Franklin Institute Library 
 
 FHIL/lbELFHm 
 
 Class 66 GSbooIcB.^ 1.5 Accessipn 5Z.O/j3 
 
CERAMIC TECHNOLOGY 
 
ABERDEEN UNIVERSITY PRESS. 
 
CERAMIC TECHNOLOGY 
 
 BEING 
 
 SOME ASPECTS OF TECHNICAL SCIENCE 
 AS APPLIED TO POTTEEY 
 MANUFACTUPE 
 
 EDITED BY 
 
 C H A E ii.H\\B\: ^js 
 
 LONDON 
 
 PUBLISHED AT THE OFFICES OF THE POTTERY GAZETTE 
 
 BY 
 
 SCOTT, GREENWOOD & CO. 
 
 19 LUDGATE HILL, E.G. 
 
§■07 
 
TABLE OF CONTENTS. 
 
 PAGK 
 
 Preface vii 
 
 Introduction . . ix 
 
 Chapter I. 
 
 The Chemistry of Pottery 1 
 
 Chapter II. 
 
 Analysis and Synthesis 10 
 
 Chapter III. 
 
 Potters' Clays ; and their Components 29 
 
 Chapter IV. 
 
 The Biscuit Oven . , . . .. , . . .43 
 
 Chapter V. 
 
 Pyrometry . . ' 56 
 
 Chapter VI. 
 
 Glazes and their Composition 64 
 
 Chapter VII. 
 
 Colours and Colour Making 78 
 
 Index 99 
 
PREFACE. 
 
 In compiling this work the publishers have en- 
 deavoured to keep clearly before them the purpose 
 of assisting, in as simple and intelligible a way as 
 possible, those who desire to attain to some fuller 
 understanding of the manifold problems of the 
 Potter. 
 
 The book is, confessedly, imperfect. Science 
 is making rapid strides, and to keep pace with 
 these is no easy task. 
 
 Since the days of Simeon Shaw nothing 
 dealing with the technical science of pottery 
 has been published in England, and the pro- 
 prietors of the Pottery Gazette, in offering the 
 present work to their clients, feel that, while they 
 could not refrain from making a beginning, it is 
 only a first step that has been taken. 
 
 The authors wish to express their indebtedness 
 to the published works of Brogniart, Salv^tat, 
 Seger, Langenbeck and Percy, and also to the 
 pages of Die Thonindustrie Zeitung and the Hand- 
 book to the Pottery in the Geological Museum. 
 
Vlll PREFACE. 
 
 They have attempted to gather and sift scientific 
 information from every possible source, and if they 
 have used any such v^ithout acknowledgment they 
 can only crave absolution. 
 
 Much, after all, depends upon the use that is 
 made of knowledge. To a wise man a hint is 
 sufficient, and it is hoped that those who read this 
 volume will find somewhat whereby to profit. 
 
 Strict scientific terms have been purposely 
 avoided. To the learner they often cause con- 
 fusion, to the learned they are unnecessary, so 
 that those who are disposed to smile at what they 
 regard as the simplicity of the writers are entreated 
 to remember that all men are not equally wise. 
 
 19 LuDGATE Hill, 
 July, 1897. 
 
INTRODUCTION. 
 
 The advances made in technical education during 
 recent years, more especially in Germany and the 
 United States, render it imperative that Great 
 Britain should be roused to an appreciation of 
 the fact that her commercial position is seriously 
 assailed. 
 
 It would have been possible a few years ago 
 to assert that the British workman was still 
 supreme, but in an authoritative publication it is 
 stated that " the great industries of to-day depend 
 more and more upon the successful application of 
 recent discoveries to ordinary manufacturing, pro- 
 cesses, and less and less upon the presence of 
 coal, iron, and raw materials. Cheaper and more 
 speedy means of transit are placing all countries 
 more nearly on a level as regards natural resources. 
 Improved tools and labour-saving machinery are 
 rapidly rendering the manual skill and dexterity 
 of the individual workman of minor significance, 
 and in the industrial race in which all are engaged 
 nearly all the advantages upon which Great Britain 
 prided herself in the past are possessed in a greater 
 
 or less degree by her rivals, and count for little as 
 
 6 
 
X 
 
 INTRODUCTION. 
 
 compared with scientific knowledge and its ready 
 application to the needs of the manufacturer."* 
 These are wise words, and it will be well if they 
 are laid to heart by all who are interested in the 
 commercial welfare of this country and her in- 
 habitants. Germany long ago made strenuous 
 efforts for the complete scientific and technical 
 education of her people, and so far from relaxing 
 those eff'orts to-day she is continually exerting all 
 her powers in the same direction. 
 
 From the United States comes the report that 
 at one of the Universities a class in ceramics has 
 been established with the view of turning out, 
 after a two or a four years' course, men competent 
 to take charge of any department in practical 
 potting. These things evidence the opinion fixed 
 in the minds of all thoughtful men that education 
 is the foundation of progress. 
 
 Potters in England are not, perhaps, worse 
 than other manufacturers, except than in their 
 particular work there is unlimited scope for the 
 operations of science, but it is to potters that 
 these lines are addressed and therefore their short- 
 comings are more especially alluded to. 
 
 In former years a certain monopoly of enter- 
 prise and skill founded and extended the Staff'ord- 
 
 * Report on a visit to Germany with a view of ascertaining 
 the recent progress of technical education in that country. 
 London : Eyre & Spottiswoode, 1896, p. 10. 
 
INTEODUCTION. 
 
 XI 
 
 shire potteries, but the path was thus made clear 
 to any one who wished to follow it, and however 
 complete may have been the monopoly in the past 
 it has most certainly been broken down. The 
 competitor, excellently trained, and provided with 
 all the resources of modern science, has come to 
 the front, and, incredible as it may seem, the 
 British Parliament has, in the Merchandise Marks 
 Act, afforded him abundant advertisement free of 
 charge. 
 
 It may not be possible for England to regain a 
 monopoly, from the consumer's point of view it 
 may not even be desirable ; but it is not yet too 
 late for British manufacturers to arise and, by a 
 develoj^ment of the powers which they undoubtedly 
 possess, to smite the foe with a great discomfiture. 
 
 It would appear, at first, to be necessary to 
 arouse those who think, to the importance of 
 education, and it is partly for this purpose that 
 this book has been published. It is hoped that by 
 a perusal of its pages some may be led to under- 
 stand that, even if they do not agree with what 
 has been written, there are at least two sides to 
 the question. But in the present state of rivalry 
 existing amongst potters, what possibility is there 
 of discussion ? What likelihood is there that a 
 well-conceived and unanimously supported scheme 
 of advancement will be formulated ? The require- 
 
xu INTEODUCTION. 
 
 ment seems to be, SiS Si preliminary, a central re- 
 search laboratory, to which any accredited person 
 may have access, and where may be duly published 
 the results of investigations. It would not be 
 difficult for any practical potter to put two or 
 three questions, the examination of which would 
 keep an investigator at work for months, and of 
 which the satisfactory solutions would be the 
 means of saving large sums of money to the pot- 
 ting industry. At present the rivalry of trade 
 seems to prevent any one seeking the advice of a 
 possible competitor, and until the initiative is 
 taken by some executive authority there will be 
 but little hope of progress. 
 
 The old days of laissez faire are passing away. 
 Every competing nation is keenly alive to the 
 necessity for education, and England must not be 
 the last. 
 
 The problems of profitable potting are numerous 
 and important, and if they cannot be solved single- 
 handed English potters must combine for their 
 mutual benefit. 
 
 An attempt, however imperfect, has been made 
 in the present volume to throw Hght upon some 
 difficulties, in the ardent hope that the day may 
 soon dawn when the troubles and losses which now 
 chng as a nightmare to the shoulders of the ceramic 
 industry of Great Britain shall vanish as a dream. 
 
CERAMIC TECHNOLOGY. 
 
 CHAPTER I. 
 
 THE CHEMISTRY OF POTTERY. 
 
 Chemistry is the science which deals with the 
 composition of substances rather than with their 
 condition. It also defines the relationship in 
 which one substance stands to another and 
 governs their mutual reactions. 
 
 The ceramic chemist has, however, almost as 
 much concern with physical as with chemical laws, 
 inasmuch as he is face to face continually with the 
 effects and operations of fire. 
 
 The success of the potter depends largely upon 
 the skill with which he can combine his materials 
 so that they may yield him the result for which he 
 strives and may do so with the smallest possible 
 loss. This skill should be founded upon expert 
 knowledge, which is, after all, only another term 
 for science. 
 
 Much has been said and written upon chemistry 
 as applied to pottery, which, to the average 
 
.2 
 
 CEEAMIC TECHNOLOGY. 
 
 manufacturer, is perfectly useless by reason of the 
 fact that the authors assume a degree of information 
 not possessed by the reader. The attempt will be 
 made here to avoid this error in the belief that it is 
 better, all things considered, to be too simple than 
 to be too abstruse. 
 
 Chemical knowledge is of use to the potter 
 throughout the whole of his operations. It 
 controls the mixtures of his body, glaze and colour, 
 the purity of his materials, and the composition of 
 gases in his kilns. It will enable him to lay his 
 finger upon weak points and to arrest losses, 
 besides putting into his hand a new power of 
 I>roducing combinations hitherto unknown. 
 
 An attempt will be made in the present chapter 
 to indicate the functions of the materials in general 
 use, and some of the more simple empirical tests 
 will be alluded to. Chemical analysis is not 
 always within the reach of the manufacturer, and, 
 even if it be available, it either teaches nothing or 
 it teaches too much. To take advantage of the 
 story of an analysis other things must be taken 
 into account, and it is hoped that what is said here 
 will prove of practical value. 
 
 I. — Materials. 
 
 Ceramic composition is a matter of com- 
 promise. Certain constituents in a body are 
 
THE CHEMISTRY OF POTTERY. 
 
 3 
 
 plastic, certain are " short ". Others are easily 
 melted, others again are infusible. It is the 
 judicious combination of these qualities that makes 
 the best mixture. 
 
 At the head of the materials used by potters all 
 over the world china clay (kaolin) and china stone 
 (petuntse) stand supreme. Geologists have not 
 yet accurately determined the relations between 
 these two substances, but that the clay is in some 
 sort dependent upon the stone is almost generally 
 accepted. 
 
 The pure form of kaolin would be a silicate of 
 aluminium,'" but in practice this does not exist. 
 Mechanically associated with it are minute portions 
 of foreign substances, quartz, lime, alkalies, and 
 occasionally a little magnesia. The potter has 
 usually nothing to do with the preparation of the 
 clay. It comes to him ready for use. It is, at the 
 same time, important that every consignment 
 should be properly sampled, and instructions on 
 the best methods of accomplishing this will be 
 found in the chapter on clays. 
 
 Petuntse or Cornish stone needs a somewhat 
 different method of testing. The quality varies 
 
 *The pure hydrated silicate of aluminium has been named by 
 Messrs. Johnston and Blake kaolinite," and is assumed to be 
 the basis of all clays. It corresponds to the formula AI2 O3 2 Si O2 
 + 2 H2 0. 
 
4 
 
 CERAMIC TECHNOLOGY. 
 
 considerably, and for many years a stone much 
 harder to break and softer to fire has been 
 extensively used. The main difference between 
 this and the yellow crumbling stone is in the 
 amount of alkalies contained. It is probable that 
 all the stone was once in the hard blue state, but 
 the action of water has resulted in a partial decom- 
 position and solution of the combined alkaline 
 salts. These being washed away the stone begins 
 to disintegrate, and it is found that the loss of 
 alkali ranges from 3 to 4 per cent, or even higher. 
 This, of course, constitutes a serious difference in 
 the fusibility of the stone, and each new consign- 
 ment of each kind of stone should be carefully 
 tested. The main points to be considered are 
 fusibility and colour. A small piece of the stone 
 fired at top biscuit heat should show very few of 
 the hard yellow particles so often found, and the 
 felspathic parts should have melted almost clear. 
 A portion of the stone may also be finely ground 
 and mixed with a little clay. Of course a definite 
 quantity of both stone and clay must be taken. 
 This can then be cast or pressed into small cups 
 for firing in the biscuit oven. The translucency of 
 the fired mixture will afford a good criterion of the 
 fusibility of the stone. All fire tests are, however, 
 useless unless the degree of heat can be ascertained 
 and uniformly adhered to. For this purpose there 
 
THE CHEMISTRY OF POTTERY. 
 
 5 
 
 is nothing better than the pyrometric cones, to be 
 presently dealt with. 
 
 Bone is a material that is more easily tested 
 in the sample than in the mixture. It may be 
 taken as a general rule that the best bone is the 
 cheapest. Bone ash usually contains a consider- 
 able proportion of foreign matter, and this not only 
 displaces a certain amount of the bone which 
 should enter the mixing tank, but also exerts a 
 deleterious influence upon the body, both in the 
 matter of colour and fusibihty. A rough and 
 ready test as to the comparative values of coarse 
 and fine bone may be readily made by sifting a 
 small quantity through a fine sieve and then 
 mixing two trials of body clay, using in one case 
 the coarse bone, and in the other the fine. The 
 body mixed from the fine bone will be neither so 
 white nor so hard as the other. Thus, though the 
 first cost of good unbroken bone be greater, the 
 ware made with it is much better, and there is less 
 oven loss. 
 
 Felspar and flint are both fairly uniform in 
 character and composition, and when any momen- 
 tous change takes place in the quality of a con- 
 signment, the appearance of the raw material will 
 indicate it. But some manufacturers purchase 
 ground spar and flint, and to such it must be 
 important to secure purity. Any deleterious sub- 
 
6 
 
 CEBAMIC TECHNOLOGY. 
 
 stance in the flint may readily be detected by 
 simply firing a small portion in the highest avail- 
 able heat. If the flint stand this without sign of 
 fusion it is suitable for any purpose. Fired at the 
 heat of china biscuit oven, pure spar should melt 
 down almost to transparency. It should exhibit a 
 pearly white colour and be entirely free from spots.* 
 The effect of spar in a mixture may be tested in 
 the manner described for stone. To a pure china 
 clay felspar will impart translucence at a sufficient 
 heat, and the colour of the mixture and amount of 
 fusion will at once indicate the quality of the 
 material. 
 
 II. — Materials in Combination. 
 
 A knowledge of the function fulfilled by each 
 material in a mixture is of the highest importance 
 to the practical potter. Without this knowledge 
 all his work is more or less hap-hazard, and 
 success is more a matter of good luck than good 
 guidance. A pottery " body " exhibits a very close 
 analogy to the body of a living creature in that its 
 several constituents fill the position of flesh, blood, 
 and bones. The flesh is, of course, the clay. This 
 
 * It will, of course, be understood that the tests spoken of 
 here are only those that could be readily undertaken in an 
 average manufactory. Where doubt exists as to purity a 
 chemical analysis should be made. 
 
THE CHEMISTKY OF POTTERY. 7 
 
 it is which gives texture and mobility. This, 
 enables a mixture to be called a " body " at all^ 
 and gives it form and feature. The bones are 
 supplied by the actual bone in porcelain or by flint 
 in earthenware, the former being chosen on account 
 of the stiff infusibility which gives to porcelain its 
 framework and enables it to stand the fierce fire. 
 The functions of the blood are fulfilled by the 
 fusible ingredients, stone or spar. These give life, 
 translucency, vitrification, ring, and fracture. It is 
 these that give to a body its quality and superior 
 nature. These fusible ingredients melt, and flow 
 through the particles of flesh and bone, . uniting 
 them into one, and endowing them with a living 
 personality. 
 
 It is essential then that these three principles 
 should be combined with due regard to their 
 special offices. The clay, whether as kaolin or 
 ball clay, wdll constitute the working properties. 
 Clay is not altered in the fire beyond the fact 
 that its water of combination is driven away, but 
 the fire leaves it in a porous state, and this porosity 
 must be overcome, at least to some extent, by 
 the addition of fusible matter. Without such 
 addition the clay will exhibit no strength or 
 quality, its fracture will be similar to that of a 
 tobacco-pipe, and it will be ill fitted for hard 
 wear. Either stone or spar will supply the 
 
8 
 
 CERAMIC TECHNOLOGY. 
 
 necessary flux. In the former case a larger 
 proportion will be needed, as the percentage of 
 alkali is smaller, and it is this alkali— usually 
 potash — in both stone and spar which does the 
 work.* According, then, to the amount of vitri- 
 fication needed the fusible material is added, but 
 the presence of the alkali immediately calls for 
 a corrective. It is of the highest importance 
 that the potter's work should maintain its shape 
 in the fire, and this it will not do if the softening 
 effect of the flux remain unchecked. For earthen- 
 ware and so-called semi-porcelain flint will serve. 
 The silica of which this is composed is infusible at 
 low temperatures, and besides the dense nature of 
 the flint itself helps in the strength of the body. For 
 higher temperatures bone is generally used. Upon 
 the intense fire the quality of porcelain depends, 
 and, therefore, it is necessary to render the body 
 sufficiently stiff to stand a severe heat. This the 
 bone does, and, while it upholds the ware, the fire 
 drives the fluxing influence of the potash in stone 
 or spar through the whole mass of the body, giving 
 quality, character, and translucence. While this is 
 the function of bone, and indicates its relationship 
 towards the other constituents of the body, there 
 
 * In practice it will be found that to equal 100 parts of stone 
 it is necessary to take 34 parts of spar, 31 of china clay and 35 
 of flint. 
 
THE CHEMISTRY OF POTTERY. 
 
 9 
 
 are, of course, in the structure of the bone itself 
 certain advantageous features. The cellular form 
 of the animal organism is not entirely broken down in 
 grinding, and this imparts to the porcelain the light- 
 ness which is so greatly valued. Pure bone is also 
 very white, perhaps the whitest of all the materials 
 used, and this must be considered an important 
 advantage. To what extent these combinations of 
 materials are subject to chemical laws is as yet but 
 imperfectly understood. The fact that all the 
 reactions take place in the glowing recesses of the 
 oven renders them extremely difficult of investiga- 
 tion, and the apparent impossibihty of securing 
 absolute uniformity of conditions, time after time, 
 for the purpose of continuous experiment, renders 
 the work of the scientific potter tedious and 
 inconclusive. Experiments are often misleading, 
 but, at the same time, every search for light musfc^' 
 begin in the laboratory. The investigator must be 
 prepared for disappointment, and must, with 
 inexhaustible patience, subject his ideas to the test 
 of time, and tabulate his results by means of a long 
 series of averages. Experimenting in pottery is 
 costly work, but without it there can be no possi- 
 bility of progress. 
 
