. (Plumbum) 207 Lithium. Li. 7 Magnesium. Mg. 24 Manganese. Mn. 55 Mercury. Hg. (Hydrargyrum) 200 Molybdenum. Mo. 96 Nickel. Ni. 58.8 Niobium. Nb. 94 Osmium. Os. 199.2 Palladium. Pd. 106.6 Platinum. Pt. 197.4 Potassium. K. (Kalium) 39- 1 Rhodium. Rh. 104.4 Rubidium. Rb. 85.4 Ruthenium. Ru. 104.4 THE METALLIC ELEMENTS, 15 COMBINING NAMES. SYMBOLS. WEIGHTS. Silver. Ag. {Argentum) IOS Sodium. Xa. {Natrium) 23 Strontium. Sr. 87.6 Tantalum. Ta. lS2 Terbium. Ter. 148.5 Thallium. Tl. 204 Thorium. Th. 235 Tin. Sn. ( Statmum) Il8 Titanium. Ti. 50 Tungsten. W. ( Wolframium 184 Uranium. U. 240 Vanadium. V. 51-2 Yttrium. Y. 92 Zinc. Zn. 65.2 Zirconium. Zr. 89.6 Of these, only about fifteen are employed in true metallic condition. These are : Antimony, Lead, Aluminum, Magnesium, Bismuth, Mercury, Copper, Nickel, Gold, Platinum, Iridium, Silver, Iron, Tin, Zinc. Twelve are more or less extensively used in medi- cine, and in the arts as coloring pigments and for alloying purposes. These are : Arsenic, Barium, Cadmium, Calcium, Chromium, Cobalt, Lithium, Manganese, Potassium, Sodium, Titanium, Uranium. l6 DENTAL METALLURGY. The remaining twenty-five are as yet of little or no practical use in the metallic state. Seven of the metals play a more or less important part in the maintenance of animal and vegetable life. These are : Aluminum, Manganese, Calcium, Potassium, Iron, Sodium. Magnesium, The metallic elements are divided by metallurgists into two classes, — the noble and base metals. The first are those which are capable of being separated from combinations with oxygen by merely heating to red- ness. The base metals are those whose compounds with oxygen are not decomposable by heat alone. The noble metals are ten in number, as follows : Mercury. Hg. 200 Silver. Ag. 108 Gold. Au. 197 Platinum. Pt. 197.4 Palladium. Pd. 106.6 Rhodium. Rh. 104.4 Ruthenium. Ru. 104.4 Osmium. Os. 199.2 Iridium. Ir. 198 Davyum. Da. (?) The base metals are further subdivided according to their affinity for oxygen and other chemical prop- erties. CHAPTER III. PROPERTIES OF THE METALS. A METAL may be defined as an elementary sub- stance, usually solid at ordinary temperatures,* insoluble in water, fusible by heat, and possessing a peculiar luster, commonly spoken of as a " metallic luster," an expression sometimes used in describing the appearance of substances which present a similar condition of surface. To these qualities must be added those of conducting heat and electricity, which the metals possess to the greatest extent, and the power of the metals of replacing hydrogen in chemi- cal reactions ; as, when zinc is placed in contact with hydrochloric acid, it displaces the hydrogen and unites with the chlorin to form zincichlorid (chlorid of zinc), thus : Zn+2HCl = ZnCl 2 4-H 2 liberated. Another characteristic of the metals is their basic properties when united with oxygen. Arsenic and tellurium are by some regarded as intermediate links between the metallic and non- metallic bodies. Watts, in his " Dictionary of Chem- istry," says of tellurium that "this element, though decidedly metallic, must be classed as a member of * Mercury is an exception, being fluid at the ordinary temperature. It freezes at — 40 F. 17 1 8 DENTAL METALLURGY. the sulfur family," probably in consequence of its poor conducting qualities and the acid character of its oxids. Bloxam does not regard arsenic as a metal, and states that, though "some authorities class it as such on account of its metallic luster and property of con- ducting electricity, yet it is lacking in the quality of forming a base with oxygen, a property common to all the true metals ;" and asserts that "the chemical character and composition of its compounds connect it in the closest manner with the phosphorus group." On the other hand, we find some of the non-metallic bodies possessing the chemical but not the physical properties of the metals. Thus, the real nature of hydrogen has long been an interesting point of dis- cussion among chemists, some supposing it to be a metal in a gaseous form. Dumas and others prophe- sied that ' ' if ever the means of liquefying hydrogen is found, it will present the appearance of quicksilver, ' ' and their grounds for this belief are its uniformly basic properties. Others contend that it is a neutral sub- stance, possessing both the basic properties of a metal and the chlorous qualities of a gas. In 1869 it was announced that Professor Graham, an eminent English chemist, had discovered the metallic hydrogen. ' ' This new metal, baptized ' hydroge- nium,' was white, magnetic, of a specific gravity about 2, and appeared to have some analogy to mag- nesium." This discovery excited much speculation. Upon verification, however, the new metal was found to be a compound of palladium and hydrogen, in which the former had absorbed 700 or 800 times its bulk of the latter. PROPERTIES OF THE METALS. iy Again, the existence of a hypothetic compound metal called ammonium, and having the constitution NH 4 , has been assumed as the only method of ex- plaining the perfect analogy that exists between the salts of ammonium and those of some of the metals, actual experiments having already strengthened this theory, at first founded only on analogy.* The metals are all quite opaque, with the single exception of gold, which, however, is only transpa- rent in leaves of a highly attenuated condition, when it transmits green light, f The Color of the metals ranges from the pure white of silver to the bluish hue of lead. Between these two the major part of the others may be found. About five run from light yellow to deep red. These are, barium and strontium, pale yellow ; calcium, somewhat deeper in color ; gold, when pure, of a rich yellow ; and copper, the only red metal. It was at one time supposed that the mineral titanium, well known to dentists as a dark red (copper-colored), crystalline substance, used in a finely divided state as a coloring pigment in the manufacture of por- celain teeth, was a metal. It was so pronounced by Wollaston. Wohler and Deville, however, demon- strated that the red mineral is an oxid, and they verified their statement by producing the metal it- self, which is of a steel-gray color. The color of the metals is modified by alloying. J Luster. — This characteristic of the metals is prob- * E. Miller, Treatise on Chemistry. f It is by some believed that the absence of transparency in the other metals may only depend upon our inability to obtain them in a sufficiently attenuated condition. \ See chapter on "Alloys." 20 DENTAL METALLURGY. ably the result of perfect opacity., by which the rays of light are reflected from the surface. Odor and Taste are possessed by some few of the metals. The greater number, however, are destitute of these qualities. Iron, copper, and zinc, when heated, evolve peculiar odors, and one means of de- tection of arsenic is the odor of garlic observed when that metal is exposed to an elevated temperature. Odor and taste may depend upon voltaic action. The former may 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 temperature, seems to promote the elec- trical action. Fusibility. — All metals admit of being reduced to a liquid state by the application of heat, but the tem- perature at which they melt differs widely. Thus, mercury retains its liquid form to 39 F. below zero, and is always fluid at ordinary temperatures. Po- tassium and sodium fuse below the boiling-point of water ; tin, lead, and antimony below redness. Gold, silver, and copper require bright redness. Iron, nickel, and cobalt fuse at white heat, while platinum, iridium, rhodium, titanium, etc., become fluid only when exposed to a powerful voltaic current or the flame of the oxyhydrogen blow-pipe. PROPERTIES OF THE METALS. 21 Table of Fusing-points of the Principal Metals. o5 u '55 0) rt en o • 2i o r &2 g^ ^ CWD CD c o a o *•* '•a a-j CD >> > tin Mercury Rubidium Potassium Sodium Lithium Tin Cadmium Bismuth Thorium Lead . Tellurium, rather less Arsenic, unknown. Zinc Antimony, just below FAHR. -39° +101.3 144-5 207.7 356 442 442 497 56i 617 fusible than lead CENT. — 39-44 c +38.5 62.5 97.6 180 227.8 227.8 258 294 325 Silver . Copper Gold . Cast Iron Pure Iron Cobalt Manganese Palladium Molybdenum Uranium Tungsten Chromium Titanium Cerium Osmium Iridium Rhodium Platinum Tantalum redness. 773 FAHR. 1873 I996 20I& 2786 412 CENT. IO23 I09I 1 102 I530 22 DENTAL METALLURGY, Capacity for Heat. — The metals, in common with other bodies, have their specific heat. This consists in the amount of heat required to raise equal weights of different metals from the same to another given temperature. Thus, if we express by i the quantity of heat necessary to raise a weight of water from o° C. to i° C, that which must be supplied to elevate the same weight of the following metals to that tem- perature would be as follows : * Mercury . 0.03332 Gold . 0.03244 Iron . 0.1 138 Nickel • 0.1086 Cobalt 0. 1070 Zinc . 0.0956 Copper 0.0952 Palladium , 0.0593 Silver . 0.0570 Cadmium . 0.0567 Tin . . 0.0562 Antimony . 0.0508 Lead . 0.0314 Platinum . 0.0311 Bismuth . 0.0308 Now, if we should take equal bulks of these metals and expose them for the same length of time to exactly the same heat, and then place them simul- taneously upon a cake of wax, we would observe those of the above table with the highest figures, such as iron, instantly melting their way through the wax, while those of the lowest capacity for heat, such as bismuth, would remain on the surface. * Phillips's Metallurgy, p. 13. PROPERTIES OF THE METALS. 23 Expansion by Heat. — Metals expand when heated, but this property is not uniform, some possessing it to a greater or less extent than others. Within cer- tain limits of temperature this takes place propor- tionately to the amount of heat to which they are exposed. Zinc possesses a rather high degree of expansibility, and is consequently useful for the pur- pose of making dies for swaging metal plates for artificial dentures. By many dentists it was for- merly thought that a metal, to be well suited for this purpose, should be entirely destitute of this property, so that after casting the die should not, in returning to its former condition in cooling, be smaller than the plaster model, the objector se being to have the plate fit the plaster cast perfectly ; whereas, the real pur- pose should be to make the plate fit the mouth closely, the plaster model being only a means to that end. Plaster expands in setting. From the impression to the model two expansions are gone through before the fac-simile of the mouth in plaster is obtained ; hence, a plate made to fit such a model perfectly must necessarily be somewhat larger than the mouth, — a condition unfavorable to atmospheric adhesion. On the other hand, a plate made to fit the zinc will not be found too small for the mouth, but will, pro- vided the impression is a good one and represents perfectly the conformation of the mouth, afford a very close-fitting plate. Even better results might be expected where the plate is somewhat smaller than the mouth, because such a condition would, in entire upper dentures, throw any undue pressure upon the alveolar ridge, while that portion of the plate covering the palatine arch would barely be in contact 24 DENTAL METALLURGY. with the tissues ; the pressure along the ridge would quickly promote absorption of the remains of the alveoli, and a uniform adaptation of the plate to the mouth would soon follow. On the contrary, if the plate be made to fit the plaster cast, and is a trifle larger than the mouth, the pressure will be thrown upon the palatine arch at the back edge of the plate, at a region not likely to change by absorption, as is the case with the alveolar ridge, and hence the margin of the plate will imbed itself in the tissues and cause much discomfort and impair the usefulness of the denture. Much time and thought have been expended in the effort to discover some alloy which, in connection with the properties of hardness and fusibility, shall possess that of non-expansibility when heated. Har- ris's " Principles and Practice of Dentistry" gives no less than nine different formulae. The author is satis- fied that the property of expansibility in zinc as used in the dental laboratory constitutes one of its most valuable qualities, as it gives us the means of com- pensating for the yielding of the tissues and the absorption along the ridge which nearly always fol- lows the first insertion of an artificial denture. Table of Expansion of Metals for each degree from o° C. to ioo° C* Gold .... 0.00155155 Silver .... 0.00190868 Platinum . . . 0.00099180 Palladium . . 0.00100000 Copper . . . 0.00171733 Iron .... 0.00123504 Lead .... 0.00284836 Tin 0.00193765 Zinc (cast) . . 0.00294167 " (ham'r'd) . 0.00310833 Bismuth . . . 0.00139167 Antimony . . 0.00108333 * Phillips's Metallurgy. PROPERTIES OF THE METALS. 25 Power of Conducting Heat. — The metals are the best conductors of heat among the solid bodies. The quality of transmitting heat is possessed by them in variable degrees. The following table shows the relative approximate ratio of conductivity of heat of each of the metals commonly used in the mechanic arts : Silver 100 J11VC1 ...... Copper 1UU 73-6 Gold ... . . 53-2 Brass* 23.6 Tin 14.5 Iron .... 11. 9 Steel ... 11. 6 Lead • • 8.5 Platinum .... . . 8.4 German Silver 6.3 Rose Fusible Metal 2.8 Bismuth .. • 1.8 Power of Conducting Electricity. — Metals conduct electricity nearly in the ratio of their capacity of transmitting heat. Davy, Becquerel, and Dr. Matthiesen have, at different times, more or less extensively experimented upon this characteristic quality of the metals. Among the results of Dr. Matthiesen' s investigations are the facts that debas- ing a metal or alloying it greatly diminishes its con- ducting power, and that elevation of temperature has the same effect, and that between 32 and 212 F. (or o° and ioo° C), great diminution takes place, — not uniformly, however, as some lose it more in pro- portion than others.f *Zinc is probably between brass and tin. t Makins's Metallurgy-, p. 17. 26 DENTAL METALLURGY. A rough means of determining the relative con- ducting power of metals consists in connecting the poles of a voltaic battery by a wire through which the current will pass freely. Now, if the wire be too small for the transmission of the electricity supplied to it, the obstruction will be manifested by the wire becoming red-hot.* Hence the relative capacity of metals for this pur- pose may be observed by employing equal battery- power upon wires of the same diameter of different metals, and noting the length of the portion of each which can thus be heated. Conversely, the same means may be employed to indicate the quantity of electricity, or the capacity of the battery itself. In this case the wire is made to demonstrate the power of the battery by the length of wire which the battery is capable of rendering incandescent. A good demonstration of the relative conducting power of different metals may be made by construct- ing a chain of alternate links of platinum and silver wire. This will show, while the current of electricity is passing, red-hot platinum links alternating with cool silver ones. Platinum, being much the inferior conductor, offers such an impediment to the passage of the current that great elevation of temperature results, while the silver, being a good conductor, offers no check to the free passage of the electricity. The power of metals of conducting electricity is * This was fully shown in some of the early experiments with the electro-magnetic mallet, in which the wire was too small for the accom- panying battery power. They worked very well for 7.. Gold • l A oz Silver . 2 " Silver . . 2 " Tin . • 3 " Tin . • 3 " Platinum . • X " Platinum • 3# c dwts. 6 4 DENTAL METALLURGY. No. 4. Gold . Silver Tin . Platinum Gold . Silver Tin . Pure Zinc oz. i& No. 5. 3/4 3 dwts. Xz oz. 3 dwts. Has always shown good results, but exhibits surface discoloration. No. 6. Gold . Silver Tin . Pure Zinc Gold. Silver Tin . Pure Zinc No. 7. v-i oz. 4 dwts. 12 dwts. \}i oz. 3X " 8 dwts. No. 8. Gold Silver Tin . Pure Zinc 14 dwts. 2 oz. 3 " 3 dwts. Results uniformly good. No. 9. Gold . Silver Tin . Pure Zinc Gold Silver Tin . Pure Zinc No. 10. 14 dwts. 1% oz. *y A « 3 dwts. 16 dwts. 2 oz. 3 " 3 dwts. Too soft, and suffered some loss by surface waste. No. 11. Gold . Silver Tin . y. oz. From actual experience with Nos. 5 and 8, the author can testify to the excellence of their general working qualities and their durability. They afford, however, very quick amalgams, and should not be mixed with mercury until the cavity is quite ready for the recep- tion of the filling ; or, if found to set too quickly, that quality may be modified by using less gold in the formula. AMALGAMS. 65 Palladium and mercury are said to make a very good filling. According to Mr. Thomas Fletcher, however, all alloys into which palladium enters as a constituent are utterly worthless ; combined with tin and silver, it has been found to be unsatisfactory. Mr. Fletcher gives the results of a number of experiments with tin, silver, and palladium.* "The alloys given below were made from chemically pure metals. They were melted first at a high temperature, under a layer of charcoal, in a clay crucible, with constant stirring. They were then poured quickly into a thick and cold iron ingot-mold ; broken up and remelted three times, to insure uniformity as far as possible. Pd. 1, Ag. 5. Result powdery and unmanage- able. Pd. 1, Ag. 5, Sn. 1. Result ditto. Both readily combined with Hg. Pd. 1, Ag. 5, Sn. 2. Result same as above. Pd. 1, Ag. 5, Sn. 3. Result very dirty to mix ; makes a leaky plug. Pd. 1, Ag. 5, Sn. 6. Result similar to last. Pd. 1, Ag. 3, Sn. 5. Result very dirty ; does not combine properly with mercury. Pd. 1, Ag. 6, Sn. 5, Au. 1. Result similar to last. Pd. 1, Sn. 4. Result very dirty ; does not set at all." In consequence of the uniformly bad results of these combinations, Mr. Fletcher discontinued further ex- perimentation in palladium alloys for dental amal- gams. The claim, however, has been made for palladium, f * British Journal of Dental Science. tDr. Bogue, in Proceedings of New York Odontological Society, Dental Cosmos, 1884, p. 403. 66 DENTAL METALLURGY. that when used as a single metal amalgamated with mercury, like Sullivan's amalgam (copper and mer- cury, see chapter on " Copper"), it does not change in form or bulk, or discolor the teeth, and though it turns quite black, it is a thoroughly reliable filling pro- vided the cavity be properly prepared. It is said that the union of palladium and mercury is a true chemical one, and is accompanied with the phenomena usual in such cases of heat and incandescence. Qualitative and Quantitative Examinations of Amalgam Alloys. — It is important for the dentist employing filling- materials of this class to be well in- formed as to their composition, and it is also often desir- able to be able to determine the composition of old amalgam fillings, the constituents of which, besides tin and silver, are unknown. With a compound belonging to the latter class the first step, after weighing, would be to free it from mercury by heating to redness, the loss of weight at the second weighing indicating the amount of mercury which was present. After this it is to be treated as an alloy, one gramme of which may be placed in a test-tube or other suitable glass vessel, and acted upon by a sufficient quantity of chemically pure nitric acid. The silver is dissolved and converted into argentic nitrate. The tin is oxidized and becomes metastannic acid (5Sn0 2 , ioH 2 0). The latter, in the form of a white powder, settles to the bottom of the vessel. If gold forms part of the alloy, it will be recognized, even in the smallest quantity, by the very decided purple color which it imparts to the precipitate, due to the for- mation of purple of Cassius. Platinum may be AMALGAMS. 67 readily detected by the presence in the test-tube of the finely divided metal, quite black in color. At this point in the experiment, if neither gold nor platinum is present, the quantitative estimation of the alloy is not difficult, and may be accomplished as follows : The solution should be rendered neutral by evaporation, and diluted with a large quantity of dis- tilled water. The oxidized tin will be found at the bottom of the vessel. After pouring off the solution, this should be washed, dried, and rendered anhydrous by heating to redness, when it is ready for weighing, every 100 parts indicating 78.66 of tin. Returning to the solution, the silver is next precipitated by sodium chlorid or hydrochloric acid, and may be collected by filtering. If cadmium is present in the remaining solution, sulfuretted hydrogen will throw it down in the form of a yellow powder (sulfid of cadmium), the color distinguishing it from zinc sulfid, which is white. The alkalies, potassa, soda, and ammonia throw down the oxid of cadmium as a white hydrate. Amnionic carbonate also produces a white precipitate, which is insoluble in excess of the precipitant. The latter quality also distinguishes it from zinc, which is soluble under similar conditions. Zinc may be precipitated from a solution by potas- sic carbonate, added in sufficient quantity to decom- pose any ammoniacal salts, if present, as such would prevent the precipitation of the carbonate of zinc. The whole is now to be evaporated to dryness, hot water added, the zinc salt boiled, collected by filter- ing, and lastly, after washing, ignited to drive off the carbonic acid, when oxid of zinc remains. In quan- 68 DENTAL METALLURGY. titative estimation, zinc is always weighed in this form, the oxid being calculated as containing 80. 24 per cent, of the metal. If copper be present, as it frequently is in amalgam alloys, it will be instantly detected by the greenish hue which it imparts to the solution, after the ' ' break- ing-up" by nitric acid. Indeed, the appearance of the contents of the test-tube will, after the student has acquired some experience, serve to convey a pretty clear idea of the composition of the alloy. For in- stance, take one gramme of Arrington's alloy, com- posed of pure tin and silver, and act upon it with nitric acid ; the contents of the test-tube will be a colorless liquid and a perfectly white powder. In another test-tube dissolve some of Lawrence's alloy, and the nitrate, instead of being perfectly colorless as in the preceding experiment, will show a decidedly green tinge, and the presence of copper is verified by the blue color developed by the addition of am- monia. Gold and platinum will be detected in alloys containing these metals by the appearances already described. The presence of copper having been verified, that metal may be precipitated as cupric sulfid, by sul- furetted hydrogen. It may then be collected by filtering, and after washing and drying may be oxi- dized by nitric acid, and again be precipitated by potassa, and may then be weighed as an oxid. Should copper and cadmium both be present, the precipitate obtained by sulfuretted hydrogen will consist of sulfids of cadmium and copper. Boiling in dilute sulfuric acid, however, dissolves the cad- mium so that the copper may be collected by filtering, AMALGAMS. 69 after which the cadmium may be again thrown down by sulfuretted hydrogen. The merest trace of copper in solution may also be detected by placing a drop of the latter on a strip of clean platinum foil and touching it with a point of zinc. A spot of reduced copper will instantly appear. For the quantitative analysis of an alloy containing tin, silver, gold, and platinum, the first step should be to remove the tin by deflagration. This is accom- plished by placing the alloy, after weighing, in a small crucible with some borax, and heating to bright redness. While at that high temperature small por- tions of potassium nitrate are added. This is de- composed ; its oxygen unites with the tin, converting it into stannic oxid (SnO a ). After cooling, the crucible may be broken, and the remaining button, together with any small globules which may adhere to the sides of the crucible, collected and weighed, the loss indicating the amount of tin which was present. If the deflagration has been thoroughly performed, the button will be entirely freed of tin, and it then remains to separate the silver from the gold and platinum. This may be accomplished either by nitric or sulfuric acid. But, as the former will dissolve a considerable portion of the platinum along with the silver, sulfuric acid, which does not thus affect the platinum, affords more accurate results, and is the agent usually employed in parting operations where the alloy consists largely of silver, with an appreci- able percentage of platinum. The button, now consisting of silver, gold, and platinum, should be rolled into a thin ribbon, and 70 DENTAL METALLURGY. then placed in a glass or platinum vessel with at least two and a half times its weight of concentrated sul- furic acid. This is boiled, during which strong action is evinced by copious disengagement of sul- furous anhydrid, and the silver is converted into a sulfate. The boiling is continued until all the silver is dissolved, when the gold and platinum will be found at the bottom of the digester. The liquid is now poured off, and the silver recovered from the sulfate solution by precipitation with plates of copper, which reduce it in a more or less crystalline state. The re- maining alloy, now consisting of gold and platinum, should be thoroughly washed, dissolved in nitro- hy- drochloric acid, neutralized by evaporation, then dis- solved in a large quantity of distilled water. It is then ready for precipitation with oxalic acid, by which the gold is thrown down, and the platinum remains in solution for subsequent treatment. The gold, which is now easily collected, should be washed, dried, and heated to redness, when it is ready for weighing. Lastly, the platinum may be recovered from the solution by precipitation with ammonic chlorid. When washed and dried it will be ready for weighing, and every ioo parts may be considered as containing 44. 