Porcelain. during the process of baking. It is this infusible sub- compositions of pastes, which are very tenacious, and Porcelain. stance which is to be considered as the substitute for the which are capable of being easily worked and well kaolin of China, and which possesses the property of con- baked, and in the process of baking which acquire the siderably contracting its dimensions, while it unites with requisite degree of hardness and density; which are inthe fusible material. According to the observation of fusible, and capable of resisting the effects of sudden Macquer, if it be subjected to the most decisive trial, changes of heat and cold, and, in short, which possess namely, the action of a violent fire, which is capable of all the qualities of the most excellent porcelain, exceptmelting every porcelain composed only of fusible mate- ing its whiteness and beauty. Materials fit for the comrials, it appears as the result of numerous experiments, position of such porcelains, it will appear, may be found that it remains infusible, unless it be exposed to a heat abundantly in most countries; but the difficulty in the which is also capable of melting the best and most per- manufacture of this ware is to unite beauty and goodfect porcelain of Japan. The Saxon porcelain, there- ness in one composition. The materials fit for the mafore, is not to be confounded with porcelain manufac- nufacture of the finer and more perfect porcelains, seem tured of vitreous and fusible materials; for it seems to to be sparing productions of nature; and therefore the be equally excellent as that of Japan, and in some of its best kind of porcelain, it is presumed, will always be reproperties perhaps superior, as will appear from an ex- garded as a valuable and high-priced commodity. amination of the qualities which constitute the peculiar excellence of porcelain. and with kaolin, Reaumur seems also to have taken an erroneous view regard to of the nature of the Chinese kaolin. According to his the Chinese account, this matter is a fine talky powder, from the mixture of which with petuntse, the porcelain of the east is manufactured. It is not impossible, it has been observed, that a porcelain similar to the Chinese might be produced from a talky substance of this nature mixed with petuntse; but it is well known to those who are at all familiar with the manufacture of any porcelain, that no vessels can be formed, unless the paste of which they are made possess that degree of ductility and tenacity which renders them fit for being worked upon the lathe, or fashioned in the mould. But substances of a talky nature, to whatever degree of fineness they may be reduced, never acquire the requisite ductility and tenacity which clays of all earthy substances only possess. But as it appears that the Chinese porcelain has been turned upon the lathe, it is obvious that they must have been formed of a very tenacious paste; and hence it is concluded, that kaolin is not purely a talky matter, but mixed with clay, otherwise the petuntse and kaolin, according to the supposition of Reaumur, are not the only ingredients which enter into the composition of Chinese porcelain; but the addition of a certain proportion of some matter of a tenacious quality is absolutely requisite. 20 3. Peculiar Properties of Porcelain. It may be worth while now to consider the properties which constitute the perfection of porcelain; and here it is necessary, carefully to discriminate between the qualities which are to be regarded as only contributing to the external decoration, and the intrinsic and essential Essential properties in which the fabric and perfection of porcequalities of lain consist. Those who have been occupied in experiPorcelain. ments on this subject, have not found it difficult to form compositions which are very white, beautifully semitransparent, and covered with a shining glazing; but which are extremely deficient in the more essential properties, as it appears they cannot be subjected to the necessary operations for want of a proper degree of tenacity; are not sufficiently compact; are quite fusible, subject to break by the sudden application of heat or cold, and from the softness of the glazing, which cracks and becomes rough, are soon deprived of their lustre. On the other hand, it is by no means difficult to form 2 30 It may be observed, that the potteries called stone- Nature of ware, possess all the essential qualities of the Japanese stoneware. porcelain; for, excepting the whiteness, on which alone the semitransparency depends, if we compare the properties of Japanese porcelain with those of our stoneware, little difference is found to exist between them. Both seem to possess the same granular texture; both have the same sonorous quality, when struck with a hard body; both have the same density; they possess also the same hardness, by which they strike fire with steel; they can resist the effects of the heat of boiling liquors without breaking, and are equally infusible when subjected to violent heat. Hence it is inferred, that if the earth which enters into the composition of stoneware, were free from foreign colouring matters, which prevent the whiteness and semitransparency, and if the vessels were carefully formed and coloured with a fine glaze, they would not be less perfect than the porcelain of the east. Earths fit for the production of the more perfect kinds of porcelain, are supposed to be more rare in Europe than in Japan and China; and hence probably it has happened, that, from the want of these earths, the first manufacturers of the porcelain in Europe confined themselves to an external imitation, by employing only