CHAPTER II. 
 
 ANALYSIS AND SYNTHESIS. 
 
 I. — Analysis. 
 
 As a preliminary step to a complete knowledge 
 of his material, the potter should make himself 
 thoroughly acquainted with the chemical composi- 
 tion of each one. Without this knowledge an 
 elaborate analysis teaches him nothing, and he is 
 unable to keep touch with the variations constantly 
 occurring in every substance that he uses. In 
 other chapters will be found some particulars as 
 to the function and testing of clays and other 
 materials, but here it is proposed to sum up the 
 scientific researches of ceramic chemists in various 
 parts of the world by giving in tabulated form 
 analyses of clays and rocks, followed by those of 
 bodies compounded in every country. It has not 
 been thought necessary to name the author of each 
 analysis. Let it suffice to say that the figures given 
 
 are all from trustworthy sources. 
 
 (10) 
 
ANALYSIS AND SYNTHESIS. 
 
 11 
 
 1. Kaolin* or China clay is found in many 
 parts of the world, the English supply coming from 
 Cornwall (hence the name "Cornish clay"). It 
 undergoes a considerable amount of preparation at 
 the place of origin, and is delivered at the manu- 
 factory ready for use. Kaolin is called by 
 geologists a " primary " clay because it is in the 
 condition of original deposit. It will be seen from 
 the analyses given that the average amount of 
 impurity in the clay is very small. 
 
 KAOLIN. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Alkalies. 
 
 Mag- 
 nesia. 
 
 Ferric 
 Oxide. 
 
 Water. 
 
 English 
 
 47-96 
 
 38-29 
 
 -43 
 
 •56 
 
 
 •90 
 
 1276 
 
 )) 
 
 47-10 
 
 37-33 
 
 •14 
 
 •20 
 
 •47 
 
 1-11 
 
 13-45 
 
 )) 
 
 48-11 
 
 36-77 
 
 1-50 
 
 1^50 
 
 •54 
 
 •44 
 
 11^80 
 
 !) 
 
 43-32 
 
 39-74 
 
 •36 
 
 
 
 •27 
 
 12-67 
 
 !) 
 
 46-38 
 
 38-60 
 
 3^47 
 
 l'77 
 
 trace 
 
 
 9-08 
 
 )J 
 
 45-52 
 
 40-76 
 
 2-17 
 
 1^90 
 
 trace 
 
 
 9-61 
 
 German 
 
 46-87 
 
 38-56 
 
 trace 
 
 •06 
 
 trace 
 
 •83 
 
 12^73 
 
 American - 
 
 46-47 
 
 38-82 
 
 -28 
 
 •75 
 
 •25 
 
 •89 
 
 13-34 
 
 » 
 
 47-66 
 
 37-56 
 
 •20 
 
 •25 
 
 •36 
 
 1^39 
 
 13-47 
 
 
 48-50 
 
 38-31 
 
 •10 
 
 •76 
 
 •14 
 
 •59 
 
 12-24 
 
 )) 
 
 46-47 
 
 38-82 
 
 •28 
 
 ■48 
 
 •25 
 
 •89 
 
 1334 
 
 )) 
 
 46-13 
 
 39-17 
 
 •22 
 
 •40 
 
 •30 
 
 1-00 
 
 1310 
 
 )) 
 
 45-22 
 
 39-11 
 
 
 •97 
 
 •28 
 
 121 
 
 13-20 
 
 French 
 
 48-00 
 
 37-00 
 
 
 2^50 
 
 
 
 13-10 
 
 J) 
 
 49-00 
 
 3600 
 
 
 1^60 
 
 
 
 12-60 
 
 
 48-00 
 
 36-00 
 
 
 2^00 
 
 
 
 13-00 
 
 Chinese 
 
 50-50 
 
 33-70 
 
 
 190 
 
 
 
 11-20 
 
 Next in importance to Kaolin and closely 
 
 * The term Kaolin is said to be derived from Kaouling (lofty 
 ridge), the name of a hill in China from which some of the 
 substance was obtained. Geolog. Mus. Handbook, 
 
12 
 
 CERAMIC TECHNOLOGY. 
 
 related to it is stone or Cornish granite (named 
 by the Chinese " Pe-tun-tse," and by mineralogists 
 " Pegmatite "). It will be seen that this differs 
 from Kaolin in that the proportions of silica and 
 alkalies are much larger, rendering the substance 
 more fusible and less plastic. Stone is liable to 
 very great fluctuations in composition, as will be 
 seen from the analyses given. 
 
 CORNISH STONE. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Fluo- 
 rine. 
 
 Alka- 
 lies. 
 
 Water. 
 
 English 
 
 73-39 
 
 16-50 
 
 -50 
 
 •31 
 
 trace 
 
 -74 
 
 7-66 
 
 1-25 
 
 m-50 
 
 17-85 
 
 2-66 
 
 -12 
 
 
 •71 
 
 7-98 
 
 1-30 
 
 
 71-66 
 
 18-79 
 
 1-70 
 
 •35 
 
 
 -14 
 
 6-60 
 
 •91 
 
 
 73-57 
 
 16-47 
 
 1-17 
 
 •21 
 
 •27 
 
 
 5-84 
 
 2^45 
 
 
 72-99 
 
 17-58 
 
 1-25 
 
 -37 
 
 •15 
 
 
 6-20 
 
 1-77 
 
 
 74-55 
 
 17-37 
 
 1-68 
 
 -54 
 
 •26 
 
 
 3-68 
 
 2^04 
 
 
 73-77 
 
 16-05 
 
 1-14 
 
 -22 
 
 •23 
 
 
 7-52 
 
 1^78 
 
 American (Texas) 
 Chinese (Yaou-ko) 
 
 68-88 
 
 16-77 
 
 -99 
 
 •17 
 
 -83 
 
 
 6-77 
 
 5-79 
 
 75-90 
 
 13-90 
 
 •40 
 
 trace 
 
 -70 
 
 
 5-17 
 
 2-7 
 
 Limoges 
 
 74-30 
 
 18-30 
 
 ■40 
 
 •30 
 
 
 
 6-50 
 
 •30 
 
 To the earthenware manufacturer Ball clay is 
 of the highest importance. This is termed by 
 geologists a " secondary " clay, inasmuch as it is 
 believed that the deposits are not in their original 
 condition, but have been washed up and redeposited 
 or, possibly, derived from the decomposition of 
 stratified rocks as the primary clays are from 
 crystalline. 
 
ANALYSIS AND SYNTHESIS. 13 
 BALL CLAY. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Alkalies. 
 
 Water. 
 
 Teignmouth 
 
 52-06 
 
 29-38 
 
 -43 
 
 •02 
 
 2-37 
 
 2-29 
 
 12-70 
 
 Poole - 
 
 48-99 
 
 32-11 
 
 •43 
 
 -22 
 
 2^34 
 
 3-31 
 
 11^60 
 
 Florida 
 
 45-39 
 
 39-19 
 
 •51 
 
 •29 
 
 •45 
 
 -83 
 
 14-01 
 
 New Jersey- 
 
 46-18 
 
 39-08 
 
 •42 
 
 •35 
 
 1^11 
 
 •51 
 
 1304 
 
 Missouri 
 
 48-51 
 
 35-18 
 
 1-01 
 
 1^47 
 
 •92 
 
 2-30 
 
 10-72 
 
 Strasbourg - 
 
 66-70 
 
 18-20 
 
 
 •60 
 
 1^60 
 
 
 12-00 . 
 
 Echassieres- 
 
 49-20 
 
 34-00 
 
 
 
 
 
 16-40 
 
 It will doubtless be noticed that some of these 
 analyses differ only in minute degree (except as to 
 the iron and alkalies) from Kaolin itself, and it is 
 difficult to say why these clays should be so much 
 stronger and more plastic than Kaolins. Perhaps 
 the fact that Kaolin is manipulated and prepared 
 after mining has something to do with its com- 
 parative lack of tensile strength. 
 
 Fire clay, otherwise called Stourbridge clay or 
 Marl, is largely in demand on account of its fire- 
 resisting properties. The potter is dependent upon 
 it for his supply of saggers and, naturally, looks to 
 the refractory nature of the clay to diminish his 
 losses. It must, however, be remembered that 
 infusibility is not the only quality to be sought for. 
 Saggers must bear great weight, and are exposed 
 to rapid influences of heating and cooling besides 
 coming directly into contact with the disintegrating 
 action of the flames and kiln gases. Unfortunately 
 an analysis does not afford much real indication of 
 
14 CEEAMIC TECHNOLOGY. 
 
 the lasting qualities of a fire clay, because so much 
 depends upon mechanical structure and, perhaps, 
 still more upon preparation. 
 
 FIRE CLAY. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Alka- 
 lies. 
 
 Water 
 
 and 
 Organic 
 Matter. 
 
 English (Stourbridge) 
 
 65-10 
 
 22-22 
 
 •14 
 
 -18 
 
 1-92 
 
 •18 
 
 9-86 
 
 „ (Brierley Hill) 
 
 51-80 
 
 30-40 
 
 1-46 
 
 -50 
 
 4-14 
 
 
 1311 
 
 51-10 
 
 31-35 
 
 1-34 
 
 1-54 
 
 4-63 
 
 
 10-47 
 
 J) )' 
 
 47-55 
 
 29-50 
 
 1-66 
 
 -71 
 
 9-13 
 
 
 12-01 
 
 )) n 
 
 48-55 
 
 30-25 
 
 1-76 
 
 1-91 
 
 4-06 
 
 
 10-67 
 
 !) )7 
 
 5111 
 
 30-40 
 
 
 trace 
 
 4-91 
 
 
 12-29 
 
 !) )> 
 
 72-28 
 
 17-75 
 
 
 2-30 
 
 2-43 
 
 
 6-94 
 
 J! >' 
 
 83-29 
 
 810 
 
 
 2-99 
 
 1-88 
 
 
 3-64 
 
 J! J7 
 
 69-25 
 
 17-90 
 
 
 1-30 
 
 2-97 
 
 
 7-58 
 
 United States - 
 
 74-93 
 
 17-19 
 
 -29 
 
 •46 
 
 •79 
 
 i-ei 
 
 5-44 
 
 The felspar most usually employed in pottery 
 is known mineralogically as orthoclase,* and en- 
 tering largely into the composition of pegmatite, is 
 generally considered to be the parent of Kaolin. 
 Chemically, orthoclase is a double siHcate of alumi- 
 nium and potassium, but small quantities of foreign 
 substances are found associated with it from time 
 to time. It will be seen from the analyses that its 
 composition is fairly uniform ; and, as the fusibility 
 of the spar depends upon the contained alkali, an 
 analysis may be considered an accurate test of 
 quality. 
 
 * A term arising from the existence, in the mineral, of two 
 cleavages, at right angles to each other. Geo. Mus. Handbook. 
 
ANALYSIS AND SYNTHESIS. 15 
 
 FELSPAR (Orthoclase). 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Potash. 
 
 Soda. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Water. 
 
 German 
 
 spar - 
 
 64-32 
 
 19-41 
 
 12-90 
 
 2-10 
 
 
 •35 
 
 •14 
 
 
 J) 
 
 j> 
 
 65-24 
 
 18-57 
 
 14-02 
 
 1-25 
 
 -34 
 
 •10 
 
 
 
 Swedish 
 
 >) 
 
 65-85 
 
 19-32 
 
 14-10 
 
 
 ■56 
 
 •08 
 
 •24 
 
 
 Scotch 
 
 )) 
 
 65-40 
 
 19 04 
 
 11-26 
 
 3-63 
 
 -22 
 
 
 trace 
 
 •20 
 
 French 
 
 
 64-00 
 
 20-56 
 
 14-99 
 
 
 •38 
 
 
 
 
 Spanish 
 
 
 62-06 
 
 19-61 
 
 16-07 
 
 
 -38 
 
 •16 
 
 
 l"ll 
 
 A variety of felspar, of which the base is not 
 potash, but soda, is occasionally used. It is named 
 " Albite " by mineralogists, and its composition is as 
 follows : — 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Potash. 
 
 Soda. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 "Albite" or Soda spar 
 
 66-81 
 
 21-09 
 
 
 9-64 
 
 203 
 
 •10 
 
 -33 
 
 V )) 
 
 68-82 
 
 19-75 
 
 
 9-15 
 
 1-64 
 
 •17 
 
 
 ); )> 
 
 68-00 
 
 20-00 
 
 -68 
 
 10-88 
 
 -35 
 
 
 
 » J) 
 
 67-63 
 
 20-48 
 
 
 10-26 
 
 •65 
 
 
 
 Flint and bone are the principal remaining 
 materials in use, but, as the first is practically 
 pure silica, and the second should be nearly pure 
 calcium phosphate, it will be unnecessary to give 
 analyses. Some particulars as to the behaviour of 
 all the materials in combination will be found upon 
 another page. It is now proposed to place before 
 the reader a series of analyses of notable bodies of 
 every description, which will illustrate the uses to 
 which the materials have been put, and, perchance, 
 open up new fields to the scientific potter. 
 
16 CERAMIC TECHNOLOGY. 
 
 At one end of the ceramic scale will be found 
 the rude wares produced in ancient times by the 
 majority of savage tribes, and at the present day 
 by such peoples as the Egyptians and Algerians ; 
 following upon these will be given certain analyses 
 representing an advance in knowledge until the 
 perfection of hard porcelain is reached. Some clue 
 as to the origin of each ware will be found in the 
 first column. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Iron. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Water. 
 
 Carbon. 
 
 Scandinavian pottery 
 
 63-90 
 
 12-76 
 
 10-24 
 
 1-04 
 
 •52 
 
 9-98 
 
 1-02 
 
 Peruvian „ 
 
 6704 
 
 10-83 
 
 10-17 
 
 3-24 
 
 -28 
 
 7-07 
 
 1-00 
 
 Etruscan „ 
 
 64-00 
 
 12-51 
 
 8-00 
 
 3-51 
 
 1-83 
 
 8-15 
 
 2-00 
 
 Samian v^are - 
 
 64-00 
 
 17-77 
 
 10-23 
 
 4-86 
 
 
 2-23 
 
 )) 
 
 Egyptian „ - 
 
 81-00 
 
 13-50 
 
 1-00 
 
 3-00 
 
 )> 
 
 1-90 
 
 )) 
 
 From the presence of water and carbon it will 
 be seen that these wares have been fired at a very 
 low temperature. 
 
 The next series consists of ware of the same 
 class, but as both the water of combination and 
 the carbon have been expelled, a greater heat is 
 indicated. 
 
 
 Silica. ' 
 
 Alumina. 
 
 Iron. 
 
 Lime. 
 
 Magnesia. 
 
 Early French 
 Roman - 
 Greek - 
 Greek - 
 
 70-27 
 56-00 
 55-49 
 60-00 
 
 14-17 
 26-00 
 19-21 
 13-63 
 
 10-29 
 7-00 
 16-55 
 19-00 
 
 1-49 
 9-00 
 7-48 
 5-91 
 
 •98 
 2 00 
 
 1-27 
 1-46 
 
ANALYSIS AND SYNTHESIS. 
 
 17 
 
 The bodies in the next range contain large 
 quantities of calcium carbonate, and it will be seen 
 that these wares were uniformly coated with 
 opaque tin glazes. M. Salv^tat states that the 
 calcium carbonate was introduced " to give to the 
 glaze rendered opaque by tin the resistance neces- 
 sary for culinary uses," and adds that this same 
 quality " could have been obtained by many other 
 means, such as the employment of alkaline fritts, 
 but it is evident that the addition of lime was the 
 most economical ". 
 
 The presence of large quantities of carbonic 
 acid indicates that the fire could not have been a 
 severe one, as had the silica been enabled to com- 
 bine with the calcium all the carbonic acid would 
 have been liberated and, in all probability, the body 
 would have melted and effervesced. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Carbonic 
 Acid. 
 
 Luca della Eobbia - 
 
 49-65 
 
 15-50 
 
 22-40 
 
 -17 
 
 3-70 
 
 8^!58 
 
 Majolica - 
 
 48-00 
 
 17-50 
 
 20-12 
 
 1-17 
 
 3-75 
 
 9^46 
 
 Early Spanish - 
 
 46-04 
 
 18-45 
 
 17-64 
 
 •87 
 
 3-04 
 
 13^96 
 
 Delft - 
 
 49-07 
 
 16-19 
 
 1801 
 
 -82 
 
 2-82 
 
 1309 
 
 Persian 
 
 48-54 
 
 12-05 
 
 19-25 
 
 -30 
 
 3-14 
 
 10^72 
 
 Nevers 
 
 56-49 
 
 19-22 
 
 14-96 
 
 -71 
 
 2-12 
 
 0-00 
 
 Rouen 
 
 47-96 
 
 15-02 
 
 20-24 
 
 •44 
 
 407 
 
 12^27 
 
 Paris - - - - 
 
 61-50 
 
 12-99 
 
 16-24 
 
 -15 
 
 301 
 
 6 10 
 
 As quality improved, it was found that the wares 
 
 contained more or less alkali, while still in some 
 
 cases retaining the lime and magnesia. The iron 
 
 2 
 
18 CEEAMIC TECHNOLOaY. 
 
 generally diminishes in quantity, remaining high 
 only in the darker coloured examples, and as a 
 consequence the paste becomes harder and capable 
 of enduring a greater heat. The colour also is 
 sensibly lighter, and the opaque tin glaze is no 
 longer indispensable. It will be seen that there is 
 no very great difference between the English and 
 foreign wares of this description. 
 
 
 Silica. 
 
 Alu- 
 mina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron, j 
 
 Alkalies. 
 
 Palissy - 
 
 67-50 
 
 28-51 
 
 1-52 
 
 
 1 
 
 2-05 
 
 traces 
 
 D'Oiron - 
 
 59-10 
 
 40-24 
 
 
 
 
 
 Luneville 
 
 67-39 
 
 16-00 
 
 13-16 
 
 l-'62 
 
 2-61 
 
 traces 
 
 St. Amancl 
 
 7500 
 
 22-10 
 
 -25 
 
 
 1-00 
 
 -84 
 
 Oreil 
 
 66-10 
 
 32-20 
 
 •14 
 
 
 •55 
 
 1-10 
 
 English - 
 
 7610 
 
 20-45 
 
 -75 
 
 •14 
 
 1-00 
 
 1-60 
 
 
 72-60 
 
 2410 
 
 
 
 1-10 
 
 2-20 
 
 >! 
 
