X. 260.

44

acid. The manganese contained in this residuum is said to communicate a blackish appearance like that of bronze, which, says the author, is far from being disagreeable to the eye. This glazing he employed several times by way of trial, first fusing it with sand in a potter's furnace, throwing it into cold water to facilitate its division, and grinding it in a mill, that it may be more completely diffused in water. This glazing is attended. with the advantage of being free from those dangerous qualities so common in all preparations made from the oxides of lead. Whatever colouring matters are employed, they are finely ground with gum water, or with some essential oil, in which state they are fit to be employed for the painting of porcelain with figures of flowers, or any other design with which it is intended to be adorned.

and gilding. In gilding porcelain, the oxide or calx of gold (B) is employed, and it is applied nearly in the same manner as the coloured enamels. The gold, which is in the state of very minute division, is mixed with gum water and borax, and in this state is applied to the clean surface of the porcelain with a fine camel's hair pencil. The painted and gilded porcelains are then exposed to such a degree of heat in the furnace as is capable of fusing the vitreous matter with which the metallic colours are mixed. The gold is fixed by means of the borax undergoing the process of vitrification, and thus strongly adhering to the porcelain. Most of the metallic colouring matters exhibit all their beauty when the porcelain is taken from the furnace; but to bring out the lustre and beauty of the gold, those parts of the porcelain which have been gilt are afterwards subjected to the operation of burnishing.

45

celain.

Use of plaThe use of platina in porcelain painting has been retina in co- commended by Klaproth; and experiments have been louring por-made on the subject by that celebrated chemist, with the view of ascertaining its effects for this purpose. The following is the conclusion of his observations. "The process which I employ in the application of platina to painting on porcelain is simple and easy it is as follows:-I dissolve crude platina in aqua regia, and precipitate it by a saturated solution of sal ammoniac in water. The red crystalline precipitate thence produced is dried, and being reduced to a very fine powder, is slowly brought to a red heat in a glass retort. As the volatile neutral salt combined with the platina in this

"The colour of platina burnt into porcelain in this manner is a silver white, inclining a little to a steel gray. If the platina be mixed in different portions with gold, different shades of colour may be obtained; the gradations of which may be numbered, from the white colour of unmixed platina to the yellow colour of gold. Platina is capable of receiving a considerable addition of gold before the transition from the white colour to yellow is perceptible. Thus, for example, in a mixture of four parts of gold and one of platina, no signs of the gold were to be observed, and the white colour could scarcely be distinguished from that of unmixed platina: it was only when eight parts of gold to one of platina were employed that the gold colour assumed the superiority.

"I tried in the like manner, different mixtures of platina and silver; but the colour produced was dull, and did not seem proper for painting on porcelain.

"Besides this method of burning-in platina in substance on porcelain, it may be employed also in its dissolved state; in which case it gives a different result both in its colour and splendour. The solution of it in aqua regia is evaporated, and the thickened residuum is then applied several times in succession to the porcelain. The metallic matter thus penetrates into the substance of the porcelain itself, and forms a metallic mirror of the colour and splendour of polished steel."

The same substance has been applied as a glazing to porcelain in some of the English manufactories, but however valuable and important the application of platina to this purpose may be, the scarcity of that metal, and its consequent high price, must always prevent it from coming into very general use.

46

We have already noticed the establishment of the ma- Manufacnufacture of porcelain in Derby. The following is a ture of por short detail of the method of conducting that manufac- celain at Derby. ture. After the paste has been properly prepared, by grinding and other necessary operations, it is delivered to the workmen, by whose dexterity the shapeless mass is converted into various beautiful forms. Vessels of a round form are usually made by a man called a thrower, by whom they are worked on a circular block moving horizontally on a vertical spindle. They are next carried to the lathe; and being fixed to the end of a horizontal spindle, they are reduced to the proper form and thickness.

plaster, imbibe the water contained in the mixture, so Porcelain. that the paste soon becomes sufficiently hard to part freely from the mould. The different parts of figures, as the head, arms, legs, &c. are cast in separate moulds, and after being dried and repaired, they are joined by a paste of the same kind, but of a thinner consistence. The porcelain pieces thus formed are then conveyed to the furnace, and after being subjected for a proper length of time, to a regular and continued heat, they come out extremely white and delicate.