28 of platinum. The composition of some of the principal dental amalgam alloys now in use, is shown in the following tables : AMALGAMS. 71 , £ ■30 od ii a o (J £ 5 6 o" o" /. ir,o "JO 10 pi pi •* ri t^. o rOO\ l/"> v~l p-i lo t^vo m-OOOOOOOO woo PJ ■<*• o U") ►* PI 0\ rO O POVO O >0 ") t^. PO -* O OsOO >-i tJ- O IO lO^C 10 10 lO "~j Tf ■* "<*" i/^^O "2 "^-O ^ri/)i/01OfO^roiO ■g-^o w. to i/"> 10 IO ■>*• lO ^-. °^ — 5 c rt rt W — BQhi rt Si £ - -00 = =<<^-- C ,, ^ 5 U " ™ ■" °1 'C &«-p-p 2 2 2 « <«WUUUU'JU 3 ° as ■P°p III 9. VC< — =^< u.2 t; 2o - — <3- n I* p ^ = -"*o en (/; 5£ •J) S. JC Od ^2 CJU — r*- - « o i2 in t,?2«) « E.E£' ." r* «J =" O O 3 p -"> - w -x fc U . > > .a ~ u £ « rt « o „ ^ « " » 09 •a-q-p p. g 0^ = ?li E o c 0.= .=: tooooo .- p p - 1 " d c« « £ £ rt - - 72 DENTAL METALLURGY. • S 13 .2 ft"0 5 2 - a o U ft C O HH t-H o .^ tsi tn .9 ,_ §§§§.2SO c c c rtt/jO c OOOOmdlD Ifi cfl tf> CO- 1 "- ^ c C C C-£ ££> jSJSJS^i en c £ 5 < & ££r2j2P h "in "(M i-rctfi j» > <- -5 rt :~ £ft 5^3 3) tfi ^ ^< T-J rQ - U> pj g . o^a"-^^3-M n?2- c ««h «3t:t: c e-s2 s £ S Si? 5 5 5 js'S *> ^ ftftc^c/jcnc^c/rc^HE- 1 ft C X 9OI. I TO Off °'3 For, as £ is a gain in weight, it must be deducted. Hence, .r — 901.1 — 0.092 — 0.3. = 900.708. And by the second formula 901. 1 — 0.3 X ~ T j_ _o^_i_ 1 1 1000 = 900.708. Gold Leaf and Foil.— -Gold designed for manufac- ture into leaf is variously alloyed, according to the color required. The following is a list showing the proportions of alloy per ounce required to produce certain tints : Color of Leaf. Proportions of Gold. Proportions of Silver. Proportions of Copper. Grains. Grains. Grains. Red. 456.460 — 20.24 Pale Red . 464. — 16. Extra Deep Red 456. 12 12. Deep Red 444. 24 12. Citron 440. 30 IO. Yellow 408. 72 — Pale Yellow . 384. 96 — Lemon 360. I20 — Green or Pale . 312. 168 White 240. 24O . GOLD. . l6l For rilling teeth nearly pure gold in the form of foil is used. It is generally prepared by beating, but some of the heavier numbers are produced by rolling. There are two varieties, cohesive and non-cohesive, exten- sively used in the United States at the present time, the methods of manipulating which are widely differ- ent. In the former the characteristic quality of co- hesiveness, which is greatly diminished by compression of the fibers in beating, is restored by heating to red- ness, and this is best effected by placing the foil upon a sheet of mica, which is held over a spirit-lamp. The habit common among dentists, of taking up the foil on the point of the plugger and passing it through the flame of a spirit-lamp, is not productive of the best results, the gold being made harsh by contact with the flame, whereas it should, in addition to co- hesiveness, possess at least some of the kid-like soft- ness of the non-cohesive variety. Some of the manu- facturers of gold foil for dental purposes produce a 4 ' soft' ' variety, in which cohesiveness cannot be de- veloped by heating. This quality may be attained by alloying, or by depositing carbon upon the surface, as the latter cannot be driven off by heating. The statement has been made by one of the most experienced manufacturers of dental foils in this country, that he makes the two varieties of cohesive and non-cohesive foils from the same ingot. As his non-cohesive or "soft" foil is unsurpassed in the qualities which are desirable for such a foil, it seems proper to assume therefore that the development of these qualities is due either to some treatment of the surface or to mechanical management during lamina- tion, and not to alloying. l62 DENTAL METALLURGY. The manufacture of non-cohesive gold foil properly so called is not, it would seem, understood by every gold-beater . Some of them prepare only cohesive foil, while others offer for sale a so-called non-cohesive foil, which is really nothing more than an unannealed sam- ple of the cohesive type, and which is destitute of the peculiar ' ' kid-like softness' ' and toughness which permits it to be carried forward by the plugging instru- ment into deep cavities without fracture. The extent to which the characteristic qualities of cohesiveness, etc., may be modified or entirely lost by the absorp- tion of gases has yet to be fully studied. In 1866 Graham demonstrated that gold is capable of occlud- ing 0.48 of its volume of hydrogen, and 0.20 of its volume of nitrogen. Varrentrapp has also pointed out that ' ' cornets' ' from the assay of gold may retain gas if they are not strongly heated. Prof. G. V. Black deserves credit for his researches published in the Dental Cosmos, vol. xviii, p. 138, showing the influence of gases and moisture on the cohesiveness of gold foil. The corrugated gold introduced about fifteen years ago belongs to this class of foils. It is prepared by plac- ing the vsheets of gold between leaves of a particular kind of unsized paper, and tightly packing it in iron boxes, which are exposed to a temperature sufficiently high to carbonize the paper. These are then allowed to cool, and on opening them the gold is found to be exceedingly soft and non- cohesive, and to present a peculiar corrugated condition of surface, while it is incapable of being rendered cohesive by annealing. Non-cohesive foil is used in the form of cylinders, made by rolling a ribbon or strip of foil, or as pellets, GOLD. 163 mats, or ropes. These are introduced into the cavity by means of plugging instruments with or without serrations, the force employed being for the most part hand-pressure, though the hand-mallet is much used by many operators as a means of impacting non- cohesive foil. The advantages claimed for non-cohesive foil are that less time is consumed in its introduction, and that in consequence of the greater softness which it possesses it is capable of being more thoroughly burnished to the edges of the cavity. When non-cohesive foil is used, union does not take place between the particles of gold introduced into the cavity. They are simply made to adhere mechanically by wedging the mats, cylinders, or pellets, as the case may be, one against the other, into a cavity properly prepared to retain them. On the other hand, between particles of gold wherein the characteristic quality of cohesiveness remains unimpaired, that property mani- fests itself whenever two pieces are brought in contact, and if cohesion be facilitated by the application of sufficient force, homogeneity results. Hence, the methods of operating with the two kinds of foil must necessarily differ. Non-cohesive foil is introduced in pieces of more or less bulk ; the cohesive variety is introduced in much smaller proportions, each piece being carefully welded to the others by means of the electro-magnetic mallet, the hand-mallet, the "auto- matic" plugger, or by hand-pressure. The method of preparing cohesive foil is thus de- scribed by the late Dr. M. H. Webb, of Lancaster, Pa. : "A half-leaf of No. 4 gold foil for small, a whole sheet for medium, and two leaves for large fillings, should be taken from the book by means of the foil-carrier or 164 DENTAL METALLURGY. spatula, and placed upon a piece of spunk covered with white kid. The foil should be folded with an ivory spatula into a tape-like form, eight or ten lines in width respectively, which is then cut across into small pieces about one-twelfth of an inch in width. Heavy foils, ranging from No. 30 to No. 60, may be advantageously employed in extensive contour opera- tions. The ordinary light numbers, however, when prepared in the manner described, can more easily be impacted into small cavities, fissures, and grooves." For cohesive gold the cavity is carefully prepared, the edges of enamel being smoothly and evenly fin- ished. A groove or undercut is then made toward the cutting-edge and toward the cervical wall, and a start- ing-point drilled in the dentine toward the palatal edge for the purpose of anchorage. Into this the gold is carried by means of hand-pressure, and by the same means worked into the grooves or undercuts. The point in which to start the filling should be only suffi- ciently deep to retain the small pieces of gold first introduced while others are being built upon them. When the first pieces are firmly fixed, the electro- magnetic mallet may be employed to the end. If the cavity be small, the gold should be prepared from a half-leaf of No. 4 foil, so folded as to be equivalent to No. 12 or No. 16, and then cut into strips about one- twenty-fourth of an inch in width. Although much diversity of opinion exists among dentists regarding the relative value of cohesive and non- cohesive foils, it may with safety be stated that, skillfully directed, either is capable of affording good results. But, like all other filling-materials, each has its proper place. Thus, in a majority of crown-cavi- GOLD. 165 ties, it would be a sheer waste of time to fill exclus- ively with cohesive foil; while, on the other hand, there are many approximal cavities in which the walls have been rendered so thin by the progress of decay that probably the electro-magnetic mallet alone could be relied on to thoroughly impact the gold to walls and periphery. In such cases the cohesive foil should be folded of No. 4, and cut into narrow strips as above described, the main object being to avoid the applica- tion of much force, such as would be required in the consolidation of large mats or cylinders of non-cohe- sive gold. Indeed, it may be stated that one of the greatest advantages in filling with cohesive foil by the electro-magnetic mallet is the thoroughness and safety with which the gold may be packed against very frail walls. The merits of the cohesive and non-cohesive forms of foil are by their respective advocates often unfairly presented, but the value of each may be stated as fol- lows : Cohesive foil in very small pieces is, with the electric plugger, capable of being brought into perfect apposition with the most delicate walls of enamel with comparatively little danger of fracture ; non-cohesive foil, in much larger masses, may with less expenditure of time be introduced into cavities where the walls are sufficiently strong to withstand the force required to consolidate the mats or cylinders, and, so far as the preservation of the tooth is concerned, affords equally good results. Gold- Beating. — In all probability the art of gold- beating originated among Oriental communities, with whom the love of gold ornaments has always been a distinguishing characteristic. The art is of great 1 66 DENTAL METALLURGY. antiquity, and is referred to by Homer and Pliny. On the coffins of Theban mummies specimens of leaf-gild- ing were met with, where the gold in a very thin state resembled modern gilding. It is stated that the Incas of Peru did not understand the art of gold-beating beyond the preparation of sheets or plates, which they nailed on the walls of their temples. The process of beating gold is conducted in the following manner : The metal is first alloyed accord- ing to the color desired, and, in order to improve its malleability, it is melted at a higher temperature than is necessary for mere fusion. It is then cast into an ineot and rolled into a ribbon of a half-inch in width and ten feet in length to the ounce. After this it is annealed and cut into pieces of about six and a half grains each, and placed between the leaves of a " cutch," which is about half an inch thick and three and a half inches square, containing about one hun- dred and eighty leaves of a tough paper manufactured in France. Fine vellum was formerly much used for this purpose, and it is yet often interleaved^ in the pro- portion of about one of vellum to six of paper. The hammer employed by gold-beaters weighs about seven- teen pounds, and rebounds, by the elasticity of the skin, to such an extent that each stroke involves but little labor. It requires about twenty minutes' beat- ing to spread the gold to the size of the cutch, and if it is intended for filling teeth it is carried no further than the cutch stage. If, however, it is to be still further attenuated, each leaf is taken from the cutch and cut into four pieces, when it is put between the skins of a "shoder," four and a half inches square and three-quarters of an inch thick, containing about GOLD. 167 seven hundred and twenty skins. The shoder requires about two hours' beating with a nine-pound hammer. As the gold will spread unequally, the shoder is beaten upon after the larger leaves have reached the edges, the effect of which is that the margins of larger leaves come out of the edges in a state of dust. This allows time for the smaller leaves to reach the full size of the shoder, by which a general evenness in the size of the leaves is obtained. Each leaf is again cut into four pieces and placed between the leaves of a " mold," — an appliance composed of about nine hundred and fifty of the finest gold-beaters' skins. Its dimensions are five inches square by three-fourths of an inch thick. The management of the gold in the " mold" is the last and most difficult stage in the process of gold-beating, and the fineness of the skin and judg- ment of the workman will greatly influence the final result. The process of lamination may be thus described : During the first hour the blows of the hammer are directed principally upon the center of the mold, by which means the edges of the leaves are made to crack, but they soon coalesce and unite ; so that, after beating, no trace of the rupture is left. After having been beaten for an hour in a mold, until the leaves have attained a thinness of j-nH^o- part of an inch in thickness, green rays of light begin to be transmitted, if the gold be pure ; but, if largely alloyed with silver, rays of a pale-violet hue pass through the gold. The membrane called "gold-beaters' skin," used in the make-up of the shoder and mold, is the outer coat of the caecum or blind gut of the ox. It is im- mersed in a potash solution, and scraped with a blunt l68 DENTAL METALLURGY. knife to free it from fat. It is then stretched on a frame, two membranes are glued together, treated with camphor in isinglass, and subsequently coated with albumen, and cut into squares of five or five and a half inches, and is ready for use. It is stated that the cseca of three hundred and eighty oxen are re- quired to yield enough of the membrane to make up one mold of nine hundred and fifty pieces, only two and one-half skins being obtained from each animal. Dryness is a matter of great importance, and, as the leaves are liable to absorb moisture from the atmos- phere, they require hot-pressing . every time they are used, and if this precaution is neglected the leaf will be pierced with innumerable holes or reduced to a pulverulent state. CHAPTER IX. SILVER. Atomic Weight, ioS. Symbol, Ag (Argentum). SILVER may be classed as next to gold in its mani- fold uses and great malleability and ductility. It has been known from the earliest ages, and, al- loyed with certain proportions of copper, it has been adopted by all civilized nations for purposes of coin- age, and for articles of plate and ornamentation. Properties of Silver. — It is distinguished from all other metals by its brilliant whiteness. Its specific gravity is 10.53. in hardness it is between gold and copper. It is one of the most ductile and malleable of the metals, — indeed, when calculated by weight, it is not even surpassed by gold. For example, one grain of gold may be beaten out to the extent of 75 square inches, and the same weight of silver to 98 square inches. Taking a cubic inch of gold at 4900 grains, this gold leaf is 8 e fl x 6 s ft part of an inch in thickness, or about 1200 times thinner than ordinary printing-paper.* But the silver, though spread over a larger surface, will be thicker, owing to the differ- ence of specific gravity between gold and silver. The extent of the malleability of gold and silver has not yet been definitely determined, as the means employed *Gold has, for the sake of experiment, been beaten out to the extent given above, but the pi„ of an inch, as given on page 119, is as thin as is ever required for practical purposes. 12 169 I70 DENTAL METALLURGY. to test it have failed before there was any appearance of the malleability of either of them being exhausted.* In tenacity silver surpasses gold. It fuses at about 1873 F. , and during the fusion absorbs oxygen to the extent of about twenty- two times its own volume ; but at the instant of solidification it undergoes consid- erable expansion, while at the same time it parts with the oxygen, which makes its escape through the thin crust formed over the fluid metal, carrying with it fine globules of the metal, which may be observed ad- hering to the sides of the crucible. It is the best con- ductor of heat and electricity known. It possesses no direct attraction for oxygen ; hence it is not oxid- ized by dry or moist air at any temperature. It is, how- ever, oxidized by ozone, and tarnished by air contain- ing sulfuretted hydrogen, which blackens the surface with a superficial layer of sulfid of silver, which may be removed by a solution of cyanid of potassium. With the exception of nitric, silver is not affected by dilute acids ; but hot concentrated sulfuric acid converts it into sulfate of silver, and when boiled with strong hydrochloric acid it dissolves to a slight extent in the form of chlorid of silver, which is precipitated by the addition of water. Occurrence and Distribution. — In the middle ages Austria was the chief source from which silver was obtained, as an associate metal with lead At the present day the United States, Peru, and Mexico supply large quantities. Silver is found, first, as native silver, occurring in flat masses occasionally, and sometimes crystalline in form. In this country it occurs with native copper, * W. Chandler Roberts, Assayer Royal Mint. SILVER. 171 masses frequently being met with in which the two metals are diffused, the silver showing in specks upon the copper. Native silver is usually free from any considerable admixture with other metals, although it invariably contains traces of gold, antimony, etc. It is also found as chlorid, iodid, and bromid. The most common ores from which silver is derived are those resulting from combination with sulfur as sulfids. These may be divided into three kinds : First may be mentioned the common sulfid, of Mexico, called vitreous sulhd. It is a protosulfid, is very fusible, and readily yields silver when made to give up its sulfur. Another sulfid, closely resembling the first, called brittle silver ore, is found in South America and in some parts of Europe. It is readily decomposed by heat, and, during exposure to high temperatures, evolves fumes of arsenic and antimony. A third sulfid, found in nearly all silver mines in the form of ruby-colored, transparent crystals, is called red silver ore, and is associated, to some extent, with oxids The composition of this ore has been given as follows : Silver, 56 to 62 ; antimony, 16 to 23 ; sulfur, 11 to 14 ; oxygen, 8 to 10. The chlorid or native horn-silver is quite an abundant ore of South America (Chili). It is a true chlorid, and, like precipitated chlorid of silver, darkens when exposed to sunlight. Its composition is given as, silver, 75.3 ; chlorin, 24.7. Methods of Separating Silver fro?n its Ores. — As much of the silver of commerce is extracted from ores too poor to admit of its economical separation by any process of melting or fusing, even in regions where 172 DENTAL METALLURGY. fuel is plenty, recourse to the method known as "amal- gamation" is necessary. This depends simply upon the easy solubility of silver and associated metals in mercury. The ore is crushed to powder, mixed with a sufficient quantity of common salt, and roasted at a dull-red heat in a suitable furnace. By this treatment any sulfid of silver contained is converted into chlorid. The mixture, which consists of much earthy matter, metallic oxids, soluble salts, silver chlorid, and metallic silver, is sifted and placed in barrels arranged to revolve on axes. Scraps of iron and water are added, and the whole agitated together for the purpose of reducing the silver chlorid to the metallic state. A sufficient quantity of mercury is then added, and the agitation continued until the metallic particles are dissolved, forming a fluid amalgam which is readily separated from the mud or earthy matter by subsidence and washing. It is then strained through a strong linen cloth or other suitable fabric to separate the fluid mer- cury from the more solid portions of amalgam. These latter are subsequently exposed to heat in a retort, by which the remaining mercury is distilled off. The silver, more or less impure from admixture with other metals contained in the ore, is thus obtained. In order to prevent loss during the amalgamation process, in consequence of a tendency on the part of the mercury to combine with sulfur, oxygen, etc., technically known as " flouring," in which condition it may be washed away, together with the silver it has taken up, from one to two per cent, of sodium is added to the mercury. The great affinity of sodium for sul- fur and oxygen prevents ' ' flouring' ' of the mercury. Considerable quantities of silver are obtained from SILVER. 173 argentiferous galena,''' and, indeed, it may be stated that nearly every specimen of native lead sulfid will be found to contain traces of the nobler metal. When the proportion of the precious metal present is suffi- ciently large to insure its profitable separation, the ore is reduced as usual, t the silver remaining with the lead, and is then treated according to a process discovered by Mr. Pattinson, by whom it was found that, when lead containing a considerable amount of silver is fused and carefully stirred while it is allowed to cool slowly, crystals much less rich in silver than the mass before melting will form, and separate and subside to the bottom. These crystals of poorer lead are removed by means of perforated ladles. The silver is thus concentrated. This method of separating silver from lead, as practiced on a large scale, is thus described by Mr. Makins in his " Manual of Metallurgy :" "A series of iron pots, from nine to twelve in num- ber, are employed. These are hemispherical, about five feet in diameter, and calculated to hold a charge of about nine tons of metal each. They are set in brick furnaces adjacent to one another, but with quite distinct flues, furnaces, dampers, etc. The lead, as- sorted according to its richness in silver, is then placed in the pots in the following order : Some lead contain- ing ten ounces of silver per ton having to be worked, nine tons of it would be placed in the fifth pot and melted. After complete fusion it is skimmed with a perforated ladle, which removes the dry oxids for sub- sequent reduction, while it permits the fluid lead to run ♦Silver is invariably present in this form of lead ore, but not always in paying quantities. fSee chapter on " Lead." 174 DENTAL METALLURGY. back into the pot. The fire is then drawn, and the metal stirred while it slowly cools until it begins to thicken. The workman at this stage of the operation employs an iron ladle of eighteen inches in diameter by five inches deep, perforated with half-inch holes, and furnished with a very long handle. This handle he raises above his head, sinking the bowl into the lead until it reaches the bottom. Then, by using the handle as a lever, and depressing it as far as possible, the ladle full of crystals is brought into view, and, by means of a hook and chain fastened to a crane, is sus- pended and left to thoroughly drain, after which the crystals are turned into the fourth pot. This opera- tion is continued until two-thirds of the lead in the fifth pot has been passed over in crystals to the fourth pot, under which a fire is made and the crystals again melted. The remaining three tons of molten lead in the fifth pot, which by the separation of the crystals contains silver equaling twenty ounces per ton, is now ladled into the sixth pot. The results of the preceding operations may be summed up as follows : In pot 5, nine tons of ten-ounce lead equals ninety ounces of silver, of which six tons of five ounces (thirty ounces silver) works into pot 4 ; and three tons of twenty ounces (sixty ounces silver) is ladled into pot 6. The work now proceeds until all the pots are in operation. Three tons of five-ounce lead would be added to the six tons passed into pot 4, while six tons of twenty- ounce lead would be carried into pot 6. Six tons from pot 6 would work into number 5, and three tons in bottoms will be put back into the same pot from num- ber 4, filling it again without the addition of pig-lead. The bottoms or portions which remain after the ladling SILVER. 