 7400 
 
 22-04 
 
 ""•60 
 
 "•17 
 
 2^00 
 
 1-06 
 
 )) 
 
 * German - 
 
 74-00 
 
 19-00 
 
 -62 
 
 
 4-25 
 
 1-30 
 
 Chinese - 
 
 62-00 
 
 22-00 
 
 •50 
 
 trace 
 
 14-00 
 
 1-00 
 
 Japanese 
 American 
 
 62-04 
 
 20-30 
 
 1-08 
 
 trace 
 
 15-58 
 
 trace 
 
 67-40 
 
 29-00 
 
 •60 
 
 
 2-00 
 
 •60 
 
 Staffordshire - 
 
 66-49 
 
 26-00 
 
 1-04 
 
 •is 
 
 6-12 
 
 -20 
 
 * Salt glazed stoneware. 
 
 The distance separating these hard earthen- 
 wares and stonewares from true porcelain is not a 
 great one. Purer clays are, of course, used, as 
 shown by the diminished amount of iron. In the 
 Sevres pastes M. Salv^tat, who made the analysis, 
 explains that the lime was purposely added to pre- 
 vent crazing, and that he had always found that 
 the addition of a small quantity of chalk to the 
 
ANALYSIS AND SYNTHESIS. 19 
 
 body cured this defect. The large proportion of 
 magnesia in the pastes of Worcester and Pi^mont 
 seems to point to the use of steatite. The alkalies 
 evidently owe their presence to the use of felsimr 
 or Cornish stone. 
 
 Porcelains. 
 
 China - 
 
 „ less fine - 
 Japan - 
 Meissen 
 Vienna 
 Berlin - 
 Limoges 
 
 Sevres - - - 
 Sevres biscuit, 1838 
 Worcester - 
 Paris - 
 Piemont 
 Bristol 
 
 
 Silica. 
 
 Alumina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 1 Iron. 
 
 Alka- 
 lies. 
 
 
 69 00 
 
 23-60 
 
 •30 
 
 •02 
 
 1-20 
 
 6^20=;= 
 
 
 70-00 
 
 22-20 
 
 •80 
 
 trace 
 
 1^30 
 
 6^30* 
 
 
 78-80 
 
 17-80 
 
 •20 
 
 
 -60 
 
 2-20+ 
 
 
 58-50 
 
 35-10 
 
 •30 
 
 trace 
 
 •80 
 
 5 00 
 
 
 59-60 
 
 34-20 
 
 1^70 
 
 1-40 
 
 •80 
 
 200 
 
 
 64-30 
 
 29-00 
 
 -30 
 
 •45 
 
 •60 
 
 3-65 
 
 
 70-20 
 
 24-00 
 
 •70 
 
 -10 
 
 •70 
 
 4-30 
 
 
 58-00 
 
 34-50 
 
 4-50 
 
 '-01 
 
 
 3-00 
 
 
 67-17 
 
 26-01 
 
 3^02 
 
 )) 
 
 3-09 
 
 
 82-00 
 
 9-10 
 
 1^30 
 
 7 40 
 
 
 
 71-20 
 
 22-00 
 
 •80 
 
 
 •80 
 
 4-50 
 
 
 69-80 
 
 10-40 
 
 2^00 
 
 17-60 
 
 
 
 
 62-92 
 
 33-16 
 
 1^28 
 
 
 
 2-64 
 
 * Mainly potash. + Mainly soda. 
 
 For the sake of comparison with the foregoing 
 hard paste porcelains there are appended a few 
 analyses of early soft pastes. These are now con- 
 sidered out of date, but their luscious tenderness of 
 paste and glaze has never been surpassed. It will 
 be noticed that these mixtures are wonderfully low 
 in alumina, and it is a matter of surprise that with 
 this infinitesimal portion of clay the body should 
 have been at all workable. The large proportions 
 of lime and alkalies denote a low fusing-point. It 
 will be seen that in the pastes of Chelsea and Bow, 
 bone was used in considerable quantity. 
 
20 
 
 CERAMIC TECHNOLOGY. 
 
 
 Silica. 
 
 Alumina. 
 
 Lime. 
 
 Mag- 
 nesia. 
 
 Iron. 
 
 Alka- 
 lies. 
 
 Phos- 
 phoric 
 Acid. 
 
 Sevres - 
 
 72-00 
 
 3-00 
 
 15-00 
 
 trace 
 
 
 8-10 
 
 
 
 78-36 
 
 1-00 
 
 12-73 
 
 trace 
 
 
 6-46 
 
 
 )j 
 
 77-00 
 
 1-20 
 
 16-36 
 
 trace 
 
 ... 
 
 5-40 
 
 
 )) 
 
 Tournay 
 
 76-45 
 
 7-35 
 
 11-20 
 
 trace 
 
 
 500 
 
 20-30 
 
 Chelsea - 
 
 40-20 
 
 8-40 
 
 27-40 
 
 
 1-20 
 
 1-90 
 
 Bow 
 
 40-00 
 
 16-00 
 
 24-00 
 
 
 ... 
 
 , 1-90 
 
 17-30 
 
 II. — Synthesis or the Decomposition of 
 Mixtures from Analyses. 
 
 An elaborate analysis is of little use unless 
 those who consult the figures are capable of 
 reversing the operation gone through in analysing, 
 thus reproducing in as accurate a form as possible 
 the original recipe. Fortunately to achieve this it 
 is only necessary to have recourse to figures, and 
 thus the practical potter can avail himself of the 
 researches of the chemist should he desire to do so. 
 
 The principle upon which this conversion is 
 accomplished is that every substance has its own 
 combining or equivalent weight, a definite weight 
 representing the proportionate amount of work 
 done by the substance itself. This combining 
 weight is founded upon the molecular weights of 
 the respective elements contained. 
 
 Taking then the items in an analysis one by 
 one, each is divided by its own combining weight 
 in order to reduce the whole to an equal basis. 
 
ANALYSIS AND SYNTHESIS. 
 
 21 
 
 For example, if the analysis of Sevres porcelain 
 given on page 19 be taken, the procedure would 
 be as follows : — 
 
 Silica .... 58-00 30 = 1-933 
 
 Alumina . . . 34-50 51 = 0-676 
 
 Lime .... 4-50 28 = 0-160 
 
 Alkalies .... 3-00 -h 44 = 0-068 
 
 Here the figures in the second column are the 
 respective combining weights of the constituent 
 substances, and the third column gives as quotients 
 the equivalent amount of each. In the case of 
 mixed alkalies as above a compromise must be made 
 between potash and soda. The combining weight 
 of potash is 47, of soda 31, the mean being there- 
 fore 39. When, from the nature of the case, potash 
 appears to predominate, the combining weight will 
 be higher than the mean, and of course when soda 
 is the greater, the reverse. Now in the case of the 
 Sevres porcelain under consideration either potash- 
 felspar or Cornish stone would be used, and there- 
 fore the balance is placed in favour of potash. 
 
 Then the substances being reduced to their 
 equivalence the operation may be continued. 
 
 The first point to be dealt with is the alkali, 
 which must be taken, as already mentioned, either 
 in the form of felspar or stone. On account of the 
 variable composition of the latter it will be found 
 
22 
 
 CEEAMIC TECHNOLOGY. 
 
 preferable to employ the spar. Each equivalent 
 of felspar contains one equivalent of alkali, mainly 
 potash, one of alumina and six of silica. It is 
 necessary from the figures arrived at to take 0*068 
 equivalent of alkali, and with this must perforce 
 be taken the same amount of alumina and 
 0 068 X 6 = 0*408 equivalent of silica. The amount 
 of alumina required must now be made up by 
 kaolin, which contains two equivalents of silica to 
 each one of alumina. There is required alto- 
 gether 0'676 equivalent of alumina, but of this 
 0'068 equivalent has already been taken in the 
 spar. The remainder, therefore, 0*676 - 0*068 = 
 0*608, will give the amount of kaolin. Again, in 
 this kaolin must be taken 0*608 x 2 =1*216 
 equivalents of silica, which, added to the amount 
 taken in the felspar, will be 1*216 + 0*408 = 1*624. 
 The whole amount of silica required is, however, 
 1*933 equivalents, which will leave 1*933 - 1*624 = 
 0*309 to be supplied. This can be given in pure 
 form as ground flint. 
 
 There now remains only the lime, which can be 
 dealt with simply by adding 0*160 equivalent of 
 either marble, chalk, or whiting, which are all pure 
 calcium carbonate, each equivalent of calcium 
 carbonate containing one equivalent of calcium 
 oxide. 
 
 The formula will therefore stand thus : — 
 
ANALYSIS AND SYNTHESIS. 
 
 23 
 
 Felspar ...... 0-068 
 
 Kaolin . 0-608 
 
 Flint 0-309 
 
 Whiting 0-160 
 
 In order to bring these amounts to figures that 
 can be utiHsed, the combining weights of these new 
 substances must be employed and each equivalent 
 weight multiplied by the respective combining 
 weight. 
 
 The combining weight of felspar is 278 4, of 
 kaoHn 129, of flint 30, and of whiting 50. The 
 figures will therefore work out thus : — 
 
 Felspar . 0-068 x 278-4 
 
 Kaolin . 0-608 x 129 
 
 Flint . . 0-309 X 30 
 
 Whiting . 0-160 x 50 
 
 = 18-77 parts by weight. 
 
 As a second example it will be well to take a 
 body in which bone has been used. The analysis 
 of Bow porcelain given on page 20 will serve. 
 This is : — 
 
 Silica .... 40-00 ^ 30 = 1-333 
 
 Alumina . . . 16-00 51 = -313 
 
 Lime . . . . 24-00 ^ 28 = -857 
 
 AlkaHes . . . 1-90 ^ 44 = -043 
 
 Phosphoric Acid . . 17*30 ^ 71 = -243 
 
 As in the previous analysis, the combining 
 weight of each substance is placed in the second 
 column and the resulting equivalent in the third. 
 
 Proceeding as before, in order to supply the 
 
24 
 
 CEEAMIC TECHNOLOGY. 
 
 alkalies there must be taken 043 equivalent of 
 felspar, which will contain also '043 equivalent of 
 alumina and -043 x 6 = -258 equivalent of silica. 
 To make up the required amount of alumina there 
 must now be taken -313 - -043 = '270 equivalent 
 of china clay, which will include '270 x 2 = -540 
 equivalent of silica. This, added to the silica taken 
 with the felspar, will give '258 + '540 = 798 
 equivalent of sihca out of the 1-333 required, 
 leaving 1-333 - -798 = -535 equivalent to be added 
 as flint. The bone is calculated in a similar 
 manner. There is to be supplied '243 equivalent 
 of phosphoric acid. Each equivalent of bone ash 
 will contain one equivalent of phosphoric acid and 
 three equivalents of lime, therefore -243 equivalent 
 of bone will contain also -243 x 3 = -729 of lime, 
 leaving only -857 - '729= '128 equivalent to be 
 supplied, but as both china clay and felspar 
 contain traces of lime, this small amount may fairly 
 be disregarded. Multiplying now each equivalent 
 by the proper combining weight the mixture is 
 completed. 
 
 Felspar ' . . -043 x 278-4 = 11-97 parts. 
 
 Kaolin . . -270 x 129 = 34-83 „ 
 
 Flint . . -535 X 30 = 16-05 ,, 
 
 Bone Ash . -243 x 171-5 = 41-67 „ 
 
 The same analysis could be worked out on the 
 basis of Cornish stone instead of felspar, allowing 
 
ANALYSIS AND SYNTHESIS. 
 
 25 
 
 to each equivalent of stone one of alkali, two of 
 alumina, and eighteen of silica. The composition 
 of stone is, however, so variable that it is impos- 
 sible to fix exact equivalents or an accurate com- 
 bining weight. 
 
 A glaze analysis may be worked out upon the 
 same lines, but here the modern method, intro- 
 duced by the German ceramists, is to separate the 
 glaze constituents into three divisions, bases, 
 alumina and acids, and in order to institute an 
 exact comparison and to adjust the balance of 
 j)ower, the bases are first added together and 
 multiplied by some number, which will bring the 
 total to 10 or 100, treating, of course, the other 
 constituents in the same way. 
 
 An average analysis of a good lead glaze would 
 be for example : — 
 
 Silica 
 
 50-00 - 
 
 - 30 = 
 
 1-666 
 
 Alumina 
 
 12-00 - 
 
 - 51 = 
 
 -235 
 
 Lime 
 
 1-25 - 
 
 - 28 = 
 
 •044 
 
 Soda 
 
 5-25 - 
 
 - 31 = 
 
 -169 
 
 Boracic Acid 
 
 6-50 - 
 
 - 35 = 
 
 -185 
 
 Oxide of Lead . 
 
 25-00 - 
 
 - 111-5 = 
 
 •225 
 
 The boracic acid and the soda being, of course, 
 in an anhydrous ^condition, the smaller combining 
 w^eights are used. 
 
 Dividing now the bases and acids into groups, 
 and placing the alumina in its intermediate posi- 
 tion, the formula will stand thus :— 
 
26 
 
 CERAMIC TECHNOLOGY. 
 
 Lead Oxide PbO 225 
 Lime CaO 044 
 Soda Na^O 169 
 
 1 
 
 Alumina AI2O3 -235 
 
 Silica SiO.3 1-666. 
 Boracic Acid B.^Og 
 
 •185. 
 
 •438 
 
 Now, it will be seen that the bases only 
 amount to "438 equivalent all together, and, there- 
 fore, to bring this to unity, each figure must be 
 multiplied by a particular number. This number 
 in the present case is found to be 2-283, and the 
 formula will then be resolved into the following : — 
 
 The calculation will then proceed somewhat 
 upon the Hues laid down for bodies, but a new sub- 
 stance, boracic acid, has to be dealt with. This is 
 most conveniently taken in the form of borax, each 
 equivalent of which contains two equivalents of the 
 acid and one equivalent of soda. Therefore, •2111 
 equivalent of borax will contain all the boracic acid 
 needed and 2111 equivalent of soda, leaving 
 •3858- -2111 = ^1747 to be taken as soda ash. The 
 alumina will be supplied as before by china clay, 
 and the remaining silica by flint. The figures will 
 now read :— 
 
 1^0 
 
ANALYSIS AND SYNTHESIS. 
 
 27 
 
 White Lead 
 Whiting 
 Borax . 
 Soda Ash 
 China Clay 
 Flint . 
 
 ■5137 
 •1005 
 •2111 
 •1747 
 •5365 
 2-7304 
 
 But it becomes necessary to incorporate some 
 of these substances in a fritt, and, as the borax, 
 soda ash, whiting, and china clay, all have water 
 or carbonic acid to lose, allowance must be made 
 accordingly. Soda ash and whiting lose their 
 carbonic acid in the fire, and, therefore, become 
 subject to the combining weight of sodium oxide 
 and calcium oxide respectively. For the fritt then 
 there would be taken : — 
 
 Borax -2111 equivalent 40-10 parts melting to 21-32 parts 
 
 Soda Ash -1747 
 Whiting -1005 
 China Clay -1365 
 Flint -7304 
 
 9-25 
 5-02 
 17-60 
 21^91 
 
 5-41 
 2-81 
 15-21 
 21-91 
 
 66-66 
 
 And to complete the glaze there would be 
 ground together the charge of fritt, which, of 
 course, should be re-weighed, and the balance of 
 the materials, thus : — 
 
 Fritt .... 66-66 parts. 
 White Lead ^5137 equivalent 66-26 
 China Clay -4 51-60 ,, 
 
 Fhnt 2-0 „ 60-00 ' 
 
28 CERAMIC TECHNOLOGY. 
 
 By these means any analysis can be reduced to a 
 workable condition, and can be made of real assist- 
 ance to the practical potter. 
 
 TABLE OF COMBINING WEIGHTS. 
 
 In order to assist those desiring to work out 
 calculations for themselves, the following table of 
 the combining weights of the substances in most 
 general use has been compiled : — 
 
 Alumina Al.^Og 
 
 Bone Ash CaCOo3Cao2PO, 
 
 Boracic Acid B^Og 
 
 Borax NagB^O- 
 
 IOH2O 
 
 Calcium Oxide CaO . 
 
 ,, Carbonate CaCOg 
 Phosphate CagSPO^ 
 Perric Oxide Pe203 . 
 Felspar KgOAl.OgBSiO^ . 
 
 Kaolin Al2Q32Si02 + 2H2O 
 
 Petuntse or Pegmatite, very irregular 
 Lead Oxide PbO 
 White Lead PbH2022PbC03 
 Magnesium Oxide MgO . 
 Potassium Oxide K2O 
 
 „ Carbonate K2CO3 
 Quartz (Fhnt) SiOa . 
 Sodium Oxide NagO . 
 
 Carbonate Na,COo 
 
 Hydrated . 
 
 Anhydrous 
 j Hydrated . 
 1 Anhydrous 
 
 51 
 171-5 
 . 62 
 35 
 190 
 101 
 28 
 50 
 155 
 80 
 278-4 
 129 
 
 1 Calcined 111-5 
 probably 680 to 690 
 111-5 
 129 
 20 
 47 
 69 
 30 
 31 
 53 
 
CHAPTER III. 
 
 POTTEKS' CLAYS ; AND THEIR COMPONENTS. 
 
 I. — The Components of Clays. 
 
 The necessary constituents of potters' clays are 
 very limited in number. 
 
 Ahanina, the chief, is in itself highly infusible ; 
 but when mixed with silica is more or less fusible, 
 according to the proportion of each ingredient. 
 
 Silica or silicic acid is present in clay both in a 
 free state in the form of sand or quartz, and also 
 in combination with the alumina. The fusibility 
 of the mass is greatly affected by the condition 
 in which the silica occurs, and for this reason 
 chemical analysis gives but little indication of 
 the cj[ualities of a clay, apart from empirical 
 tests. 
 
 Water is present in all plastic clay, and the 
 plasticity of a clay is in direct ratio to the amount 
 of combiried water contained in it. 
 
 In addition to the combined water there is 
 
 usually some amount of water in a free state which 
 
 (29) 
 
30 
 
 CERAMIC TECHNOLOGY. 
 
 may be expelled by drying at a temperature below 
 boiling-point. It is always desirable to eliminate 
 this portion of the water from samples of clay 
 before commencing an analysis. 
 
 In addition to the components already named, 
 most clays contain some proportion of basic earths 
 and alkalies. Of the former, lime and magnesia 
 are the most important. 
 
