Pottery decorating : a description of all the processes for decorating pottery and porcelain

Front matter

R. HAINBACH

BOOKS ON POTTERY MANUFACTURE

THE MANUAL OF PRACTICAL POTTING.

Preface

TX/" HERE AS tlie methods of manufacturing the body for

? the finest ceramic ware, namely, fine earthenware, hard and soft porcelain, have attained the highest stage of develop¬ ment during the past half-century, the methods relating to the artistic decoration of the ware are still in process of evolution — probably because chemical science is now in an advanced condition and scarcely a day passes without some new discovery in the domain of chemical technology.

Ceramists are constantly endeavouring to produce new colour effects on their ware, and combine these with effects of reflection and lustre ; and surprising results have been obtained in the decoration of ceramic products with iridescent colours, lustres, and the noble metals. The high state of the transfer printing method applied to glazed ware has enabled articles to be put on the market, decorated in a highly artistic manner and at prices out of all proportion to those which must be paid for ware of identical appearance that has been produced by hand.

Photography, too, has now been pressed into the service of the ceramist and has enabled perfect photographic pictures to be reproduced on porcelain, the results surpassing all other methods in durability.

The author has also succeeded in applying the sand blast — previously confined to the glass industry — to the decoration

VI

Preface of porcelain, and in combining this process with electroplating for the decoration of ceramic objects.

In accordance with the title selected for the present work, the author has set himself the task of describing all the methods of ceramic decoration, by means of glazes, engobes, colours, metals, and lustres, in a manner that will enable any professional ceramist to obtain any desired effect in this class of decoration.

Special consideration has also been devoted to describing the methods of preparing the glazes, etc., in such a way that failure is impossible if the instructions given are carefully followed. The author therefore is filled with the pleasant hope that his work will be found a reliable handbook and work of reference on all practical questions in connection with its subject.

THE AUTHOR.

Part I. Glazes And Engobes

CHAl>' PAGE

INTRODUCTION 1

I. GLAZES AND ENGOBES GENERALLY .... 6

II. GLAZES AND THEIR COMPOSITION .... 8

III. GLAZE MATERIALS ...... 23

IV. THE PREPARATION OF GLAZES . . . .31

V. COLOURED GLAZES ...... 54

VI. ENGOBES AND GLAZES FOR SAME . . . .70

VII. PORCELAIN GLAZES ...... 77

Part II. Ceramic Colours

I. GENERAL REMARKS ON CERAMIC COLOURS . . .93

II. THE CERAMIC COLOURS IN PARTICULAR . . .97

HI- PREPARATION OF THE PURE COLOURS . . . 100

IV. UNDERGLAZE COLOURS ..... 107

V. APPLYING THE COLOURS ON EARTHENWARE . .127

VI. GLOST FIRE COLOURS . . . . . .152

vii. muffle colours ...... 155

viii. muffle colours — continued . . . . .162

IX. DECORATING PORCELAIN WITH METALS . . .189

X. DECORATING PORCELAIN BY ELECTROPLATING . . 202

XI. LUSTRE DECORATING ON PORCELAIN .... 206

XII. FIRING MUFFLE COLOURS ..... 223

XIII. THE IMITATION OF PAINTINGS ON PORCELAIN . . 239 vii

PART I.

Introduction

Glazes and Engobes.

rpiIE attempt at shaping plastic materials into the form of vessels and utensils was the first symptom of industrial activity evinced by man in the course of his intellectual development. Since the clay occurring in great abundance on various parts of the earth’s surface exhibits the property of forming an unusually plastic mass when mixed with water, and setting hard again when dried, it is highly probable that the first vessels made were of this material. The circumstance that vessels made solely of clay are extremely fragile will sufficiently account for the absence of any existing remains of these products at the present time.

Accident may have taught man that exposure to a red heat will convert clay into a hard, resistant body : and it is to the discovery of this fact that we owe the existence of numerous souvenirs of clay modelling dating from prehistoric times. The remains of the lake dwellers’ habitations always contain, in addition to fragments of textiles and weapons of I horn and stone, vessels of more or less well-burned clay in a good state of preservation.