Porcelain. thickness. They are afterwards finished, and handled by other persons, if that should be necessary, and are then introduced into a stove, where the moisture is entirely evaporated, and they become fit for the process of baking. Vessels of an oval figure, such as tea-pots, tureens, &c. acquire their form by being pressed with the hand into moulds of plaster or gypsum. The pieces of porcelain being thus prepared, are put into the saggars or cases, which are of various sizes and dimensions, and these are set in the kiln or furnace, one upon the other, till they are filled up nearly to the top, in the manner already described. The furnace being full, the ware is baked, and after this first baking, the porcelain is in the state of biscuit.

47

Glazing,'

48

Painting.

49

Gilding.

Biscuit fi

gures.

The next process is the glazing, which, according to the description already given, is performed by dipping the pieces of porcelain in glaze of the consistence of cream. They are then conveyed to the glaze furnace, where they are again baked, but in a degree of heat inferior to that necessary for the first baking.

If the pieces of porcelain are to receive the additional ornaments of painting and gilding, they are next delivered to another set of workmen. The colouring matters, as already noticed, are extracted from mineral bodies, and after proper preparation, they are applied to the ware by the painters, in the form of landscapes or figures, according to the requisite pattern. After this process the ware is again conveyed to the furnace, and the colours are vitrified, to give them the proper degree of fixation and lustre. After every coat or layer of colour, a fresh burning is necessary. In the common kind of porcelain, once or twice is found sufficient for the ornaments it requires; but in the finer decorations, the colours must be laid on several times, and as often subjected to the action of heat, before the full effect can be produced. This completes the process for those articles of porcelain in which glazing and painting only are required.

But when the pieces of porcelain are to be farther decorated with gilding, they are pencilled with a mixture of oil and gold, dissolved or thrown down by quicksilver with the aid of heat, and are again introduced to the furnace. Here the gold returns to its solid state, but comes out with a dull surface; and to recover its lustre and usual brilliancy, it is burnished with bloodstones, and other polishing substances. Much care and attention are necessary in the latter part of the process; for if the gold be not sufficiently burnt, it will be apt to separate in thin flakes, and if it have been exposed to too great a heat, it is not susceptible of a fine polish. In this manufactory, when pieces of porcelain are to be finished in the highest style, they are frequently returned to the enamel furnace, where the colours are fluxed six or seven different times; and having gone through the processes now described, the porcelain is fit for the market.

White ware, or biscuit figures, are made at this manufactory, which are supposed to be equal in beauty and delicacy to any European productions of a similar kind. In this kind of porcelain, the lathe is of no use, for the figures are cast in moulds of plaster or gypsum. The materials of which they are composed being properly prepared, and previously reduced to a liquid of the appearance and consistence of thick cream, are poured into the moulds, which from the absorbent property of the

Porcelain manufactories have been long established at ManufacTournay in Flanders; one of these manufactories fur-tory at nishes all Flanders with blue and white porcelain. At Tournay. this manufactory they have a particular process in forming cups and other vessels, which is somewhat similar to that now described. They are neither turned on the lathe, nor is the clay compressed in a mould; but after being diluted in water, and when the liquid has acquired a proper consistency, the workmen pour it into moulds, two or three hundred of which are arranged together. When they have filled them all, they return to the first in the row. The liquid part is drawn off by a gentle inclination; the surplus adheres to the side of the vessel, and thus forms the piece which it is intended to make. The piece is detached from the mould by means of a slight stroke, and after being sufficiently dried, is conveyed to the furnace, to undergo the process of baking.