 Lime is infusible when pure, but very fusible 
 when combined with silica, alumina, or alkaline 
 silicates. A very small percentage will greatly 
 affect the character of a clay. 
 
 Magnesia occurs in small proportion in many 
 clays. It is infusible when pure, and when com- 
 bined with other materials tends to reduce the 
 plasticity of the body. It is largely present in 
 magnesite, an important ingredient in some 
 Spanish porcelains. 
 
 The Alkalies — soda and potash — add greatly to 
 the fusibility of a body when in combination with 
 the other materials, readily vitrifying with silica, 
 lime, or barytes. 
 
 Other ingredients and some metallic oxides 
 occur at times in clays. Amongst these 
 
 Ferric Oocide is by far the most general. Its 
 presence adds greatly to the fusible qualities of the 
 clay, and it imparts a colour to the mass, buff, 
 yellow, red, grey, or black, according to the 
 
potters' clays ; and their components. 31 
 
 proportion present or the conditions under which 
 it is fired. 
 
 The Testing of Clays. 
 
 It is always needful to ascertain the e^act 
 nature of a clay to be used for potting purposes 
 before adopting it. At first sight it might be 
 supposed that in the case of clays intended for use 
 in coarse wares only this would be an unimportant 
 preliminary. But this is by no means the case, for 
 the following reasons : — 
 
 The clays for Jine wares undergo washing, sifting 
 and intimate admixture in the slop state. But in the 
 case of coa7'se pottery the clay must be used in its 
 natural condition. The large expense of such pre- 
 paration obviates the possibility either of removal 
 of foreign and deleterious matter or of obtaining a 
 perfect combination with other ingredients which 
 might assist to correct any faulty qualities. 
 
 The knowledge of these qualities must there- 
 fore be accurate, and it is seldom worth while to 
 make a test at all unless a definite conclusion can 
 be deduced. If the conditions are not definitely 
 known, and the comparative tests are not made 
 under identical circumstances, the results can 
 only be misleading. Many disastrous losses have 
 accrued to manufacturers from want of accurate 
 data on these points. 
 
32 CERAMIC TECHNOLOGY. 
 
 It is equally important that the tests should be 
 exhaustive. It must be borne in mind that many 
 clays are inconstant in their qualities, varying 
 greatly according to the position in which they 
 occur. A single sample is, therefore, no criterion 
 of the bulk. A series of samples should in all 
 cases be taken over a large area — definitely located 
 and identified, or thoroughly mixed together before 
 testing, if an average result be sought for. 
 
 It is hardly necessary to add that the tests 
 should be systematic. Results are often important 
 for reference in the future, and if not uniformly 
 marked, in numerical rotation, with clearly distinc- 
 tive method, hopeless confusion ensues. Arbitrary 
 choice of letters or signs soon passes from the mind, 
 and tabulation of data becomes unmanageable, if 
 not impossible. 
 
 Passing on to deal with the nature of the tests, 
 it will be observed that they are either physical 
 and technical or chemical and analytical. That is 
 to say, the potter may either examine the natural 
 properties and the physical condition of the 
 particles and of the mass by empirical methods ; 
 or may proceed to ascertain the components by a 
 laboratory analysis, determining the accurate pro- 
 portions of each. 
 
 Both these operations are really necessary for 
 success, but it is certainly true that chemical 
 
POTTEES' CLAYS ; AND THEIR COMPONENTS. 33 
 
 analysis alone is not snfficient to guide a potter in 
 the selection of his clays. 
 
 It is, therefore, important that physical and 
 technical tests should be efficiently carried out ; 
 and the following observations have for their 
 object the assistance of the manufacturer in 
 obtaining such results. 
 
 The conditions under which clay is worked 
 and the purposes for which it is used are so various 
 that it is difficult to lay down any general instruc- 
 tions which will apply to all classes of tests and all 
 kinds of materials. In every case, however, it will 
 be desirable to obtain information on the following 
 points : — 
 
 1. Plasticity, or formative quality. 
 
 2. Tenacity, or binding properties. 
 
 3. Uniformity of texture and homogeneity. 
 
 4. Shrinkage at various temperatures. 
 
 5. Porosity after firing at these temperatures. 
 
 6. Vitreousness, or density after firing. 
 
 7. Colour when fired and glazed. 
 
 8. Glazing properties. 
 
 For the purposes of definite comparison it is very 
 important that the methods adopted should be 
 reducible to figures. 
 
 1. Plastkity.—The plasticity of a clay will be 
 
 found to depend largely upon the condition and 
 
 quantity of the silica contained in it. The coarser 
 
 3 
 
'34 CERAMIC TECHNOLOGY. 
 
 the sand particles, and the greater their quantity, 
 the less the plasticity. Obviously, however, those 
 clays, which are coarse and open in body, are more 
 absorptive, so that they readily take up water ; but 
 with the addition of the same quantity of water a 
 plastic clay is far more resistant to displacement 
 than a non -plastic clay. 
 
 Making use of this fact, it is possible to obtain 
 comparative tests of plasticity in two ways. By 
 the first method, the sample of clay is first 
 thoroughly dried and pounded, a measured quantity 
 of water being added to a given weight of clay 
 sufficient to render it plastic enough for ordinary 
 work. Having thoroughly mixed the mass, a 
 small portion of it is pressed tightly into a metal 
 ring, the upper surface being struck off perfectly 
 level with a spatula. 
 
 To gauge the plasticity for comparison with 
 other samples it is now needful to ascertain the 
 resistance of the mass to the penetration of a steel 
 wire of a given size under a known pressure. 
 
 It is by no means difficult to devise a method 
 of supporting the needle in such a way that the 
 distance of its descent in a given time can be read 
 off on a vertical index, and if the behaviour of a 
 standard clay under these circumstances be first 
 noted, the comparison of other samples similarly 
 treated can be easily accomplished. 
 
potters' clays ; AND THEIE COMPONENTS. 35 
 
 Such an apparatus has already been devised for 
 cement testing, and is termed a " Vicat " needle. 
 
 A second, and simpler, though less accurate, 
 method, is to take a sample of the dry ground clay 
 moistened as before with a known proportion of 
 water, and to form the mass into a cyhnder by 
 forcing it tightly into a metal tube. It is then 
 carefully expelled from one end by means of a 
 plunger exactly fitting the tube. 
 
 The cyhnder may now be supported horizontally 
 in a semicircular channel of the same diameter, and 
 so arranged that a wire can be strained across it at 
 right angles. By attaching a known weight to 
 the end of the wire and recording the time occupied 
 in severing the cylinder a comparison of the 
 plasticity of the clay may be fairly accurately 
 attained. 
 
 2. Tenadtif. — The tenacity or binding property 
 of a clay does not always follow its plasticity, nor 
 is the tenacity greatest in the " white hard " state. 
 In most cases, however, it will be sufficient for 
 practical purposes to ascertain the tenacity in this 
 condition. 
 
 It is possible approximately to estimate this by 
 the following method. A strong rectangular metal 
 frame V deep 1" wide and 3" long is obtained, and 
 into this a piece of the clay to be tested, previously 
 moistened to a plastic state and carefully mixed, is 
 
36 CEEAMIC TECHNOLOGY. 
 
 inserted. This clay should first be cut as nearly as 
 possible to the size of the frame, which must be 
 previously oiled slightly inside. The frame being 
 now placed on a flat slab of plaster is completely 
 filled with the sample of clay, the superfluous 
 portion being carefully struck off level with the top. 
 
 On removing the clay from the mould, great 
 care must be taken to prevent distortion. 
 
 The moulded block is now thoroughly dried, 
 and for the purpose of testing should be supported 
 on two bars of metal ^ x ^ in section, fixed 
 horizontally and parallel to each other on a wooden 
 stand at a distance of exactly 2 " apart. 
 
 The amount of weight which when applied to 
 the centre of the moulded block causes fracture, 
 will furnish data of comparison as to tenacity. 
 The result may be expressed in ounces per square 
 inch of section. In this case also the apparatus 
 employed for testing cement will be found suitable, 
 and this, moreover, may be readily obtained with- 
 out special manufacture. 
 
 The calculation must, of course, be made on the 
 sectional area of the block when dry, otherwise the 
 comparison may be aff'ected by the variation in 
 shrinkage of the diff'erent blocks. 
 
 3. U7iifo?'miti/ of Texture. — The importance of 
 this quality is greatly increased in the case of such 
 clays as are intended for coarser manufactures in 
 
potters' clays ; and their components. 37 
 
 which it is impossible to afford the outlay in 
 preparation necessary to render them homogeneous. 
 
 To obtain a knowledge of this, it will be most 
 useful to take test-pieces of the clay from different 
 portions of the bulk, and these being first 
 thoroughly dried should, if possible, be shaped 
 with a knife to a uniform rectangular size. 
 
 The condition of the surface, texture, and 
 colour should be noted before and after firing, 
 comparison being made of the amount of variation 
 between the different pieces. 
 
 The occurrence of granules of quartz, iron 
 oxide, lime, lignite and other vegetable matter will 
 need careful attention, especially in those instances 
 where the clay is intended to be used without 
 washing. 
 
 4. Shrinkage. — To obtain accurate data as to 
 shrinkage requires some care. The shrinkage of a 
 clay is chiefly due to two causes, viz. : — 
 
 The evaporation of the water in the mass during 
 the process of drying, and 
 
 The parting with the combined water, together 
 with other chemical changes which take place 
 during the firing. 
 
 It is most desirable to ascertain the amount of 
 shrinkage due to each of these causes separately. 
 
 To ascertain the shrinkage in drying, a portion 
 of the pounded clay is mixed with 29 per cent. 
 
38 CEEAMIC TECHNOLOGY. 
 
 weight of water, and made up into a slab 7" long in 
 a plaster mould. Upon the face of this a length of 
 6" is marked. The piece is then removed from 
 the mould and allowed to dry steadily. The 
 length marked on the clay will now be less than 
 6". A second 6" length is marked off as before, 
 and the difference in length between the two is 
 carefully noted. Multiplying this difference by 
 2 the amount of shrinkage per foot length is 
 obtained. 
 
 By setting off the shrinkage marks in centi- 
 metres instead of inches the results may be more 
 easily converted into percentage, if so desired. 
 
 The marked slab of clay must now be passed 
 through the biscuit oven, ^nd after firing a new 
 6" length is marked parallel to the others on 
 the burnt ware. The differences are carefully 
 noted and multiplied separately by 2, as before. 
 The shrinkage from plastic to biscuit and from 
 white hard to biscuit will now be indicated on the 
 clay itself. 
 
 This method also affords an accurate compari- 
 son of the ratio of shrinkage in drying to that of 
 shrinkage in firing. 
 
 It is necessary to bear in mind that the shrink- 
 age of a clay does not always increase in the 
 same ratio as the firing temperature. Test-pieces 
 should therefore be fired in different positions and 
 
potters' clays ; and their components. 39 
 
 in different kilns to ascertain the relative shrink- 
 ages at different heats. 
 
 5. Porosity, — The porosity after firing at a given 
 temperature may be arrived at by noting the 
 weight of water absorbed by a piece of burned 
 clay when thoroughly saturated. The piece to be 
 tested should be first accurately weighed and then 
 placed in a shallow vessel of water for twenty-four 
 hours. It is better to leave the upper surface 
 exposed, to allow the air to escape, which, if the 
 whole surface were immersed, might remain im- 
 prisoned in the pores. 
 
 The saturation will be more easily effected if 
 the water is allowed to soak through a fractured 
 end, as the surface of the fired clay is often less 
 porous than the interior. 
 
 After soaking the piece is removed from the 
 water and the surface wiped dry from superfluous 
 moisture. The weight is again carefully taken and 
 the increase observed. Multiplying this difference- 
 by 100 and dividing by the original weight, the 
 quotient will be found to give the absorption per- 
 cent. 
 
 6. Density. — The density of a clay may be as- 
 certained by its specific gravity, which is usually 
 compared with that of water. 
 
 The burnt clay is first weighed accurately, and 
 then weighed again while immersed in pure water 
 
40 
 
 CEEAMIC TECHNOLOGY. 
 
 at 60° F, by hanging the piece from the scale on a 
 fine horse-hair, taking care that all attached air- 
 bubbles are removed. The piece will now weigh 
 lighter than before, and by deducting the latter 
 weight from the former and dividing the remainder 
 into the weight of the piece in air, the quotient 
 will give the specific gravity. Thus : — 
 
 Weight in air - - - 200 
 Weight in water - - 120 
 
 200 80 = 2-5 specific gravity. 
 
 For all practical purposes it may be assumed 
 that the specific gravity will give sufficient indica- 
 tion of the relative vitreousness of diff'erent sam- 
 ples of clay. 
 
 7. Colour after firing. 
 
 The colour of clay is aff'ected. 
 
 By presence of metallic oxides. 
 
 By the presence of earthy substances, chiefly 
 lime. 
 
 By the conditions of the firing process. 
 
 Oxide of iron is the most frequent colorant of 
 clays, but is so greatly influenced in its action by 
 the quality of the fire (whether oxidising or reduc- 
 ing), as well as by the temperature attained, that 
 it is important to verify results by a series of tests 
 in different parts of the oven, and, if possible, in 
 different ovens. 
 
potters' clays ; AND THEIR COMPONENTS. 41 
 
 Minute variations in colour can be better appre- 
 ciated when the biscuit ware is glazed with a thin 
 transparent glaze of known composition. Impuri- 
 ties in the body are also more easily detected by 
 this means. 
 
 The presence of other earths will also affect the 
 colour of the fired clay. Thus barytes will whiten, 
 while lime will, in the presence of iron, tend to im- 
 part a yellowish buff tint. 
 
 8. Glazing properties {and co-efficient of expan- 
 sion). 
 
 The clay may in all other respects be suitable, 
 but when fired under glaze the latter may craze, or 
 it may splinter, or it may shatter the ware itself 
 It is therefore necessary to test its glazing pro- 
 perties by dipping in a glaze of known composition 
 ^nd noting the result after firing. 
 
 It is, however, very undesirable to form conclu- 
 .sions on this point too precipitately. Additional 
 thickness of glaze will often cause test-pieces to 
 show crazing when the manufactured ware might 
 prove sound. 
 
 On the other hand a test-piece may come from 
 the oven sound, and the crazing or other defect 
 may only appear after a long period of time. 
 Great caution should therefore be used in arriving 
 .at conclusions on this point. 
 
 Many other facts in connection with clays and 
 
42 CEEAMIC TECHNOLOGY. 
 
 their qualities might doubtless prove interesting to 
 examine, and some are important in special manu- 
 factures, but the tests indicated will be found to 
 suffice for all general purposes of pottery manu- 
 facture. 
 
CHAPTER IV. 
 
 THE BISCUIT OVEN. 
 
 I. The Oven. — The best known form of pottery 
 oven in England is the up-draught, built with 
 dome and hovel. The fire is applied in eight or 
 more mouths, which enter the firing chamber 
 through the 'bags" built up inside. There is no 
 doubt that this type of oven is wasteful of heat and 
 difficult to control, but so strong are the prejudices 
 of firemen, and so great is the dislike of the manu- 
 facturer to experiment with matters relating to 
 firing, that the up-draught oven holds the field. 
 
 The down-draught oven is more correct in 
 theory, as the products of combustion pass 
 through the firing chamber twice instead of once, 
 but this oven needs a far stronger current of air. 
 In other words, a chimney-stack is necessary to 
 secure good results. The longer distance traversed 
 by the flame renders a strong draught indispens- 
 able. Fuel is economised, but, on the other hand, 
 it is difficult to get sufficient heat on the bottom 
 
 of the oven. For this reason ovens of the latter 
 
 (43) 
 
44 
 
 CERAMIC TECHNOLOGY. 
 
 type are built somewhat low in the crown, so as to 
 provide for a more thorough diffusion and deflec- 
 tion of the heat through the mass of ware. 
 
 There are other methods of constructing ovens 
 which are but little known in this country. The 
 ancient Chinese built their ovens one above an- 
 other upon a hill side, and, beginning at the bot- 
 tom, they carried the waste heat through the 
 upper kilns. Some principle of the kind has been 
 adopted at Limoges in the two-storied ovens, but 
 the most complete method is the arrangement of 
 six or more ovens in a circle, each communicating 
 with an annular flue and, through this, with a 
 central chimney-stack. By an arrangement of 
 dampers the waste heat can be conducted from 
 any one oven to its neighbour on either side, thus 
 heating the newly-placed oven to redness before 
 an ounce of coal is used. So far as we are aware 
 this method has not been successfully employed 
 in England, but the theory seems sound, and 
 ought to be tried. 
 
 After all, it is hardly to be expected that such 
 advice as can be given in a work of this kind will 
 induce any manufacturer to build new ovens ; and 
 the most useful course to adopt will be to point 
 out what may be done to economise with the ex- 
 isting plant and to save loss in firing. 
 
 2. Placing. — Good placing is the foundation of 
 
THE BISCUIT OVEN. 
 
 45 
 
 good ware. The bed influences the shape of the 
 piece in a very large degree. With hollow ware 
 the placer has not very much to do. He must 
 see that covers where fired with the bottoms or 
 middles are set on true, with a little flint on the 
 verge, and that over-hanging parts are bedded up 
 well. This applies both to China and earthen- 
 ware, though more especially to the former. In 
 cases where the piece may be expected to sus- 
 tain its own weight it is better without a bed. 
 The unequal resistance of flint or sand often 
 causes crookedness where the unbedded piece 
 might have kept its shape. Lengthy pieces, such 
 as tubes, are often better hung than laid flat ; but 
 this can only be accomplished when there is on 
 the piece a flange or collar to support the 
 weight. 
 
 Vases and similar ornamental pieces will 
 generally, if properly made, stand firing with 
 but little support, but it sometimes happens that 
 a more than usually elaborate handle, or, perhaps, 
 a lightly modelled figure, will need special assist- 
 ance. Much, of course, depends upon the body in 
 which such pieces are made, but it will be found 
 useful to cast a thick " chum " or cylinder to sur- 
 round the delicate piece and to prop the parts 
 from the sides and top of this. The props and 
 chum are, of course, made from the same clay as 
 
 ■ 
 
46 
 
 CEEAMIC TECHNOLOGY. 
 
 the piece to be supported, and for tins purpose 
 scraps, too dirty for best use, may be worked up. 
 