In proportion as civilisation developed, the art of making clay vessels advanced to higher stages. Thus we find in ancient Assyrian buildings beautifully coloured bricks in general use for decorative purposes ; and the Egyptians had already learned the art of making earthenware vessels in different colours and coating them with glaze. Among the

ancient Greeks and Etruscans the potter’s trade reached the dignity of a fine art. The handsome vases of Grecian and

Etruscan origin that adorn our museums afford striking

testimony that these peoples had brought this branch of

human artistic skill to a height that compels our admiration at the present day.

As the civilisation of the antique world declined, the art of the potter, like all the other arts, receded to a low ebb, and it was only about the eighth century of our era that it began to flourish once more, this time more particularly in the hands of the Moors in Spain. Coloured and glazed pottery by Moorish artists of that age testify eloquently to the high position they had attained in this branch of art ; and it is probably due to their influence that potting had risen from a mere handicraft to a fine art in Italy by the fifteenth century.

A Florentine artist, Luca della Eobbia, was the first to produce the highly appreciated glazed and painted artistic earthenware known as “Majolica,” specimens of which are numbered among the costly treasures of our museums. Luca della Robbia and his successors already understood the art of coating pottery with various glazes, and of applying to the latter a diversity of colours; so that we must regard this artist as the real father of modern ceramics. The production of majolica ware spread throughout Italy, and works were established at Pesaro, Urbino, and other places, for turning out

Introduction glazed and painted earthenware. Faenza was also a seat of the industry \ and the name “ Faience,” given to pottery ware possessing certain qualities, is derived from that town.

The development of potting was also accompanied by progress in the ceramic art in Germany, France, and the Netherlands. There is no doubt that the ceramists of these countries were disciples of the Italian masters, especially as regards the beauty of their glazes and colours ; hut it should also be mentioned that German pottery had reached a high state of perfection even before the introduction of majolica ware. Germany and the Netherlands manufactured a kind of ware distinguished by extieme hardness and strength — a product still occupying an important position among ceramic products, under the name of “ stoneware.”

In England, potting remained for a long period in the state of a mere handicraft, the potters only turning out simple ware, suitable for domestic purposes, and it was not until the middle of the eighteenth century that the ceramic art took a sudden upward turn in this country, when Josiah Wedgwood succeeded in making ware of such eminent beauty, both in glaze and colour, that it rapidly acquired a world-wide reputation. It was to Wedgwood also that we owe the production of “Queen’s ware,” characterised by a uniform pure whiteness throughout the mass.

Although European ceramics had attained a high state of development, the art was still several centuries behind that of Eastern Asia, none of the Western makers being able to produce ware able to compare in beauty and strength with that imported, under the name of “porcelain,” from China and Japan. There was no lack of endeavour to produce a porcelain body equal in its properties to that of the Chinese potters, but the results were not crowned with any great success, until Johann Friedrich Bottcher, in 1709, prepared a body equal to that of Chinese porcelain.

The rapid extension of this new method of manufacture, contemporaneously with the great strides made by chemistry at the close of the eighteenth century, enabled European porcelain to overtake the Chinese product, both in point of internal properties and colouring; but it was reserved for the nineteenth century to raise the porcelain industry to its present high position. This result was obtained only after innumerable painstaking experiments on the composition of the porcelain bodies, glazes, and appropriate pigments — experiments necessi¬ tating the prosecution of exact chemical investigations as an essential preliminary to success.

These investigations were not confined solely to porcelain, but extended to other branches of ceramics as well. Whereas formerly, in the production of ordinary pottery ware, each maker worked in accordance with the teachings of practical experience, and modified the operations to suit the raw material available, it became the practice to subject the chemical composition of the materials — more particularly the clays and the substances employed for glazes — to exhaustive examination ; and this has now been pushed so far that attention is bestowed on the chemical properties and behaviour in the kiln, etc., of the clay used for making even the commonest class of ware — namely, bricks.