52

In the manufacture of utensils for chemical purposes, Utensils for where they are to be subjected to the effects of powerful chemical agents, greater attention is necessary. Vessels of this de- purposes. scription should be infusible at any degree of heat; possess a sufficient compactness of texture, to retain saline and other fluxes in fusion, without undergoing any change; and should bear sudden changes of temperature, particularly sudden heating, without cracking, or in any degree giving way. It has been found impracticable to have the three requisites now mentioned united in the same ware, so that it becomes necessary to select the kind of ware according to the purpose for which they are intended. For bearing high degrees of heat, Hessian crucibles are found to answer best; they are composed of a very refractory clay, mixed with sand, of which the finest part is separated by a sieve, and thrown away. These vessels are made by mixing the clay with a smaller proportion of water than usual, so that a stiffer mass is obtained, and the vessel brought to the requisite shape by ramming the clay strongly into an iron mould. In this way they are very compact, and for a considerable time retain saline fluxes. Ordinary crucibles, it is found, are rendered more retentive by lining them on the inside, before they are quite dry, with a thin coating of pure clay, without the addition of any other mixture. But the most refractory material known is a mixture of unburnt with burnt clay. Vessels made of this material are found capable of resisting the effects of saline fluxes longer than any other, and hence this material is employed in making large crucibles for glasshouses.

One of the most valuable qualities of porcelain ware, is to bear sudden changes of heat and cold; but in this quality some of the most perfect kinds of ware in other respects are extremely deficient, and can scarcely be subjected, without danger of cracking, to the draught of a wind furnace, even when the heat is slowly and gradually applied. This happens to the celebrated porceBb 2

lain

53 Wedgwood's porcelain.

not been able to separate it. When this earth, bow- Porcelain. ever, does not exceed five or six parts in a hundred, it appears that it is not hurtful to the quality of the pottery; but if more abundant, it renders it too fusible.

The oxide of iron, besides the inconvenience of communicating a red or brown colour, according to the degree of baking, to the vessels in which it forms a part, has the property of rendering them fusible, and even in a greater degree than lime.

Porcelain. lain fire ware invented by an enlightened and philosophi'cal manufacturer, the late Mr Wedgwood. This effect of cracking, on sudden changes of temperature, seems to depend on the hardness and closeness of texture; and the closeness of texture is found to be in proportion to the minute division of the materials before baking. The clay and flint of Wedgwood's ware are brought to a most impalpable powder before mixture, so that the texture is uncommonly hard and close. It may be worth while to mention, that Wedgwood's porcelain resists the effects of sudden heat and cold much better, by being covered with a thin coating of Windsor loam, or of a fire lute composed of coarse sand and clay, and tow or horse-dung. When crucibles are intended merely for the fusion of metals, they are greatly improved by a mixture of black lead. This substance being involved in the clay, is protected from the access of air, and is then incombustible. It has no affinity for the earths at any temperature, and being absolutely infusible, it enables the clay to bear, without melting, the greatest degree of heat. The mixture of this substance, as a material for crucibles, has another advantage, that no part of the melted metal is detained in the crucible, as is the case in the common rough ware. It also bears sudden heating and cooling better than any other.

6. General Principles of the Manufacture of Porcelain. Convinced that every accurate and scientific investigation into the nature and processes of any important art, will always be deemed of some value to the philosophic observer, or the enlightened manufacturer, we shall introduce the following observations on the principles of the manufacture of porcelain.

Observations by Vauquelin.

According to this celebrated chemist, four things may occasion difference in the qualities of earthen ware: 1st, The nature or composition of the matter; 2d, The mode of preparation; 3d, The dimensions given to the vessels; 4th, The baking to which they are subjected. By composition of the matter, the author understands the nature and proportions of the elements of which it is formed. These elements, in the greater part of earthen ware, either valuable or common, are silex, argil, lime, and sometimes a little oxide of iron. Hence it is evident that it is not so much by the diversity of the clements that good earthen-ware differs from bad, as by the proportion in which they are united. Silex or quartz Nature of makes always two-thirds at least of earthen-ware; argil the compo- or pure clay, from a fifth to a third; lime, from 5 to 20 parts in the hundred; and iron from 0 to 12 or 15 parts in the hundred. Silex gives hardness, infusibility, and unalterability; argil makes the paste pliable, and renders it fit to be kneaded, moulded, and turned at pleasure. It possesses at the same time the property of being partially fused by the heat which unites its parts with those of the silex; but it must not be too abundant, as it would render the earthen-ware too fusible and too brittle to be used over the fire.

54.

uent parts of porce

lain.