 In pressed ware, which includes thrown and 
 turned and also jollied work, cups and basins 
 sometimes cause loss by reason of defective shape. 
 A non-circular cup is an abomination, and the ring 
 is not always satisfactory. The angle at which the 
 ring is turned is a matter that is too often left 
 entirely to the discretion of the turner. As a 
 matter of fact, hardly two shapes of cup need a 
 ring of the same angle. The angle will vary from 
 120 to 170 degrees as extremes, and every manu- 
 facturer should know the exact angle required by 
 each cup he makes. It is a simple matter to con- 
 struct a gauge for the use of the turner. A strip 
 of thin steel is cut to the angle required, and this 
 gauge being applied now and then ensures the 
 correct angle being kept. Cast cups require a 
 wider angle than turned cups, but it sometimes 
 happens that a very wide angle does not exert 
 sufficient power over the cup to keep it circular. 
 In that case either the ring should be made 
 heavier, or the cup should be inverted upon the 
 ring and a small piece of sagger placed on the foot 
 as a weight. This increases the pull of the ring 
 and enables it to perform its function. Flat ware 
 as a general rule burns best on a " face " bedder, 
 or if a bedder be not necessary the ware is placed 
 
THE BISCUIT OVEN. 
 
 47 
 
 face downwards. There are some exceptions to 
 this, and obviously ware with embossed patterns 
 cannot well be placed in this way. China plates 
 have a tendency to rise at the edge when turned 
 face downward, and to prevent this a slight weight 
 either of a light ring or a ridge of flint is advis- 
 able. It will be found that there is in the section 
 of any piece of flat ware a certain line of equili- 
 brium. Pieces, the section of which is deeper 
 than this, will deepen yet more, while those that 
 are flatter will become still more flat. This must 
 be taken into account when making bedders for 
 plates and saucers. Considerable art is here 
 necessary on the one hand to form a bed tight 
 enough to keep the piece straight, and on the 
 other to allow sufficient liberty for contraction 
 and deepening. 
 
 Earthenware, on account of its comparatively 
 slight contraction and small amount of vitrification 
 is but little dependent upon the bed. Plates are 
 usually placed together a number in a sagger, each 
 forming its own bed on the top of another. Sand 
 is used in this work in preference to flint. First 
 because the greater porosity of the sand allows a 
 free movement of the ware in contraction, and 
 secondly because in drawing the kiln flint would 
 make an excessive dust. China plates being re- 
 moved one by one from the setters the flint is not 
 
 ■1 
 
48 
 
 CERAMIC TECHNOLOGY. 
 
 disturbed, but upon emptying a sagger full of 
 earthenware plates the whole of the bedding is 
 shaken out. The dust raised by sand is bad 
 enough. It would be unbearable if flint were 
 used. 
 
 The sand or flint used in making beds is 
 slightly damped to cause adhesion. This requires 
 to be carefully and evenly done, for were the bed 
 too damp in any one place the result would be a 
 stain upon the fired ware. Damp, wrongly ap- 
 plied, has a very prejudicial eff'ect upon clay, and 
 the evil is often not discernible until after firing. 
 Ware left too long placed before firing sometimes 
 experiences this. The damp of the bed will pene- 
 trate the clay and cause damage, or it may happen 
 that the bed, through standing too long, has lost its 
 solidity, in which case the slightest shake will dis- 
 integrate the flint and, as a result, put the ware 
 out of shape. 
 
 Bedders are usually made of plaster, but on 
 account of the rapid wearing action of the flint 
 attempts have been made to construct them of clay 
 and to fire them. The objections to these " pit- 
 cher " bedders are the extreme difficulty of getting 
 them exact to size and their liability to chip. It is 
 possible to coat or to harden plaster bedders so as 
 to make them more durable. The white of an eiro: 
 mixed with a little solution of potassium bichro- 
 
THE BISCUIT OVEN. 
 
 49 
 
 mate will form a varnish which hardens under the 
 effect of light, or the bedder may be saturated 
 with weak gum-water to which a little bichromate 
 has been added. Either of these methods will 
 materially lengthen the life of a bedder. 
 
 The durability of saggers and setters has a very 
 serious bearing upon the cost of firing. Five times 
 firing should be the average of a sagger's exist- 
 ence, but it is very doubtful whether this is always 
 the case. It is important that, as far as possible, a 
 sagger should receive a harder fire in the first place 
 than that which it will have to undergo in use. 
 This is not always practicable, especially when the 
 green saggers are, for the sake of economy, used to 
 contain ware. Then, to make durable saggers, the 
 clay should be aged and properly tempered. A 
 sufficiency of " grog" will help. 
 
 It is hardly possible to use too much so long- 
 as the clay is workable, but the grog must not be 
 too fine. To obtain clay that has been long ex- 
 posed is not easy, and to accumulate large stocks 
 for the purpose of weathering involves the sinking 
 of capital, so that manufacturers are generally 
 driven to take a middle course. Only, be it re- 
 membered that the older the clay the more econo- 
 mical will be the saggers. Placers are apt to find 
 fault if the setters and saggers are made at all 
 heavy, but it should be borne in mind that a strong 
 
50 
 
 CEEAMIC TECHNOLOGY. 
 
 sagger as a rule lasts longer than a very light one, 
 and it is better to tax the muscles of the kilnmen 
 than the purse of the proprietor. 
 
 3. Firing. — Around the question of the fire hang 
 most of the anxieties of the potter. He must con- 
 sign his ware to the flames, where it is not pos- 
 sible for him to follow it, and where it must take 
 care of itself without the chance of outside help. 
 
 It is to be regretted that in this matter of firing 
 there is so little interchange of ideas amongst 
 manufacturers. The keen nature of modern 
 competition prevents one seeking the advice of 
 another or the publication of the results at which 
 any investigator may have arrived. Yet there can 
 be no doubt that a comparison of methods would 
 be of the greatest mutual advantage, and the pot- 
 ting industry would reap benefit all along the line. 
 
 Staffordshire is well placed as regards her coal 
 supply. Some of the cleanest burning fuel of 
 England is found beneath her soil, and the potters 
 have taken full advantage of these stores. But to 
 possess good coal is one thing, and to understand 
 the use of it is another. It is be feared that the 
 average fireman knows very little about the 
 scientific use of his fuel, and his ignorance 
 leads to considerable loss. 
 
 In starting the fires care should be taken not to 
 fill up the mouths too rapidly. At the beginning 
 
THE BISCUIT OVEN. 
 
 51 
 
 a clear fire should be aimed at, which shall dis- 
 charge hot air and not smoke into the kiln. 
 Gradually, as the mouth and bag become heated, 
 the fuel may be cast on more liberally until, as 
 the walls begin to glow, the fire-mouth becomes a 
 generator, from which the gas passes into the fir- 
 ing chamber before its combustion is completed. 
 A biscuit oven can receive a vast amount of fire 
 with apparently no result. Every sagger and 
 every piece enclosed therein must throughout its 
 entire substance be permeated by the heat, and 
 this is a matter which must perforce take some 
 considerable time. A gradual combustion is in- 
 dispensable, and to effect this large coal is best. 
 The mouths should be kept well filled, both to 
 exclude an excess of air and also to prevent 
 a too greedy attack upon the fresh baitings. 
 If the mouth be allowed to burn half empty a 
 strong oxydising flame enters the kiln, and when 
 new fuel is supplied the keen rush of air and the 
 rapid combustion carry a new flame charged with 
 smoke and coal dust amongst the heated saggers. 
 The result is flashed and discoloured ware and 
 damaged saggers. All this is escaped if the 
 mouths are fed a little before they seem to need it. 
 
 It is to be regretted that the ovens in use in 
 England permit so little control of draught on the 
 part of the fireman. He can open or close the air 
 
52 CEEAMIC TECHNOLOGY. 
 
 holes on the mouths, but there his power seems to 
 end, and, for example, in dull weather, he has no 
 means of accelerating his draught. It would be a 
 decided advantage if the ovens could be connected 
 with a chimney-stack to get the maximum draught, 
 and that this should be brought under control by 
 means of dampers. The admission of air could then 
 be regulated by the fireman, and greater uniformity 
 would be the result. 
 
 During firing the interior of the oven is in a 
 state of low pressure. In consequence of this 
 there is an inward rush of air, under the greater 
 pressure of the atmosphere, through every crack 
 and cranny. To ascertain the amount of this out- 
 side pressure an ingenious kiln-barometer has been 
 devised. Its construction is very simple. A tube 
 is inserted through the wall of the kiln, and the 
 brickwork around it is made air-tight. The outer 
 end of the tube leads to a box, upon which a 
 graduated glass gauge is fitted. The low pressure 
 in the oven exerts a suction upon a liquid con- 
 tained in the box, and thus readings of the amount 
 of pressure can be taken. It is not necessary here 
 to give details of construction, for the principle 
 being made known, any ingenious superintendent 
 should be able to have an instrument carried out 
 to his liking. 
 
 Such a barometer would at once indicate any 
 
THE BISCUIT OVEN. 
 
 53 
 
 excess or deficiency of draught, and by this it 
 would be possible to judge whether the burning 
 fuel were employing too much or too little oxygen. 
 
 4. Loss in the Biscuit Oven. — The losses in the 
 biscuit oven may be broadly divided under two 
 heads : those caused by the maker and those 
 caused by the kiln. With the former this is not 
 the place to deal, but it may be stated that the 
 makers' losses include bad throwing, turning and 
 casting, cracked handles, bad seams, and untem- 
 pered clay. The kiln losses may be sub-divided 
 into losses of the placer and losses of the fire. 
 The placer is mainly responsible for bad shape, 
 crooked plates, saucers and cups, and for the fall- 
 ing of pieces which should have been supported by 
 the bed. To his account must also be laid ware 
 nipped and strained in the clay, a class of fault to 
 which flat ware is especially liable. The losses of 
 the fire are numerous and severe, probably 80 per 
 cent, of biscuit kiln loss is caused by the fire alone. 
 The most prolific source is fire cracking. Apart 
 from the " straining " already mentioned there 
 is a vast amount of " fire-crack " pure and simple, 
 which no amount of vigilance seems to diminish. 
 The inability of superintendents to check this 
 loss is partly due to an erroneous belief as to when 
 this cracking occurs. It is generally understood 
 that the contraction of the ware is the cause 
 
54 CERAMIC TECHNOLOGY. 
 
 This is not entirely the case. A short-fired oven 
 has invariable less fire-cracking than one which is 
 fired up, and yet even in the shortest fired oven all 
 the contraction has taken place. The fact is the 
 crack is the result of extreme tension at the time 
 of greatest vitrification. This tension is the result 
 partly of the bed and partly of the molecular 
 arrangement of the clay under fire. It can to 
 some extent be modified by a due consideration of 
 the tendency of all ware to rebound from the line of 
 equilibrium already spoken of, and also by careful 
 accuracy in firing off. The fireman can save a good 
 deal of the fire-cracking by watchfulness, for the 
 least amount of over-fire invariably means greater 
 loss. This applies, of course, in the main to china. 
 Earthenware is not subject to fire cracks to any- 
 thing like the same extent. 
 
 There is no doubt that a good deal of oven loss 
 may be avoided by a careful examination of the 
 green ware. In the clay small cracks are often 
 discernible, which would, of course, be developed 
 by the fire into fatal defects. There is, however, 
 great risk in handling green ware, and without a 
 very delicate touch the remedy becomes worse 
 than the disease. In plates the foot-crack can 
 often be checked by diminishing the number piled 
 up in one bung when green. It is mainly the 
 bottom plate that suffers, both from the weight 
 
THE BISCUIT OVEN. 
 
 55 
 
 of its companions and from the irregular surface of 
 the board. 
 
 The loss in a china biscuit oven should not 
 exceed 8 per cent. In addition to this there will 
 be a proportion of seconds and thirds, according 
 to the standard of the manufactory, but the ab- 
 solute loss may be kept well within this figure. 
 It is simply a matter of vigilant and patient care. 
 
CHAPTER V. 
 
 PYROMETRY. 
 
 The science of pyrometry concerns itself, as the 
 name implies, with the measurement of heat, and 
 has always been understood to refer to the record- 
 ing of temperatures beyond the reach of the ther- 
 mometer. 
 
 Since the days of Josiah Wedgwood but little 
 advance has been made by English potters in this 
 study. They are for the most part content to go on 
 using the rude tests of their forefathers, and it is 
 very difficult to excite any enthusiasm for a new 
 system. 
 
 Wedgwood's pyrometer consisted of a brass 
 tablet about six inches long, in which was a taper- 
 ing groove running the whole length. This groove 
 was graduated to degrees, and the instrument was 
 worked upon the principle of the contraction of 
 olays. A number of small slabs of clay were pre- 
 pared and placed in the oven, when the firing was 
 
 in progress one of these was withdrawn, and when 
 
 (56) 
 
PYROMETRY. 
 
 57 
 
 cold was passed into the brass groove, the distance 
 it would travel towards the narrow end indicated 
 the amount of contraction that had taken place, 
 and by deduction, the amount of fire which the 
 kiln had undergone. 
 
 To this method there are two great objections. 
 First, the extreme difficulty of obtaining continual 
 supplies of clay absolutely uniform in shrinkage ; 
 and second, the fact that contraction does not 
 correctly indicate the amount of firing or the de- 
 gree of heat. For example, in the china biscuit 
 oven, all the contraction will have left the body 
 long before the point of translucence is reached, 
 and further, in most, if not in all wares, after the 
 shrinkage has taken place a reaction will set in 
 under continued fire, resulting in a slight expan- 
 sion, which would entirely upset any calculation 
 based upon delicate measurements. If anything is 
 to be learned from the contraction of clay in the 
 fire, information must be gathered not only as to 
 the amount of that contraction, but also as to the 
 time at which it takes place, and the period over 
 which it extends. If in a fire-clay tube, with 
 closed end, a cylinder, about seven inches long, of 
 unburned clay be placed, and upon the end of this 
 the pressure of a spring brought to bear, the 
 length of the clay cylinder will give an appreciable 
 amount of movement during contraction, and this 
 
58 CERAMIC TECHNOLOGY. 
 
 motion may be made, by suitable mechanism, to 
 register its path, as a diagram, upon a graduated 
 scale, much in the same manner as the chart of a 
 recording barometer is produced, and while this 
 does not afford a satisfactory guide to the fireman, 
 it gives valuable data as to the progress of events 
 inside the oven. It is found, for example, that, in 
 a china biscuit oven which is under fire for thirty- 
 eight hours, the first twenty hours are occupied in 
 gathering heat. Then the contraction begins, and 
 continues for some seven hours, while the remainder 
 of the time is needed to secure complete vitrification. 
 The diagram thus recorded also shows whether the 
 shrinkage has been uniform or not, and recent 
 investigations go to show that a regular contrac- 
 tion carries with it a smaller amount of loss than 
 an intermittent, notwithstanding that the former 
 may be far more rapid than the latter. The 
 shrinkage of earthenware has been known to run 
 its entire course in less than three hours without 
 damage, while in another firing a contraction, 
 occupying twice the length of time, but with less 
 regularity, was not so satisfactory. 
 
 Many persons have a difficulty in separating 
 the question of firing from numerically expressed 
 degrees of temperature, At great heats the actual 
 number of degrees reached is more or less a matter 
 of conjecture, and after all it is of no importance. 
 
PYEOMETEY. 
 
 59 
 
 What is wanted is to get the ware right, and hav- 
 ing once secured a good oven to be able to do so 
 again. 
 
 Undoubtedly the most satisfactory method of 
 gauging ceramic fires is that introduced by Pro- 
 fessors Seger and Cramer at Charlottenburg, and 
 known as the "Normal Pyrometric Cones".'" The 
 principle is that certain mixtures of clays and al- 
 kalies fuse at different points, according to their 
 composition. These mixtures are made into pyra- 
 mids or cones, six centimetres high and one and a 
 half centimetres on each side of the triangular base. 
 One of them is placed in the oven where it can be 
 seen from outside, and when the point of the 
 pyramid bends so far as to touch the base level, 
 the required temperature is reached. This system 
 has rapidly gained ground upon the Continent and 
 in America, and it is certain that its adoption here 
 would lead to far more regularity in firing, and 
 would moreover enable the manufacturer to more 
 effectually check the operations of his firemen. 
 
 The chief difficulty in the use of these cones 
 lies in the fact that the cone mixture must be 
 adapted to the existing temperature of the oven. 
 A heat is already in use, in every manufactory, 
 which is suitable to the ware, and it becomes 
 
 * The editor is indebted for much information regarding 
 these cones to a valuable work on The Chemistry of Pottery by 
 Karl Langenbeck. Pittsburg, Pa., The Chemical Publishing Co. 
 
60 
 
 CERAMIC TECHNOLOGY. 
 
 necessary to compose a cone for each set of ovens, 
 which will exactly bend to its lowest point when 
 that oven is sufficiently fired. Moreover, in firing 
 an oven repeatedly week after week, it is not pos- 
 sible under the present system to secure absolute 
 uniformity, and therefore a cone which appears 
 satisfactory one week will seem too soft or too 
 hard the next. Patience is necessary to subject a 
 selected mixture to a long average of trials, so that 
 the exact melting point may be assured. 
 
 The normal cones range over a wide series, 
 fusing from estimated 1760' F. (960° C.) to 
 over 3000° T., but this great variety is, of 
 course, not necessary for an English manufac- 
 turer. It may be taken that felspar melts almost 
 to a clear glass at china biscuit heat, and this 
 melting point of felspar is estimated to be 1400° 
 C."" On the other hand, an earthenware printed 
 oven at its lowest point is more than sufficient 
 to fuse silver, the melting point of which metal is 
 estimated to be 960° C. It will, therefore, be safe 
 to say that in England glaze kilns range from 1000° 
 C. to 1300° C, earthenware biscuit ovens from 
 
 . * As Centigrade or, what is tlie same tiling, Celsius, tem- 
 peratures are given in almost all scientific works, it may be more 
 convenient to adopt the same scale here, giving the following 
 formulae for the conversion of Fahrenheit into Centigrade and 
 vice versa. 
 
 F. = ^ ^ ^ 5 (E. - 32) _ 
 
PTEOMETRY. 
 
 61 
 
 1200° C. to 1350° C, and china biscuit ovens from 
 1300'^ C. to 1450° C. But these figures are points 
 of only academic interest, they do not greatly help 
 the practical potter. What is wanted is a mixture 
 to fuse at each required heat. 
 
 For the softer cones a fritt must be made of, 
 say:— 
 
 Borax . _ . . 190 parts. 
 Whiting - - - - 50 „ 
 China clay - - - 50 ,, 
 Fhnt - - - - 95 „ 
 
 And for a composition bending at the heat of 
 printed glost there would be taken, say : — 
 
 Of the above fritt - - 33 parts. 
 