175 become by that process so rich in silver as to often contain six hundred ounces to the ton. This is finally submitted to cupellation, by which means the complete separation of the silver is effected." The cupel and its application may be thus briefly described : Bone-ash is mixed with water, made into a cup, in a suitable mold, and dried. This is called the cupel, * and has the property of absorbing oxids when they are combined with oxids of lead in a state of fusion . Impure silver is mixed with a certain quan- tity of lead, determined by the amount of impurity supposed to exist in the alloy. The mixture is melted in the cupel in a current of air until the whole ol the lead is converted into oxid, which, in a fused state, sinks into the porous cupel, carrying along with it the other impurities, the silver being left behind in a pure state. The whole operation is based on the absence of attraction for oxygen evinced by the noble metals even when exposed to high temperatures, and on the affinity possessed by the base metals for oxygen under similar conditions. Cupellation may be accomplished either in a muffle arranged with reference to the passage of a current of air, so that oxygen may be freely supplied to the melted metal, or it maybe performed under the oxid- izing flame of the blow-pipe. The latter operation is often employed in blow-pipe analysis. A certain amount of the alloy is mixed with about four times its weight of pure lead, and then placed on the cupel and the oxidizing flame of the blow-pipe directed on it. The oxidizing process soon begins, and in about thirty minutes all the lead will be converted into litharge, * These may be obtained at the chemists' furnishing-shops ready for use. 176 DENTAL METALLURGY. which is fusible, and is readily absorbed into the porous substance of the cupel, carrying with it all the oxidizable metals that may be present. At this point, the button having parted with every trace of the latter, assumes an exceedingly bright appearance, technically called the "brightening of the button," thus offering a certain means of ascertaining when the process of cupellation is complete. Cupellation, under the oxidizing flame of the blow-pipe, for quan- titative discrimination, requires careful management, particularly when the silver has parted with the base metals and approaches a state of purity. For it is at this stage of the operation that the well-known prop- erty of melted silver, of absorbing oxygen from the atmosphere, and then parting with it as it approaches the point of solidification, may be observed. The giving-off of the absorbed oxygen is what causes " sputtering," by which minute globules of the metal are thrown off and lost, thus rendering the assay inaccurate. Besides the method of obtaining silver above de- scribed, the metal may be obtained by converting sulfids into chlorid, the latter being easily reduced to metallic silver by the wet method. The sulfid is also sometimes converted into sulfate, when the silver may be reduced from the solution by precipitation. Another method of separating silver from its ores consists in roasting the latter with common salt to convert the silver into chlorid, which is dissolved out of the mass by means of a strong solution of chlorid of sodium ; the silver is then recovered in the metallic state by precipitating with copper. Hyposulfite of soda has also been employed to dissolve out the chlo- SILVER. 177 rid of silver, the resulting solution being precipitated by sulfid of sodium, yielding sulfid of silver, which requires roasting to drive off the sulfur and liberate the metallic silver. Compounds of Silver. — There are three compounds of silver with oxygen : the suboxid, AgO ; the oxid, Ag,0 ; and the peroxid, which is thought to have the formula of Ag,0,. The oxid is the only one having any practical importance. Being the base contained in the salts of silver, it is obtained by adding caustic potassa or baryta-water to a solution of nitrate of silver. Silver nitrate (AgN0 3 ) is prepared by dissolving silver in nitric acid by the aid of gentle heat, after which it is evaporated to dryness or until it crystal- lizes. These crystals are colorless, transparent, and soluble in an equal weight of cold and in half the quantity of boiling water. They are also soluble in alcohol. Nitrate of silver is fusible, and when poured into cylindrical molds forms the lunar caustic employed by surgeons. At high temperatures (red heat) it is decomposed, yielding pure metallic silver. Silver sulfate (Ag,S0 4 ) is prepared by boiling metal- lic silver in sulfuric acid. Silver sulfid is remarkable for being so soft and malleable that medals may be struck from it. It may be formed as a black precipitate by the action of hy- drogen sulfid (H.,S) upon a solution of silver nitrate, or it may be formed by heating silver with sulfur in a covered crucible. It is the affinity existing between these two elements which renders the combination of silver and vulcanizable rubbers impracticable. Silver sulfid is not soluble in dilute sulfuric or hydrochloric 178 DENTAL METALLURGY. acid, but is readily dissolved by nitric acid. Metallic silver also dissolves sulfid of silver when melted with it. Silver chlorid (AgCl) is the form into which silver is commonly converted in separating it from other metals or from its ores. It is a white, curdy precipi- tate, and may be obtained from a solution of the ni- trate by the addition of sodium chlorid or hydrochloric acid. When freshly prepared it is perfectly white, but soon darkens, and eventually becomes quite black by exposure to solar light, parting with a portion of its chlorin, and becoming a subchlorid (Ag 2 Cl). Silver chlorid may also be formed by suspending a silver leaf in a glass vessel containing chlorin gas, and when thus prepared it is not blackened by exposure to light. Argentic chlorid is fusible at 500 F. A much higher heat converts it into vapor, but does not decompose it. It is soluble in ammonia. Discrimination. — The chlorids and hydrochloric acid precipitate white argentic chlorid, and so delicate is the test that when one part of silver is dissolved in 200,000 times its weight of water it may be readily detected by the opalescence which is imparted to the fluid by the precipitant. This precipitate is always changed to a violet-black by exposure to light, but the presence of mercury* will prevent discoloration. It is insoluble in nitric acid, but is readily soluble in ammonia, and may be fused to a horny mass without decomposition. Sulfuretted hydrogen added to a solution contain- ing silver throws down a black precipitate of silver sulfid, which is not soluble in dilute acids, alkalies, or potassic cyanid. Sulfuric acid at a temperature of * Mercurous chlorid. SILVER. 179 212 F. will, however, dissolve it, with separation of the sulfur. Ammonia or potassa, when employed as a precipi- tant, throws down a brown oxid, insoluble in the latter, but soluble in the former, and if freely exposed to air this solution will deposit fulminating silver. The blow-pipe is frequently used in the discrimina- tion of silver compounds, which when heated on char- coal with sodic carbonate yield a bright bead of metallic silver, often accompanied by a red-colored deposit on the charcoal. Quantitatively, the estimation of silver may be ac- complished either by the usual humid process or by assaying. The first consists in precipitating the metal as chlorid, which is to be separated and weighed. The precipitation is effected as follows : The silver solution is acidulated by nitric acid ; hydrochloric acid or sodic chlorid, slightly in excess, is then added ; but, as silver chlorid is to a certain extent soluble in either of these, an undue excess must be avoided The chlorid must now be carefully and repeatedly washed and filtered in a thoroughly dry filter, previously weighed. After the solution has passed through the filter, the latter with its contents is dried and weighed, and the weight, minus the weight of the filter, will be the quantity of silver chlorid present. The dry method or assaying process consists in forming an alloy of the silver with lead, and is espe- cially applicable to ores and the sweep of the dentist's laboratory. The specimen to be treated is heated with from twelve to thirty times its weight of pure granu- lated lead in a bone-ash cupel, which is placed in a muffle so arranged that a current of atmospheric air l8o DENTAL METALLURGY. may pass freely over the vessel and oxidize the lead. This oxid of lead, being quite fusible, combines with any base metal present and oxidizes it, uniting subse- quently with the oxid as a fusible slag, while the gold or silver will be held by the unoxidized portion of the lead. In the treatment of specimens of alloys, such as plate or coin, a quantity of the specimen is accu- rately weighed and mixed with from four to five times its weight of pure granulated lead. It is then placed in the cupel and exposed to heat, as above described, until all the lead is oxidized or converted into litharge, when the remaining button assumes the brilliant appearance of surface before alluded to, which denotes that the base metals or oxidizable constituents have been oxid- ized and taken up by the lead oxid. This button is then to be weighed by means of a delicate assay balance, and the loss of weight shows the proportion of alloy that was present. Pure Silver. — Pure silver, which is reckoned as iooo fine, may be obtained from standard or other grades of silver by dissolving them in nitric acid slightly diluted with water, the solution being much facilitated by exposure to gentle heat. If gold be associated with the alloy it will be found at the bottom of the vessel, in which case it will be necessary to use a syphon to remove the argentic nitrate solution. The silver is now to be precipitated in the form of chlorid by the addition of an excess of common salt. When all has subsided the liquid is carefully poured off, and the chlorid thoroughly washed to remove all traces of acid. The chlorid is then placed in water acidulated with hydrochloric acid (an ounce of chlorid requiring six to eight ounces of water) and pieces of clean SILVER. l8l wrought iron put in it, when a copious evolution of hydrogen follows, which, uniting with the chlorin of the argentic chlorid, liberates metallic silver. The latter should not be disturbed until the last particle of it is thus reduced, when it will be found to be a spongy- mass. The undissolved iron should now be carefully removed, the ferrous and ferric chlorid carefully de- canted, and the silver washed in hot water containing about one-tenth its bulk of hydrochloric acid. This is repeated several times, and finally the silver is again thoroughly washed with pure hot water. The silver, after drying, is then ready for melting, and if care has been observed in the process it will be found to be of a fineness of 999.7 parts in 1000, the 0.3 of impurity present being due to traces of iron. The chlorids may be acidulated with sulfuric acid, and reduced with zinc instead of iron. Another method of precipitating silver in the metallic form consists in placing a sheet of copper in a solution of argentic nitrate. The metal is thrown down in a crystalline form. Silver thus obtained is never free from traces of copper. Pure silver can only be obtained from samples of a lower grade by fusing the pure chlorid with sodic carbonate. The reaction is shown in the equation : 2AgCl+Na,C0 3 =Ag 2 +2NaCl+0+C0 2 . Owing to the copious evolution of carbonic acid gas which takes place during the decomposition, some of the silver may be thrown from the crucible, and loss may occur by the absorption by the crucible of some of the fused chlorid. To avoid this the sides of the vessel should be coated with a hot saturated solution of borax. 152 DENTAL METALLURGY. A composition of ioo parts of argentic chlorid, 70.4 of calcic carbonate (chalk), and 4.2 of charcoal, has been recommended as a means of obtaining pure sil- ver. This mixture is heated to dull redness for thirty minutes, and then raised to full redness ; carbonic acid and carbonic oxid are given off ; the calcic chlorid is converted into calcic oxychlorid, underneath which, in the bottom of the crucible, will be found the button of pure silver. Alloys of Silver. — In consequence of its softness, silver in the pure state is liable to considerable loss by attrition. For all useful purposes, however, the requi- site amount of hardness may be conferred upon it by the addition of a small proportion of copper. Thus, silver for coinage and manufacturing purposes usually contains in 1000 parts from 900 to 925 of silver and from 75 to 100 of copper. The term "standard sil- ver" refers to the metal thus alloyed with copper, that of the United States coinage being silver 900 parts, copper 100. Previous to the introduction of vulcanized rubber as a base for artificial dentures, standard silver was much employed in the United States for temporary dentures, when cheapness was an important consideration. In England a much more durable alloy is used, in which the alloying metal is platinum, in the proportion of from three to ten grains of the latter to each penny- weight of silver. The advantages possessed by this alloy over ordinary standard silver may be summed up as follows : It resists wear better, and not even a suspicion can be reasonably entertained of any ill effects occurring, either locally or to the general sys- tem, from its presence in the mouth. It permits of SILVER. 183 the employment of a higher grade of solder, and it is a much more rigid alloy than ordinary standard or coin silver. Hence it makes a stronger artificial den- ture, which is less likely to have its adaptation im- paired by bending. But, while silver is improved in some respects when platinum is the sole alloying com- ponent, it must not be supposed that its affinity for sulfur is thus materially lessened, or that its tendency to blacken when brought into contact with that ele- ment or its compounds is obviated. Indeed, it may be stated that platinum added to silver in such small quantities does not wholly protect the latter from the action of its ordinary solvents. Such an alloy of sil- ver, for instance, would not only be readily dissolved by nitric acid, but the platinum also, though unaffected ordinarily by that menstruum, would readily yield to it when combined with silver. This alloy of silver, which is known in England as "dental alloy," often contains from twenty-five to thirty per cent, of platinum. To separate the latter metal from the silver the alloy is dissolved in nitric acid, which on the addition of heat will dissolve all of the silver with about ten per cent, of the platinum. On introducing a bar of copper into this solution, the silver and platinum are quickly precipitated in a metallic state. This precipitate is again placed in nitric acid, which redissolves the silver, leaving the platinum untouched, which, however, may be dissolved with the other fifteen or twenty per cent, of the plat- inum left at first, in aqua regia. Precipitating by an excess of ammonium chlorid, evaporating to dry- ness, and igniting, yields pure platinum. The silver may be recovered in the usual way by precipitation in 184 DENTAL METALLURGY. the form of a chlorid, which may be easily reduced to a metallic state by treating with a plate of zinc in acidu- lated water. It is a somewhat common belief that the putting together of silver and platinum in the formation of an alloy of this kind, owing to the infusibility of platinum and the wide difference in the fusing-points of the two, is a matter of great difficulty. It should be borne in mind, however, that between the metals more or less affinity exists, especially at high temperatures ; hence it is only necessary to introduce the platinum, rolled into thin ribbons, into the crucible containing the sil- ver in a state of complete fusion, and the platinum will be observed to quickly fuse and mix with the other metal. It is sometimes thought advisable to add larger proportions of platinum than the quantity here given. This may be done by adding the platinum until the alloy becomes infusible, and this result will be attained as soon as sufficient platinum is added to raise the fusing-point of the alloy above the capacity of the ordinary melting apparatus. Von Eckart's alloy, employed to some extent in France as a base for artificial dentures, is composed of the following proportions : silver, 3.53 ; platinum, 2.40; and copper, 11. 71. It is very elastic (which property it does not lose by annealing), and can be highly polished. Silver Solders. — When the plate to be united con- sists of pure silver alloyed with platinum, the solder may be formed of the standard metal (coin), with the addition of from one-tenth to one-sixth its weight of zinc, according to the proportion of platinum contained in the alloy. Silver solders are, however, generally 66 parts. 30 " 10 " 6 dwts. 2 " 1 dwt. 5^ dwts. 40 grs. SILVER. 185 composed of silver, copper, and zinc, or silver and brass, in variable proportions, of which the following are examples : No. 1.* Silver Copper Zinc No. 2.f Silver Copper Brass No. 3. Silver Brass wire In putting together the constituents of silver solders, the affinity for oxygen manifested by zinc, brass, and copper, when exposed to high temperatures, should be remembered, and in order to guard against loss the mode of procedure should be as follows : The silver, placed in a clean crucible, with a sufficient quantity of borax to cover it, should be thoroughly fused, and, without permitting it to cool in the least, the zinc, brass, or copper, as the case may be, should be quickly added. Before pouring, it should be shaken or agitated to insure admixture. When cool, it may be removed from the ingot-mold and rolled into plate of, say No. 27 of the standard gauge. The surface of standard silver may be whitened by being heated and immersed in dilute sulfuric acid. It is in this way that frosted silver is produced. The acid, dissolving the oxid of silver from the surface, leaves a quite pure superficial film. * Richardson's Treatise on Mechanical Dentistry. t Ibid. 13 186 DENTAL METALLURGY. Silver may be deposited upon the surface of another metal by connecting the article to be silvered with the negative (zinc) pole of the galvanic battery, and then immersing it in a solution made by dissolving cyanid of silver in a solution of cyanid of potassium. The current decomposes the argentic cyanid, and the metal is deposited upon the object connected with the negative pole. During this decomposition the cyano- gen liberated at the positive (copper or platinum) pole acts upon a silver plate with which this pole is con- nected, the quantity of silver dissolved at this pole being precisely equal to that deposited at the opposite pole ; the silvering solution is always maintained at the same strength. CHAPTER X. PLATINUM. Atomic Weight, 197.6. Symbol, Pt. PLATINUM is found in nature in flattened grains of varying sizes, more or less alloyed with pal- ladium, rhodium, ruthenium, davyum, and irid- ium.* It occurs in Brazil, Peru, Australia, and Cal- ifornia. Russia, however, furnishes the largest supply of platinum, from the Ural Mountains. It was dis- covered in 1736 by Anton Ulloa, at Choco, in South America ; but, in consequence of its infusibility and unworkable nature, no use was made of it, and its presence in mining products was considered a hind- rance. Dr. Wollaston devised the first practical pro- cess of working it, and in 1859 Deville and Debray published improved methods of fusing large quantities of platinum. Wollaston' s method, which consists of a series of chemical and mechanical processes of a rather com- plicated nature, may be thus described : The ore is first heated with nitric acid to dissolve any copper, lead, iron, or silver. It is then washed and heated with hydrochloric acid to remove any magnetic iron ore that may be present ; after which the ore is to be treated with nitro-hydrochloric acid diluted with an equal bulk of water to prevent the iridium, which is gen- * A group of rare metals only found in platinum ores, and known as the " platinum metals." 187 1 88 DENTAL METALLURGY. erally present, from being dissolved. The propor- tions of acids are one hundred and fifty parts of hydrochloric to forty parts of nitric. Three or four days' digestion, aided by gentle heat, is necessary to complete solution. The suspended matter, generally consisting of iridium, is allowed to subside, when the solution may be syphoned off. Ammonic chlorid* is next added as a precipitant, and throws down the yellow crystalline ammonio- platinic chlorid, which is readily decomposable by heat, yielding platinum in a finely-divided state. The liquid from which the precipitate is obtained will still be found to contain about eleven parts of platinum, together with all the associated metals. These are all thrown down by means of a plate of zinc, and washed carefully and again dissolved in nitro-hydrochloric acid. A small quantity of strong hydrochloric acid is added to avoid precipitation of lead or palladium, when precipitation of the remaining platinum may be again effected by ammonic chlorid. This precipitate will require careful washing in cold water to remove iridium, which during the process forms a double salt with the ammonic chlorid. The next stage in the operation consists in separat- ing the metal from the ammonia salt by ignition, and, as it is important to the success of the subsequent working that the precipitate shall remain in a finely- divided state, too high a degree of heat must be avoided, as otherwise cohesion of the particles will take place. Ignition is generally accomplished by the fol- lowing means : The precipitate is heated in a graphite crucible until nothing remains but the finely-divided * About forty parts. PLATINUM. 189 platinum. This is powdered, should it be found some- what lumpy, in a wooden mortar with a wooden pestle, sifted through a fine lawn sieve, and mixed with water to the consistence of a stiff paste. This is placed in a brass mold with a slightly tapering cylindrical cavity about seven inches in length, provided with a loosely- fitting steel stopper, which enters to the depth of a quarter of an inch. The mold is first oiled and set up in a vessel of water. The platinum mud is then intro- duced, and as it settles into the water air is displaced, and the platinum is thus made to fill every part of the mold. The water is allowed to drain, and its removal may be aided by pressure. Ultimately, however, the mold is placed in a press worked by a powerful lever, by which the mass sustains an enormous pressure, after which the plug and the column of platinum are removed by gently tapping the mold. It is then heated in a charcoal fire, in order to thoroughly dry it and to burn off any adherent oil. The next step, which depends upon the quality of welding possessed by platinum, consists in heating the porous cylinder in a blast-furnace to white heat, when it is removed, set upright on an anvil, and hammered on the ends in order to weld the particles ; after which it is coated with a mixture of borax and carbonate of potash, and again heated for the purpose of removing traces of iron, which is dissolved by the mixture, the latter being removed by immersion in dilute sulfuric acid. The bar of platinum is now ready for use, and may be rolled or hammered. It may readily be surmised that so imperfect a means of obtaining a solid bar of metal as the latter part of the operation just described cannot always be relied I90 DENTAL METALLURGY. upon for the production of a uniform and solid speci- men ; and, indeed, platinum prepared in this way, though of great purity, is liable to blister upon its surface, this being probably due to minute globules of air incased in the body of the ingot during the forging, which, during the conversion of the ingot into plate by means of rollers, are elongated and spread out in the form of blisters. The dry metallurgic operations of Deville and De- bray consist in heating in a reverberatory furnace about two-hundred-weight of platinum ore with an equal weight of galena (sulfid of lead). When the ore is sufficiently heated (to bright redness), portions of the galena are added and mixed with the ore by constant stirring. An equal quantity of litharge is next added, in order to supply oxygen to the sulfur of the lead ore, which passes off as sulfurous anhydrid, reducing all of the lead which combines with the platinum. After remaining in a state of fusion for a short time the upper portion is ladled off, and will be found to consist of an alloy of lead, platinum, and smaller portions of palla- dium and silver, the latter being introduced from the galena, which always contains more or less silver. The heavier metals of the platinum group, by their greater density, subside to the bottom. Cupellation is now resorted to in order to separate the platinum from the lead. This consists of two distinct operations. The first is performed at the ordinary furnace-temperature, and is continued until by loss of lead the fusing-point of the remaining alloy rises to such an extent that a state of fusion can no longer be maintained. The second and final opera- tion is performed in an apparatus which serves the PLATINUM. 191 purpose of both furnace and cupel. It is formed of blocks of thoroughly burned lime. In form it may be described as a sort of basin or concavity with a similar piece for a cover. The lower part is intended for the reception of the metal ; through the center of the upper portion or cover pass the tubes for the oxyhy- drogen jet, while the lower portion is provided with a lip or spout for pouring the melted metal. The tubes which pass through the top for the transmission of the two gases are generally formed of copper, with plat- inum tips. The outer and lower tube carries hydro- gen, while the inner and upper one carries a jet of oxygen, into the middle of the flame. The tubes are furnished with stop-cocks, so that the supply may be regulated. When the object is merely to fuse some scraps of platinum, the lime-furnace is first put to- gether, the hydrogen jet is lighted, oxygen is then turned on, and the interior of the apparatus soon becomes heated. The platinum is then introduced in pieces through a small hole at the side, and quickly fuses after entering the furnace. When used as a cupel the lime absorbs the impuri- ties, and the platinum is kept in a state of fusion until all the lead is oxidized, when the metal may be poured from the lime-cupel into an ingot- mold formed of coke or plates of lime. Some difficulty may be experienced at the moment of pouring, in consequence of the dazzling white surface of the molten metal. From seven to eight pounds may be melted in this way in from forty to sixty minutes. Although such metals as palladium, osmium, gold, silver, and lead are volatilized at the intense heat used, it has been found that platinum obtained by the Deville- 192 DENTAL METALLURGY. Debray method is not as pure as that obtained by Wollaston's plan. Properties. — Platinum is somewhat whiter than iron. It is exceedingly infusible, requiring the flame of the Fig. 19. ■=5*=^ compound blow-pipe (oxyhydrogen) to render it fluid. In both the hot and cold states it is exceedingly mal- leable and ductile.