Hitherto it has not been proved by experience that lime is necessary in the composition of pottery and if traces of it are constantly found in that substance, it is because it is always mixed with the other earths, from which the washings and other manipulations have

55

As some kinds of pottery are destined to melt very Mode of penetrating substances, such as salts, metallic oxides, prepara. glass, &c. they require a fine kind of paste, which is tion. obtained only by reducing the earths employed to very minute particles. Others destined for melting metals, and substances not very penetrating, and which must be able to support, without breaking, a sudden transition from great heat to great cold, require for their fabrication a mixture of calcined argil with raw argil. By these means you obtain pottery, the coarse paste of which resembles breche, or small-grained pudding-stone, and which can endure sudden changes of tempera

ture.

The baking of pottery is also an object of great im- Baking. portance. The heat must be capable of expelling humidity, and agglutinating the parts which enter into the composition of the paste, but not strong enough to produce fusion; which, if too far advanced, gives to pottery a homogeneousness that renders it brittle. The same effect takes place in regard to the fine pottery, because the very minute division given to the earths reduces them nearly to the same state as if this matter had been fused. This is the reason why porcelain strongly baked is more or less brittle, and cannot easily endure alternations of temperature. Hence coarse porcelain, in the composition of which a certain quantity of calcined argil is employed, porcelain retorts, crucibles, tubes, and common pottery, the paste of which is coarse, are much less brittle than dishes and saucers formed of the same substance, ground with more la

bour.

57

The general and respective dimensions of the differ- Dimensions ent parts of vessels of earthen-ware have also consider- of the dif able influence on their capability to stand the fire.

In some cases the glazing or covering, especially when too thick, and of a nature different from the body of the pottery, also renders them liable to break. Thus, in making some kinds of pottery, it is always essential, 1st, To follow the best proportion in the principles; 2d, To give to the particles of the paste, by grinding, a minuteness suited to the purpose for which it is intended, and to all the parts the same dimensions as far as possible; 3d, To carry the baking to the highest degree that the matter can bear without being fused; 4th, To apply the glazing in thin layers, the fusibility of which ought to approach as near as possible to that of the matter, in order that it may be more intimately united.

C. Vauquelin, being persuaded that the quality of good pottery depends chiefly on using proper proportions of the earthy matters, though it might be of importance, to those engaged in this branch of manufacture, to make known the analysis of different natural clays employed for this purpose, and of pottery produced by some of them, in order that, when

a new earth

ferent parts of vessels.

Porcelain is discovered, it may be known by a simple analysis whether it will be proper for the same object, and to what kind of pottery already known it bears the greatest resemblance.

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Porous

ware.

59

Raw kaolin 100 parts.-Silex 74, argil 16.5, lime 2, water 7. A hundred parts of this earth gave eight of alum, after being treated with the sulphuric acid. Washed kaolin 100 parts.-Silex 55, argil 27, lime 2, iron 0.5, water 14. This kaolin, treated with the sulphuric acid, gave about 45 or 50 per cent. of alum. Petuntzé. Silex 74, argil 14.5, lime 5.5, loss 6. A hundred parts of this substance, treated with the sulphuric acid, gave seven or eight parts of alum. But this quantity does not equal the loss sustained.

Porcelain of retorts.-Silex 64, argil 28.8, lime 4.55, iron 0.50, loss 2.77. Treated with the sulphuric acid, this porcelain gave no alum.

There is a kind of earthen vessels, called Alcarrezes, used in Spain for cooling the water intended to be drunk. These vessels consist of 60 parts of calcareous earth, mixed with alumina and a little oxide of iron, and 36 of siliceous earth, also mixed with alumina and the same oxide. The quantity of iron may be estimated at almost one hundredth part of the whole. This earth is first kneaded into a tough paste, being for that purpose previously diluted with water; formed into a cake of about six inches in thickness, and left in that state till it begin to crack. It is then kneaded with the feet, the workman gradually adding to it a quantity of sea-salt, in the proportion of seven pounds to a hundred and fifty after which it is applied to the lathe, and baked in any kind of furnace used by potters. The alcarrezes, however, are only about half as much baked as the better kinds of common earthen ware; and being exceedingly porous, water oozes through them on all sides. Hence the air, which comes in contact with it, by making it evaporate, carries off the caloric contained in the water in the vessel, which is thus rendered remarkably cool.

Observations of Brongniart.