 Whiting - - - - 30 „ 
 
 Felspar - - - - 50 ,, 
 
 China clay - - - - 10 ,, 
 
 Flint - - - - 60 „ 
 
 Ferric oxide - - - 15 ,, 
 
 For a cone to bend at earthenware biscuit heat 
 
 the mixture would be something like this, the 
 
 fritt being unnecessary : — 
 
 Felspar - - - - 85 parts. 
 
 Whiting - - - - 35 „ 
 
 China clay- - - - 30 ,, 
 
 Fhnt 80 „ 
 
 And for a cone to suit the china biscuit oven 
 
 there should be taken : — 
 
 Felspar - - - - 85 parts. 
 
 Whiting - - - - 35 „ 
 
 China clay - - - - 80 ,, 
 
 Flint - - - - - 200 „ 
 
62 
 
 CEEAMIC TECHNOLOGY, 
 
 It will, of course, be understood that these 
 mixtures are only suggestions, but they are based 
 upon the actual compositions used by Professor 
 Seger. It will be necessary for each manufacturer 
 to modify them according to the heat at which 
 he is firing, and this modification can usually be 
 effected by simply diminishing or increasing the 
 amount of flint. 
 
 For the manufacture of the cones themselves it 
 will be necessary to model a cone in stiff clay, and 
 to have it moulded in the ordinary manner, if laid 
 face downwards a " single " mould can be run, 
 which will answer every purpose. It will be well, 
 however, to have half a dozen of these moulds 
 before beginning to make the cones. 
 
 The proper mixture must be well ground on a 
 small pan, and when dry a portion is taken and 
 mixed with about one-tenth of its bulk of dextrine 
 powder. Water is now added, and the mass 
 worked up into a stiff clay. This can then with- 
 out difficulty be pressed into each of the moulds 
 and taken off level with a knife or wire. For con- 
 venience of reference each different make of cone 
 should have a number impressed upon it before it 
 leaves the mould. If some old type is at hand 
 these numbers can be made sufficiently bold and 
 deep to be easily read after firing. It will, per- 
 haps, be found that the dextrine is apt to clog the 
 
. PYEOMETRY. 
 
 63 
 
 mould and to cause adherence. This may be par- 
 tially relieved by dusting the mould with some of 
 the dry mixture ; but, in any case, it is desirable to 
 gently detach the cones from the moulds by means 
 of a fine tool, otherwise the drying of the clay will 
 cause breakage at the finer end. The use of the 
 dextrine is to harden the cones so that when dry 
 they may be handled with ease. 
 
 In the use of these cones it will be found 
 necessary to bed them in a little soft fire-clay to 
 prevent a possible fall, which would, of course, 
 render the test entirely useless. Some difficulty 
 may be experienced at first in seeing the cones 
 when in the oven, but a little contrivance will 
 obviate this. It is desirable to bring the cone as 
 near to the front of the proof sagger as possible, 
 and, of course, a cone must be set at each proof- 
 hole if the kiln is to be fired by their means. In 
 all probability it will be necessary for some time 
 to use the usual tests in addition to the cones. A 
 fireman can no more change his skin than can a 
 leopard his spots, and it is not to be expected that 
 an important and far-reaching alteration is to be 
 made at once. 
 
CHAPTER VI. 
 
 GLAZES AND THEIR COMPOSITION. 
 
 Pottery glazes are of two descriptions, raw and 
 fritted. The raw glaze is the simpler, and some- 
 times consists merely of a wash of lead or of 
 pulverized galena sprinked upon the moist clay. 
 Such a practice is, however, most undesirable, 
 and can hardly form part of the procedure of a 
 reputable manufacturer. 
 
 A glaze may be used in its raw or unfritted 
 condition when it contains no soluble salts such as 
 borax or carbonate of potash, or when the alkaline 
 earths such as lime and barytes are only present in 
 small quantities. Under other conditions a fritt 
 or glass must first be prepared. The soluble salts 
 must be rendered insoluble by combination with 
 other materials, and the earths must be more per- 
 fectly fused than could possibly be done in the 
 ordinary heat of the glost oven. 
 
 It must be borne in mind here that the use of 
 
 a fritted or a raw glaze is also governed by the con- 
 
 (64) 
 
GLAZES AND THEIE COMPOSITION. 
 
 65 
 
 siderations under which it is to be worked and the 
 na,ture of the body upon which it is to be used. 
 Some wares require to be burned and glazed at 
 one and the same fire, and it will be found easier 
 to adjust a fritted glaze to a biscuit body and a 
 raw glaze to a clay body ; neither does it follow 
 that without alteration of mixtures the conditions 
 can be successfully reversed. 
 
 In most cases fritting is accomplished in a 
 specially designed furnace, where upon a sloping 
 bottom of fire-brick the materials are melted to- 
 gether, to be run out when ready into a tank of 
 water. Some fritts are, however, so hard that 
 they refuse to flow, and some, especially if con- 
 taining lead, exert too great an erosive action upon 
 the kiln. In such cases the mixture must be put 
 into saggers or crucibles, and either placed in one; 
 of the ovens or in a special kiln. 
 
 1. Materials. — The ingredients from which 
 glazes can be composed are comprised in the fol- 
 lowing list, arranged approximately in order of 
 fusibility : — 
 
 Kaolin. White lead. 
 
 Quartz or sand. Eed lead. 
 
 Cornish stone. Carbonate of soda. 
 
 Felspar. Carbonate of potash. 
 
 Fluor spar. Nitre. 
 
 Carbonate of Ihne. Common salt. 
 
 Carbonate of barytes. Borax. 
 
 Oxide of bismuth. Boracic acid. 
 
_66 CEEAMIC TECHNOLOGY. 
 
 In addition to these there are various sub- 
 stances used for the coloration of glazes, which 
 may be grouped respectively as earthy colorants, 
 simple metallic oxides, or compound salts. 
 
 Kaolin or china clay supplies the largest part 
 of the alumina in glazes, and is also useful for 
 thickening the mixture in dipping, thus preventing 
 the heavier particles from settling out. 
 
 Quartz, sand, or flint affords the usual source 
 of free sihca. For the preparation of fritts, white 
 sand is in many respects the most suitable, as 
 it requires no preliminary treatment. Flint and 
 quartz need to be calcined and ground before use, 
 and although the composition of these varies but 
 slightly, their qualities differ sufficiently to make 
 it important to avoid substitution when possible. 
 FHnts are prepared by either wet or dry grinding, 
 and those picked from the beach on some parts of 
 the Sussex coast are by many considered the best. 
 
 Cornish stone is beheved to be produced from 
 decomposed granite and is valuable in a glaze, 
 partly as a source of alumina and partly on ac- 
 count of the silicate of potash it contains. As the 
 composition of the stone varies considerably, it is 
 important that tests should be made, as described 
 upon another page. 
 
 Felspar is found pure in many parts, sometimes 
 in large masses. It is liable to contain small 
 
GLAZES AND THEIR COMPOSITION. 67 
 
 quantities of lime, soda, and occasionally mag- 
 nesia, relsjmr enters largely into the composition 
 of glazes used for porcelain and stoneware, as 
 well as those for glazed bricks. At the ordinary 
 heat of the porcelain fire it melts into a semi- 
 opaque glassy mass, and it is often usual to trust 
 solely to this fusibility as a test of quality. This 
 is, however, not entirely satisfactory, as the pro- 
 portions of Kme and alkali are somewhat variable. 
 It is, therefore, desirable to make, if possible, an 
 analysis. In the chapter on analysis the composi- 
 tion of a variety of spars from different sources is 
 given. 
 
 . Fluor spar is much more fusible than felspar and 
 is greatly used in some parts of Germany. Quite 
 insoluble in water, it occurs in crystalline form and 
 of various colours, often amongst veins of lead. 
 
 Carbonate of lime is usually introduced in the 
 form of whiting, but a finer quality is sold as 
 " Paris white ". It combines readily with borax 
 and lead to form fritts, which melt at a low^ 
 temperature, but it is only with difticulty fusible 
 with flint. Whiting is much used in the composi- 
 tion of felspathic glazes for stoneware and bricks. 
 
 Carbonate of barytes is sometimes used in 
 glazes in which it is desired to avoid lead. Some 
 colours are benefited by its influence, but it is not 
 a completely satisfactory substitute. 
 
68 CERAMIC TECHNOLOGY. 
 
 Oxide of bismuth is fusible at a high tempera- 
 ture and, with silica, acts as a powerful flux. 
 
 White lead and red lead are important fluxes 
 and indispensable to the English glazeniaker. 
 They are readily fusible and quite insoluble in 
 water. Both materials can easily be obtained of 
 sufficient purity, but commercial white lead at all 
 times contains some proportion of hydrate, besides 
 being liable to adulteration with sulphate of bary- 
 tes. The deleterious properties of lead render it 
 advisable to fritt many glazes which might other- 
 wise be used raw, it having been clearly demon- 
 strated that glazes in which the lead has been 
 fritted are not injurious to the health of the 
 workers. 
 
 Carbonate of soda is a most important in- 
 gredient in glaze making, although in a humid 
 atmosphere it is liable, if used in excess, to 
 effloresce from the glaze. In proper combination 
 with lead, flint, or barytes it will produce satisfac- 
 tory results, and the alkalinity which it imparts 
 to the glaze is necessary for the production of 
 some colours. 
 
 Soda tends to render glazes pure and trans- 
 parent, and is most conveniently used in the form 
 of soda ash, though this is sometimes found to be 
 adulterated with sulphate of soda. 
 
 Carbonate of potash is best used in the form of 
 
GLAZES AND THEIR COMPOSITION. 69 
 
 pearl ash, but both the nitrate and the bicarbonate 
 are available. Mixed with soda it produces fritts 
 which are more fusible than those obtained from 
 either salt when used alone. 
 
 Pearl ash is hable to vary in its composition, 
 and should, therefore, be carefully analysed. It 
 also attracts moisture rapidly, and the amount of 
 this should be determined before use. 
 
 Nitre is used when greater purity is required, 
 instead of pearl ash, for the introduction of potash 
 into the glaze. Being rich in oxygen, it exerts a 
 powerful influence in the elimination of any or- 
 ganic substances in the glaze mixture. It also 
 adds to the brilliancy of the colour of some 
 enamels. Nitre is a natural efflorescence from the 
 soil in some countries ; it is also produced artificially 
 in France from refuse, and in other places from 
 ammonia. Most of the supply now used in this 
 country comes from the East Indies. 
 
 Common salt is often used in the production of 
 stanniferous enamels. It is an extremely power- 
 ful flux and hence the presence of the smallest 
 quantity is liable to considerably alter results. 
 Salt is used for i3roducing the glaze on salt-glazed 
 stoneware, as well as in the composition of some 
 smears and flows. 
 
 Borax is of the greatest importance in glaze 
 composition. It owes its value largely to the 
 
70 CERAMIC TECHNOLOGY. 
 
 presence of boracic acid, of which it is the chief 
 source, though the simple acid is sometimes em- 
 ployed. Its use in glazes makes them harder in 
 surface and more brilliant. In large proportion it 
 exerts a solvent action upon under-glaze colours. 
 Borax alters the coefficient of expansion of a glaze 
 in the same direction as silica, but affords a result 
 which fuses at a lower temperature. It is oc- 
 casionally rendered impure by the presence of the 
 sulphates of soda and ammonia. 
 
 Borax is imported in its crude state from the 
 East Indies under the name of tincal, while boracic 
 acid comes from Tuscany, or is manufactured from 
 crude salts found in Peru. 
 
 Occupying an intermediate position between 
 the foregoing substances, which form the glaze 
 itself, and the colouring eai'ths and oxides, stand 
 the white oxides of tin and zinc. 
 
 Oxide of tin is most valuable in the production 
 of opaque glazes and enamels. Its chief property 
 lies in the fact that it does not dissolve in the 
 glaze, but is dispersed throughout the mass in 
 white particles, which remain in suspension, thus 
 forming a milk-white covering, as in Italian 
 majolica. 
 
 Oxide of zinc is also white, but less opaque than 
 the oxide of tin. It is an indispensable material 
 for toning the shades of various colours. In this 
 
GLAZES AND THEIR COMPOSITION. 71 
 
 capacity it is used in the production of yellows^ 
 greens, blues, and browns. 
 
 Of the earthy colorants, the most important are 
 red and yellow ochre, umber and sienna. Ochres 
 are largely used in producing the brown dip on the 
 upper portion of salt-glazed vessels, and, indeed^ 
 for all purposes where a cheap form of iron oxide 
 is required. All these substances are liable to 
 such considerable variation in the proportion of 
 iron present that they need to be constantly tested 
 and analysed to maintain uniformity. 
 
 Umber contains about a fifteenth part of man- 
 ganous oxide and about one-third of oxide of iron. 
 Its use in glazes is hardly desirable, as it usually 
 contains a proportion of sulphate of lime wdiich 
 would prove detrimental. 
 
 The simple metalhc oxides in use for colouring 
 glazes are those of manganese, copper, cobalt, iron,, 
 chromium, nickel, and antimony ; while the com- 
 pound salts embrace the chromates of iron, barium,, 
 and lead. The effects of these colorants under 
 different conditions will be dealt with in the 
 chapter upon colour making. 
 
 2. The constructio7i of glazes. — It is not diffi- 
 cult for the potter, after a long course of practice^ 
 to compose glazes from materials with the nature 
 of which he has become famihar, provided that the 
 glazes are to be used under known and tried con- 
 
72 CERAMIC TECHNOLOGY. 
 
 ditions. When, however, a glazemaker finds him- 
 self compelled to employ new substances in his 
 glazes, to adapt them to entirely new bodies or to 
 tire them under novel conditions, scientific know- 
 ledge will alone enable him accurately and 
 promptly to avail himself of the teaching of his 
 experience. 
 
 He may, it is true, patiently build up a new 
 fund of empirical experience, but, meif^nwhile, 
 much valuable time is lost. 
 
 It is for this reason that some knowledge 
 of the principles underlying glaze composition 
 becomes necessary as an adjunct to the usual 
 practical experience with which, hitherto, so many 
 glazemakers have been willing to remain satisfied. 
 
 Glazes are the result of the chemical combina- 
 tion of certain components under the influence of 
 intense heat. The fusing together of these materials 
 is the result of reaction between the silicic or 
 boracic acids and the alkalies, alkaline earths or 
 lead oxides, which act the part of bases, as in 
 other chemical combinations. In addition to the 
 bases and acids, it has been found necessary to 
 introduce certain components which occupy a 
 neutral position, but which, nevertheless, exer- 
 cise an important influence upon the glaze. Fore- 
 most amongst these stands alumina, without which 
 it seems impossible to construct a satisfactory 
 
GLAZES AND THEIR COMPOSITION. 73 
 
 glaze, and it has been found that ferric oxide 
 exerts a similar power. The office of these sub- 
 stances is that they prevent injury to the glaze by 
 prolonged and repeated firing, and greatly assist 
 the perfect adjustment of glaze to body. 
 
 If the component parts of a glaze are grouped 
 in these three divisions — ^base, neutral, and acid — 
 it becomes possible to construct a formula which is 
 capable of comparison and adjustment without 
 difficulty. 
 
 Glazes are subject to two great evils which 
 overshadow many minor ones. These evils are 
 crazing " and " moulting ". The former is caused 
 when the glaze on cooling contracts more than the 
 body. The latter when the body contracts more 
 than the glaze. This degree of expansion or con- 
 traction has nothing to do with the usual shrink- 
 age of the clay in firing, but is the ordinary expan- 
 sion under heat, followed, of course, by an equal 
 contraction on cooling, which is a property of 
 every substance. The constituent parts of the 
 body and glaze and the fire undergone in biscuit 
 and glost affect the degree of this expansion, and 
 therefore it becomes necessary not only to assign 
 a certain glaze to a certain body, but also to pre- 
 scribe the degree of heat at which each shall be 
 fired. 
 
 It is well known that a short-fired body will 
 
74 
 
 CERAMIC TECHNOLOGY. 
 
 cause the glaze to craze, but, on the other hand, an 
 over-fired biscuit will also cause crazing, the reason 
 of which is probably the slight permanent enlarge- 
 ment or swelling which takes place in over-fired 
 wares. This seems to have the elFect of diminish- 
 ing the degree of expansion of the body, and craz- 
 ing is the natural result. 
 
 Short-fired glaze will also be liable to craze, 
 because the glaze is not complete until it has 
 united itself with the body. At a full fire it does 
 so, absorbing a portion of silica and alumina from 
 the surface of the ware. When the fire fails to 
 reach the point at which this can be accom- 
 plished, the union between body and glaze is 
 incomplete and the modicum of clay is not taken 
 up. This union must always be allowed for in 
 composing the mixtures, for the more perfect it 
 becomes in the fire the better will be the manu- 
 factured article. 
 
 It follows, therefore, that if a small amount of 
 silica, preferably in the form of flint, be added to a 
 glaze, it will serve as an antidote to crazing, but 
 always with the effect of making the glaze harder. 
 Boracic acid, as has already been remarked, exerts 
 a similar influence, but increases the fusibility of 
 the mixture. 
 
 Moulting is the opposite to crazing, but is not 
 nearly so prevalent. The fact that this disease 
 
GLAZES AND THEIE COMPOSITION. '75 
 
 almost always appears immediately after the fire 
 renders it more easy to conquer. The usual cause 
 of moulting is an excess of silica, either in the 
 body or the glaze or both, and the cure is obvious. 
 
 A good glaze for under-glaze colours is sought 
 after by every manufacturer, and in composing 
 such it must be borne in mind that boracic acid on 
 the one hand, and lead on the other, exert a 
 solvent influence. These materials may be used 
 in moderation, but the colours always suffer to 
 some slight extent. Carbonate of barytes is some- 
 times helpful, but its use is often productive of 
 other evils. Carbonate of calcium may also be 
 employed without disadvantage, except that the 
 fire needed for its fusion is fatal to some tints. 
 The point to be aimed at in making a glaze for 
 colours is to get the maximum of hardness with 
 the minimum of fire. The heat should be only 
 that which will give the glaze a bright surface. 
 The glaze should be such as will melt without 
 moving. 
 
 3. Glo.^t Jiving. — Glost-oven losses are most 
 aggravating to the potter, and are by no means 
 easy to prevent. The faults most liable to occur 
 in glost firing are enumerated below, and the en- 
 deavour will be made, if not to suggest a remedy, 
 at least to point out the cause of each. 
 