vitrifiable matters with fusible salts, and a small quantity of white earth, from which fusible and vitreous porcelains were composed. Such might not improperly be denominated false porcelains; but great improvements have taken place since the first introduction of the manufacture of porcelain into Europe. Genuine white porcelains have been long ago produced in Germany, and especially in Saxony. These porcelains are in no respect inferior to those of China or Japan. They are found even to be considerably superior in beauty and whiteness to the productions of the eastern manufactories of modern times; for in these qualities the porcelains of the latter have greatly degenerated. And in one of the most valuable qualities of porcelain, namely, the property of resisting the effects of sudden changes of heat and cold, the European porcelain exceeds that of China or Japan. The quality of porcelain, it is to be observed, is not to be judged of by a slight trial; for as numerous circumstances concur to render a piece of porcelain capable or incapable of resisting the effects of heat or cold, boiling water may be at the same time poured into two vessels, one of which is good porcelain, and the other of an opposite quality, it is not impossible that the former may break, and the latter may remain entire. The true method of discover ing Japanese porcelain reckoned the most perfect, 32 but not superior to that of Saxony. It has long been a very general opinion, that the Japanese porcelain is the most perfect; it has indeed continued to be the object of admiration and emulation, and has been held up as a model for the European manufacturer; a model which has not yet been equalled, and which, according to the opinion of some, cannot be equalled. In deciding on this subject, the Saxon porcelain is considered as inferior to the Japanese, on account of its greater smoothness, lustre, and less granular aspect of its internal texture, qualities in which it ought really to be regarded as superior to the porcelain from Japan. This surface has a near resemblance to that of glass, and it is supposed that this similarity has suggested the opinion; and it would be well founded, if the density and lustre of the European porcelain depended on the fusible and vitreous property of the ingredients of which it is composed but this not being the case, and the Saxon porcelain being equally fixed and infusible as that of Japan, its superior density must be admitted as a valuable property. For in the comparison of different porcelains which are equal in other properties, that which is most firm and compact certainly claims the superiority. Hence it is that the internal texture of the Japanese porcelain is held in greater estimation, because it possesses a greater degree of density, compactness and lustre, than the European porcelain, which is composed only of vitreous sand or fritt. For a similar reason the superior density of the Saxon porcelain ought to obtain for it a preference to that which is imported from the east. It is supposed besides, that it would be no difficult matter to communicate to the Saxon porcelain the granular texture of the Japanese, by mixing with the paste a certain proportion of sand or siliceous earth. But in this point, in producing by these means a nearer resemblance to the Japanese porcelain, those who conducted and brought to perfection the Saxon manufactures, were not insensible that their porcelain would sink in its valuable properties. 4. Porcelain Manufactories in different parts of Europe. Manufactories of porcelain have been long established in almost every country of Europe. Besides that of Saxony, which was the first established in Europe, porcelain is made to a considerable extent at Vienna, at Frankendal, and in the neighbourhood of Berlin, and in other places of the German states. The German porcelains are similar to those of Saxony, and are composed of similar materials, although from differences in the proportions, or in the modes of managing the manufactories, considerable differences arise in the porcelains manufactured at different places. Italy also is celebrated for its porcelain manufactures, the chief of which, it is said, are carried on at Naples. When M. de la Condanine travelled into Italy, he visited a manufacture of porcelain established at Florence, by the marquis de la Ginor, who was then governor of Leghorn. The French traveller was particularly struck with the large size of some of the pieces of this porcelain. Statues, and even Porcelain. groups of figures half as large as nature, and modelled from some of the finest antiques, were formed of it. The furnaces, he observed, in which the porcelain was subjected to the process of baking, were constructed with a great deal of ingenuity, and were lined with bricks made of the same materials as those which entered into the composition of the porcelain itself; and hence they were able to resist the effects of high degrees of heat. The paste of the porcelain manufactured at Florence appeared to be extremely beautiful, and to possess all the qualities of the best oriental porcelain. The glazing mployed in this manufactory seemed to be inferior in whiteness, a circumstance which is supposed to be owing to the desire of using those materials only which are found in the country. 