In addition to carefully testing the clay forming the funda¬ mental substance of all ceramic labours, it was found advis¬ able to examine the substances used in preparing the glazes, experience having shown that, in order to ensure successful glazing, the substance of the ware must bear a certain relation to that of the glaze. This applies equally to the pigments used, whether these be incorporated with the body or glaze, or applied to the surface of the latter

Unless the requisite relation be maintained between the funda¬ mental substance of the ware and the applied coatings — which may be divided into two main categories, glazes and engobes —

Introduction the production of a perfect article will be solely a matter of chance, and the majority of the pieces will be more or less impaired by defects which we shall consider in detail later. In view of this fact it becomes essential for every ceramist to study most carefully the relation between the different com¬ ponents — shard, engobe, and glaze, or shard and glaze only — going to make the complete ware, so that he may he in a position to turn out a perfect article and to remedy any defects that may arise.

CHAPTER I. Glazes and Engobes generally

T) Y the term glaze we understand in ceramics a thin vitreous ^ coating that attaches itself firmly to the body of the ware, in order to render the latter impervious to moisture, and impart a gloss and smoothness to the surface.

In the majority of instances the glaze is colourless, so that the colour of the underlying body shows through. Occasionally, however, the glaze is intentionally mixed with pigments or with substances rendering it more or less opaque, in which event the colour of the shard is no longer visible.

In addition to glazes, earthenware is sometimes coated with engobes, consisting of coloured clay, for the purpose of covering the surface of the ware with a coloured layer of predetermined thickness. These engobes are analogous in composition to the body of the ware, differing therefrom mainly in respect of colour.

Considerable importance also attaches to the coloured fluxes, which are closely allied to the glazes, inasmuch as their composition resembles that of the colourless glazes in many respects, though differing essentially in containing an admixture of pigment. For certain kinds of ware, pigments known as underglaze colours are applied before glazing, and these are more nearly related to the engobes than the glazes.

Apart from the increased resistance to external influences, imparted to the ware by glaze, by the vitreous nature of the latter, the application of glazes and engobes is chiefly for

Glazes and Engobes generally artistic purposes, with the object of imparting lustre, smoothness, and colour to the surface of the ware. In an aesthetic sense the maximum of possibility has now been achieved, since there is no known degree of lustre or shade of colour that cannot be reproduced on ceramic ware ; and, by means of suitable engobes, it is possible to produce the finest colouring on pieces made of ordinary ugly clay. This is a circumstance of special importance in many cases, where the problem of imparting a superficial imitation of fine ware to articles of inferior character, such as stove tiles, and facing bricks or tiles for building purposes, is in question. In this connection, also, a very high degree of perfection has now been attained, and the potter’s handicraft raised to the dignity of a fine art in this branch too.

CHAPTER II. Glazes and their Composition

A CCORDIHG to their composition, glazes may be classified into three chief groups :

1. Salt glazes ;

2. Earthy glazes ; and,

3. Fused or fritted glazes.

The first group, salt glazes, may be dismissed in a few words, their production requiring no special study or preparation. They are applied while the ware is in the kiln, by throwing a certain quantity of common salt into the kiln at a certain stage of the baking process.

The reaction produced is as follows: — The common salt (sodium chloride) is vaporised, and decomposes on coming into contact with the water vapour present in the furnace gases> to form sodium oxide and hydrochloric acid. The oxide combines with the silicates of which the earthenware consists to form a double silicate, a refractory glass, constituting a very thin layer firmly adhering to the substratum and forming a coating that is only slightly attacked by the strongest chemicals, glass being one of the most resistant substances known. Salt glaze is applied to the majority of stoneware articles, especially such as are used in factories where chemical products are made.

The second group, or earth glazes, consist of bodies insoluble in water. The chief point in their preparation is

to mix the ingredients as intimately as possible, and bring them into an extremely fine state of division. This is effected by grinding the mixtures to an exceedingly fine powder.