* It is the heaviest substance in nature, its specific gravity being 21.5, and it is ex- * Wollaston, in endeavoring to substitute platinum for the spider's web usually employed in micrometers, made platinum wire finer than had hitherto been obtained. This was accomplished by forming a coating of silver upon a platinum wire, and then passing it through the draw-plate, after which he dissolved the silver, leaving the platinum the I of an inch in diameter, a mile of which, notwithstanding the high specific gravity of the metal, would only weigh a single grain. PLATINUM. 193 ceeded in tenacity only by iron and copper. No single acid attacks it, and it is unaffected by air or moisture at any temperature. It is therefore of great value in the construction of chemical vessels. At bright-red heat platinum welds quite readily, and injured vessels may be repaired in this way. In the finely-divided state, as obtained by Wollaston's pro- cess, it may be made into small vessels by pressing the pulverulent metal into suitable molds, heating and hammering to complete the welding of the particles. Platinum possesses the remarkable property of in- ducing chemical combination between oxygen and other gases. Even in the compact condition it pos- sesses this quality, as demonstrated by the familiar experiment of suspending a coil of platinum wire in the flame of a spirit-lamp, and suddenly extinguishing the flame as soon as the metal becomes entirely heated, wnen, by inducing the combination of the vapor of the spirit with oxygen, the wire will continue to glow. An instantaneous light apparatus has been made in which a jet of hydrogen is thrown upon a ball of spongy platinum ; the latter induces combination be- tween the oxygen condensed between its pores and the spirit-vapor, and ignition takes place. Platinum-black, in which the metal exists in an ex- ceedingly fine state of division, possesses this power of promoting combination of oxygen with other gases to the highest degree. In this form it is capable of absorbing eight Hundred times its volume of oxygen. No combination, however, takes place between the two, the gas being merely condensed within the pores of the metal ready for combination with other bodies ; hence, if a jet of hydrogen be thrown upon a small 194 DENTAL METALLURGY. lump of this powder, ignition ensues. During the operation of melting, platinum absorbs oxygen and gives it off in cooling, ''sputtering," as silver does under like conditions. The proper solvent for platinum is nitro-hydrochloric acid, the chlorin evolved being the active agent. Alloyed with silver, however, platinum will be dis- solved in nitric acid, and when platinum is found in gold as an alloy it may be separated by quartation with silver. Alloys. — Equal weights of platinum and gold afford a malleable alloy ; the brilliancy of appearance char- acteristic of gold is, however, much lessened by the admixture. The two metals, combined in the propor- tions of i part of platinum to 9.5 of gold, form an alloy of the same density as platinum. An excess of platinum with gold yields an alloy which is infusible at ordinary furnace -heat The tenacity of gold is very greatly increased by admixture of platinum, while at the same time it is rendered more elastic. Platinum and silver may be combined in all propor- tions, constituting alloys of greater hardness than either of their constituents, while the color is between the color of silver and that of platinum. Hot sulfuric acid will dissolve the silver from an alloy of this kind, and when one part of platinum is alloyed with ten parts of silver both metals may be dissolved by nitric acid. Platinum and mercury do not amalgamate readily, and combination can only be effected by rubbing finely- divided platinum, such as is reduced from the ammonio-chlorid, in a heated mortar with mercury PLATINUM. 195 moistened with water acidulated with acetic acid. By this means an unctuous amalgam is obtained, which has been employed in platinizing metallic objects in a manner similar to that known as fire-gilding. The use of platinum as a constituent in alloys for dental amalgams has been almost entirely abandoned. The author found, as the result of a large number of experiments, that it rendered the alloy very brittle ; and, while its presence seemed to retard amalgama- tion, it increased the capacity of the alloy for mercury (see page 54). Iridium confers upon platinum great hardness and tenacity ; indeed, the alloy resulting from this combi- nation is so rigid that it is with the greatest difficulty it can be swaged into plates. It is, nevertheless, an alloy of great value to the mechanical dentist, as it affords a means of obtaining greater strength in arti- ficial dentures of the "continuous-gum" class, and it has been used in the author's laboratory since 1870 in connection with vulcanized rubber, the plate being constructed of iridio-platinum, with the teeth, single or in sections, attached by means of rubber. The swaging requires the use of the zinc counter-die, and when the ridge is very prominent it is best not to attempt to carry the plate entirely over it, rather allow- ing the rubber to take its place. An artificial denture constructed in this way has no superior in point of strength and durability. Nothing but pure gold should be used as a solder in uniting two pieces of platinum or iridio-platinum. Indeed, so feeble is the union between the latter and an ordinary gold solder that two pieces united by its agency may be readily torn apart with the pliers. The I96 DENTAL METALLURGY. addition of iridium is also of value in the construction of platinum vessels for experimental laboratory use, as the metal is thereby rendered more resistant to high temperatures, and less susceptible to the action of chemicals. Platinum combines with tin in all proportions, and the resulting alloy is hard, brittle, and more or less fusible. Between the platinoid metals and tin, at the moment of fusing together, phenomena very suggest- ive of true chemical union have been observed, and if tin and platinum foils be rolled together and heated under the blow-pipe, combination takes place explo- sively. It is in consequence of this affinity that two such metals, one of which is infusible at ordinary furnace-temperature, while the other is readily fusible at a low degree of heat, may, with the greatest facility, be melted together to form an alloy.* Oxids. — Platinum unites with oxygen to form two compounds, — the monoxid or platinous oxid (PtO), and the dioxid or platinic oxid (Pt0 2 ). The first is obtained as a black powder by digesting the dichlorid with caustic potash. The second (PtO a ) may be pre- pared by adding barium nitrate to a solution of platinic sulfate. Barium sulfate and platinic nitrate are thus formed, and from the latter caustic soda precipitates one-half of the platinum as platinic hydrate, a bulky brown powder, which, when gently heated, becomes black and anhydrous. It is also formed when platinic chlorid is boiled with an excess of caustic soda, and acetic acid added. It combines with bases and dis- solves in acids. Platinic oxid with ammonia forms * See chapter on "Alloys." PLATINUM. 197 an explosive compound, which detonates violently at about 400 F. Both oxids of platinum are reduced to the metallic state by heating to redness. Platinic chlorid (PtCl 4 ) is the most useful salt of the metal, and is the one from which all the platinum com- pounds are obtained. It may be prepared by dis- solving scraps of platinum in a mixture of four meas- ures of hydrochloric acid with one of nitric acid, one hundred grains of platinum requiring the presence of two ounces of hydrochloric acid. After complete solution the liquid is evaporated at a gentle heat to a syrupy consistence, redissolved in hydrochloric acid, and again evaporated to expel excess of nitric acid. The syrup-like fluid solidifies on cooling to a red- brown mass, which is deliquescent and readily dissolves in water or alcohol. Spongy platinum is prepared by heating the yellow crystalline precipitate obtained by the addition of ammonic chlorid. Platinous chlorid (PtCl 2 ) may be formed by heating platinic chlorid to a point somewhat above 450 F. A very high temperature reduces it to the metallic state. Sulfids. — The compounds PtS and PtS, are pro- duced by the action of hydrogen sulfid, or the hydro- sulfid of an alkali metal, on the dichlorid and tetra- chlorid of platinum respectively. They are both black, insoluble substances. Discrimination of Platinum Salts. — 1st. A blackish- brown precipitate, insoluble in nitric or hydrochloric acid singly, will be thrown down by the addition of hydrogen sulfid (H,S). 2d. Ammonia or potash throws down a yellow crystalline precipitate. 198 DENTAL METALLURGY. 3d. A brown hydrated platinic oxid is precipitated from the salts of platinum by the addition of soda, and it should be remembered that the precipitate is soluble in an excess of the soda. 4th. A deep-brown color is imparted to solutions of platinum salts by the addition of stannous chlorid, but no precipitate is obtained. Quantitatively, platinum may be separated from other metals with which it is likely to be associated by precipitating with ammonium chlorid. This is added to the platinum solution, followed by a little alcohol. The precipitate is collected, washed with alcohol, and dried, when it is ready for weighing. Every 100 parts will contain 44.28 of platinum. CHAPTER XI. IRIDIUM. Atomic Weight, 198. Symbol, Ir. IRIDIUM, named from Iris, the rainbow, because of the varied colors of its compounds, has already been mentioned as occurring in the insoluble alloy from the platinum ores, and disseminated in small, hard points throughout the substance of California gold. It is also obtained when crude platinum is dis- solved in nitro-muriatic acid. A gray, scaly, metallic substance is found at the bottom of the vessel, which has entirely resisted the action of the acid. This is osmiridium, a native alloy of iridium and osmium. The subsequent treatment for the separation of the two metals consists in reducing them to powder, com- bining them with an equal weight of dry sodium chlorid, and heating to redness in a glass tube, through which moist chlorin gas is allowed to pass. The tube is connected with a receiver containing a solution of ammonia. The gas is quickly absorbed, and iridium chlorid and osmium chlorid are formed. The first remains combined with the sodium chlorid, while the osmium chlorid, being a volatile substance, passes into the receiver. The contents of the tube, consisting of iridium and sodium chlorids, when cold, are dissolved with water, and mixed with an excess of sodium car- bonate and evaporated to dryness. After ignition in a crucible, boiling in water, and drying, the metal may be reduced by hydrogen (at a high temperature), and 199 200 DENTAL METALLURGY. heating- successively with water and strong hydro- chloric acid to free it of the alkali and iron. What remains consists of metallic iridium in a finely- divided state. Properties. — Iridium is an exceedingly hard and brittle metal, nearly white in color, and fusible only by the oxyhydrogen blow-pipe, by which means it has been converted into a white mass having some- what the appearance of polished steel. It is hard^ and brittle while cold, but it is rendered somewhat malleable at red heat. Its density is about the same as that of platinum. It has been obtained in tolerably compact masses by compressing some of the metal in a very fine state and then heating to the highest point attainable in a forge-fire. The density of a sample of iridium prepared in this way will not exceed 16.0. When reduced by hydrogen at low temperatures it dissolves in nitro-hydrochloric acid, but is rendered insoluble in all acids by exposure to white heat. It may again be dissolved by igniting it with a mixture of the chlorid of potassium and sodium in a current of chlorin. Mr. John Holland, of Cincinnati, devised an in- genious process for the preparation of larger pieces of iridium than are generally found in nature. Some of the ore is heated to whiteness with phosphorus in a Hessian crucible, by which means complete fusion is obtained. Phosphor-iridium, as this compound may be called, is as hard as the iridosmine from which it is * Its hardness is such that the hardest file will make no impression on it. In working California gold, which often contains disseminated through it small grains of the native alloy of osmium and iridium, the file is sensi- bly injured by contact with it, while in coining operations much incon- venience and injury are caused by its presence. IRIDIUM. 20I prepared, and is used for making points for the Mackinnon stylographic pen. Alloys. — Platinum containing a small quantity of iridium is rendered more rigid, and is of great value as a means of strengthening continuous-gum work, by forming the backing of it, especially in partial lower sets, where, in addition to the two pieces of pure platinum covering the gums back of the natural (front) teeth, an extra piece of platinum alloyed with iridium, No. 26, may be used. It also answers well in combination with vulcanizable rubber, * for either entire or partial cases, and though decidedly the most refractory of the various alloys employed in the dental laboratory, it may be perfectly swaged by the use of a zinc counter-die. In the construction of partial cases it may be employed for clasps, but wherever it is necessary to unite two pieces by soldering nothing but pure gold should be employed as the solder. Ordinary gold solders do not afford a strong union. Iridium unites with oxygen, sulfur, chlorin, and iodin, to form oxids, sulfids, chlorids, and iodids. DiscriminaMon. — With ammonium or potassium chlorid, iridium solutions afford a dark reddish- brown crystalline precipitate of ammonium or chlor- idium, the color of which, and the fact that it is reducible to soluble chlor-iridium when treated with hydrogen sulfid, distinguishes it from the correspond- ing platinum precipitate. * See chapter on " Platinum." 14 CHAPTER XII. PALLADIUM. Atomic Weight, 106.5. Symbol, Pd. PALLADIUM, rhodium, iridium, ruthenium, os- mium, and davyum* constitute a group of metals which possess many properties in common. They are also closely allied by natural association. Crude platinum is a native alloy of platinum with these metals, and is the source whence they are usu- ally obtained. f Palladium is, however, occasionally found native, in a comparatively pure state, intermixed with platinum, from which metal it may be readily distinguished by the fibrous appearance of its grains. It is obtained at the present time chiefly from the solution of crude platinum, after that metal has been separated by precipitation with ammonic chlorid. The remaining liquid is neutralized by sodium car- bonate, and mixed with a solution of mercuric cyanid ; palladium cyanid separates as a whitish, insoluble substance, which, on being washed, dried, and heated * This metal, named in honor of Sir Humphry Davy, was discovered by Kern in 1877, m platiniferous sand. It was obtained from the mother liquors after the separation of platinum, palladium, osmium, and iridium, by heating them with an excess of ammonium chlorid and nitrate. A deep-red precipitate was obtained, which after calcination at a red heat left a grayish mass resembling platinum sponge. This, when fused by the oxyhydrogen blow-pipe, furnished a silver-white metal of great hardness, though malleable at a red heat, having a density of 9.39; soluble in aqua regia, but only slightly acted upon by boiling sulfuric acid. f Palladium was, at one time, obtained in considerable quantities from Brazilian gold, with which it was associated as an alloy, but this means of supply having failed, it has become too expensive for employment in the industrial arts. 202 PALLADIUM. 203 to redness, yields metallic palladium in a spongy state, which admits of welding into a solid mass in the same manner as platinum. In appearance palladium resembles an alloy of platinum and gold wherein the proportion of the former greatly exceeds that of the latter. It possesses the qualities of malleability and ductility, but in those properties it is probably inferior to platinum. Its density differs very much from platinum, being only 11.8. It is more oxidizable than platinum, and when heated to redness in the air, especially when in a finely-divided state, it acquires a superficial film of oxid of a bluish color, which may be again reduced at a high temperature. It does not, however, oxidize in the air unless its temperature is raised to red heat, and on account of its unalterability in the air and its bright silver-white color, which is not affected by ex- posure to sulfuretted hydrogen, it is used for prepar- ing the graduated surfaces of astronomical instruments and for coating silver goods. It is the most fusible of the platinum metals, and requires about the heat needed to fuse malleable iron to reduce it to a state of fluidity, in which condition it absorbs oxygen, parting with it again in cooling in the same way that silver does. It is dissolved by nitric acid, but its best sol- vent is nitro-hydrochloric acid. It is attacked by iodin, and may be distinguished from platinum by heating a drop of tincture of iodin upon it, when it will show a stain, while platinum similarly treated is quite unaffected. Alloys. — In a finely-divided state it unites readily with mercury, and there would appear to be some chemical affinity between the two, the union being 204 DENTAL METALLURGY. accompanied by evolution of heat. As the amalgam cools it sets and becomes tolerably hard, and it is stated that it expands in hardening.* Palladium Precipitate. — The form in which t he metal is generally used in amalgams seems to lose its affinity for mercury after long exposure to the air, and combination can only be effected by the assistance of gentle heat. Apparatus for determining the expan- sion or contraction of amalgams has been employed to test the expansibility of palladium amalgams. One of these, consisting of a trough having a movable end with a screw micrometer capable of noting the most minute change, indicated in one specimen of palladium amalgam an expansion of i in 25f of its diameter, though the amount of solid matter in this class of amalgams is very small, as it requires three parts of mercury to one of precipitated palladium. Mr. Coleman J states that palladium amalgam sets very rapidly, and when mixed in large enough quan- tities to fill good-sized cavities all the phenomena of true chemical affinity are observed. He also gives the following description of the manner of mixing and applying the amalgam : ' 'About as much mercury as would fill the cavity to be treated is placed in the palm of the hand, and the palladium powder very gradually added. It requires some careful rubbing with the fore- finger before the two become incorporated, when it should be divided into smallish pellets, and these rapidly carried, one after another, to the cavity, each piece * Hitchcock on Dental Amalgams, page 33, Transactions of the New York Odontological Society, 1874. f British Journal of Dental Science, Vol. IX, Part II, July, 1888. \ Manual of Dental Surgery and Pathology, page 129. PALLADIUM. 205 being - well compressed, and rubbed into the irregu- larities of its walls with a burnisher or compressing instrument." Mr. Coleman further states that this is probably the most durable of all the amalgams, but the most difficult to manipulate. Its surface changes to a black color, but, as a rule, it does not stain the structure of the tooth. Palladium has been used as a constituent in dental alloys (amalgams), and when added to a gold, silver, and tin alloy it probably has about the same effect as does platinum. It has been observed, however, that dental alloys in which it is a constituent blacken to a greater extent than when it is omitted. Mr. Fletcher, * after a series of experiments, finally abandoned its use in dental amalgams. Silver and palladium unite in all proportions, form- ing alloys which retain an exceedingly brilliant sur- face. Gold and palladium, in equal proportions, form a hard, gray alloy. Indeed, all the alloys formed of these two metals are exceedingly hard. Palladium and platinum form a hard alloy, which fuses below the melting-point of palladium. Palladium combines with antimony, bismuth, zinc, tin, iron, and lead, forming brittle alloys. With nickel it forms malleable alloys which are susceptible of high polish. It is quite probable that palladium, either alone or alloyed with other metals, might be advantageously employed in prosthetic dentistry, but its high price practically excludes it from the dentist's laboratory. Palladium enters into combination with chlorin *See chapter on "Amalgams." 206 DENTAL METALLURGY. (PdCL), oxygen (PdO and Pd0 2 ), and sulfur (PdS). When heated to redness in a stream of hydrogen gas, it possesses the remarkable power of absorbing about 643 times its volume of the gas, whereby its specific gravity is reduced to 11.06. The absorption of the gas is attended with evolution of heat, and this fact, together with the supposed constancy of com- position of the hydrogenized palladium, led Graham to suppose that the product was a definite chemical compound, to which he assigned the formula PdH 2 . Troost and Hautefeuille, however, found the correct composition to be Pd 2 H * Discrimination. — Mercuric cyanid is the chief test for palladium. f It precipitates a yellowish-white cyanid, soluble in hydrochloric acid, which is easily reduced by heat to metallic palladium, when it is ready for weighing. * Watt's Dictionary of Chemistry. f There are other reagents employed in the discrimination of palladium, — sulfuretted hydrogen, potash, ammonia and its carbonates, ferrous sulfate, and stannous chlorid. Mercuric cyanid, however, is the one generally employed in the quantitative estimation of palladium. CHAPTER XIII. IRON. Atomic Weight, 56. Symbol, Fe (Ferrum). IRON is present in nearly all forms of rock, clay, sand, and earth. It is the most widely diffused of the natural coloring ingredients, and its presence may be readily distinguished by the color which it imparts. It is found in varying proportions in plants and the bodies of animals, the blood of the latter con- taining about 0.5 per cent, of iron associated with its coloring-matter. Iron seems to have been known very early in the world's history, and at a remote period instruments of agriculture and war were manufactured of it.'* Its chief ores are oxids, carbonate, and sulfids. Metallic iront is met with in nature in the meteorites or metallic masses of unknown origin which occasionally fall to the earth. J The carbonates and oxids are the ores from which iron is chiefly obtained. Their reduction — the oxids especially — is exceedingly simple, and consists in merely heating them in contact with carbonaceous compounds, by which means the metal is liberated. * Archaeologists distinguish a Bronze Age in prehistoric times inter- mediate between those of Stone and Iron. I Metallic iron, though of exceedingly rare occurrence, has been found at Canaan, in Connecticut, forming a vein about two inches thick, in mica slate. I Isolated masses of soft iron, sometimes of large dimensions, have been found upon the surface of the earth in South America ; they are supposed to have had a similar origin. 207 208 DENTAL METALLURGY. Properties. — Pure iron is white in color, extremely- soft and tough, and has a specific gravity of 7. 