 Crazing and moulting have already been 
 
76 
 
 CERAMIC TECHNOLOGY. 
 
 touched upon as belonging to the glaze itself. 
 Pinholes are very troublesome in some glazes, but 
 this fault belongs more especially to the body. 
 An over-fired biscuit, too vitreous a ware, too 
 finely ground body materials, or too hard a glaze, 
 each will cause pinholes, and in each case there is, 
 of course, an obvious remedy, but the cure of 
 pinholes after glazing is a most difficult matter. 
 They may be rubbed down and stopped, but the 
 remedy is almost worse than the disease. Preven- 
 tion is better than cure, and there is little doubt 
 that this evil will invariably be found to arise from 
 one of the causes above mentioned. 
 
 The so-called " sulphuring " is an oven fault 
 pure and simple. It commonly takes two forms. 
 Either the glaze is dulled on the surface and 
 darkened in colour as if by smoke or it appears 
 to be covered with a minute crystalline diaper. 
 In the first case it is probable that the cause is 
 the presence of a reducing atmosphere in the oven, 
 or, in other words, there has not been enough air 
 admitted to the mouths. A reducing atmosphere 
 is that which is charged with carbon rather than 
 with oxygen. This carbon has the effect of with- 
 drawing the combined oxygen from the surface of 
 the melted glaze and so disturbing the relations in 
 which the constituent parts of the glaze ought to 
 stand to each other. 
 
GLAZES AND THEIR COMPOSITION. 77 
 
 Sulphur has, in fact, nothing to do with the 
 case, except that a considerable amount of sulphur 
 in the coal may cause excessive clinker, and thus 
 check the draught. The remedy is a clear passage 
 for the air and good luting, but the cure of defec- 
 tive pieces has so far been found impracticable. 
 A certain amount of smoke is inseparable from 
 coal firing, but a careful fireman will see that no 
 baiting is put on too near the time of firing off. 
 While the glaze is in a molten state a clear finish- 
 ing fire will help to restore the bright surface, but 
 if the ware be allowed to cool in a carbonaceous 
 atmosphere the result will be disastrous. 
 
 The second evil which commonly goes by the 
 name of sulphur, appears to be actually due to 
 the presence of sulphur in the coal. It is not 
 nearly so prevalent as the first-named trouble, but 
 is caused, in the opinion of some who have investi- 
 gated the matter, by a directly opposite influence. 
 Too much air seems to have the effect of allowing 
 any sulphur vapour that the combustion may liber- 
 ate to attach itself to the glaze, and from this con- 
 clusion it would seem that a middle course, in 
 the matter of the regulation of draught, will give 
 the most satisfactory result, and that the periodical 
 evolution of reducing gas as each baiting is per- 
 formed is not without a useful effect upon the con- 
 tents of the oven. 
 
CHAPTER VII. 
 
 COLOURS AND COLOUR MAKING. 
 
 There is no more fascinating branch of ceramic 
 chemistry than that which concerns itself with 
 colouring agents and their behaviour under the 
 varying conditions of the manufactory. 
 
 There are, doubtless, some to whom this is a 
 field almost unexplored. If so, it is greatly hoped 
 that the perusal of these lines may lead any such 
 to enter upon investigations which may, possibly, 
 prove as profitable to the potting industry as they 
 will, undoubtedly, be interesting to the inquirer. 
 
 Colours may be classed under certain heads 
 according as they have to undergo more or less 
 severe fires, and by reason of the distinction be- 
 tween the ovens in which the products are burned 
 the lines of demarkation are perfectly apparent, for 
 although the same oxide may produce the same 
 colour at every temperature, the conditions attend- 
 ing the profitable use of any colorant differ with 
 
 every variation in mixture or fire 
 
 (78) 
 
COLOURS AND COLOUR MAKING. 
 
 79 
 
 Colouring matter can be used in either of the 
 following ways : — 
 
 (a) In the clay, ^-Biscuit fire 
 
 (o) Upon the clay as a dip. ) 
 (c) In the glaze. ) qj^^^ 
 
 ((i) Under the glaze. ^ 
 
 (e) Upon the glaze. Enamel or hard-kiln fire. 
 
 But instead of describing the colours used in 
 each class, it will be more convenient to proceed 
 by dealing with the principal colorants one by one, 
 explaining their behaviour in the presence of other 
 substances and under different circumstances. 
 
 Before examining the colouring salts in detail 
 it will be well to notice the requirements of each 
 class of treatment above mentioned, that the mind 
 may be prepared to grasp the conditions which, in 
 each case, the materials will be called upon to 
 fulfil. 
 
 Coloured bodies are employed mainly for the 
 cheaper grades of wares, such as jugs, teapots, and 
 the like. Whether used as the main substance or 
 as a dip, matters little as to the colouring matter, 
 except that in the latter case the effect of some 
 oxides in altering the contraction of the clay must 
 be taken into account. Most bodies are now 
 mixed in slip, and, therefore, it will be found 
 more accurate to employ the stain also in slip, 
 adhering, of course, to a definite weight for the 
 pint, just as in the case of clays. If the 
 
80 
 
 CEEAMIC TECHNOLOGY. 
 
 amount of stain be not sufficient to allow of this it 
 is well to dilute the oxide with a proportion of 
 Hint or stone, which must, of course, be allowed 
 for in the mixing. In staining china bodies regai'd 
 must be had to the presence of bone, which often 
 exerts a detrimental influence upon colours. The 
 greater fire of the china also renders it more diffi- 
 (;ult to produce a satisfactory colour. In mixing 
 a coloured body notice should be taken whether, in 
 firing, the ware has a tendency to vitrify on the 
 surface, and the endeavour should be made to 
 maintain the coloured clay, when fired, of as nearly 
 as possible the same porosity or density as the un- 
 stained clay. This is, of course, a matter for ad- 
 justment, but it does not present any insuperable 
 difficulty. 
 
 Coloured glazes are seen at their best when 
 quite transparent and when fused upon a white 
 body. Any undulations in the modelling then 
 give light and shade, and a rich mellow effect is 
 produced. The practice of making black ware by 
 means of a blue glaze upon a brown clay is com- 
 mended for the sake of cheapness, but the special 
 advantages of both coloured body and coloured 
 glaze are lost. 
 
 Several distinct palettes of colours in glazes 
 may be produced from the same colorants by 
 simply altering the constituents of the glaze, but 
 
COLOURS AND COLOUR MAKING. 81 
 
 this alteration is to a great extent dependent upon 
 the composition of the body. Thus it is scarcely 
 possible to obtain an alkaline glaze upon a vitreous 
 body except at an extremely hard fire. In prepar- 
 ing coloured glazes it will be found most satisfac- 
 tory to add the colouring matter to the fritt and to 
 run this down in saggers rather than upon the fritt 
 kiln. A more perfect incorporation of the color- 
 ant is the result, with less probability of disturbing 
 the quality of the glaze. 
 
 Under-glaze, or rather glaze-kiln colours, for 
 some of them are best used upon the glaze at 
 glost fire, consist, in the main, of colouring 
 oxides without flux, but containing a proportion of 
 clay or similar material as a diluent. Colours 
 used upon the glaze in hard kiln or enamel kiln 
 must have as an integral part of their composition 
 a flux more or less soft, which shall agree with the 
 glaze upon which it is to be used. The salts used 
 in this flux exert great influence upon the tint of 
 the colour itself, so that an extensive knowledge of 
 the behaviour of each material is necessary. 
 
 1. Colouring agents. — The following metals, in 
 the form of oxides, are employed as colour pro- 
 ducers or chromogens : chromium, iron, man- 
 ganese, zinc, cobalt, antimony, copper, tin, and 
 nickel. In addition to these oxides there are 
 
 certain salts of the same metals, as : chromat& 
 
 6 
 
82 CERAMIC TECHNOLOGY. 
 
 of iron, chromate of barium, cliromate of lead, 
 chloride of silver, and purple of cassius ; while for 
 some colours use is made of earths such as umber 
 and ochre. 
 
 Chromium is one of the most valuable of color- 
 ants in pottery ; it may be used in every class of 
 colour from- clay to enamel. In coloured bodies 
 the oxide of chrome will give various tints of 
 green, and by combination with cobalt almost any 
 tone may be produced. The greens, however, are 
 not brilliant, but subdued and greyish. Chrome 
 used in excess is liable to spread through the ad- 
 jacent white parts of the clay, and for this reason 
 it is not easy to make a strong green dip, but on 
 the other hand this substance exerts little or no 
 softening action upon the clay, and will stand the 
 hardest fire. 
 
 In glazes chrome is less satisfactory. The 
 particles refuse to dissolve, but remain suspended 
 and retain their opacity, thus the chief beauty of a 
 coloured glaze is lost. Used in minute proportion 
 some salts, such as the chromate of lead, will im- 
 part a pale yellowish green to a glaze, but this 
 cannot be considered a desirable colour. 
 
 For under-glaze colours chrome is invaluable ; 
 its refractory nature enables it to stand, and the 
 readiness with which it will combine with cobalt 
 permits the colourmaker to produce greens of 
 
COLOURS AND COLOUR MAKING. 83 
 
 every shade. The best of these, however, can only 
 be brought out by the use of a considerable amount 
 of flux, and this causes some uncertainty of tint 
 under repeated fires. 
 
 Curiously enough the only available pink for 
 under-glaze use is also make from chrome, or 
 rather from tin under chromic influence. Salv6tat 
 found only a half per cent, of oxide of chrome in 
 a sample of pink, and it will be seen on making 
 up any recipe for this colour that a very large 
 quantity of the chrome is washed away in the 
 manufacture. The purity of this pink will de- 
 pend in a very large measure upon the proper 
 calcination, grinding, and washing of the mixture. 
 With a little cobalt various tints of purples, 
 mauves, and amethysts may be produced, but 
 these colours are always liable to a certain amount 
 of separating action exerted by the glaze. The 
 cobalt is dissolved, the tin only suspended, hence 
 if a slight excess of fire be given the glaze carries 
 a portion of the blue away from the pink. This 
 may to some extent be obviated by a thorough 
 calcination of all the ingredients, but it is never 
 entirely got rid of. Chromium oxide may often be 
 introduced with advantage in the form of a salt 
 such as potassium bichromate or lead chromate, 
 but allowance must of course be made for the 
 other elements thus unavoidably included. 
 
84 CERAMIC TECHNOLOGY. 
 
 For enamel and hard-kiln greens of all shades 
 chrome is used. With a suitable flux, and dark- 
 ened or lightened with cobalt or Naples yellow, 
 almost any tint is possible, but usually the painter 
 prefers to mingle these at the point of his pencil, 
 so that his requirements are mainly one good dark 
 green and one light. 
 
 Iron is an important and indispensable colour- 
 ing agent. It is more widely distributed in nature 
 than any other stain, as red clays and yellow marls 
 alike owe their tones to its influence. Some com- 
 mercial and natural products, such as ruddle, 
 crocus, and rouge, are used by potters. They all 
 owe their colouring power to the presence of ferric 
 oxide, but it is not well to depend upon their 
 purity or uniformity. Ferric oxide is readily 
 made by calcining copperas, ferrous sulphate, at a 
 red heat. For bodies and glaze-kiln work the 
 degree of temperature matters little, but for 
 enamel reds the heat is everything. 
 
 Upon clays iron exercises a softening in- 
 fluence in proportion to the amount used, but 
 this can to some extent be obviated by the 
 increased use of kaolin. Even then the body 
 containing much iron will not stand a great 
 fire. 
 
 In glazes iron supplies a considerable range of 
 fawns and browns, and even, under the influence 
 
COLOUES AND COLOUE MAKING. 86 
 
 of a reducing flame, the green tones so valued in 
 the Chinese celadons. The brown shades are most 
 favoured by lead glazes, the yellow and green by 
 lime and soda. 
 
 For under-glaze colours iron is chiefly valuable 
 in the form of the chromate, which is largely used 
 in most glaze-kiln browns. The darker tones are 
 obtained by the addition of manganese, and the 
 lighter by the employment of zinc and alumina. 
 These more delicate tints are, however, somewhat 
 uncertain, and will not stand severe or repeated 
 fires. Ferric oxide is sometimes added to yellow 
 base to produce orange, but this is not a colour in 
 general use, inasmuch as the majority of under- 
 glaze colours are now required mainly for print- 
 ing. An under-glaze red seems to be, under 
 modern conditions, out of the question. The pure 
 red found upon Rhodian ware was made with a 
 natural clay under an alkaline glaze ; the brilliant 
 colour, even if the earth could now be found, 
 would be utterly destroyed by lead. 
 
 In the enamel kiln palette iron is extensively 
 used. Reds of all shades are obtained by calcin- 
 ing copperas at different temperatures. Sal v^ tat 
 gives eight distinct tints, ranging from orange-red 
 to deep violet, which are all dependent absolutely 
 upon the heat employed. These colours are not 
 fritted, but the oxide, being obtained of the right 
 
86 CEEAMIC TECHNOLOGY. 
 
 tone, and being thoroughly washed with boiling 
 water to remove every trace of unaltered sulphate, 
 is ground with a sufficient amount of the proper 
 flux. The chestnut-brown from iron chromate 
 may also be obtained in the enamel kiln, and the 
 strong colorant may be modified by the use of 
 zinc oxide. In this case the whole colour must be 
 fritted at a gentle heat. 
 
 Manganese is constantly employed in the pro- 
 duction of dark browns at every degree of heat. 
 In bodies it is most useful, and is sometimes found 
 in such materials as umber, where its influence is 
 utilised without recognition. 
 
 Manganese is mainly used in the form of black 
 oxide, and this almost always contains some pro- 
 portion of iron. The iron is not objectionable, but 
 the percentage contained should always be ascer- 
 tained in order to secure the repetition of any 
 desired tone. 
 
 In glazes the most remarkable feature of this 
 colorant is that it supplies, in the presence of 
 alkalies, purple and claret tints. To obtain these 
 the absence of both lead and iron is impera- 
 tive, and hence the carbonate of manganese is the 
 most suitable salt for the purpose, being more 
 generally free from iron than the oxide. To 
 obtain the required conditions is not easy, but 
 upon such soft earthenwares as those of Japan, 
 
COLOURS AND COLOUR MAKING-. 87 
 
 which are glazed with silicates of soda and 
 lime, beautiful manganese purples are found, and 
 M. Deck of Paris was very successful in the 
 same direction. 
 
 Browns for j^rinting under-glaze are largely 
 dependent upon manganese for their strength 
 and uniformity under varying fires. Iron is 
 used as already explained, mainly as chromate, 
 but manganese is the great resource of the 
 maker of printing browns. To enamel-kiln 
 browns the same remarks apply, except that 
 the necessity for the use of a strong colorant 
 is not so apparent. 
 
 Oxide of zinc is colourless, and is not much 
 used for any work at high temperatures. It has 
 already been mentioned in connection with iron 
 browns, and will be touched upon again under 
 cobalt. It is valued on account of the mellowing 
 influence it exerts upon other oxides. 
 
 Oxide of cobalt is the most widely known and 
 the most universally used of all metallic color- 
 ants. Absolutely indestructible by fire, it sup- 
 pKes the blue tints of the potter's palette from top 
 to bottom. In clays it must be employed with 
 caution, as it exerts a softening influence, but in 
 moderation it may be relied upon to produce 
 almost any shade of purplish blue. In glazes 
 cobalt gives rich if somewhat monotonous efl'ects, 
 
•88 
 
 CERAMIC TECHNOLOGY. 
 
 .as it dissolves readily in the glaze substance, and 
 the result is a rich transparent blue. Carbonate 
 of barytes has the effect of enriching this blue, but 
 is sometimes productive of other evils. In under- 
 glaze printing and groundlaying cobalt is widely 
 used. Blues of all strengths are supplied by it 
 from the famous hleu du Roi to the simple 
 ^' Brosely " and cheap " flow blue ". Combined 
 with alumina it produces mat blue, and it can, 
 moreover, be combined with greens, pinks, and 
 browns in every possible way. 
 
 Enamel blues are mainly produced from cobalt 
 with the aid of oxide of zinc, which has the effect 
 of both softening and brightening the colour. The 
 Erench chemists state that cobalt and alumina can 
 b)e precipitated together to give an ultramarine 
 colour, but though this appears very beautiful in 
 powder, there is extreme difficulty in fluxing it 
 satisfactorily. Nothing is easier than to make a 
 blue from cobalt, few things are harder than to 
 make a good one and to use it without loss. 
 
 Oxide of antimony is colourless in itself, but 
 is valuable in combination with lead. The anti- 
 moniate of lead (Naples yellow), used sometimes 
 in combination with oxide of tin, is the foundation 
 of the majority of pottery yellows. This is not 
 used, however, in clay work, and though a good 
 under-glaze yellow is not readily made, nor, in- 
 
COLOURS AND COLOUR MAKING. 89 
 
 deed, greatly wanted, the yellow base, fluxed for 
 the hard kiln or enamel kiln, gives a variety of 
 tints. Oxide of iron can be added to the lead 
 and antimony to produce orange, or the yellow 
 may be used with chrome for making Hght greens. 
 All these finer tints are in demand when under- 
 glaze painting is in progress, but for printing 
 stronger colours are preferable. 
 
 Raised paste for use under gold is also made 
 from this base, the tin giving sufficient body for 
 the purpose. 
 
 Oxide of copper is a substance which, under 
 clever management, yields wonderful results, but 
 in body mixing it is practically useless. It will 
 not stand a great heat, and, besides, acts upon 
 clay as a flux. Neither can it be used to any 
 great advantage in under-glaze colours, but for 
 the coloration of glazes a great variety of tints are 
 obtainable. With an ordinary lead mixture copper 
 will afford greens of varying tone and strength. 
 With alkalies a rich turquoise blue glaze can be 
 made. This colour was, it is said, discovered in 
 ancient times, when it was produced by heating a 
 mixture of sand, carbonate of soda, and copper 
 filings. A French chemist has, however, shown 
 that it may also be made without soda, for he 
 succeeded in obtaining a fine blue from sand, lime, 
 and copper. Boracic acid seems to destroy the 
 
90 GEEAMIC TECHNOLOGY. 
 
 fine blue tones and the extreme difficulty of satis- 
 factorily glazing English wares with soda alone 
 accounts for the non-appearance of the fine colour 
 in this country. The glaze made by Deck, and 
 already mentioned as developing the purple tones 
 of manganese, was capable also of giving a fine 
 turquoise with copper. Oxide of copper also 
 supplies, under the influence of a reducing flame, 
 the wonderful red colour or glaze known as sang 
 de hoeuf. For this it would appear that a fels- 
 pathic or alkaline glaze mixture is also necessary, 
 for any lead would be reduced at the same time as 
 the copper. The general opinion is that the red 
 colour is caused by finely-divided particles of 
 metallic copper being distributed through the 
 glaze. 
 