33 tures of porcelain in France. In France a greater number of manufactories of por- Manufac celain has been established than in any other country; and it must be allowed that the French have had wonderful success in the improvement and perfection of this manufacture. Some time even before Reaumur commu. nicated the result of his inquiries, porcelain was manufactured at St Cloud, and in the suburb of St Antoine at Paris. This porcelain indeed was of the vitreous or fusible kind, but at the same time possessed no inconsiderable degree of beauty. Since the period to which we allude, extensive manufactories of porcelain have been established at Villeroy, Chantilly, and Orleans, and at those places the manufacture has been brought to a great degree of perfection. But the productions of the celebrated porcelain manufactory at Sevres, on account of the pure shining white, the fine glazing and coloured grounds, the splendour and magnificence of the gilding, and the elegance and taste displayed in the shape and figures, are universally allowed to surpass every thing of the kind which has yet appeared. In speaking of the French porcelain, we may notice the result of some researches which were made on this subject by Guettard, and of which an account appeared in the Memoirs of the Academy of Sciences for the year 1765. In the neighbourhood of Alençon, M. Guettard discovered a whitish argillaceous earth, in which mica considerably predominated. This earth he employed as a substitute for kaolin. The substance which he used in place of the petuntse, he obtained from a hard stone, which is described as a quartzose gritt stone, very abundant in that country, and with which the streets of Alençon are paved. With these materials Guettard instituted a series of experiments on porcelain, previous to the year 1751, and was associated in his in quiries with the duke of Orleans. For many years the count de Lauraguais, a member of the Academy of Sciences, was keenly engaged in prosecuting experiments to discover the true nature of porcelain, and the means by which the manufacture might be improved and perfected. To obtain the object of his researches, which was to produce porcelain that in its essential qualities might be equal to that of eastern countries, be spared no trouble or expence; and it would appear that he was not unsuccessful in his labours; for in the year 1766, when he exhibited some species of porcelain from his manufactory to the members of the Academy of Sciences, the persons who were appointed by that learned body to examine their properties, delivered it as their opinion, that of all the porcelain made in France, that Porcelain. of the count de Lauraguais approached most nearly in the essential properties of solidity, texture and infusibility, to that of China and Japan. It is, said, however, that it was considerably deficient in whiteness and lustre, when compared with the ancient porcelain of Japan. 34 In England. 35 In Staffordshire The manufacture of porcelain has been brought to a great degree of perfection in England. In many of the essential qualities, and particularly in the beauty and richness of the paintings, as well as in the elegance of the forms, the English porcelain is little inferior to that of any other country. Manufactories of this ware have been established in different parts of England. This manufacture was first established at Derby about the year 1750, by a Mr Duesbury, who is said to have been a very ingenious artist. Since his death the manufactories received very considerable improvement, and chiefly in the judicious methods pursued in the preparation of the paste, and increasing the beauty of the ornaments. The ware itself is said not to equal in fineness that which is manufactured in Saxony and France, although it is greatly superior in respect of decoration and workmanship. The paintings in general are rich, and executed with taste, and the gilding and burnishing are extremely beautiful. The body of the semi-vitreous kind, which is formed of a fine white clay, in combination with various proportions of different fusible matters, has obtain ed the name of porcelain. The best kind is wholly infusible, and is glazed with a vitreous substance which has not a single particle of lead in its composition. The most famous manufactory of stone-ware, as well as of other kinds of pottery, is at Burslem in Staffordshire. This can be traced with certainty at least two centuries back; but of its first introduction no tradition remains. In 1686, as we learn from Dr Plot's Natural History of Staffordshire published in that year, only the coarse yellow, red, black, and mottled wares, were made in this country; and the only materials employed for them appear to have been the different coloured clays which are found in the neighbourhood, and which form some of the measures or strata of the coal-mines. These clays made the body of the ware, and the glaze was produced by powdered lead-ore, sprinkled on the pieces before firing, with the addition of a little manganese for some particular colours. The quantity of goods manufactured was at that time so inconsiderable, that the chief sale of them, the Doctor says, was "to poor cratemen, who carried them on their backs all over the country." About the year 1690, two ingenious artisans from Germany, of the name of Eller, settled near Burslem, and carried on a small work for a little time. They brought into this country the method of glazing stoneware, by casting salt into the kiln while it is hot, and some other improvements of less importance; but finding they could not keep their secrets to themselves, they left the place rather in disgust. From this time various kinds of stone-ware, glazed by the fumes of salt in the manner above mentioned, were added to the wares before made. The white kind, which afterwards became, and for many succeeding years continued, the staple branch of pottery, is said to have owed its origin to the following accident. A potter, Mr Astbury, travelling to London, perceived something amiss with