8. Iron may be regarded as possessing a greater number of valuable qualities than any other metal ; hence it occupies the highest place in the useful arts. Although possessing nearly twice as much strength as the strongest of the other metals, it is yet one of the lightest, and is therefore peculiarly fitted for use in the construction of bridges, ships, etc. It is rendered so ductile by heating that it may be rolled into very thin sheets or drawn into the finest wire ; and yet at ordinary temperatures it is the least yielding of the metals in common use, and may always be relied upon to afford a rigid support. An iron wire one-tenth of an inch in diameter is capable of sustaining seven hun- dred and five pounds. It is very difficult of fusion, and before becoming liquid passes through a soft or pasty condition. Pieces of iron pressed or hammered while in this state cohere or weld together. The fusing-point of iron has been estimated at 2900 F. It is soluble in nitric, dilute sulfuric, and hydro- chloric acids, but is not much affected by strong sul- furic. Chlorin, iodin, and bromin attack it readily. Under certain circumstances it is not acted upon by strong nitric acid. If a piece of platinum wire be kept in contact with it, it will remain in this acid for many weeks without being acted upon. Its crystalline form is supposed to be a cube. When rolled into bars or drawn into wire it possesses a fibrous texture, upon the perfection of which much of its strength and value depend. It is the most tenacious of all the metals. At red heat iron decomposes water evolving hydro- gen, and is changed into the black oxid. It is a IRON. 209 strongly magnetic metal, but loses this quality when heated to redness. Iron does not oxidize in dry air at ordinary tem- peratures, and it may be immersed in water from which the air has been carefully excluded without change. Contact with a more electro-positive metal will also prevent oxidation. Thus, fine steel instru- ments are sometimes packed for exportation by wrap- ping in thin sheet zinc. For a description of the compounds of iron and the reagents employed in its discrimination, the student is referred to Fownes's " Elementary Chemistry." The value of iron does not depend alone upon its physical properties, for it enters into a large number of compounds which are of great use in the arts, and its chemical relation to carbon is such that the addi- tion of a small quantity of that element converts it into steel, harder and more elastic than iron, while a larger quantity of carbon produces cast iron, which is so fusible that many useful articles may be made of it by casting. Steel. — Herodotus states that among the most precious gifts presented by the Indian monarch Porus to Alexander the Great was a pound of steel, the value of which at that time has been estimated at about two hundred dollars. At a later period the manufacture of steel in its application to warlike instruments was carried to a great state of perfection in India and in the south of Europe. Steel differs from iron in possessing the property of becoming very hard and brittle, if, when heated to bright redness, it is suddenly cooled by being plunged into water. Steel is simply iron chemically combined 2IO DENTAL METALLURGY. with the precise amount of carbon which will produce the condition referred to, together with additional toughness. It does not, however, become decidedly steel-like until the carbon amounts to 0.3 per cent. The hardest steel contains about 1.2 per cent, of car- bon, and when that proportion is exceeded it begins to assume the properties of cast iron. There are several processes by which steel may be produced. Bars of iron imbedded in charcoal powder in a suitable crucible or chest made of some substance capable of resisting the fire are, after several hours' exposure to heat, converted into steel, the iron taking up the requisite amount of carbon. The product of this operation is called blistered steel, and is far from uniform, either in composition or texture, as portions of the bars thus produced will be found to contain more carbon than others, and the interior to be more or less porous. For the purpose of improving its quality the bars are cut into short lengths, made up into bundles, heated to the welding-point, and placed under a powerful tilt-hammer, which consolidates each bundle into one mass. This is called shear steel. Fusing and casting steel is another process for the treatment of the blistered form, by which is produced the best and most homogeneous variety. It consists in fusing about thirty pounds of broken fragments of blistered steel in a plumbago crucible, the surface being protected from oxidation by glass melted upon it. When perfectly fluid the steel is cast into ingots, and when it is desirable to form a very large ingot, several crucibles are simultaneously emptied into the same mold. Cast steel is superior in density and hardness to shear steel, and is the form best adapted IRON. 211 to the manufacture of fine cutting-instruments. It is, however, somewhat brittle at red heat, and much care and skill are required in forging it. The addition to it while fused of one part of a mixture of charcoal and oxid of manganese affords a fine-grained steel, which may be cast into a bar of wrought iron in the ingot-mold, in order that the tenacity of the iron may be an offset to the brittleness of the steel when forged together, while it affords an economical compound in the manufacture of cutting-implements, the iron form- ing the back and the steel the edge of the instrument. Bessemer* steel is produced by forcing atmospheric air into melted cast iron. The carbon, which is oxidized more readily than the iron, escapes in the form of carbon monoxid, combustion of which takes place on coming in contact with atmospheric air, and sufficient heat is thus generated to keep the tem- perature above the melting-point of steel during the operation. The current of air is stopped as soon as the decarburation has progressed far enough, when a quantity of white pig-iron containing manganese is added to the fluid metal for the purpose of assisting the separation of gas from the melted metal. It is then ready for casting. Some New Iron Alloys. — The combinations of iron with aluminum, chromium, copper, manganese, nickel, silicon, and tungsten constitute a class of alloys which are essentially new and are termed steels, and are usually designated with a prefix of the name of the particular element present, as nickel steel, chrome steel, etc. * For other methods of producing steel, see Percey's, Phillips's, or Makins's works on Metallurgy. 212 DENTAL METALLURGY. Aluminum Steel. — The addition of aluminum slightly increases the tensile strength, and propor- tionately the elastic limit, in rolled and cast steel when the amount added is not greater than one per cent. Chrome Steel. — Chromium increases the hardness, tensile strength, and elastic limit of iron, but lessens its weldability. Ferro- chrome is made by heating the mixed oxids of iron and chromium in brasqued crucibles, adding powdered charcoal and fluxes. Chrome steel is then produced from ferro-chrome by melting it with wrought iron or steel in graphite cru- cibles. It has been stated that the presence of chro- mium in steel renders it more susceptible to oxida- tion by exposure to air and moisture than ordinary steel. It was thought that chrome steel would ad- mirably fill certain special requirements where great hardness and toughness is needed, as the manufacture of dental instruments, projectiles, cuirasses, etc., but there are other steel alloys coming into use which are so much better that it is probably only a ques- tion of time when it will be superseded. Copper Steel. — M. Henry Schneider, of Creusot, France, obtained patents for the manufacture of alloys of iron and copper and steel and copper. This alloy can be made in crucible, cupola, or open-hearth furnace. The furnace is charged with copper scraps and cast iron mixed between layers of coke, or if a cupreous coke be employed then the cast iron is laid in alternate layers with it, and a layer of anthracite is laid over the whole. The alloy thus formed contains gener- ally from five to twenty per cent, of copper, according to the purpose for which it is to be employed, and it is remarkable for its great strength, tenacity, and malle- IRON. 213 ability, properties which may still further be developed by chilling or tempering. Nickel Steel. — United States patents 415,657 and 415,655, November 19, 1889, were granted M. Henry Schneider, of Creusot, France, for the manufacture of alloys of cast iron and nickel and steel and nickel re- spectively. The alloy of cast iron and nickel con- tains from five to thirty per cent, of nickel, and is re- markable for its great elasticity and strength, proper- ties which admit of further development by the usual chilling or tempering. The alloy of steel and nickel usually contains about five per cent, nickel, and is especially suitable for use in the construction of ordnance, armor-plate, gun- barrels, and projectiles. Some specimens of nickel steel recently produced by Carnegie, Phipps & Co., for the U. S. Navy Department, containing three- sixteenths per cent, nickel, showed, when tested, the following results : Elastic limit, 59,000 and 60,000 pounds per square inch ; ultimate tensile strength, 100,000 and 102,000 pounds per square inch. It is stated that the presence of manganese in nickel steel is most important, as it appears that without the aid of manganese in proper proportions the best re- sults could not be obtained. Nickel steel is said to be less liable to corrode in salt water than ordinary steel, which, it maybe proper to observe in this connection, is more readily acted upon by sea-water than are the more impure grades of iron. The success of the nickel-steel armor-plate at the recent test by the U. S. Navy Department, in which the nickel-steel plates alone withstood the eight-inch chrome-steel projectiles without cracking, has had a 214 DENTAL METALLURGY. most important influence upon the manufacture of that alloy. Manganese Steel. — The maximum of strength, toughness, and hardness is probably reached in this alloy, when about fifteen per cent, of manganese is added to steel. The chief obstacle to the commercial production of manganese steel is its extreme hard- ness. The working of some of the grades of this ma- terial by the ordinary methods is almost impossible. Manganese steel is usually made by adding ferro- manganese to molten Bessemer or open-hearth steel. The extreme point of brittleness in this alloy occurs in specimens containing from four to five per cent, of manganese. Extremes of atmosphere, 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 cool- ing in air. Ha?'dening and Tempering. — Hardening of ordi- nary carbon steel is effected by subjecting the object to extremes of temperature. The common practice is to first coat the surface of the metal with some car- bonaceous substance, such as soap, to prevent scaling and oxidation of the surface. Ferro-cyanid of potas- ' sium has also been used for surface-hardening. This salt contains cyanogen (C 2 N 2 ), a gas consisting of twelve parts by weight of carbon and fourteen of nitrogen. This is decomposed at the high tempera- ture which is employed, and supplies carbon to the surface of the metal. This salt is, however, better suited to the process known as case-hardening, while IRON. 21 in re-tempering dental instruments soap answers every requirement. The metal is next heated to the point of full red- ness, and then suddenly plunged into cold water, oil, tallow, or mercury, or, in the case of small objects, is merely placed on a large piece of cold metal. It is thus rendered very hard, while at the same time it increases slightly in volume. If hardened steel be heated to redness, and allowed to cool slowly, it is again converted into soft steel, but it may be proportionately reduced by heating to a temperature short of redness, the proper point of which may be ascertained by noting certain colors which appear on the ground or brightened surface of a steel instrument when held over a flame. This dis- coloration is due to the formation of a thin film of oxid, and as the temperature rises the film becomes thicker and darker, and the instrument softer. It is therefore necessary to plunge the instrument into a cold menstruum the instant the color indicating the desired degree of hardness is reached. The follow- ing table indicates the tempering heats of various instruments : Temperature. Color. Use. 430 to 450 F. Light yellow. Enamel chisels. 47o° F. Medium yellow. Excavators. 490 F. Brown-yellow. Pluggers. 5io° F. Brown-purple. Saws, etc. 520 F. Purple. Wood-cutting tools. 530 to 570 F. Blue. When elasticity is desired. In "letting down" or tempering dental instruments the flame of a spirit-lamp may be employed, the in- 2l6 DENTAL METALLURGY. strument being placed in it ; the flame should strike, however, some distance from the cutting-end, and when the proper color reaches the end it should be thrust into water. Another very convenient means of effecting the same result consists in heating an iron bar to redness at one end, and then fixing it in a vise. The object to be tempered is placed in contact with this until the desired tint appears. Steel when fractured shows a fine silky appearance of the broken surface. Overheating, however, de- prives it of carbon, when the fractured surface pre- sents a coarse, granular condition, showing that it is unfit for use for fine cutting-instruments. Case-hardening consists in conferring the hardness of steel upon the external surface of iron objects which are to be subjected to considerable wear, such as gun-locks, etc., and is accomplished by heating them in some substance rich in carbon (such as bone- dust, cyanid of potassium, etc.), and afterward chill- ing in water. The body of the piece so treated retains the toughness of iron. Malleable iron is produced by a process the reverse of that employed in case-hardening. It consists in heating the object, usually made of cast iron (when great softness and tenacity are required), for some hours in contact with oxid of iron or manganese, by which its carbon and silicon are removed. A steel instrument may be readily distinguished from iron by placing a drop of nitric acid upon it, a dark stain being produced upon steel by the separa- tion of the carbon. CHAPTER XIV. MERCURY. Atomic Weight, 200. Symbol, Hg (Hydrargyrum). MERCURY (quicksilver) has been known from a very early period. It is the only metal which is liquid at ordinary temperatures, but it be- comes solid at 39 F. below zero. It is frequently found in nature in the metallic state, and it is proba- ble that the first supplies of the metal were from this source. The ancients distinguished between such mercury and that obtained by reduction from the ores by designating the native metal as argentum vivum and the reduced as hydrargyrum, — a fact which has led some to doubt whether they were believed to be the same metal. The sources of mercury are cinnabar, or mercuric sulfid (the ordinary ore from which the metal is ob- tained) ; horn quicksilver, or native calomel ; native amalgam of silver and mercury, sometimes found trickling from crevices in the ores. It is also occa- sionally found in globules disseminated through the native sulfid. Occurreiice. — It is found in considerable quantities in Almaden in Spain, Idria in Austria, and has been found in great abundance and of remarkable purity in California and Australia. The metal is extracted from the sulfid at Idria by roasting the ore in a kiln, which is connected with an extensive series of con- densing-chambers built of brick-work. The sulfur 15 217 2l8 DENTAL METALLURGY. is converted by the air in the kiln into sulfurous acid gas, while the mercury passes off in vapor, and is condensed in the chambers. The greater portion of the mercury of commerce is produced at the Aus- trian mine at Idria, whence it is exported in bottles of hammered iron, containing seventy-five pounds each, in a state often nearly, though never quite, pure. It is, however, frequently, when purchased in small quantities, adulterated with tin and lead. The pres- ence of debasing metals such as these may be detected by scattering a little upon a clean glass plate, when it * ' tails' ' or leaves a track upon the glass . Lead, which is its most frequent impurity, may be removed by ex- posing the mercury in a thin layer (in a broad, shallow dish) to the action of nitric acid diluted with two parts of water, which should cover its surface and be allowed to remain in contact with it for several days, with occa- sional stirring. The lead is thus oxidized by the acid, and after digestion may be washed away. Distillation is the means most frequently recom- mended for the purification of mercury, but it is now regarded as an uncertain one, since, if zinc or bismuth be present, they will be sure to distil over into the re- ceiver with the mercury. In distilling or redistilling mercury a strong glass retort should be used, in size corresponding with the quantity of the metal to be operated upon,* and filled one-third with mercury, upon the surface of which is placed a layer of clean iron filings. The retort should be imbedded in a sand-bath, with the neck made to incline so as to dip below the surface of the water, * For quantities exceeding two or three pounds iron retorts are em- ployed. MERCURY. 219 with which the receiver is half filled ; and, in order to facilitate condensation of the metallic vapor, the re- ceiver should be surrounded by cold water. Should there be a film of oxid upon the surface of the distilled mercury, a small quantity of hydrochloric acid will dissolve it, when the mercury may be washed and dried at a moderate heat. Another and better method is to substitute coarsely-powdered cinnabar for the iron filings. The sulfur liberated from the former con- verts foreign metals into sulfids, while the mercury present is liberated. Probably the most certain means of obtaining mer- cury free from admixture of other metals is to operate upon one of the salts of mercury ; as, for instance, the red oxid, the bichlorid (corrosive sublimate), or pure vermilion. These are readily decomposed by heat, and when the red oxid is employed, simple distillation is sufficient. Should the metal after distillation show a film of oxid, the latter may be easily removed with dilute nitric acid slightly warmed. Where vermilion or the chlorid is to be reduced, the distillation will be greatly facilitated by the addition of one part of lime. It is important that the mercury employed for dental purposes should be quite pure, and doubtful specimens may be effectually freed of the presence of the adulterating metals named by the following very simple means : A small quantity of mercurous nitrate is dissolved in water, and into this the impure mercury is placed. The salt will be decomposed, the adulter- ating metals oxidized, and they will be replaced by the mercury of the salt. The same result may also be attained by digesting in a solution consisting of one part of nitric acid and eight parts of water for several 220 DENTAL METALLURGY. hours at a temperature of 130 or 140 F. The mer- cury should be placed in a flask or glass dish with a broad, shallow bottom, and will require frequent shaking or stirring. The dilute acid has little or no effect upon the mercury, while it readily dissolves and combines with the impurities. Another very simple means, said to have been de- vised by Dr. Priestley, has frequently been employed in purifying mercury. It consists in placing the metal with some fine-ground loaf-sugar in a bottle, which, after being securely corked, is vigorously shaken for a short time ; it is then opened and air blown in by means of a bellows ; again stopped and shaken as before, and, after repeating this treatment three or four times, the mercury is filtered through a cone made of smooth paper, with a fine pin-hole at its apex. The sugar, to which the oxids of foreign metals adhere, remains behind. The method of filtering mercury through chamois- leather is to most dentists a familiar means of exclud- ing a superabundance of mercury from amalgams, but is not to be relied on as a means of affording pure mercury. Properties. — As before stated, mercury is fluid above a temperature of — 39 F. Below that point, however, it solidifies, and may be hammered or welded like other metals. During solidification it contracts and exhibits octahedral crystals. When pure, the fluid metal is exceedingly lustrous. It boils at about 66o° F. It is volatile even at the ordinary temperatures, and this property is greatly increased by heat. There have been curious illustrations of the disposition to assume a state of vapor which this MERCURY. 221 metal evinces. * It is readily soluble in strong nitric acid, but is dissolved in sulfuric acid only by the assistance of heat. Hydrochloric acid has no effect upon it When shaken in air, or rubbed with grease, turpen- tine, or other substances, such as sugar, chalk, etc., mercury loses its metallic appearance, and is converted into a gray mass, as in mercurial ointment, etc. This was formerly regarded as an oxidation of the metal, and the belief was probably favored by the fact that mercury, in a state of fine division, becomes active for therapeutic purposes. The microscope, however, has revealed that such preparations are composed of metallic globules of about j±q of a line in diameter. Mercury amalgamates more or less readily with gold, silver, tin, zinc, lead, bismuth, cadmium, potas- sium, etc., and with a greater degree of difficulty with platinum, palladium, and copper. In some instances combination between mercury and other metals takes place with considerable violence, as in the case of potassium, in which light and heat are developed. Many of the amalgams become solid and crystalline, as illustrated in dental amalgams, and for the most part they may be looked upon as definite compounds. Indeed, a native compound of mercury and silver has been found crystallized in octahedra and other forms of the regular system. The liquid amalgams are probably in many instances solutions of definite com- pounds in an excess of mercury ; if these are pressed through chamois-leather the mercury is excluded, * Burnet relates an instance in which some mercury in leather bags, forming; part of the cargo of a ship, escaped into the bilge of the vessel. An elastic Huid was soon evolved, which covered every metallic article on board, while the crew, to a man, were salivated. 222 DENTAL METALLURGY. carrying with it but a small portion of the other metals, while a solid amalgam, frequently of definite atomic constitution, remains behind. Compounds. — Mercury unites with oxygen to form mercuric and mercurous oxids, both of which are highly poisonous. With chlorin it forms two com- pounds, mercuric chlorid (HgCl 2 ) commonly called corrosive sublimate, and mercurous chlorid (Hg 2 Cl 2 ), familiarly known as calomel. Mercuric chlorid is a valuable germicide, and as such is extensively used by dentists in the treattnent of devitalized teeth. With iodin, mercury forms two compounds, mercuric iodid (Hgl 2 ), and mercurous iodid (Hg 2 I 2 ). The former is also regarded as a valuable antiseptic agent. With sulfur, however, mercury combines to form sulfates and sulfids. Of the latter we have mercuric sulfid (HgS), a compound of great interest to dentists in consequence of its extensive use as a coloring pigment in vulcanizable rubbers and celluloid. It is the most common ore of mercury, and, as such, is termed cinnabar ; when produced artificially it is known as vermilion. The best quality of the latter is made by the Chinese. Their process of manufacturing, for a long time a secret, consists in stirring a mixture of one part of sulfur and seven parts of mercury in an iron pot ; chemical union takes place, the result of the combination being a black powder. This is divided into small lots, which are emptied separately into suitable subliming pots, heated to redness. When a sufficient quantity has been placed in the pots they are covered up, and the heat is continued for thirty-six hours, with occasional stirring by means of an iron rod passed through the lid. Lastly, the pots are broken, MERCURY. 223 and the vermilion adhering to the upper portions levi- gated and dried. It may also be formed by rubbing three hundred parts of mercury with one hundred and fourteen parts of flowers of sulfur, moistened with a solution of caustic potash. The resulting product, which is black, is then digested at about 120 F. with seventy-five parts of hydrate of potash and four hun- dred of water, until it acquires a fine red color. Vermilion may be adulterated with red lead, disulfid of arsenic (As,S,), ferric oxid, brick-dust, or any cheaper substance of a similar color ; and the discom- fort sometimes caused by wearing vulcanized rubber artificial dentures may be in part due to the presence of such deleterious substances as the arsenic and lead salts referred to. Vermilion is an inert mercurial compound,* and is quite insoluble in either nitric, sulfuric, or hydrochloric acid. It is unaffected by water, alcohol, or the alkalies. Nitro-hydrochloric acid (aqua regia), however, dis- solves and converts it into corrosive sublimate (HgCL), and by exposure to a temperature of 6oo° F. vermilion is decomposed, and the reduced metal may be col- lected in globules by condensing the vapor. Pure vermilion, in combination with rubber, is not likely to produce deleterious effects when worn in the mouth, nor is it probeible that this compound can be decomposed chemically and converted into a poisonous salt of mercury by mere contact with the saliva. The mechanical dentist will, however, do well to avoid the use of nitro-hydrochloric acid in removing tin foil from the surface. * In the less civilized countries the ancients used cinnabar to paint their bodies, without any bad effects. Makins, p. 128. 224 DENTAL METALLURGY. Regarding the presence of free mercury in rubbers before or after vulcanizing, Prof. Austen stated that the researches of Prof. Johnston with the microscope, and of Prof. Mayr by chemical analysis, failed to dis- cover the slightest trace in samples of that which had been used by him for several years. Prof. Wildman observed that sulfur sublimed during vulcanization, but did not find the smallest trace of free mercury.* Prof. Austen failed by mechanical force to press out any metallic globules, and during his entire experience with indurated rubber as a base for artificial dentures never, even with the microscope, detected the slight- est particle of metallic mercury upon the surface of any finished piece. If it is true, as some assert, that free mercury has occasionally been observed in rubber, then its presence must have been due to the use of an imperfect quality of vermilion ; but that the latter, when pure, is ever reduced during the process of vulcanizing, or by wear- ing in the mouth, is not at all probable. The modified condition of that portion of the sur- face of the mouth in contact with the rubber plate, often accompanied by a sensation of heat, has been attributed to an electrical action, due to the fact that rubber, "like sealing-wax, is a powerful negative electric, "f The real solution of the question, however, will probably be found in the following conditions : ist. The non-conducting quality of the substance. 2d. The rough condition of the surface, due to careless- ness or want of skill in construction. 