The fritted or fused glazes forming the third group are really the most perfect of all, and are prepared in a merely fritted or sintered mass, or as a completely fused mass of vitreous character. Owing to the high uniformity obtainable by fusion, this operation is sometimes applied to the earthy glazes as well , so that there is no sharp line of demarcation between these two classes. Fusion, however, is an essential feature of the fused glazes, because some of the ingredients are soluble in water and therefore cannot be ground wet. They also present so many other advantages over the earthy glazes that the latter are being more and more displaced, even for cheap ware. True, the fusing of the glazes, in the first place, increases their cost, but this is more than outweighed by the greater convenience in application.

It is clear that when earthy glazes are used, the actual glaze is not formed until the ware is baked, and in these circum¬ stances it is easy for bubbles to he formed which greatly impair the appearance of the pieces. Hence, in order to obtain uniform glazing, it is essential that the ingredients of the glaze should be ground as finely as possible and mixed together so intimately that a uniform coating is produced when the ware is fired.

When, as is frequently the case with earthy glazes, a lead compound is present in the mixture, it may happen that the heavy lead compound settles down out of the mixture, owing to its higher, specific gravity, in which case the glaze will be of irregular character.

This is one of the reasons why earthy glazes are going out of use, except for very common articles, and for the same reason even common glazes are now generally fused, this operation yielding a uniform mass which does not need to be so finely ground as the earthy glazes, though it will give a perfectly even coating, free from bubbles, when fired.

Owing to the difficulty, on the one hand, of avoiding the use of lead in glazes for common ware, and on the other to the poisonous character of the soluble lead compounds, care must be taken that the lead compounds are contained in such a form in the glaze that they are not easily attacked by acid foods. The only way to ensure this result is to endeavour to obtain a lead-alkali silicate that contains sufficient silica to withstand the action of acids. Since, in the case of cooking utensils, acetic acid is the most usual form of acid encountered, the hygienic regulations of various countries now prohibit the use of lead glazes that are attacked by 4 per cent, acetic acid Avlien boiled with the same for half an hour.

In compounding glazes care must he taken that the glaze has a lower fusing point than the ware to which it is to be applied. This condition is easily fulfilled, since even common clay has a fairly high fusing point ; but if the two fusing points were very close together, the pieces would inevitably be dis¬ torted in the glost kiln. The same applies to engobes, which must have a higher fusing point than the glaze, and be fired at a temperature just sufficient to cause them to adhere firmly to the ware, without melting. Engobed ware is therefore always dull when fired, and only acquires its lustre by sub¬ sequent glazing.

The fusing-point of the glaze is a most important factor for the success of the glazing, and is correct when the fired glaze melts to a liquid which distributes evenly over the entire surface of the piece and forms a vitreous coating when cold. If the glaze be too refractory, the finished ware will be dull in patches, showing where the glaze has been unable to flow properly. Conversely, glaze that is too fusible will appealthick n some places, and thin in others.

i i

FUSING POINT OF GLAZES.

The fusing point of a glaze depends on the nature of the ingredients used, just as much as on the relative quantities employed. The glazes are glasses, the acid constituent of which is usually silica exclusively, more rarely accompanied by boric acid, the acid or acids being combined with basic substances to form silicates or boro-silicates. In these com¬ pounds the fusing point of the glazes is determined —

1. By the equivalent proportion between acids and bases;

2. By the proportion of alumina in relation to the bases and acids ;

3. By the ratio between the silica and boric acid.

The richer a glaze is in silica the less readily does it fuse, and therefore the modification of the proportion of silica in a glaze affords an excellent means of regulating its fusibility. Conversely, when a glaze contains too little silica, it is gener¬ ally too fusible, and then either sinks into the porous sub¬ stratum, or runs off the surface of the ware. An excessive silica content, on the other hand, results in the glaze either not fusing uniformly or else decomposing and becoming “ devitrified” and dull, on prolonged exposure to the firing heat.

The bases present in the glaze have also a certain influence on the physical character of the product, a considerable analogy existing in this respect between them and the components of glass metal. Most of the ordinary kinds of glass consist of calcium silicate in combination with potassium or sodium silicate. A glaze containing only one or other of these alkali silicates together with the calcium silicate is always more infusible than one in which, whilst retaining the same relative proportion between the silica and the basic constituents, both potassium and sodium silicate are present with the calcium silicate.