 At the enamel fire copper is used for the 
 manufacture of cheap simple greens, which pos- 
 sess great transparency, but here also the great 
 value of this colorant is found in the turquoise 
 blues, of which it is the only possible source. 
 The finest turquoise is considered to be that pro- 
 duced on the soft paste of Sevres. Many attempts 
 have been made to rival this colour, but so far 
 without success. It would seem impossible to 
 construct an enamel colour without lead, and yet 
 every particle of lead is so much loss to the purity 
 of tint in the turquoise. 
 
COLOUES AND COLOUR MAKING. 
 
 91 
 
 Oxide of tin is of use mainly in the enamel 
 kiln for the production of white enamels. These 
 owe their opficity to the fact that the white 
 oxide is not dissolved but only suspended in the 
 flux. In making enamel the main difficulty is to 
 secure a pure brilliant white. The least particle 
 of iron is fatal, and the tin itself sometimes 
 assumes a pink cast, apparently without reason. 
 The preparation of the oxide has a great deal to 
 do with its fitness for use in enamel. It is far 
 better made by calcining pure tin in a current of 
 air than by the simpler method of digestion in 
 nitric acid. 
 
 Oxide of nickel is more employed in coloured 
 bodies than in glazes or colours, but its chief 
 advantage lies in the fact that it is cheap and can 
 be simply used. In glazes it is liable to cause 
 cloudy separations, and in every case the tones 
 which it supplies are dull and uncertain. Nickel 
 is rarely used in enamel colours. 
 
 2. Colour making. — For the stains used in col- 
 oured bodies or dips there is not much " making " 
 in the sense of manipulation. The important 
 point is to see that the mixture of clays is not 
 destroyed in its nature by the addition of colour. 
 If natural colorants can be procured of good and 
 uniform quality it is often better to employ them, 
 as, for example, good marl cannot be surpassed as 
 
92 
 
 CERAMIC TECHNOLOGY. 
 
 a stain for yellow or cane-coloured clay, but this is 
 a somewhat exceptional case, in that the sup^^ly 
 of marl is abundant and the colour very uniform. 
 Where several oxides are used together in the 
 production of a stain, it is best to mix them well 
 and to calcine the whole at a heat at least as great 
 as the finished body will have to undergo. The 
 mass may then be thoroughly ground and mixed 
 with the clay. 
 
 Many metallic oxides, in the cross combina- 
 tions brought about by heat, resolve themselves 
 into partially soluble salts. These should always 
 be removed by washing with hot water until 
 nothing but pure water comes off. This washing 
 may, of course, have the effect of altering the 
 expected tint of the colour, but it is the only 
 way to secure purity and uniformity. It is most 
 important for the maker of tints to be able to 
 match them again accurately, and for this reason 
 no material or process should be relied upon that 
 cannot be adojDted a second time. These remarks 
 apply with even greater force to the maker of 
 under-glaze and enamel colours. 
 
 The use of coloured glazes is not so extensive 
 as it once was, a fact which must be regretted by 
 all who value the special qualities of ceramic 
 colours. Probably the keen demand for uni- 
 formity has contributed to the disparagement of 
 
COLOUES AND COLOUR MAKING. 
 
 93 
 
 tinted glaze, for the accidental irregularities which 
 are so attractive to the artistic eye, constitute a 
 fatal objection in the opinion of a certain section 
 of the public. 
 
 When possible the stains for coloured glazes 
 should be fritted with a portion of the glaze mix- 
 ture or of the fritt which enters into it. This 
 fritting is best accomplished in flinted saggers or 
 crucibles at a special heat. The more fusible 
 numbers of the " Seger " pyrometric cones will 
 serve admirably to record the temperature at 
 which any fritt will melt, and this point should be 
 rigidly adhered to if a repetition of the result be 
 desired. If coloured glazes are to be dipped, it is 
 best not to fritt the whole mass, but to reserve 
 certain parts of the insoluble materials to be 
 ground with the fritt, and thus to assist in the 
 working both at the mill and in the dipping- 
 house. 
 
 Under-glaze colours are not difficult to make if 
 the right glaze is at hand. It must be remem- 
 bered that in the glaze kiln one flux — the glaze 
 itself— has to serve for each and every colour, and, 
 therefore, the modifications that give rise to varia- 
 tion of tint must be made in the colour itself. 
 
 Too much stress cannot be laid upon the 
 importance of calcining, grinding, and washing. 
 With pure materials and due attention to these 
 
94 
 
 CERAMIC TECHNOLOGY. 
 
 points success may be attained by any intelligent 
 worker, but there is no room for inattention to 
 any single detail. 
 
 Regard must, of course, be had to the purpose 
 for which the colour is to be used. At the pre- 
 sent day nine-tenths of the under-glaze colours in 
 use are required for printing, and this demands 
 special qualities. Printing colours must be solid 
 and rich, somewhat opaque and not liable to fade 
 away in the lighter parts. The printing-press can 
 make no allowance for the weakness of any colour, 
 and, therefore, the maker of the colour must elimi- 
 nate the weakness. For grounds the principal 
 glaze-kiln colour is mazarine blue, which is, gener- 
 ally, almost pure oxide of cobalt. For the ad- 
 vantage gained in working it is best, however, , to 
 manipulate the colour by calcining with a propor- 
 tion of hard flux, afterwards grinding and washing 
 the mixture. 
 
 Hard-kiln and enamel colours, if they are to be 
 produced in their full beauty, tax the skill and 
 patience of the colourmaker to the utmost. Fore- 
 most amongst these stand the gold colours, ma- 
 rone, ruby, carmine, purple, and rose colour. The 
 base of all these is " Purple of Cassius," a rich 
 coloured precipitate of gold thrown down by tin. 
 The difference of tint and shade in the colours 
 is a matter of the amount and character of flux 
 
COLOURS AND COLOUR MAKING. 95 
 
 and of the addition of more or less chloride of 
 silver. 
 
 The success of these colours hinges entirely 
 upon details of manipulation. The makers who 
 supply the markets of the world have the experi- 
 ence of many years behind them, and it may be 
 doubted whether, even if their recipes were made 
 j)ublic property, any one else could at once obtain 
 as good results. 
 
 Briefly, the procedure followed is something 
 like this. A given weight of brown gold is dis- 
 solved in aqim regia (nitro-hydrochloric acid), and 
 the proper proportion of pure tin is also dissolved, 
 either by slowly adding it to the gold solution or by 
 taking it up with additional acid. The two solu- 
 tions are then mingled, and the precipitate is 
 thrown down by the addition of large quantities of 
 water. The crimson colour is then collected and 
 fluxed, of course without being fritted, to produce 
 the required colour. This is admittedly a very 
 crude description of the process, but it is enough 
 to indicate the difiiculties lying in the path of the 
 inquirer. Let difficulties deter no man, however, 
 for there is a sure market for fine gold colours. 
 
 Enamel reds are also colours which need 
 delicate treatment. It has already been ex- 
 plained under the head of " Iron " that the colour 
 is produced by the calcination of copperas. This 
 
96 CERAMIC TECHNOLOGY. 
 
 should not be trusted to a kiln fire, but should be 
 accomphshed in the laboratory, over a clear fire, 
 upon an iron plate. The difficulty is to keep the 
 whole of the salt at one temperature, for some is 
 likely to be over-roasted while parts are yet green. 
 The washing' water should be retained, as it may 
 be evaporated to recover the dissolved iron, or the 
 floating hydrate may be collected and employed. 
 Keds are not fritted with the flux, but simply 
 ground for use. 
 
 In most other colours the flux and base are 
 melted together, and in this case the chief diffi- 
 culties lie — first in the preparation of the base, 
 and secondly in the selection of a flux. 
 
 A very great similarity exists between glaze- 
 kiln colours and enamel-kiln bases. Any of the 
 former would, if fluxed, produce some sort of a 
 colour in the enamel kiln. But owing to the 
 greater delicacy of the enamel fire a greater range 
 of tints is possible. The manipulative processes 
 are, however, much the same. Calcining, grind- 
 ing, and washing are the important points, 
 and these should be carefully attended to before 
 the flux be added at all. It may seem useless 
 labour to grind and wash a substance that has to 
 be calcined and ground over again, but the extra 
 work will be abundantly repaid by the superior 
 quality of the resulting colour. The last grinding 
 
COLOUES AND COLOUR MAKING. 97 
 
 sliould be as perfect as possible. The colour 
 should be reduced to an impalpable powder, and 
 the subsequent drying must be managed with very 
 gentle heat. Few things are more aggravating 
 than to have the working properties of a fine 
 colour destroyed by careless drying. Every 
 particle of the colour should be so soft that it 
 can be pulverised between finger and thumb, and 
 if this be the case a sieve will reduce the whole 
 to the condition beloved of the artist. 
 
 7 
 
INDEX 
 
 Albite, analysis of, 15. 
 Algerian pottery, 16. 
 Alkali, action of, 8. 
 
 — in pottery, 17. 
 Alkalies in clay, 30. 
 Alumina in clay, 29. 
 American pottery, 18. 
 Analysis, 10. 
 
 Ball clay, analysis of, 13. 
 Barometer for kilns, 52. 
 Barytes, carbonate of, 67, 75. 
 Bedders, 48. 
 Berlin porcelain, 19. 
 Biscuit oven, the, 43. 
 
 — — loss in, 55. 
 Bismuth, oxide of, 68. 
 Body, pottery, 6. 
 Bone, 5. 
 
 — function of, 8. 
 Boracic acid, 68. 
 
 — — calculation of, 26. 
 Borax, 68. 
 
 Bow porcelain, 20. 
 
 recomposition of, 23. 
 
 Bristol porcelain, 19. 
 
 Calcium, carbonate of, 75. 
 
 — — in pottery, 17. 
 Carbon in pottery, 16. 
 Carbonate of barytes, 67, 75. 
 
 — of lime, 67, 75. 
 
 — of potash, 69. 
 
 — of soda, 69. 
 
 Carbonic acid in pottery, 17.- 
 Ceramic composition, 2. 
 Chelsea porcelain, 20. 
 Chemistry of pottery, the, 1. 
 Chinese porcelain, 19. 
 
 — pottery, 18. 
 Chromium, 82. 
 
 A "l^Ji^W "tfCkr?>roJ)*s;.^;'« 'J* 
 
 — secondary, 12. 
 foniRoneul^Df, 2Sr.. 
 
 Cobalt, 87. 
 
 Coefficient of expansion, 41, 68, 73 
 Colours and colour making, 78. 
 
 — classification of, 79. 
 
 — grinding of, 96. 
 
 — washing of, 92. 
 Colour making, 91. 
 
 — of clays, 40. 
 Colourants of glazes, 66. 
 Colouring agents, 81. 
 Coloured bodies, 79. 
 
 — glazes, 80. 
 Combining weights, use of, 20. 
 
 — — list of, 28. 
 Combined water in clay, 29. 
 Components of clays, 29. 
 Cones, normal pyrometric, 59. 
 Construction of glazes, 71. 
 Contraction of clays, 37. 
 Copper, 89. 
 
 Cornish clay, 11. 
 — granite, 12, 66. 
 (99) 
 
100 
 
 INDEX. 
 
 Cramer, Prof., 59. 
 Crazing, 41, 73, 74. 
 
 — antidote to, 74. 
 Crooked ware, 53. 
 Cup rings, 46. 
 
 Delft ware, 17. 
 Density of elaj's, 39. 
 Dextrine, use of, in cones, 62. 
 D'Oiron ware, 18. 
 Down draught ovens, 42. 
 
 Early French pottery, 16. 
 
 — Spanish pottery, 17. 
 E xrthenware placing, 47. 
 
 — use of Ayit ill) 8. 
 ;Eiyptr^6jli^ttj3ryj J*Cj 
 '"Enamei'oftlbiHse, 04i : .. 
 English pottery, 18. 
 "E^tijiifl^fi'-iMiof/AJir^ 47.1 1 1 I 
 fiqtiivaletvt; wiigjit/,'Pist i)f ,:2| 
 
 — — use of, 20. 
 Etruscan pottery, 16. 
 
 Felspar, 14, 66. 
 
 — analysis of, 15. 
 
 — alkali in, 22. 
 
 — alumina in, 22. 
 
 — quality of, 5. 
 
 — silica in, 22. 
 Ferric oxide in clay, 30. 
 Fire-clay, analysis of, 14. 
 Fire-crack, 53. 
 
 — causes of, 54. 
 Firing, 50. 
 
 Flint, 5, 15, 66. 
 
 — in Earthenware, 8. 
 
 — in placing, 48. 
 Fluor spar, 67. 
 Formula for glaze, 25. 
 Fritt, arrangement of, 27. 
 
 — for cones, 61. 
 Fuel, use of, 51. 
 
 German method in glaze ar- 
 rangement, 25. 
 
 German pottery, 18. 
 
 Glaze, recomposition of, 25. 
 
 Glaze, separation of constituents, 
 25. 
 
 Glaze, tin, 17, 18. 
 
 Glazes and their composition, 64. 
 
 — raw and fritted, 64. 
 
 — materials used in, 65. 
 
 — construction of, 71. 
 
 — evils in, 73. 
 
 Glazing properties of clays, 41. 
 Glost firing, 75. 
 
 — oven losses, 75. 
 
 — — colours, 81. 
 .■Cri'e^k'Jpttery, 16. 
 '•(jrri«I>ding of colours, 96. 
 
 Hard kiln colours, 94. 
 Hard porcelain, 19. 
 
 Iron, 84, 
 — chromate of. 
 
 85. 
 
 Japanese porcelain, 19. 
 
 — pottery, 18. 
 
 Kaolin, 3, 66. 
 
 — derivation of term, 11. 
 Kaolinite, 3. 
 
 Kaolins, analysis of, 11. 
 Kiln-barometer, 52. 
 Kiln losses, biscuit, 53. 
 
 — — glost, 75. 
 
 Lead, white and red, 68. 
 Lime, carbonate of, 67, 75. 
 
 — in clay, 39. 
 Limoges porcelain, 19. 
 Loss in biscuit oven, 53. 
 
 — — glost oven, 75. 
 Lucca della Robbia ware, 17. 
 Luneville pottery, 18. 
 
INDEX. 
 
 101 
 
 Magnesia in clay, 30. 
 Majolica ware, 17. 
 Manganese, 86. 
 Materials, 2. 
 
 — functions of, 7. 
 
 — for glazes, 65. 
 
 — in combination, 6. 
 Meissen porcelain, 19. 
 Metallic oxides in glazes, 71. 
 Moulting, 73. 
 
 — cure of, 75. 
 
 Nevers pottery, 17. 
 Nickel, 91. 
 Nitre, 69. 
 Normal pyrometric cones, 60. 
 
 Ochre, 71. 
 
 Opaque tin glaze, 17, 18. 
 Oreil pottery, 18. 
 Orthoclase, derivation of term, 
 14. 
 
 Orthoclase, analysis of, 15. 
 Ovens, control of, 51. 
 
 — up and dow^n draught, 43. 
 
 Palissy ware, 18. 
 Paris porcelain, 19. 
 
 — pottery, 17. 
 " Paris white," 67. 
 Pearl ash, 69. 
 
 Pegmatite, composition of, 12. 
 Persian ware, 17. 
 Peruvian pottery, 16. 
 Petuntse, 3. 
 
 — analysis of, 12. 
 Phosphoric acid, calculation of, 
 
 24. 
 
 Piemont porcelain, 19. 
 Pinholes in glaze, 76. 
 Placing, biscuit, 44. 
 Plasticity in clays, tests of, 33, 34. 
 Porcelain, hard, 19. 
 
 — - soft, 20. 
 
 Porosity of clays, 39. 
 Potash, carbonate of, 69. 
 
 — felspar, 15. 
 Primary clay, 11. 
 Principles of combination, 7. 
 " Purple of Cassius," 95. 
 Pyrometer, Wedgwood's, 56. 
 Pyrometric cones, normal, 60. 
 Pyrometry, 56. 
 
 Quartz in glazes, 66. 
 
 Eed lead, 68. 
 Rings for cups, 46. 
 Eoman pottery, 16. 
 Rouen pottery, 17. 
 
 Saggers, requirements of, 13. 
 
 — durability of, 49. 
 St. Amand pottery, 18. 
 Salt, 70. 
 
 Samian ware, 16. 
 Sand in placing, 48. 
 Scandinavian pottery, 16. 
 Secondary clay, 12. 
 Seger, Prof., 59. 
 " Seger " cones, 59. 
 Sevres porcelain, hard, 19. 
 
 — — soft, 20. 
 
 — — recomposition of, 21. 
 Shrinkage of clays, 37. 
 Shivering, 41. 
 
 Silica in clay, 29. 
 Soda-felspar, 15. 
 
 — carbonate of, 69. 
 Soft porcelain, 19, 20. 
 Spanish ware, early, 17. 
 Specific gravity of clays, 40. 
 Staffordshire pottery, 18. 
 Stone, Cornish, 12. 
 
 — testing of, 4. 
 
 — materials to replace, 8. 
 Stoneware, 18. 
 Stourbridge clay, 13. 
 
102 
 
 INDEX. 
 
 Straining ware, 53. 
 Sulphuring, 76. 
 
 Synthesis or recomposition, 20. 
 
 Tenacity in clays, tests of, 35. 
 Testing of clays, 31. 
 Tests, chemical and physical, 
 32. 
 
 Tin glaze, 17, 18. 
 
 — oxide of, 70, 91. 
 Tournay porcelain, 20. 
 
 Umber, 71. 
 
 Under-glaze colours, 93. 
 
 — — — glaze for, 75. 
 Uniformity of clays, 36. 
 
 Vienna porcelain, 19. 
 
 Water in clay, 29. 
 
 — in pottery, 16. 
 Wedgwood's pyrometer, 56. 
 White lead, 68. 
 Worcester porcelain, 19. 
 
 Zinc oxide, 70, 87. 
 
il 
 
 lllllliilllllllilliiliilllllil£ 
 
 
 BOOKS ON 
 
 Mechanics, Engineering, 
 
 Electricity, Trades, &c. 
 12 Cortlandt Street, N. Y. 
 
 = 
 
 in 
 
 Illllllllllllliltltllltttllll 
 
 n;