one of his horse's eyes; an hostler at Dunstable said he could soon cure him, and for that purpose put a common black Porcelain. flint stone into the fire. The potter observing it, when taken out, to be of a fine white, immediately conceived the idea of improving his ware by the addition of this material to the whitest clay he could procure: accordingly he sent home a quantity of the flint stones of that country, where they are plentiful among the chalk, and by mixing them with tobacco-pipe clay, produced a white stone-ware much superior to any that had been seen before. Some of the other potters soon discovered the source of this superiority, and did not fail to follow his example. For a long time they pounded the flint stones in private rooms by manual labour in mortars; but many of the poor workmen suffered severely from the dust of the flint getting into their lungs, and producing dreadful coughs, consumptions, and other pulmonary disorders. These disasters, and the increased demand for the flint powder, induced them to try to grind it by mills of various constructions; and this method being found both effectual and safe, has continued in practice ever since. With these improvements, in the beginning of the present century, various articles were produced for tea and coffee equipages. Soon after attempts were made to furnish the dinner table also; and before the middle of the century, utensils for the table were manufactured in quantity as well for exportation as home consumption. But the salt glaze, the only one then in use for this purpose, is in its own nature so imperfect, and the potters, from an injudicious competition among themselves for cheapness, rather than excellence, had been so inattentive to elegance of form and neatness of workmanship, that this ware was rejected from the tables of persons of rank; and about the year 1760, a white ware, much more beautiful and better glazed than ours, began to be imported in considerable quantities from France. 36 wood. The inundation of a foreign manufacture, so much Improved superior to any of our own, must have had very bad ef. by Wedgfects upon the potteries of this kingdom, if a new one, still more to the public taste, had not appeared soon after. In the year 1763, Mr Josiah Wedgwood, who had already introduced several improvements into this art, invented a species of earthen ware for the table quite new in its appearance, covered with a rich and brilliant glaze, bearing sudden alternations of heat and cold, manufactured with ease and expedition, and consequently cheap, and having every requisite for the purpose intended. To this new manufacture the queen was pleased to give her name and patronage, commanding Queen's it to be called Queen's ware, and honouring the inven- ware. tor by appointing him her majesty's potter. The common clay of the country is used for the ordinary sorts; the finer kinds are made of clay from Devonshire and Dorsetshire, chiefly from Biddeford; but the flints from the Thames are all brought rough by sea, either to Liverpool or Hull, and so by Burton. The convenience of plenty of coals, which abound in that part of the country, is supposed, and with good reason, to be the chief cause of the manufacture having been. established here. The flints are first ground in mills, and the clay prepared by breaking, washing, and sifting, and then they are mixed in the requisite proportions. The flints are bought 37 Porcelain. bought first by the people about the country, and by ed a proper degree of hardness and transparency. To Porcelain. them burnt and ground, and sold to the manufacturers ascertain this point, a good deal of attention is necesby the peck. sary; and this is done by taking out of the furnace from time to time, and examining, small pieces of porcelain placed for that purpose in the cases, which have lateral openings to render them accessible. When it appears from the examination of those pieces, that the porcelain is sufficiently baked, the fire is no longer to be supplied with fuel; the furnace is allowed to cool gradually, and the porcelain is afterwards taken out. In this state the porcelain has the appearance of white marble, having nothing of that shining surface which it acquires by covering it with a vitreous composition known by the name of glazing, a process which is afterwards to be described; but in the mean time we shall speak of the infusible porcelains. 38 Vitreous or celain. The mixture is then laid in large quantities in kilns to evaporate the moisture; but this is a nice work, as it must not be too dry: next it is beaten with large wooden hammers, and then is in order for throwing, and is moulded into the forms in which it is to remain; this is the most difficult work in the whole manufacture. A boy turns a perpendicular wheel, which by means of thongs turns a small horizontal one, just before the thrower, with such velocity, that it twirls round the lump of clay he lays on it into any form he directs it with his fingers. There are 300 houses, which are calculated to employ upon an average twenty hands each, or 6000 in the whole; but of all the variety of people that work in what may be called the preparation for the employment of the immediate manufacturers, the total number cannot be much short of 10,000, and it is increasing every day. Large quantities are exported to Germany, Ireland, Holland, Russia, Spain, the East Indies, and much to America; some of the finest sorts to France. 