The author of the following observations is superintendant of the celebrated porcelain manufactory at Sevres in France. The extensive views he has taken of the subject, and the general principles which he has advanced, will, we doubt not, be favourably received by the intelligent manufacturer, and meet with attention and consideration adequate to their importance and utility. "The art of employing metallic oxides for colouring the anci by fusion different vitreous matters, is of very great antiquity every body knows that the ancients manufactured coloured glass and enamel, and that this art was practised in particnlar by the Egyptians, the first people who in this manner imitated precious stones. practice of this art in modern times has been carried to a high degree of perfection: but the theory has been

Enamel of

ents.

The

neglected; it is almost the only one of the chemical arts Porcelain. in which no attempt has yet been made to apply the new principles of that science.

"It is well known that all vitrifiable colours have for their basis metallic oxides; but all the metallic oxides are not proper for this purpose: besides, as they are not vitrifiable by themselves, they can scarcely ever be employed alone.

66

60

colours for

porcelain.

Highly volatile oxides, and those which adhere little Metallic to the great quantity of oxygen they contain, either oxides emcannot be employed in any manner, as the oxide of ployed as mercury and that of arsenic, or are employd only as agents. The colour they present cannot be depended' on, since they must lose it in the slightest heat by losing a part of their oxygen: such are the puce-coloured and red oxides of lead, the yellow oxides of gold, &c. Oxides in which the proportions of oxygen are susceptible of varying with too much facility are rarely employed :the oxide of iron, though black, is never employed for that colour, and the green oxide of copper is, under many circumstances, very uncertain. I have said that oxides alone are not susceptible of fusion: however, as they are destined to be applied to thin strata on vitrifiable substances, they may be attached to them by a violent heat. But, except the oxides of lead and bismuth, they would give only dull colours. The violent heat, often necessary to fix them, would change or totally destroy the colours. A flux then is added to all metallic oxides.

"This flux is glass, lead, and silex; glass of borax, or a mixture of both. Its general effect is, to give splendour to the colours after their fusion; to fix them on the article which is painted, by promoting more or less the softening of its surface; to envelope the metallic oxides, and to preserve their colour by sheltering them from the contact of the air: in a word, to facilitate the fusion of the colour at a low temperature not capable of destroying it.

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the sub

stances to

“I shall speak here only of the application of metal-Nature of lic colours to vitreous bodies or to vitreous surfaces. These bodies may be divided into three classes, very which they distinct by the nature of the substances which compose are applied. them, the effects produced on them by the colours, and the changes they experience. These classes are: rst, Enamel, soft porcelain, and all crusts, enamels, or glass, that contain lead in a notable quantity. 2d, Hard porcelain, or porcelain which has a crust of feldspar. 3d, Glass in the composition of which no lead enters, such as common window-glass.

"I shall here examine in succession the principles of the composition of these colours, and the general phenomena they exhibit on these three kinds of bodies.

"It is well known that enamel is glass rendered opake by the oxide of tin, and exceedingly fusible by the oxide of lead. It is the oxide of lead, in particular, contained in it, that gives it properties very different from those of the other excipients of metallic colours. Thus all glass and glazing that contain lead will participate in the properties of enamel; and what we shall say of one may be applied to the rest with very trifling differ

Porcelain.

62

Flux for soft porcelain colours.

63. Colours applied to

lain.

to be softened by introducing into it more siliceous or Porcelain. calcareous flux, according to the nature of the feldspar. This method has succeeded; and for about a year past the colours might be exposed two or three times to the fire without scaling, if not overcharged with flux, and if not laid on too thick.

"The third sort of excipient of vitrifiable metallic colours is glass without lead.

"Enamel or soft porcelain colours require less flux than others, because the glass on which they are applied becomes sufficiently soft to be penetrated by them. This flux may be either glass of lead and pure silex, called rocaille, or the same glass mixed with borax. Montamy asserts that glass of lead ought to be banished from among the enamel fluxes; and he employs only borax. He then dilutes his colours in a volatile oil. On the other hand, the painters of the manufactory of Sevres employ only colours without borax, because they dilute them in gum; and borax does not dilute well in that substance. I have found that both methods are equally good; and it is certain that Montamy was wrong to exclude fluxes of lead, since they are daily employed without any inconvenience, and as they even render the application of colours easier.