3d. Want of * Harris's Principles and Practice of Dentistry, p. 682. f Ibid., p. 681. MERCURY. ^D care on the part of the wearer, in not frequently clean- ing the piece of portions of food and the secretions of the mouth, which are likely to undergo chemical change by long confinement in contact with the tissues, and thus become irritants. The author has frequently noticed this inflamed con- dition where the denture was of gold or silver, but always in cases where the plate was seldom removed or cleansed. It is true, however, that the trouble referred to is more common in rubber or celluloid work ; but in both of these there are more conditions favoring such a result than are found in a metallic denture. The facts that the symptoms are not con- stant, and that by far the greater number of mouths in which rubber or celluloid is worn are not in the least affected by it, would seem to confirm this view. Discriminatioyi. — The presence of the soluble salts of mercury may be detected by Reinsch's test, * which consists in placing a clean strip of copper in the solu- tion ; metallic mercury will be immediately deposited upon it, giving it a silvery-white appearance. The strip of copper is then heated in a glass tube, by which the mercury is sublimed, and may be detected in the form of minute globules adhering to the sides of the tube after condensing. Insoluble compounds of mercury may be detected by placing a small portion in a glass tube and cover- ing with a layer of dry sodium carbonate, and then heating. If mercury be present, it will separate and condense in globules in the cool parts of the tube. This test is based on the fact that all mercurial com- pounds are decomposable by a temperature of ignition. *See works on chemistry. 226 DENTAL METALLURGY. Mercury is also precipitated from its soluble com- binations by a solution of stannous chlorid used in excess. Hydrogen sulfid and ammonium sulfid produce in solutions, both of mercuric and mercurous salts, black precipitates insoluble in ammonium sulfid. The quan- tity of the reagent should be sufficient to produce complete decomposition ; otherwise, a white precipi- tate will be formed, consisting of mercuric sulfid, with the original salt. An excess of hydrogen sulfid, how- ever, instantly turns the precipitate black. This reac- tion is regarded as characteristic of mercuric salts. Caustic potassa or soda, when added to solutions of mercuric salts, produces a yellow precipitate. Ammonia or ammonium carbonate produces a white precipitate insoluble in excess. Potassium or sodium carbonate causes a red-brown' precipitate. Potassium iodid throws down a bright scarlet pre- cipitate, soluble in excess either of the mercuric salt or of the alkaline iodid. CHAPTER XV. COPPER. Atomic Weight, 63.4. Symbol, Cu (Cuprum). COPPER is a metal with which mankind has been acquainted from the most remote periods, and probably the first metallic compound employed was copper alloyed with tin (bronze), of which many relics in the form of arms, ornaments, and domestic implements, evidently belonging to an early period in prehistoric times, are still to be found. * It is proba- ble, however, that the production of the pure metal is an operation of a more recent date. Copper ores are found in many parts of America and Europe. In some parts of the United States the native metal is found in immense masses many hun- dred pounds in weight, sometimes slightly intermixed with silver. Nothing is certainly known of the origin of these, but they are supposed to have been formed from the cupric sulfid, which, by exposure to air and moisture, was converted into sulfate, and then, by electro-chemical agency, reduced to the metallic state. There are several ores which yield copper. The one most commonly employed, however, is copper pyrites, a combination of sulfid of copper and iron. The blue and green carbonates, known respectively as azurite and malachite, are beautiful minerals, exten- sively used in Russia and Bohemia in the manufacture * The epoch marked by the use of bronze is known in archaeological chronology as the Bronze Age. 227 228 DENTAL METALLURGY. of ornamental objects. They contain upwards of fifty per cent, of copper. The process of obtaining copper from an ore, such as copper pyrites, may be thus briefly described : The ore is heated in a reverberatory furnace for the pur- pose of converting the iron sulfids into oxid. The copper, which remains unaltered, is then heated with a siliceous sand, which combines with the iron oxid to form a slag, and separates from the heavier (copper) compound. By repeating this process the iron is finally got rid of, when the copper sulfid begins to decompose in the flame-furnace, parting with its sul- fur and absorbing oxygen. The resulting oxid is, however, reduced by the aid of carbonaceous matter and a high degree of heat. Properties. — Pure copper may be obtained by decomposing a solution of pure sulfate of copper in the galvanic current. If the negative wire be attached to a copper plate immersed in the solution, the pure metal will be deposited on it, and may be readily stripped off. The chief value of copper in the useful arts is due to its great malleability, in which quality it is only exceeded by gold and silver. It fuses at about 2000 F. It expands in solidifying, and absorbs oxygen very much in the same manner as silver does under similar conditions. In tenacity copper ranks next to iron, as a copper wire of one-tenth of an inch in diam- eter will support about 385 pounds. Its power of con- ducting electricity is nearly equal to that of silver, while in the transmission of heat it is surpassed only by silver and gold. It is readily soluble in nitric acid, but in sulfuric acid only with the assistance of heat. COPPER. 229 Hydrochloric acid attacks it slowly, and in vacuo is inactive. The specific gravity of copper is 8.93. For the compounds of copper with the non-metallic ele- ments the student is directed to Fownes's, Bloxam's, and other works on chemistry. Amalgams. — Copper does not readily unite with mercury without the assistance of heat. There is, however, an amalgam of pure copper and mercury ex- tensively used in Europe under the name of Sullivan's amalgam. Its preparation is as follows : Pure copper in a finely-divided state is obtained by boiling a con- centrated solution of cupric sulfate with distilled zinc until the blue color of the salt disappears, when the zinc should be removed. The copper, which will be found in a pulverulent mass at the bottom of the ves- sel, should be washed with dilute sulfuric acid, sub- sequently in hot distilled water, and dried. It is then moistened with a solution of nitrate of mercury, by which means the copper becomes completely coated with mercury. The mercury is then added to it to the extent of twice the weight of copper (3 of copper to 6 of mercury). It is then rolled into small, lozenge- shaped pieces, which become quite hard, and are supplied to the profession in bottles containing an ounce or more. This amalgam possesses the property of softening with heat and again hardening, and when employed as a filling-material one of the lozenge- shaped pieces is placed in a small iron spoon made and sold for the purpose, and heated over the flame of a spirit-lamp until small globules of mercury are driven to the surface, when it is placed in a small glass or porcelain mortar and rubbed into a smooth paste. Some recommend washing with a weak solution of 230 DENTAL METALLURGY. sulfuric acid, or soap and water, and lastly with clean water alone, to remove the last traces of either acid or soap, and finally squeezing through chamois-leather to exclude surplus of mercury, when it is ready to be introduced into the cavity. It requires several hours to harden. Mr. Fletcher says of this amalgam that "it is an absolutely permanent filling, as the copper salts permeate and perfectly preserve the tooth." It is said to be quite insoluble in the mouth. It, how- ever, becomes intensely black, and imparts a most ob- jectionable stain to the teeth. According to Watt's "Chemical Dictionary," the specific gravity of pure copper amalgams is the same after hardening as before, ' ' hence the presence of copper in amalgam alloys lessens their contractibility. ' ' It is said* that the tendency of copper amalgam to discolor may be lessened by careful attention to its preparation. The older way of preparing it was by precipitating copper from a solution of cupric sulfate, with mercury at the bottom of the vessel that con- tained it, by stirring the fluid with a bar of zinc ; but a better way, and one now employed, is to substitute a clean iron bar for the zinc, and leave it from twelve to twenty-four hours in a jar containing the solution. The iron bar becomes covered with a dull red floccu- lent precipitate of copper. When a sufficient quantity of the precipitate is formed it is collected into another jar, and well washed with a stream of cold water until it becomes quite clean. It is then ground in a mortar until it begins to amalgamate, the amalgamation being hastened by hot water slightly acidulated with sulfuric acid, which will also remove traces of iron. It should * Mr. E. P. Collett, British Journal of Dental Science, April 15, 1890. COPPER. 23I next be washed in liquor ammonia to neutralize traces of acid, and must then be thoroughly triturated in a mortar until thorough amalgamation has been effected. The amalgam should be rolled into small pellets, and allowed to set for twenty-four hours before using. The expectations of valuable therapeutic qualities in copper amalgams have not been realized. Drs. C. D. Cook and W. St. G. Elliott, of London, found by experiment that copper amalgams shrunk more than simple alloys of tin and silver, and the opinion seems to be gaining ground among those who have recently somewhat eagerly adopted it as a filling- material, that it is a very uncertain agent, and that "whatever antiseptic influence it has, it does not prevent decay from beginning and progressing directly in contact with it"* Alloys. — Copper unites readily with all other metals, and many of the resulting alloys are of great value in the industrial arts, — of even more value than the pure metal. It is added to silver for the purpose of con- ferring sufficient hardness upon the latter to enable it, in the form of coin or plate, to withstand the attrition to which such articles are exposed. The formation of a perfectly uniform alloy of silver and copper is a process attended with some uncertainty, owing to a tendency on the part of the copper to separate and pass off toward the edges as the ingot solidifies. Thus, in silver coins one portion of the piece will frequently be found to contain more copper than another. The decimal proportions of copper and silver in * Dr. Howe, in discussions of J. Allen Osmun's paper entitled " Some Observations on the Use of Copper Amalgams," read before the New York Odontological Society, published in the International Dental Journal for July, 1892. 900, ' 100. 925, 75. 947, ' 53- 811, ' 189. 283, ' 717. 232 DENTAL METALLURGY. standard silver (coin) of several different nationalities are as follows : Of the United States . . silver 900, copper 100. " France .... " England .... " Indian rupees " Germany — Prussian thalers " Prussian silver groschen The properties conferred upon gold by the addition of copper are similar to those imparted to silver. These have already been alluded to on page 181. The decimal proportions in the gold coins of the United States, France, and Holland are : gold, 900 ; copper, 100 ; while English coins are composed of 916.6 of gold and 83.3 of copper. In coin-gold malleability is not greatly interfered with . Gold may, however, be rendered brittle by large proportions of copper, or when the latter is impure. Copper and platinum form an alloy, when the pro- portions are equal, of nearly the same specific gravity and color as gold. Copper also unites with palladium to form a light, brassy alloy. By admixture with lead or bismuth copper is rendered quite brittle. The principal alloys in which it forms a leading ingredient are brass, bronze, and German-silver. Aluminum bronze is formed of pure copper alloyed with from 2.5 to 10 per cent, of aluminum. It is quite malleable, and has a fine, rich, golden color. Phosphor-bronze is copper combined with from three to fifteen per cent, of tin and from one-quarter to two and a half per cent, of phosphorus. Other metals, such as silver, nickel, cobalt, antimony, and bismuth, frequently enter into the composition of bronzes. COPPER. 233 Copper in small quantities (from 5 to 7 per cent.) is said by Mr. Fletcher to confer upon amalgams the quick-setting property obtained by the addition of platinum. It is, however, considered inferior to plati- num as a constituent in dental alloys ; but in the absence of platinum, amalgams are improved by the addition of a small proportion of copper. It is stated* that an alloy of tin 10, silver 8, gold 1, copper 1, has been extensively used (in England prob- ably) under the names of gold amalgam and platinum amalgam. Hydrogen sulfid (H 2 S) and ammonium sulfid, when added to a copper solution, afford a brownish-black cupric sulfid. Caustic potash throws down a pale-blue precipitate of cupric hydrate, which changes to a blackish- brown anhydrous oxid on boiling. Ammonia also gives a blue precipitate, soluble in excess, affording a deep purplish blue solution. Potassium ferrocyanid gives a red-brown precipitate of cupric ferrocyanid. It may also be detected in very weak solutions by placing a drop on a slip of clean platinum foil. A point of zinc is then dipped in so as to touch the foil, and instantly a spot of reduced copper appears. A green line is imparted to the oxidizing flame of the blow-pipe when a copper salt is heated in it. It also communicates a green tint to borax when heated with it. There are several methods which may be advan- tageously employed for the estimation of copper. The operations of the dentist, however, are chiefly ♦Fletcher. 16 234 DEXTAL METALLURGY. confined to the examination of amalgam alloys. The alloy should first be acted upon by nitric acid ; silver. if present, may then be recovered in the form of chlorid ; after which the copper may be precipitated from the remaining solution either as oxid, sulfid. or in the metallic state. When attempting the estima- tion of an alloy, a qualitative examination should first be made (page 66), and if the solution to be examined is found to contain no other metal whose oxid is thrown down bv caustic potassa, an excess of that agent is to be added. In the resulting precipitate, when boiled, washed, dried, and weighed, every one hundred parts mav be estimated as containing 79.85 per cent, of metallic copper. When hvdrogen sulfid or ammonium sulfid is emploved as the reagent, the resulting cupric sulfid is usually oxidized by nitric acid, and again precipitated by potassa, so as to estimate as oxid. The estimation as metallic copper is accomplished as follows : Place in the solution contained in a plat- inum dish a piece of zinc, adding also a little hydro- chloric acid. The electrolyzing action instantly com- mences, and continues until the solution is colorless and the zinc completely dissolved. The finely-divided metallic copper will be found at the bottom of the vessel. This is to be well washed, dried, and weighed. CHAPTER XVI. ZINC. Atomic Weight, 65.2. Symbol, Zn. THE ancients were undoubtedly acquainted with an ore (probably cadmia*) which they employed with copper to form brass. Many objects of ancient manufacture, analyzed at different times, have been found to contain zinc.f The extraction of the metal itself, however, is probably a modern discovery. Metallic zinc is never met with in nature. The principal ores are the red oxid — the sulfid of zinc (blende) and the native carbonate (calamine). The latter is the most valuable of the zinc ores, and is preferred for the extraction of the metal. It is first roasted to expel water and carbonic acid ; then mixed with fragments of coke or charcoal, and distilled at a full red heat in an earthen retort. Carbon monoxid escapes, while the reduced metal volatilizes and is condensed by suitable means. Properties. — Zinc is a brittle, crystalline metal, with a density varying from 6.8 to 7.2. Until about the commencement of the present century the valuable property possessed by this metal, of becoming quite malleable between 248 and 302 F. , was not known ; hence, prior to that discovery it was but little used in the industrial arts. Between these degrees of heat it * An ore used by the ancients, containing cadmium and zinc, f Phillips made a number of analyses of such objects, all of which showed the presence of zinc. 235 236 DENTAL METALLURGY. may be rolled or hammered without the least danger of fracture. Sheet zinc of commerce is manufactured by this means, and it retains its malleability when cold. Zinc fuses at 773 F. (below red heat). At a bright red heat it boils and volatilizes, and if heated in air combustion takes place, during which it unites with the atmospheric oxygen with brilliant incandescence. At 410 F. zinc is so brittle that it may be powdered in a mortar. Alloys. — With mercury zinc forms an exceedingly brittle amalgam. The two combine in the cold state, but union is greatly facilitated by heating. Zinc is occasionally employed as a constituent in dental alloys. . An amalgam has been suggested, the proportions of which are " approximately" given as, tin, 50 odd ; silver, 30 ; gold, 5 to 7 ; zinc, 2 to 4; and recent experiments with it have proved so satisfactory that it has to a certain extent taken the place of platinum in dental amalgams. Added to silver, in the proportion of 2 of zinc to 1 of silver, a nearly white, malleable alloy results. The color of gold is heightened by the addition of zinc, while its malleability is greatly impaired. Makins states that gold rendered standard by zinc is a green- ish-yellow, brittle alloy, with a specific gravity above the mean. Combination between zinc and platinum or palladium may be effected at a comparatively low temperature, and it is accompanied by evolution of light and heat. It is stated that an alloy of 16 parts of copper, 7 of platinum, and 1 of zinc closely resembles 16-carat gold, is quite malleable, does not tarnish in air, and is capable of resisting cold nitric acid. ZINC. 237 Zinc and lead mix with each other to a very limited extent. If equal parts of the two metals are melted together and allowed to cool, they will be found to have separated into two layers, the upper, and conse- quently the lighter one, zinc, retaining 1.2 percent, of the lead, while the lower layer consists of lead alloyed with 1.6 per cent, of zinc. The necessity of carefully keeping these two metals separate in all mold- ing operations in the dental laboratory will readily be appreciated, as a failure to observe precaution in this direction will be followed by vexatious consequences. If by accident lead becomes mixed with the zinc used for dies, the lead, by its greater specific gravity, will settle to the bottom and fill up the deeper portions of the sand matrix representing the alveolar ridge, the most prominent part of the die. This may not be discovered until an attempt to swage is made, when the die will be found to be totally unfit for the purpose. In such cases the mixed metal should be discarded and new zinc substituted. Zinc and tin unite in all proportions without diffi- culty. Alloys of zinc and tin are frequently employed in casting dies for swaging plates. Richardson* gives a formula for an alloy consisting of zinc 4 parts, tin 1 part ; which, he states, fuses at a lower temperature, contracts less in cooling, and has less surface-hardness than zinc. Fletcher, however, states that all alloys of zinc and tin are superior to zinc alone for dies. The impression from the sand he believes to be much finer, and the shrinkage in cooling greatly reduced. Zinc 2, tin 1, is given as the best proportion, t Makins states * Mechanical Dentistry. t Practical Dental Metallurgy, p. 69. 238 DENTAL METALLURGY. that zinc and tin, when combined in equal proportions, form a white, hard alloy, not very malleable or ductile, which is capable of being worked as readily as brass. Zinc and copper unite in various proportions to form many different grades of brass, known respec- tively as pinchbeck, Manheim-gold, similor, Bath- metal, Prince Rupert's metal, Muntz'ssterro, Gedge's and Aich's metals. German silver and the Chinese alloys known as pacfong and tutenag are also alloys of zinc and copper, with the addition of nickel. Dies and Counter- Dies. — Zinc is the metal most commonly employed in the formation of dies for swaging plates, and is superior to any of its alloys.* Another important application of zinc is in the formation of counter-dies. The die is placed in the iron ring when a Bailey flask is employed, or in- vested in the molding-sand and then surrounded by a suitable iron ring in the old-fashioned way. The zinc is then heated and poured in upon the zinc die just at the moment of complete fusion. Should the metal be accidentally allowed to remain on the fire too long, and thus reach a higher temperature than is necessary, it should not be poured until it begins to solidify at the edges. The belief seems to be pretty general that melted zinc cannot be poured upon a zinc die without causing cohesion, f but if the necessary precaution regarding the proper tempera- ture at which the metal is poured is observed, it is * The author has not found the alloys of zinc and tin to be, in any respect, superior to zinc alone for dies. t If the melted metal be poured, at a temperature of 8oo° F., upon a die having a temperature of 70 F., the fused zinc, by contact with the iron ring and by radiation, will lose heat enough to cause its temperature to fall far below the fusing-point, and it will probably not impart to the die more than 400 F. ZINC. 239 impossible for union to take place, and when cool the die and counter-die will separate quite as readily as though the latter was of lead. It seems strange that this valuable expedient for the dental laboratory has not found a place in the text-books on mechanical dentistry. It frequently occurs that the zinc die and lead counter-die are totally inadequate to bring a plate (particularly if the latter is of platinum-gold or irid- ium-platinum) into perfect adaptation to all parts of a model, especially where the palatal arch is very deep and the rugae are prominent. The zinc counter- die is also of especial service in partial cases where a number of teeth remain. These are cut off from the plaster model previous to mold- ing within one-sixteenth of an inch of the margin of the gum, so that a sufficiently distinct impress may be made in the plate to serve as a guide in filing the latter to fit around the natural teeth. Where the swaging is likely to be attended with difficulty, at least three sets of dies and counter-dies should be made. The most imperfect of these should be furnished with a lead counter-die, and used as a preliminary die upon which to start the plate. The next in quality may be used with the zinc counter-die, and the nearest perfect of the three, with a lead coun- ter, reserved as a finishing-die. When the plate, by means of the horn or wooden mallet and some prelimi- nary swaging with a light hammer, has been made to assume somewhat the form of the die, and has been carefully carried past the stage when pleating or wrinkling of the plate is likely to occur, it should be trimmed to the proper dimensions, annealed, and placed between the die and zinc counter-die, and at 240 DENTAL METALLURGY. first gently tapped with a hammer until the die passes well into the counter- die, when one or two sharp blows with a heavy hammer, either upon the die or its counter, will carry the plate into perfect adaptation to all parts of the former. Some slight compression, however, of the prominent points of the die is likely to occur in the use of the zinc counter, so that it will be necessary to anneal and give the plate two or three sharp blows between the finishing-die and its lead counter- die ; after which it will be found to perfectly fit the mouth, without any attempt to compensate for contraction of the zinc* It will be seen that the zinc counter-die is not intended to supersede, but is merely used as an adjunct to, the lead counter, and there is probably no better means of carrying the plate to the deep parts of the model, and of obtain- ing a sharp, well-defined impress of the rugae and prominent parts of the model. Zinc will, under favorable conditions, unite with iron, and it frequently attacks the cast-iron ladle in which it is melted, and may penetrate the side and escape into the fire. Accidents of this kind, how- ever, may be avoided by coating the inside of the ladle with whiting. Compounds of Zi?ic. — The oxid and the chlorid are the compounds of this metal most frequently employed by dentists. The first forms the chief ingredient in the plastic filling- materials known as oxychlorids and oxyphosphates. Zinc oxid is a white powder, the product of the combustion of the metal. It turns yellow on heating, but resumes its *The subject of shrinkage of zinc when used for dies in forming metallic plates has been fully referred to on page 23. ZINX. 24I pure white color on cooling. Chlorid of zinc, pre- pared by acting upon the metal with hydrochloric acid or by heating metallic zinc in chlorin, is a fusible, deli- quescent substance, quite soluble in water and alcohol. Oxychlorid of zinc, the well-known filling-mate- rial, consists of a powder and a fluid. The first is prepared by various formulas. One in common use is as follows : Grind together in a mortar borax 2 grains, fine silex 1 grain, oxid of zinc 30 grains. When thoroughly mixed these are placed together in a small crucible and heated to bright redness. This is called the frit, and when cool requires grinding to again reduce it to a pulverulent state. It is then thoroughly mixed with three times its weight of cal- cined oxid of zinc. The fluid usually employed with the powder consists of chlorid of zinc diluted with water in the following proportions : Deliquesced chlorid of zinc, 1 ounce ; water, 5 or 6 drams. The oxyphosphate powders are similar mixtures.