5. Different Processes in the Manufacture of Porcelain. The basis of those porcelains which are known by the fusible por- name of vitreous or fusible, and sometimes false porcelain, is denominated by the workmen a fritt. This is a mixture of sand or powdered flints, with a saline substance, capable of bringing it to a state of fusion when the mixture is exposed to a sufficient degree of heat. The fritt is then mixed with a proper proportion of clay or argillaceous earth, so that it may have such a degree of tenacity as to make it capable of being worked upon the wheel. The whole mixture is, after being well ground in a mill, to be made into a paste, which is to be formed, either upon the wheel or in moulds, into pieces of such forms or figures as may be required. Each of the pieces, when it is sufficiently dried, is put into a case made of earthen ware, and placed in the furnace, that it may be subjected to the process of baking. These cases are known among the English potters by the name of seggars or saggars, and they are generally formed of a coarser kind of clay, but this clay must possess the property of resisting the action of heat necessary for the baking of porcelain, without being fused. The porcelain contained in the cases is thus protected from the smoke of the burning fuel: the whiteness of the porcelain depends greatly on the purity of the clay of which it is made, so that being of a more compact texture, the smoke is more effectually excluded. These cases are arranged in the furnace or kiln in piles, one upon the other, to the very top of the furnace. The furnaces are chambers or cavities of various forms and sizes, and they are so constructed that the fire-place is situated on the outside, opposite to one or more openings, which have a communication with the furnace internally. The flame of the fuel is drawn within the furnace, the air of which being rarefied, determines a strong current of air to the inside, as is the case in other furnaces. A small fire is first made, that the furnaces may be gradually heated, and it is to be increased more and more, till the process of baking is completed; that is, till the porcelain shall have acquir 39 The materials which enter into the composition of the infusible porcelains, and such as approach to the nature of stone ware, are first to be ground in a mill, and the earths or clays being well washed, are next to be carefully mixed and formed into a paste. The pieces at first receive a rude form from the wheel or lathe of the potter, according to their nature and magnitude. As the wheel and lathe are the principal machines employed in the manufacture of porcelain or pottery, we shall here give a short description of their construction. The potter's wheel, which is used for larger works, consists principally in the nut, which is a beam or axis, whose foot or pivot plays perpendicularly on a free-stone sole or bottom. From the four corners of this beam, which Potter's does not exceed two feet in height, arise four iron bars, wheel. called the spokes of the wheel; which forming diagonal lines with the beam, descend, and are fastened at bottom to the edges of a strong wooden circle, four feet in diameter, perfectly like the felloes of a coach-wheel, except that it has neither axis nor radii, and is only joined to the beam, which serves it as an axis, by the iron bars. The top of the nut is flat, of a circular figure, and a foot in diameter; and on this is laid the clay which is to be turned and fashioned. The wheel thus disposed is encompassed with four sides of four different pieces of wood fastened on a wooden frame; the hind-piece, which is that on which the workman sits, is made a little inclining towards the wheel; on the fore-piece is placed the prepared earth; on the side pieces he rests his feet, and these are made inclining to give him more or less room. Having prepared the earth, the potter lays a round piece of it on the circular head of the nut, and sitting down turns the wheel with his feet till it has got the proper velocity; then, wetting his hands with water, he presses his fist or his finger-ends into the middle of the lump, and thus forms the cavity of the vessel continuing to widen it from the middle; and thus turning the inside into form with one hand, while he proportions the outside with the other, the wheel constantly turning all the while, and he wetting his hands from time to time. When the vessel is too thick, he uses a flat piece of iron, somewhat sharp on the edge, to pare off what is redundant; and when it is finished, it is taken off from the circular head by a wire passed under the vessel. 40 The potter's lathe is also a kind of wheel, but more Potter's simple and slight than the former: its three chief memlathe. bers are an iron beam or axis three feet and a half" high, and two feet and a half diameter, placed horizon tally Porcelain. tally at the top of the beam, and serving to form the vessel upon and another large wooden wheel, all of a piece, three inches thick, and two or three feet broad, fastened to the same beam at the bottom, and parallel to the horizon. The beam or axis turns by a pivot at the bottom of an iron stand. The workman gives the motion to the lathe with his feet, by pushing the great wheel alternately with each foot, still giving it a greater or lesser degree of motion as his work requires. He works with the lathe with the same instruments, and after the same manner, as with the wheel. The mouldings are formed by holding a piece of wood or iron cut in the form of the moulding to the vessel, while the wheel is turning round; but the feet and handles are made by themselves and set on with the hand; and if there be any sculpture in the work, it is usually done in wooden moulds, and stuck on piece by piece on the outside of the vessel. The lathe is employed for smaller works in porcelain. 