Glazes containing considerable amounts of alumina can only be used when the glaze material is fused at a very high temperature, since it is only under these conditions that alumina forms fusible compounds with silica. In such cases, therefore, alumina can be employed to make the glaze more refractory and check its tendency to run.

Just as in silica and alumina we have means available for raising the fusing point of glazes, so also in boric acid we have a means of reducing that fusing point. It may be remarked here, that silica and boric acid are mutually replaceable ; con¬ sequently, when a portion of silica is replaced by boric acid, this must be done in such a manner that the total amount of acid remains unaltered.

From the foregoing remarks a series of prescriptions can be laid down for correcting the fusing point of a glaze.

The fusing point of a glaze is raised —

1. By increasing the proportion of silica in the mixture, with¬ out changing that of the bases;

2. By replacing a portion of the boric acid by silica, in the case of glazes containing both these substances ;

3. In glazes containing lead oxide, by replacing a certain part of the lead oxide by equivalent quantities of potash, soda, lime or magnesia, leaving the other constituents unchanged ;

4. By replacing potash-soda by potash or soda alone, or taking lime or magnesia alone in place of lime-magnesia, without altering the acid content ;

5. By introducing alumina into the composition, when the glaze is intended to fuse at a high temperature only.

When a too refractory glaze is to have its fusing point lowered, the precisely opposite course must be pursued in each case.

THE CONSTITUTION OF DEFECTIVE GLAZES.

Sometimes it happens that glazed ware preserves its original fine appearance for a considerable time, but afterwards alters

*3 in such a way as to clearly indicate defective constitution. To guard against the very unpleasant consequences that may ensue from sending out large quantities of such defective ware, it is essential to subject the glaze, or glazed test objects, to a series of tests, which they must pass before the glaze can be regarded as properly compounded. These tests will be de¬ scribed later on.

A glaze is properly compounded when it will stand con¬ siderable fluctuations of temperature without undergoing altera¬ tion. A glazed piece of earthenware, e.rj. a pot glazed inside and out, consists of three strata : the two external thinner layers being composed of glaze, whilst the third and thicker one is the body or shard of clay. It is essential that, when the vessel is exposed to a heightened temperature, the glaze and body should exhibit exactly the same coefficient of expansion, so that in proportion as the shard expands, the glaze will follow suit, the two contracting equally on re-cooling, and the appearance of the vessel consequently remaining unaltered.

If the glaze, however, possesses a different coefficient of expansion from that of the body of the ware, then, as the temperature changes, a strain will be set up between the two substances, which strain may proceed so far as to overcome their mutual cohesion, and either loosen or separate them entirely.

In addition to these purely physical evils, others may arise that originate in the chemical composition of the glaze. We will deal first with the physical alterations, which manifest themselves by the following phenomena : —

The glaze cracks (“crazes”) all over, the surface exhibiting a complete network of lines as fine as a hair, some portions becoming detached and falling away, probably carrying with them small fragments of the body of the ware. In extreme cases the untouched pieces have been known to burst with a considerable report into a number of pieces, scattered to a distance — probably in consequence of a slight change in temperature. In many cases the glaze is found to be cracked only at the edges, but these cracks will gradually extend over the whole surface.

When the coefficient of expansion of the glaze is higher than that of the body of the ware, it will expand more rapidly than the latter on heating; and since the latter, on account of its greater thickness and the low heat-conducting properties of the clay, cools more slowly than the glaze, this in turn contracts the more rapidly of the two, with the result that its cohesion is destroyed, and it breaks into a number of innumerable small fragments, bounded by hair cracks. The latter eventually get filled with dust and show up more and more clearly as dark coloured lines.

These cracks are frequently observed on old Chinese porce¬ lain, the pieces then being classed as “vieux craquete.” At one time this class of ware had a certain vogue among col¬ lectors, and numerous imitations were produced ; as a matter of fact, the frequent appearance of crazing on old Chinese ware is merely a proof that the old makers used glazes of defective composition.