* The fluid, however, is prepared by dissolving in pure water some glacial phosphoric acid, and then evaporating until the solution attains the consistence of glycerin. The presence of zinc in solution is distinguished by the following reactions : A white precipitate soluble in excess of the alkali is obtained by the addition of caustic potash, soda, or ammonia, and zinc is distin- guished from all other metals by ammonium sulfid, which precipitates white sulfid of zinc, insoluble in caustic alkalies. * Oxid of zinc, 200 parts, silex 8, borax 4, ground-glass 5, levigated under water to insure complete admixture, then dried by evaporation, calcined at a white heat, and pulverized, has been found to be equal in durability and working qualities to any of the numerous oxyphosphates now in the market. CHAPTER XVII. CADMIUM. Atomic Weight, 112. Symbol, Cd. OADMIUM, a metal closely allied to zinc, was dis- \y covered by Stromeyer and Hermann in 18 1 7. It does not occur in the metallic state, and there is only one definite mineral known which contains it in quantity, namely, the sulfid, or greenockite, which is found in Renfrewshire, Scotland. This contains 77.7 per cent, of cadmium and 22.3 per cent, of sulfur. The production of cadmium is confined to a very few localities in Belgium and England. It occurs in zinc blende to the extent of about 0.2 per cent. In the calcination of the blende the cadmium, volatilizing at a lower temperature than the zinc, passes off before the latter assumes the form of vapor. The oxid of cadmium is collected in condensing-tubes, and is sub- sequently reduced to the metallic state by heating with carbon. Cadmium is a white metal with a slight bluish tinge. It is somewhat lighter in color than zinc or lead. It is susceptible of a high polish. It has the fibrous fracture characteristic of soft, tough metals. It differs from zinc in its crystalline form, that of cadmium being the octahedral, while that of zinc is rhombohedral. It is harder than tin, but not so hard as zinc, and is sufficiently malleable to admit of rolling into thin sheets. Its specific gravity after fusion is 8.6. Its electric conductivity is 22.10, — somewhat lower than 242 CADMIUM. 243 that of zinc. It melts at a temperature below redness ( 3 i5 to 3 20 C.). Cadmium has been used in the formation of dental alloys, but its employment as a constituent in amal- gams is now so generally condemned that it is seldom used for that purpose. Probably the best test for cadmium is the color afforded when it is volatilized and oxidized under the blow- pipe flame. This is a reddish-brown, zinc under the same conditions giving a deposit which is a bright yellow while hot, becoming white on cooling. It may also be detected by precipitation from an acid solution as a yellow sulfid, and may in this way be distinguished from zinc, as zinc sulfid does not separate except from neutral or alkaline solutions. In quantitative analysis cadmium is always estimated as oxid, being separated from solution as carbonate by precipitation with carbonate of sodium, which is converted into oxid by calcination. It may also be separated from its solution in acids by means of zinc, which precipitates it in a dendritic form, like the well- known lead tree. CHAPTER XVIII. ALUMINUM. Atomic Weight, 27.4. Symbol, Al. (OCCURRENCE. — Aluminum is never found in the metallic state. Of all the metals the sources of alumina are the most numerous and abundant. Its chief combinations are with silicon and other bases. These substances undergoing atmospheric changes form clays and soils, which under the influence of heat and moisture become fruitful. It would seem that its presence is not necessary to the maintenance of animal or vegetable life, since no traces of it have been found in either. Some of the compounds of aluminum are quite unattractive, but there are a number possessing great hardness and extraordinary beauty. The follow- ing with their formulae are a few examples of the latter : Ruby- A1 2 3 . Sapphire A1 2 3 . Garnet . . (Ca Mg Fe Mn) 3 Al 2 Si 3 12 . Cyanite Al 2 Si0 5 . As early as 1760 Guytori de Morveau called the sub- stance obtained by calcining alum alumina. Lavoisier, sixteen years later, suggested the existence of metal- lic bases of the earths and alkalies, and alumina was thought to be an oxid of a metal which was called aluminium ; and thus it was named long before it was isolated. * Corundum, the ruby, and the sapphire have the same chemical formula, A1 2 3 . 244 ALUMINUM. 245 In 1807 Sir Humphry Davy tried to decompose alumina by means of an electric current, and again to reduce the metal by vapor of potassium, in both of which experiments he failed. In 1827 Wohler isolated the metal by decomposing aluminium chlorid by potassium. The metal first isolated by Wohler was a gray powder, taking under the burnisher the appearance of a highly polished metal. Later, in 1845, Wohler obtained the metal in small malleable globules by making a vapor of alum- inum pass over potassium placed in platinum vessels, and from these specimens he was able to determine the properties of the metal with some degree of accuracy. The credit of the reduction of aluminum in a state of purity and the determination of its true properties belongs to H. St. Clair Deville, who in 1854 brought it from the rank of a mere laboratory curiosity to that of the useful metals. In 1854 the Emperor Napoleon III, in the hope that aluminum might be used in the construction of armor and helmets for the French cuirassiers, author- ized experiments on a large scale to be carried on at his own expense. In 1855 the Emperor put the neces- sary funds at the disposal of Deville, whose experi- ments were continued for four months, the result being that in August of the same year aluminum was placed on thejmarket in Paris at 300 francs a kilo. The first article known to have been made of alum- inum was a baby-rattle for the infant Prince Imperial, for which purpose it was well fitted on account of its sonorousness ; but application of the metal to the manufacture of cuirasses and helmets was decided to be impracticable, and the idea was abandoned. 246 DENTAL METALLURGY. Reduction of Aluminum. — A mixture of the double chlorid of aluminum and sodium, or the double fluorid of aluminum and sodium (cryolite), is heated to red- ness with the metal sodium, when energetic chemical action takes place, during which chlorid of sodium is formed and the metal aluminum separated. Aluminum may be separated by electrolysis. The electric current from a ten- cell battery, provided with carbon poles, is passed through the fused salt. The metal appears at the negative pole in large globules. In 1882 the cost of aluminum was materially les- sened by inventions of Webster, of Birmingham, England. This inventor's method of producing the metal consisted in reducing sodium compounds in cast- iron pots from a fused bath of caustic soda. By this means the yield of sodium is much greater than by the method of Deville, while the temperature required in the operation is considerably less. In 1859 Mr. Chas. M. Hall, of Ohio, obtained letters-patent for an electrolytic method which is superior to any that preceded it. The principal fea- ture of this process is the electric decomposition of alumina suspended or dissolved in a fused bath of the salts of aluminum, the current reducing the alumina without affecting its solvent. Mr. Hall has succeeded in producing the metal aluminum as an article of com- merce at $2.00 per pound. Aluminum is nearly the color of new zinc. It is very malleable and ductile, and admits of rolling into thin sheets, or it may be drawn into fine wire. It is highly sonorous, and has the power of conducting heat and electricity in about the same degree as silver. It is onlv two and a half times heavier than water (four ALUMINUM. 247 times lighter than silver). Its specific gravity is 2.56, and it melts at a red heat. Aluminum does not oxidize in air, and is not attacked by sulfur compounds. It is not attacked by strong nitric acid, and is insoluble in dilute sulfuric acid, but it may be readily dissolved in either dilute or strong hydrochloric acid, which is its true solvent. The metal is easily dissolved in solutions of caustic potash or soda. As the result of the invention of the electrical fur- nace of the Messrs. Cowles, of Cleveland, Ohio, aluminum bronze is made directly from corundum (A1 2 3 ). Twenty-five pounds of the crushed ore is mixed with about fifty pounds of copper and twelve pounds of a mixture of charcoal and electric- light carbon, and placed in a rectangular box of fire-brick, lined with limed charcoal to prevent loss of heat by radiation and to protect the fire-brick from disintegra- tion. The charge is surrounded on all sides by a layer of charcoal to prevent the alloy from being contami- nated with calcium from reduction of the lime present. The cast-iron slab forming the cover of the furnace is then securely luted on, and the current from a power- ful dynamo-electric machine is passed into the furnace by means of two large electric-light carbons which pass through the ends of the furnace and into its con- tents. It requires about five hours of exposure to the intense heat afforded by the electric current to reduce the aluminum from its ore. When the furnace has cooled sufficiently the product of the reduction will be found to consist of about fifty pounds of a copper alloy containing from 15 to 35 per cent, of aluminum, which may be brought to the usual 10 per cent. 248 DENTAL METALLURGY. standard of aluminum bronze by remelting it with the proper proportion of copper. The reaction which takes place in the process, which is aided by the intense heat of the electric current, is probably as follows : The carbon unites with the oxy- gen from the corundum, forming carbon monoxid ; a small percentage of the aluminum remains free, mixed in small particles with the charcoal, while the greater portion unites with the copper to form the alloy. Nearly all the oxid is reduced and the charcoal is changed to graphite. Some of the aluminum unites with carbon to form the carbid of aluminum. The fusing-point of 10 per cent, aluminum bronze is some- what below that of pure gold. Aluminum may be melted in an ordinary clay cruci- ble. No flux need be used. Borax is not only useless but is actually hurtful, as aluminum readily attacks the glasses. Biederman* recommends dipping the scraps which are to be melted together in benzine before put- ting in the crucible. Should any of them be contam- inated with solder it may be removed by immersing in nitric acid, which does not act upon the aluminum. Annealing. — Aluminum may be softened by heat- ing to redness and chilling suddenly by dropping into water. Richards recommends rubbing the piece to be annealed with tallow and then heating until the fat is carbonized, when at the moment the last trace of black disappears from the metal it may be dropped into water. Alloys. — Aluminum forms alloys with nearly all the metals. That with copperf is the most important, *" Aluminum: Its Properties, Metallurgy, and Alloys." Richards. f Aluminum bronze— alloy of copper with five per cent, of aluminum. ALUMINUM. 249 and presents a closer resemblance to gold than, per- haps, any other alloy. It is used for articles of jewelry, for mountings of astronomical instruments, and for making balance-beams. German dentists are now using aluminum bronze as a base for artificial dentures. Professor Sauer, in a paper on the application of this alloy to dental pur- poses, says, "That in the proportion of Cu. 900 to Al. 100 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 frequently, 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." He also gives the point of fusion of the alloy as above that of 18-carat gold, so that 14- or 18-carat gold solder alloyed with copper may be used upon it without difficulty. Although the alloy is highly recommended by many German dentists, the author does not hesitate to express the opinion that it will not find favor in this country. The following solders are well adapted to aluminum bronze : I. Hard Solder for 10 per cent. Aluminum Bronze. Gold 88.88 per cent. Silver 4.68 Copper 6.44 " 100.00 II. Medium Hard Solder for 10 per cent. Aluminum Bronze. Gold 5440 per cent. Silver 27.00 " Copper 18.00 " 100.00 17 250 DENTAL METALLURGY III. Soft Solder for Aluminum Bronze. Copper 70 per cent. ") _ Tin « J Brass . 14 30 per cent. Gold . 14.30 " Silver . 57.10 " Copper . 14 .30 100.00 Aluminum with tin and zinc forms a brittle alloy, and with silver it yields a hard, though workable com- pound. Aluminum amalgamates with mercury by the assistance of heat, and at the boiling-point of mercury the solution is very rapid. Aluminum may be made to unite with mercury by the intervention of a solution of caustic potash or soda. If the surface of the metal be well cleaned, or moistened with the alkaline solution, it is immediately melted by the mercury, but the affinity of the alumi- num for oxygen is greatly increased by the state of fine division, so that the amalgam when exposed to the air soon becomes covered with a white excrescence, which Watts found to be pure alumina. Aluminum is employed in the manufacture of very small weights, such as the milligram of the metric system, — a use to which, in consequence of its ex- ceedingly low specific gravity, it is particularly well adapted. Its lightness, strength, and resistance to oxygen and the sulfur compounds, are properties which would seem to point to this metal as a suitable substance as a base for artificial teeth. The readiness, however, with which it is attacked by alkaline solutions renders it unfit for use in the construction of a permanent artificial denture. ALUMINUM. 25I Aluminum, notwithstanding its extreme lightness, may be cast with great exactness. The late Dr. J. B. Bean, who patented a process for casting alum- inum, succeeded in producing castings of exquisite fineness. Indeed, it may be stated that Bean suc- ceeded in overcoming all the physical difficulties en- countered in the effort to render aluminum available in prosthetic dentistry, but its susceptibility to the action of alkaline solutions finally compelled him to abandon it. Dr. C. C. Carroll, of Meadville, Pa., has devised a means of casting aluminum, which while much sim- pler than the method of Dr. Bean, affords results equally good. The metal is melted in a plumbago crucible having the form of a thick- walled cylinder closed at one end which serves as a bottom. A channel is formed within the wall of the crucible, one orifice of which terminates within at the side close to the bot- tom. Starting from the orifice, the channel rises in the crucible wall near the top, making a sharp return upon itself, and descends in a parallel course after the manner of a syphon, and makes its exit at the base and near the side of the crucible. Here it terminates in an iron nipple that fits into a corresponding socket in the gate-way of the molding-flask. A cylindrical plug of soapstone which fits the open mouth of the crucible is provided with a central tube of brass, to the free end of which is connected by a short length of rubber tubing a large rubber bulb. When the metal has been brought to a state of fusion the crucible is connected by means of the iron nipple at its base with the gate-way of the flask, which has been pre- viously heated to redness, and the soapstone plug is 252 DENTAL METALLURGY. inserted in the mouth of the crucible. Compression of the air at this point by means of the rubber bulb forces the fluid metal out of the crucible through the syphon-like channel into the- mold, filling the most minute lines and affording an exceedingly fine casting. Carroll makes the somewhat extraordinary statement that he has found a means of controlling the contrac- tion of the metal, together with its tendency to disin- tegrate from exposure to the fluids* of the mouth, by the admixture of other metals. Richards, in his valuable work on aluminum, states that to overcome the difficulties of contraction and corrosion by the fluids of the mouth Dr. Carroll adds "a little copper, which, he says, decreases the con- traction, while the addition of some platinum and gold renders it unalterable in the mouth." Aluminum may be cast upon plain teeth with com- parative safety, provided the metal is prevented from overlapping the necks of the teeth. But when gum teeth are employed, either single or in sections, their fracture is almost certain to follow the contraction incident to the cooling of the metal. Specimens of Dr. Carroll's work have fully proved this, and it was the one difficulty which finally defeated Dr. Bean's efforts by compelling him to cast his plate separate from the teeth. For, if it had been practicable to cast the metal directly upon block teeth without dan- ger of fracture, the denture would have lasted for at least six or eight years ; but the necessity of attaching the teeth to the plate by another metal so hastened disintegration that a few months only were necessary to render the piece useless. * Demonstration before dental class, University of Pennsylvania. ALUMINUM. 253 There are two methods which have been employed in the construction of artificial dentures of this metal. The one most frequently resorted to consists in merely swaging a plate in the ordinary way. A number of countersunk holes are then made along the part cov- ering the top of the alveolar ridge, as a means of fastening the teeth, which are attached with rubber or celluloid. Sets of teeth made in this way have been known to do good service for eight or nine years, but they showed unmistakable evidence of the action of the oral fluids. In the second method the plate is cast, but disintegration in this case progresses with much greater rapidity. As the plate is cast separate from the teeth, and the latter are afterward attached by means of tin or an alloy of tin and aluminum, it is probable that the galvanic action incident to the pres- ence of the two metals greatly hastens solution of the plate. For some time the difficulty of soldering aluminum prevented the metal from being applied to useful pur- poses. The solder recommended for general use in the manufacture of articles of ornamentation is com- posed of copper, four parts ; aluminum, six parts ; zinc, ninety parts. The use of this requires some skill and experience. At the moment of fusion small aluminum tools are used, the friction of which is necessary to induce adhesion. Borax cannot be em- ployed as a flux, as it is liable to attack the metal and prevent union. Another method of uniting two pieces of aluminum with ordinary solder in conjunction with silver chlorid as a flux has recently been recommended by F. J. Page and H. A. Anderson, of YVaterbury, Conn. The finely 254 DENTAL METALLURGY. powdered fused silver chlorid is spread along the lines of junction, and the solder is melted with a blow-pipe or other device. The union thus obtained is said to be perfectly strong- and reliable.* The following alloys are also used as solders in unalloyed aluminum articles of jewelry : I. II. III. IV. Zinc . 80 85 88 92 Aluminum 20 15 12 8 In soldering with these alloys a mixture is used as a flux consisting of three parts copaiba balsam, one part Venetian turpentine, and a few drops of lemon- juice. The soldering-iron is dipped into the mixture. Mr. Wm. Frishmuth, of Philadelphia, recommends the following solders for aluminum, with vaselin as the flux : Soft Solder. Pure Block Tin . . . from 99 to 90 parts. Bismuth . . . . " 1 " 10 " Hard Solder. Pure Block Tin . . . from 98 to 90 parts. Bismuth . . . . " 1 " 5 " Aluminum . . . . " 1 " 5 " Schlosserf recommends two solders containing aluminum as especially suitable for dental laboratory use : Platinum-Aluminum Solder. Gold 30 parts. Platinum 1 " Silver 20 " Aluminum 100 " * Chemical News, iv, 81. t Richards. ALUMINUM. 255 Gold-Aluminum Solder. Gold .50 parts. Silver 10 " Copper 10 " Aluminum 20 " O. M. Thowless has patented the following solder for aluminum, and method of applying it : Tin 55 parts. Zinc 23 " Silver 5 " Aluminum 2 " The silver and aluminum are first melted together, the tin and zinc are then added in the order named. The surfaces to be soldered are immersed in dilute caustic alkali or a cyanid solution, and then washed" and dried. They are next heated over a spirit-lamp, coated with the solder, and clamped together ; small pieces of the solder being placed at the points of union, the whole is then heated to the melting-point. No flux is used. The only oxid of this metal is alumina (A1 2 3 ). It is prepared by mixing a solution of alum with excess of ammonia. The resulting precipitate (aluminum hydrate) is of a bulky, gelatinous character, and re- quires to be calcined at a high temperature ; after which it may be described as a perfectly white powder, soluble in caustic potassa or soda, and not readily acted upon by acids. Corundum and emery are nearly pure alumina. The ruby and sapphire are also transparent varieties of alumina in a crystalline state, their brilliant colors being due to oxid of chromium. The only known sources of corundum until 1869 were a few rivers in India, where it occurred in crys- 256 DENTAL METALLURGY. tals having the form of the double six-sided cone. Its cost at that time was from twelve to twenty-five cents a pound. Since that date, however, it has been discovered in inexhaustible quantities in Georgia, North Carolina, and Pennsylvania. At present it can be bought at the mines at $10 per ton. For the discrimination of the salts of aluminum, see any of the recent works on chemistry. CHAPTER XIX. LEAD. Atomic Weight, 207. Symbol, Pb (Plumbum . THE reduction of lead is effected in a reverberatory furnace, in which the broken lead ore (galena) is roasted at a dull-red heat, by which means the sulfid becomes oxidized and converted into sul- fate. At this stage of the operation the contents of the furnace are thoroughly mixed and the tempera- ture raised, which causes the sulfid and sulfate to react upon each other, producing sulfurous oxid and metallic lead. Lead is the softest metal in common use, and may be said to be the least tenacious. In fusibility it also surpasses all other metals commonly employed in the metallic state except tin, the fusing-point of lead being 617 F. = 325° C. It is quite malleable and ductile, and will admit of being rolled into thin sheets or foil, in which form it was at one time much used in filling teeth. Its chief use in the dental laboratory consists in the formation of counter-dies. Alloys. — Lead unites with tin in all proportions, the resulting alloys being more tenacious and fusible than either constituent. By the addition of bismuth the fusing-point is reduced below the boiling-point of water. Lead amalgamates readily with mercury, conden- sation accompanying the union. The noble metals are all rendered brittle and unworkable by the pres- 257 258 DENTAL METALLURGY. ence of lead. There are some properties peculiar to alloys of lead and silver which are turned to advan- tage in the separation of silver from lead when it occurs as a native alloy. Lead combined with a con- siderable quantity of silver will remain fluid at a lower temperature than other specimens containing a smaller percentage, thus affording an opportunity for the poorer lead to crystallize, when it is ladled out.* The smallest proportion of lead in gold will greatly impair the ductility of the latter. Makins states that " Hatchett found that y-gVo" of lead destroyed the coining qualities of gold." Gold reduced to standard fineness by lead is light-yellow in color, and quite brittle. The contents of the dentist's gold-drawer are always liable to contamination by small pieces of lead, the latter being much used in the form of thin sheets in the making of patterns by which the gold or silver plate is cut. As the working qualities of the precious metals are seriously impaired by its presence, means should be instituted to insure its complete re- moval. This may be accomplished by cupellation, or by melting the gold or silver in a crucible, and adding nitrate of potassium when the point of complete fusion has been reached. Lead and platinum, like tin and platinum, appear to possess considerable affinity for each other, and an alloy of the two can be formed at a comparatively low temperature. An alloy of lead and platinum is very hard and brittle. With palladium also lead forms a very hard and brittle alloy. * See chapter on " Silver." LEAD. 259 The most valuable alloys of lead are those which it forms with tin, antimony, and bismuth, constituting solders, pewter, type-metal, etc. For the discrimination of lead, the student is re- ferred to Fownes's or other standard works on chemistry. CHAPTER XX. TIN. Atomic Weight, 118. Symbol, Sn (Stannum). THE metal tin has been known for probably three thousand years. It is found in all parts of the world, chiefly as oxid. In reducing the ore it is first powdered and roasted to free it of sulfur and arsenic. It is then exposed to a high temperature with charcoal, and the metal is thus liberated. Pure tin is white in color, and is perfectly soft and malleable. It has a. density of 7.3, and its fusing- point is 458. 