41 illed bisuit. After the first application of the pieces of porcelain to the wheel or lathe, they are allowed to become nearly dry; and to give the requisite form, or a greater degree of accuracy and perfection, they are again subjected to the same operation. They are afterwards introduced into the furnace, not, however, for the purpose of baking them completely, but only to apply a sufficient heat, to give them that firmness and solidity that they may undergo the various necessary manipulations without being disfigured or broken. In this state they are ready for the process of glazing. As the pieces of porcelain, after being subjected to this moderate degree of heat, are very dry, they readily imbibe water, and it is this property of absorbing water, which greatly assists in the application of the glazing; and having received this covering, the pieces of porcelain are again put into the furnace, to complete the process of baking. The heat is gradually raised, and at last brought to that degree that all the objects within the furnace shall be white, and the cases shall be scarcely distinguished from the flame. To ascertain when the porcelain is sufficiently baked, small pieces are taken out in the manner already described, after which the fire is withdrawn, and the furnace allowed to cool gradually. If the process of baking have succeeded properly, the pieces of porcelain will, after this operation, be sonorous, compact, having a moderate degree of lustre, and covered externally with a fine coat of glaze. If this porcelain is destined to receive the ornaments of painting and gilding, these operations are performed in the manner to be afterwards described. nglazed After the porcelain has been subjected to the process orcelain of baking, and before it is glazed, it is said to be in the state of biscuit, in which it possesses various degrees of beauty and perfection, according to the nature and proportions of the materials employed. For particular purposes, the porcelain is sometimes allowed to remain in this state, and particularly when it is employed in smaller and finer pieces of sculpture, where the fineness of the workmanship and the sharpness of the figures are wished to be preserved, as it is well known that these VOL. XVII. Part I. + Cover 42 will be greatly injured by being covered with a coat of Poreclain, glazing. The celebrated manufactory of Sevres in France has been long distinguished for figures or small statues, and even for larger works, as ornamental vases, &c. which are left in the state of biscuit. The English manufactories, and particularly that of Mr Wedgwood, are probably not inferior in the delicacy and accuracy of execution of ornamental productions of this kind. The next operation in the manufacture of porcelain Method of is the process of glazing. This process consists in glazing ing the porcelain with a thin coat of vitreous or fusible porcelain. matter, which adds greatly to its beauty, by its lustre or shining appearance. In preparing and applying the materials fit for glazing porcelain, it has been found that the same kind of glass will not admit of general application; for it appears that a glass which forms a fine glazing for one kind of porcelain, will not answer the same purpose when applied to another. In the former it may have all the necessary requisites, but in the latter it may crack in many places, may have no lustre, and may contain bubbles or be apt to scale off. The first thing then is to prepare a glass which shall be suited to the nature of the porcelain for which it is intended. The glazing must be appropriated to each kind of porcelain, that is, to the ingredients which enter. into its composition, or to the degree of hardness or density of the ware. The materials of which the glazing is composed are prepared by previously fusing together all the substances of which they consist, and thus forming a vitreous mass (A). This mass of vitrified matter is to be finely ground in a mill, and the vitreous powder thus obtained is to be mixed with a sufficient quantity of water, so that the liquor shall have the consistence of cream of milk. The pieces of porcelain are to be covered with a thin coating of this matter, which is done by immersing them hastily in the liquid, and as they greedily imbibe the water, there remains on the surface a uniform covering of the glazing materials. This covering, which, it is necessary to observe, should be very thin, in a short time becomes so dry, that it does not adhere to the fingers when the pieces are handled. When they are sufficiently dry, they are replaced in the furnace in the same manner as in preparing the biscuit, and the heat is continued till the glazing be completely fused; but the degree of heat necessary for that purpose is far inferior to that which is requisite in baking the paste. The pieces of porcelain which are intended to remain white, are now finished, but those which are to be ornamented with painting and gilding must go through various other operations, of which the following is a general account. 43 The colours which are employed in painting porce- Of paintlain are similar to those which are applied in the paint-ing. ing of enamel. They are all composed of metallic oxides or calces, combined with a very fusible, vitreous matter. The different colours are obtained from different metals. The oxides of iron afford a red colour; gold precipitated by means of tin, furnishes a purple and violet colour; copper precipitated from its solution in acids by means of an alkali, gives a fine green; cobalt, Bb or (A) The proportion of the materials employed for common white pottery-ware are 60 parts of litharge, 10 of clay, and 20 of ground flint. |