When the conditions are such that the coefficient of expansion of the body of the ware is greater than that of the glaze, the latter will tear at the edges, or spring off in parts ; and in extreme cases the whole piece bursts with violence.

Apart from the foregoing defects, arising from unequal coefficients of expansion, there are others which are attribut¬ able to the chemical character of the glaze.

hen the materials used are not chemically pure, the mass contains impurities, some of which, like the sulphates and chlorine compounds, may be a source of considerable difficulty. At a very high temperature, the sulphates are decomposed by silica, though not at the temperature of the glost kiln; and the chlorides are decomposed by considerable heat, but only when water vapour is present.

In presence of a very small quantity of sulphates, the glaze becomes coated with a misty film, which reappears when wiped off ; and when the proportion of sulphates is large, the glaze is dull and void of lustre, or exhibits a number of wrinkles and hubbies. These latter are formed in consequence of the partial transformation of the sulphates into acid salts, which at higher temperatures part with the surplus sulphur trioxide, and this blows out the soft glaze in the form of tenuous bubbles. On the surface being wiped over with a cloth, these very thin bubbles are broken, leaving the surface rough to the touch. Dust gradually accumulates in the cavities, and forms dark specks. Defective glaze of this kind is sometimes known as “measly.”

The so-called drying of the glaze on the edges of plates is due to the presence of sulphates in the glaze. The liquid portion of the glaze is less readily absorbed at the thin edges, and the soluble sulphates migrate to these parts, and finally crystallise there. When the glaze is fired, the edges become covered with dull patches and bubbles.

This effect of sulphates may appear remarkable, because a sulphate, i.e. gypsum, is largely used in the preparation of many porcelain glazes. It should, however, be remembered that the porcelain kiln frequently contains a reducing atmo¬ sphere, which decomposes the calcium sulphate, liberating the volatile sulphur dioxide, whilst the lime enters into com¬ bination with silica.

In the event of glazed ware exhibiting any of»the aforesaid peculiarities, the maker must chiefly endeavour to prevent their recurrence, since otherwise his output would consist almost entirely of “ wasters.” Two ways are open : either by modifying the composition of the body so that it will have the same coefficient of expansion as the glaze, or else by adapting the composition of the glaze to the character of the body.

In the former case the composition of the body can he modified only when the latter is compounded of several substances, as happens with all fine earthenware. This is out of the question with inferior ware, since in this case the usual practice is to merely prepare the available clay in a suitable manner so that it will furnish a plastic mass; and in these circumstances a remedy must be sought by modifying the composition of the glaze in order to make its coefficient of expansion agree with that of the body.

In connection with modifying the composition of the body, certain general rules can be established. When the glaze is found to craze, a trial can be made on the following lines : —

The proportion of clay in the body is reduced and the amount of silica concurrently increased. If China clay be used, a portion of this may be replaced by a highly plastic clay. In the case of bodies containing felspar, the proportion of this substance should be diminished, or else the silica present should be more finely ground than before.

Sometimes — though rarely — crazing can be prevented by firing the unglazed ware at a higher temperature.

When the defect shows itself in the form of cracked edges or partial peeling, the reverse procedure should be adopted.

When the composition of the glaze is to be altered to effect a remedy, leaving the body unchanged, the following method is recommended, provided the defect manifests itself in the form of crazing.

The proportion of silica in the glaze is increased, thus raising the fusing point.

All the ingredients of the glaze are left in their original proportions, except the boric acid which is increased, with the result that a somewhat lower temperature can be employed in the glost kiln.

Should the defect consist in cracked edges or peeling of the glaze in places, the converse procedure must be followed.

It may happen that despite the composition of the body and glaze being mutually suitable, no handsome, uniform glaze is obtained. In such event the most important point is to see that the body is perfectly homogeneous, since, when the body contains large internal cavities, it will never glaze evenly, the irregular thickness of the body causing unequal absorption of the glaze.