6° F. (237 C). It is but slightly acted upon by air, but when heated much above its melting- point it oxidizes freely, and is converted into a yel- lowish-white powder, — the well-known polishing - putty. The action of nitric acid upon tin is to con- vert it into a white hydrated dioxid. It is dissolved by hydrochloric acid, assisted by heat, and forms stannous chlorid. Nitro-hydrochloric acid acts upon tin with much energy, converting it into stannic chlorid. Alloys. — Tin is readily dissolved in mercury (see page 50). With silver it forms a malleable alloy, which is considerably harder than tin. The late Dr. Bean used tin alloyed with a small percentage of silver for lower sets, which he cast directly upon the teeth after the ordinary cheoplastic method. Alloys of tin and silver, in which the former is slightly in excess, are much used as amalgam alloys. 260 TIN. 26l Tin 10, silver 8, gold 1, is also frequently employed in filling teeth ; and tin 10, silver 8, gold i, copper 1, has, according to Fletcher, been largely used as ' ' gold and platinum" amalgam. It is stated that from 5 to 7 per cent, of copper has the property of replacing platinum in amalgams, conferring the quick-setting quality claimed for platinum.'^ Dr. G. F. Reese has formed an alloy for a base for artificial dentures, composed of 20 parts of tin, 1 of gold, and 2 of silver. f This is cast directly upon the teeth, the process being similar to the cheoplastic method. The alloys which have been used in the cheoplastic process are chiefly composed of tin, silver, bismuth, and, in some instances, cadmium and antimony. According to Makins, gold and tin form a malleable alloy, X and gold reduced to standard by pure tin re- tains its malleability. Tin and platinum in equal proportions afford a hard and quite brittle alloy, fusible at a comparatively low temperature. When it is remembered that the fusing- points of these metals almost represent extremes of temperature, it would seem that their union must be attended with difficulty, but, as has already been stated, 1 1 it is probable that some affinity exists between the two, as platinum is readily dissolved by and alloys with the fused tin. * T. Fletcher. t Alloys and Amalgams Chemically Considered," J. Morgan Howe, M.D. % A precipitated alloy of gold and tin, having the form of a black pow- der, may be formed by acting upon a concentrated solution of tnchlorid of gold with stannous chlorid. See chapter on " Alloys." 262 DENTAL METALLURGY. With palladium tin is said to form a brittle alloy. With lead tin forms the chief part in the alloys used for soft-soldering, and in the compounds known as pewter and Britannia-metal. Tin solders are com- posed of two parts of tin to one of lead. Pewter consists of four parts of tin to one of lead, while Britannia-metal is formed by the addition of small quantities of antimony and copper. Alloys of tin and lead are harder and tougher than either metal singly, and they are more fusible than the mean of their constituents. The addition of bis- muth to such an alloy lowers the melting-point to a remarkable degree, and the fusing-point is still further reduced by the addition of cadmium. Thus, an alloy composed of 15 parts of bismuth, 8 of lead, 4 of tin, and 3 of cadmium, fuses at 145 F. = 63 C. Dr. C. M. Richmond used a fusible alloy in crown- and bridge-work which he states is as hard as zinc, andean be melted at 150 F. , and poured into a plaster impression without generating steam. The formula of this alloy is as follows : Tin 20 parts by weight. Lead 19 " " Cadmium . . . . 13 " " Bismuth . . . . 48 " " The following fusible-metal alloys are also suitable for the purpose : in. Lead. Bismuth. Melting-point of Alloy Fahr. I 2 2 2 3 6° 5 3 3 202° 3 5 8 197 Dr. George W. Melotte, of Ithaca, N. Y., to TIN. p 263 whom is due the credit of having introduced the use of fusible metal and the compound called " moldine" into bridge-work, uses an alloy of — Tin, 5, Lead, 3, Bismuth, S. Moldine, of which Melotte forms his matrix for casting, is a compound of potter's clay and glycerin. The alloy known as "Wood's metal," occasionally employed by dentists in replacing teeth on vulcanite plates, is composed of 7 parts of bismuth, 6 of lead, and 1 of cadmium, and fuses at 180 F. = 82 C, a point much below the boiling-point of water. In re- placing a broken tooth by means of Wood's metal the usual dovetail is cut in the rubber plate with a fine saw, the tooth is fitted to its place, and the fusible alloy is packed in with a spatula heated in a spirit- lamp. Lead 75, tin 5, and antimony 20 parts, is the com- position of the best form of type-metal. With copper tin affords a number of very useful alloys. Bell-metal is formed of 78 parts of copper to 2 of tin. Gun-metal is formed of 90 per cent, of copper to 10 per cent, of tin. Speculum-metal is formed of 6 parts of copper, 3 of tin, and 1 of arsenic. Babbitt-metal, named after Isaac Babbitt, of Bos- ton, Mass., is an alloy consisting of 9 parts tin, 10 parts copper, used for journal boxes {vide patent 1839). Many modifications have since been made in this alloy, but the term is still applied to any white alloy em- ployed in the construction of bearings, to distinguish it from the " bronzes" and " brasses." Mr. Fletcher recommends an alloy of copper 4 264 DENTAL METALLURGY. pounds, Banca tin 96 pounds, Regulus antimony 8 pounds. This alloy is said to be nearly as hard as zinc, while its shrinkage is much less. These quan- tities, together with the low temperature at which it fuses, entitle it to a place in the dental laboratory for the preparation of dies and counter-dies. Dr. L. P. Haskell recommends the formula, tin 72.72, copper 9.09, antimony 18.18. Bronze is an alloy of copper and tin, and some- times zinc. It is affected by changes of temperature in a manner precisely the reverse of that in which steel is affected, becoming soft and malleable when quickly cooled, and hard and brittle when allowed to cool slowly. The art of making bronze was practiced before any knowledge of the working of iron existed, and it was used at a .very early period in the manu- facture of weapons. Commercial tin is liable to contain minute quan- tities of lead, iron, copper, arsenic, antimony, bis- muth, etc. Pure tin may be precipitated in crystals by the feeble galvanic current excited by immersing a plate of tin in a strong solution of stannous chlorid. Water is carefully poured on so as not to disturb the layer of tin solution. The pure metal will be de- posited on the bar of tin at the point of junction of the water and the metallic solution. Perfectly pure tin may also be obtained by dis- solving commercial tin in hydrochloric acid, by which it is converted into stannous chlorid. After filtering, this solution is evaporated to a small bulk, and treated with nitric acid, which instantly converts the stannous chlorid into stannic oxid. This is thoroughly washed and dried, and exposed to red heat in a cruci- TIN. 265 ble with charcoal. A button of pure tin will be found at the bottom of the crucible. Pure tin in the form of foil is frequently used in fill- ing teeth, for which purpose it doubtless ranks next to gold. Tin foil is also employed in connection with non-cohesive gold in filling approximal surfaces of cavities in bicuspids and molars. Two sheets of foil, one of gold and the other of tin, are placed together and made into mats or cylinders. These are carefully packed against the cervical margins of the cavity. The frequent failure of ordinary gold fillings at this point has led some practitioners to entertain the theory that between the tooth-substance and the gold there is galvanic action, to which the lime-salts of the tooth yield, and that by the combination of two metals, whether tin and gold or amalgam and gold, the gal- vanic action is confined to the metals, the tooth-sub- stance being thus protected. The appearance of a filling formed of tin and gold would seem to confirm this theory, as it soon becomes dark in color, and presents a surface resembling amalgam, but it effectually protects the margins from decay.* Tin having but slight affinity for sulfur, is largely- used in the formation of models in the construction ot vulcanite dentures, and tin foil forms the best coating for plaster casts in the vulcanizing process. The manufacturers of miscellaneous rubber articles do not use plaster in forming the matrix in which the rubber is packed before vulcanizing ; having long since discovered that contact with plaster lessens the * Prof. James Truman, Report of Proceedings of Odontological Society of Pennsylvania, November, 1881. 18 266 DENTAL METALLURGY. toughness and elasticity of the indurated rubber, they therefore form every matrix of sheet tin, which is placed in a suitable iron box and covered tightly with dry powdered soapstone (steatite). Dr. J. S. Campbell, who introduced the vulcanizer known as the ' ' New Mode Heater, ' ' described a means whereby all parts of the matrix contained in the flask in constructing rubber dentures could be covered by sheet tin, so that after vulcanizing and the removal of the sheet tin or foil the surface of the rubber would be found to be smooth and highly polished, and if the ' ' waxing up' ' had been carefully done, little or no filing and scraping were needed. The method demonstrated by Dr. Campbell possessed precision and saved labor, and it is to be regretted that it was not generally adopted by mechanical dentists, who have not improved to any extent upon the slovenly methods employed in 1858, when indurated rubber was first employed in dental practice. When tin foil is used as a coating for plaster casts in rubber work, the foil may be removed with the finger- nail if the surface of the plaster model was smooth and hard, and this condition of the plaster surface can be obtained by using nothing as a coating for the plas- ter impression but sandarac varnish, and carefully avoiding the use of oil or solutions of soapas'a means of separating the impression from the model. If, however, in consequence of the roughened surface of the plaster cast the tin foil adheres so tenaciously that it] cannot be removed except by means of a solvent, hydrochloric acid is the only one that will accomplish that end without injury to the rubber. Both nitric and nitro-hydrochloric acids should be avoided, as TIN. 267 they attack indurated rubber with more or less en- ergy. Lower vulcanite dentures may be loaded with tin to give them additional weight, and by lessening the quantity of rubber prevent the occurrence of porosity during the process of vulcanizing. Solve7its. — Tin is readily dissolved by either of the three mineral acids. Sulfuric acid converts it into stannic sulfate. Tin dissolved in hydrochloric acid forms stannous chlorid. By the action of dilute nitric acid tin is not dissolved, but is converted into stannic oxid, which settles to the bottom of the vessel as a white powder. This, when rendered anhydrous by heating to redness, affords the well-known polishing- powder called " polishing-putty." Chlorids. — There are two chlorids of tin, — stannous chlorid or protochlorid of tin (SnCl 2 ), and stannic chlorid or bichlorid of tin (SnClJ. Stannous chlorid is prepared by dissolving tin in hydrochloric acid, the action being assisted by gentle heat. Stannic chlorid is obtained by dissolving tin in nitro- hydrochloric acid (aqua regia). These two compounds of tin are employed in the preparation of purple of Cassius, in which process stannous chlorid is added to a mixture of stannic chlorid and trichlcrid of gold (see page 149). For other compounds of tin, see works on chemistry. Discrimination. — Tin is detected before the blow- pipe by fusing the compound under examination on charcoal with sodium carbonate, when a bead of the metal is obtained. From a tin solution caustic potash and soda precipitate a white hydrate, soluble in excess. Ammonia affords a similar precipitate, not soluble in 268 DENTAL METALLURGY. excess. Hydrogen sulfid and ammonium sulfid throw down a dark-brown precipitate of monosulfid. Tri- chlorid of gold added to a dilute solution of stan- nous chlorid causes a purple precipitate (purple of Cassius). CHAPTER XXI. ELECTRO-METALLURGY. THE origin of electro-metallurgy was undoubtedly due to the early experiments of Wollaston and Davy, while the credit of its development belongs to the late Professor Daniell, who devised the particular form of battery which bears his name. A Daniell cell consists of a copper vessel containing a saturated solution of sulfate of copper. In this is placed a porous cylinder containing dilute sulfuric acid. A rod of amalgamated zinc is immersed in the acid, and on the two metals being connected, electrical action is immediately set up, and the zinc, which forms the positive element, is dissolved, with formation of sul- fate of zinc ; the sulfate of copper is reduced, and the metallic copper is deposited upon the surface of the copper vessel, which forms the negative element of the combination. It was observed that the copper thus deposited took the exact shape of the surface on which it was thrown, presenting a faithful counterpart of the slightest indentation or irregularity. De la Rue called attention to this fact in a paper published in 1836, but no practical application was made of it until 1839, when Professor Jacobi, of St. Petersburg, published his discovery of a means of producing copies of engraved copper plates by the agency of electricity. In 1840 Mr. Murray announced that an electro- deposit of metal could be formed upon almost any 269 270 DENTAL METALLURGY. material, provided its surface was rendered a conductor of electricity by a thin coating of graphite (black lead). Instead of copying the object in a metallic medium, it is only necessary to take a cast in plaster of Paris, wax, gutta-percha, or any convenient material, and then to coat the surface with finely-powdered graphite applied with a camel' s-hair pencil. The Gramme machine, a modification of the mag- neto-electric apparatus, consists of a ring of soft iron carrying a number of coils of insulated copper wire, caused to rotate between the poles of a fixed horse- shoe magnet. The currents induced in the coils are collected by two metallic disks, whence they may be drawn off for use in electro-deposition. The core being circular, the magnetization proceeds contin- uously, affording a uniform current. Both poles of the magnet are used, producing simultaneously two opposite continuous currents. These and similar sources of electricity enable the electro-metallurgist to deposit a metal upon a matrix or to coat one metal with another. The art of electro-metallurgy is divided into two branches, electrotypy and electro-plating. In the former the reduced metal is separated from the mold on which it is deposited, forming a distinct work of art, while in the latter the deposited metal forms an inseparable part of the plated object. Electrotypy is employed in producing copper duplicates of engrav- ings on wood and of any kind of type-matter for printers' use. A cast of the object is first taken in wax or gutta-percha, and the surface of this mold is brushed over with black-lead, and it is then, by means of a wire, suspended in a bath of sulfate of copper ELECTRO-METALLURGY. 271 connected with a battery. Since the introduction of dynamic electricity, perfect copies may be produced in an hour or two, and the deposited metal made thick enough to stand any reasonable amount of wear. Electro-plating, by which silver is deposited on the surfaces of objects of copper, brass, or German-silver, was introduced soon after the discovery of the art of electro- metallurgy. The article to be plated must first be thoroughly cleansed by immersing it in a hot solution of caustic potash, and also by means of the "scratch-brush," and sometimes by "pickling" it in a bath of nitric and other acids. The surface is then given a thin film of mercury, by washing the article with a solution of mercuric nitrate. This is called "quicking." The article is then rinsed with water and transferred to the silver bath, which consists of a solution of cyanid of silver in cyanid of potassium, — a very poisonous compound. Plates of silver are sus- pended from a rectangular frame connected with the positive pole, while the articles to be plated are sus- pended by wires attached to the negative pole. The quantity of silver to be deposited depends upon the requirements of the case. One ounce of silver per square foot forms for ordinary purposes a sufficiently heavy coating. Electro-gilding is effected by means similar to those employed in electro-silvering. The solution employed is generally the double cyanid of gold and potas- sium, and it is used hot, the temperature ranging from 130 F. to 212 F., according to the ideas of the operator. Nickel-plating was first introduced in 1869, by Dr. Isaac Adams, of Boston, who patented a process for 272 DENTAL METALLURGY. depositing nickel from solutions of double salts of sulfate of nickel and ammonium. Iron may also be deposited from the double sulfate of iron and ammonium. Practical application is now made of this discovery, and engraved copper plates are found to be much more durable when faced with electro-deposited iron. Plates for printing bank-notes are sometimes treated in this way. INDEX. PAGE Acid, nitric, action of on silver 177 nitric, in the quartation process of refining gold 125 nitro-hydrochloric (aqua regia) 132 sulfuric, action of on silver 170 sulfuric, in the quartation process of refining gold... 125 Acids, action of on alloys 45 Agents which may volatilize a metal 33 Alloying, influence of. 32, 39 the purposeof 35 Alloys 34 as definite compounds 36 color of 3S composition of 41 conductivity of. 26 decomposition of. 42 density of. 37 for solders 35 fusibility of 40 influence of constituent metals in 42 liquation of 43 malleability, ductility, and tenacity of. 39 Matthiesen's definition of. 37 of gold employed in dentistry as solders 141 of silver employed in dentistry as solders 185 oxidizability of 45 preparation of 44 properties of 35 specific gravity of. 38 study of • 34 table of 41 temper of 44 Von Eckart ' s 1 84 Alumina 244 Aluminum 244 273 274 INDEX. PAGE Aluminum, alloys of 248 annealing 248 bronze 41, 249 " preparing, Cowles's method of 247 " solders for 249, 250 casting, Bean's method of 251 casting, Carroll's method of. 251 Page's solder 255 reduction of. 245 solder for 253, 254, 255 solder for aluminum bronze 249 solvents of. 247 swaged plates of. 253 Amalgams 46 composition of some of the well-known 71, 72 definition of. 46 discoloration of ... 48 expansion of. 47 experiments with 49 for dental purposes 46 formation of. 46 forming alloys for 50, 59 formula for, Dr. Ambler Tees's 73 " Dr. L.Jack's 61,62,63, 64 influence of different metals in..... 50 introducing fillings of. 57 mixing, methods of. 56 palladium 65, 66 platinum 60 qualitative and quantitative examinations of 66 quantity of mercury to be used in 55 shrinkage of. 47 Sullivan's 66, 229 zinc 61 Ammonium 19 Argentiferous galena 173 Argentum (silver) 169 Arsenic in alloys 43 Bloxam's classification of. 18 INDEX. 275 PAGE Arsenic, odor of 20 Assay of amalgam alloys 66 Assaying gold 157 Atomic weights of metallic elements 14, 16 Auric chlorid 149 oxid 148 iodid 149 silicate 153 sulfid 149 Aurous chlorid 149 Aurum (gold) 116 Babbitt-metal 263 Beating gold 165 Bellows for blow-pipe 79, 80, 81 Bessemer steel 211 Black-lead crucibles 90 Blast furnaces 86, 87, 8S Blistered steel 210 Blow-pipes 76, 77, 78, 79 hot-blast 79 Knapp's S2 oxyhydrogen 190, 191, 192 supports for use with 97 Bone-ash cupels 175 Borax 96 Brass 23S Britannia-metal... 262 Brittle gold, treatment of. 129 Bromids, metallic .• 104 Bronze 264 Cadmium 242 precipitation of 67 Calamine 235 Calomel 217 Carbon, proportion of, in cast iron and steel 210 Case-hardening 216 Cast iron 209 276 INDEX. PAGE Cast steel 210 Charcoal as a reducing agent 111 Chlorids, metallic 103, in preparation of 104 reduction of. •• in Cinnabar 217 Coke for supports in soldering 99 Color of metals • 19 influence of alloying on 38 Copper 227 alloys of . 231 amalgam 229 as a constituent in amalgams. 229 discrimination of 233 properties of. 228 solvents of • 228, 229 value as a therapeutic agent 231 Copper steel 212 Corundum 255 Crucibles •• 89 Crystallization 31 Cupellation.... 175, 176 Cupels 175 Cyanids, metallic 103 Dental alloy 183 Ductility of metals 27, 28 influence of alloying on = - 39 Electro-deposit of metals 269 Electrolysis 115 Electro-metallurgy..... 269 Electrotypy, origin of 269 Elements, metallic 13 Emery 255 Flame, blow-pipe, management of 76 Fletcher's hot-blast blow-pipe 78, 79 Fluorids, metallic 105 INDEX. - 277 PAGE Flu * 151, 152 Forging platinum 193 Fulminating gold J54 silver 179 Furnace, lime, for melting platinum 192 Cowles's electrical - 109, 247 Furnaces 85, 86, 87, 88 Fusible alloys used in bridge-work 262, 263 Fusible metal 262, 263 Fusing-points of metals 21 Galena, argentiferous. 173 Gauge-plate 93 Gold 116 alloys of 136 chlorids of. 148 clasps for partial artificial dentures 140 coin 136, 137 compounds of 148 containing iridium 130 discrimina tion of 1 54 foil, cohesive • 163 foil, non-cohesive 161 in combination with tin in filling teeth 265 Lamm's shredded 134 native, formsof. 121 oxids of. 14S precipitants for 133 precipitated, different forms of 134 precipitation of, by ferrous sulfate 135 precipitation of, by oxalic acid 134 precipitation of, by sulfurous acid 135 properties, occurrence, and distribution of 117 pure, preparation of 130 quartation process of refining 124 reducing, to a higher or lower carat 143 refining 124 swaging, when alloyed with platinum 140 volatilizing 33, 120 278 INDEX. PAGE Gold, Watts's crystal 136 welding properties of 119 Gum frit 149 Gun-metal 263 Hot-blast blow-pipes 79 Hydrogenium 18 Ingot-molds 92 Iodids, metallic 105 Iridium 199 Iron 207 compounds of 209 fusing-point of 208 meteoric 207 native - • 207 properties of • 207 solvents of 208 Knapp's oxyhydrogen blow-pipe 82 Lamps, soldering 75 Lead 257 alloys of 257 amalgamation of 257, 258 desilvering of '• 173, 258 discrimination of. 259 properties of 257 reduction of. 257 Magnetic iron 208 Malleability . 27, 39 influence of alloying on 39 Melotte's fusible metal 262 Mercury 217 adulterations of 218 chlorids 222 compounds 222 INDEX. 279 PAGE Mercury, discrimination of 225 filtration of 220 native 217 occurrence of 217 ores, reduction of 217, 218 oxids of. 222 properties of 220 pure, to obtain 219 purification of, by digestion 220 purifying, Priestley's plan of. 220 quantity of, to be used in amalgams 55 redistillation of- 218 sources of 217 sulfids of 222, 223 Metallic bromids 104 Metallic chlorids 103 cyanids 103 fluorids 105 iodids 105 oxids 105 sulfids 10S Metallurgy, definition of... 9 Metals, base 16 capacity for heat of 22 color of 19 conduction of electricity of 25 conduction of heat of 25 effects of alloying on 35 elasticity and sonorousness of 32 expansion of, by heat 23 fusibility of 20 fusing-points, table of 21 luster 19 malleability, ductility, and tenacity of. 27 modes of melting 74 noble 16 odor and taste of 20 properties of 17 tableof 14 280 INDEX. PAGE Metals, volatility of. 32, 33 volatility of, agents which induce 33 Moldine 263 Nickel-plating 27 Oxids, metallic 105 reduction of 113 Palladium 202 alloys of. 203 amalgams 65, 66, 204 cost of -. 205 discrimination of 206 influence of, in amalgams 65, 204, 205 properties of 203 sources of- • ••• 202 Pewter 259 Phosphor-bronze 43, 232 Phosphor-iridium 200 Platinum 187 alloys of 194 amalgamation of.... 194, 195 associate metals with 187 chloridof. 197 discrimination of 197 melting, Deville's furnace for 190 oxids of 196 preparing, Wollaston's method of 187 properties of 19 2 pure 19 1 solvents of 194 welding 193 Polishing-putty 260 Purple of Cassius i49> I 5° Quartation process of refining gold 124 Reduction of metals..- 109, no, in, 112, 113 INDEX. 28l PAGE Refining gold 124 Reinsch's test 115, 225 Richmond's alloy for crown- and bridge-work 262 Rolling mills 93 Ruby 244, 255 Sapphire 244, 255 Scorification 157 Silicate of gold 153 Silver 169 alloys of. 1S2 chlorid of 178 compounds 177, 178 cupellation of 177 deposition of, by battery 186 discrimination of 17S estimation of 179 native 171 nitrate of.. 177 oxid of 177 precipitation of, by copper 1S1 precipitation of, by iron 180 precipitation of, by sodium chlorid 180 precipitation of, by zinc 181 properties of 169 pure 180 separation of, from ores 171 soldering 94, 95 solders for 184, 185 solvents of 170, 177 spitting of, during fusion 170 sulfate of 170 sulfid of 177 Solders for aluminum .2^9, 254, 255 gold silver 184, 185 soft. 254 Specific gravity of alloys 38 Speculum-metal 263 19 282 INDEX. PAGE Stannic chlorid < 260 Stannous chlorid. 264 Stannum (tin) 260 Steel....: 209 aluminum 212 Bessemer 211 blistered 210 cast 210 chrome 212 copper 212 discrimination of 216 hardening 214, 215 manganese 214 nickel 213 shear 210 tempering 214, 215, 216 tungsten. 211 Sulfids, metallic ....- 108 reduction of 112 Supports for use in melting and soldering 97, 98, 99 Tellurium ■ 17 Tenacity 28, 39 influence of alloying on 39 Tin, alloys of 260 amalgamation of 260 chlorids of 267 discrimination of 267 estimation of 66 foil 265 in amalgams 5° oxid of. 66 pure, preparation of- 260 refining . 264 solvents of. • 267 Titanium • x 9 Type-metal 263 Vermilion 22 3 INDEX. 283 PAGE Von Eckart's alloy 184 Welding properties of platinum 192 Wire-drawing 93 Wood's metal. 263 zinc 235 alloys in dentistry 236 alloys of 236 counter-dies 238, 239 dies for swaging plates 237 discrimination of 241 expansibility of 23 in amalgams 62, 64, 236 'oxychlorid of. 241 properties of. 235 J~Z <^J^. C ^-6 J^A^W^L, *•- ^5 tZll^C. /&. F" /I COLUMBIA UNIVERSITY LIBRARY This book is due on the date indicated below, or at the expiration of a definite period after the date of borrowing, as provided by the rules of the Library or by special ar- rangement with the Librarian in charge. DATE BORROWED DATE DUE DATE BORROWED DATE DUE fHOV i% v - w ■ ~ - C28(239)M100 IRK653 Lssig E«7 189' Dental raettalurgj COLUMBIA UNIVERSITY LIBRARIES (hsl.stx) RK 653 Es7 1893 C.1 Dental metallurgy 2002457350