"I have said that in the baking of these colours, the crust, softened by the fire, suffers itself to be easily penetrated by them. This is the first cause of the change which they experience. By mixing with the crust they become weaker, and the first heat changes a figure which appeared to be finished into a very light sketch.

The two principal causes of the changes which colours on enamel and soft porcelain are susceptible of experiencing do not depend in any manner on the composition of these colours, but on the nature of the glass to which they are applied. It follows from what has been said, that painting on soft procelain has need of being several times retouched, and of several heats, in order that it may be carried to the necessary degree of strength. These paintings have always a certain faintness; but they are constantly more brilliant, and they never are attended with the inconvenience of detaching themselves

in scales.

"Hard porcelain, according to the division which I have established, is the second sort of excipient of metallic colours. This porcelain, as is well known, has for its base a very white clay called kaolin, mixed with a silicious and calcareous flux, and for its covering feldspar fused without an atom of lead.

"This porcelain, which is that of Saxony, is much newer at Sevres than the soft porcelain. The colours hard porce-applied to it are of two kinds: the first, destined to represent different objects, are baked in a heat very inferior to that necessary for baking porcelain. They are exceedingly numerous and varied. The others, destined to be fused in the same heat as that which bakes porcelain, lay themselves flat, and are much less numerous. The colours of painting are made nearly like those destined for soft porcelain; they only contain more flux. Their flux is composed of glass of lead and borax. When porcelain is exposed to heat in order to bake the colours, the covering of feldspar dilates itself and opens its pores, but does not become soft; as the colours do not penetrate it, they experience none of those changes which they undergo on soft porcelain. It must however be said that they lose a little of their intensity by acquiring that transparency which is given to them by fusion.

"One of the greatest inconveniences of these colours, especially in the manufactory of Sevres, is the facility with which they scale off when exposed several times in the fire.

"To remedy this defect without altering the quality of the paste, I was of opinion that the crust only ought

4

64

"The application of these colours to glass constitutes and to glass painting on glass; an art very much practised some centuries ago, and which was supposed to be lost because out of fashion; but it has too direct a dependence on painting in enamel and porcelain to be entirely lost.

"The matters and fluxes which enter into the composition of the colours employed on glass are in general the same as those applied to porcelain. Neither of them differ but in their proportions; but there are a great number of enamel or porcelain colours which cannot be applied to glass, where they are deprived of the white ground which serves to give them relief.

Of Colours in particular.

"After collecting the general phenomena exhibited by each class of vitrifiable colours, considered in regard to the body on which they are applied, I must make known the most interesting particular phenomena exbibited by each principal kind of colours employed on soft porcelain and glass in a porcelain furnace.

Of Reds, Purples, and Violets, made from Gold.

"Carmine red is obtained by the purple precipitate of cassius: it is mixed with about six parts of its flux; and this mixture is employed directly, without being fused. It is then of a dirty violet, but by baking it acquires a beautiful red carmine colour: it is, however, exceedingly delicate; a little too much heat and carbonaceous vapours easily spoil it. On this account it is more beautiful when baked with charcoal than with wood.

"This colour and the purple, which is very little different, as well as all the shades obtained from it, by mixing it with other colours, really change on all porcelain and in every hand. But it is the only one that changes on hard porcelain. Its place may be supplied by a rose-colour from iron which does not change; 90 that by suppressing the carmine made with gold, and substituting for it the rose oxide of iron here alluded to, you may exhibit a palette composed of colours none of which change in a remarkable manner. This rose-coloured oxide of iron has been long known; but it was not employed on enamel, because on that substance it changes too much. As the painters on enamel, however, have become the painters on porcelain, they have preserved their ancient method.

"It might be believed that, by first reducing to a vitreous matter the colour called carmine already mixed with its flux, it might be made to assume its last tint. But the heat necessary to fuse this vitreous mass destroys the red colour, as I have experienced. Besides, it is remarked that, to obtain this colour very beautiful, it must be exposed to the fire as few times as possible.

"The carmine for soft porcelain is made with fulminating gold slowly decomposed, and muriate of silver; no tin enters into it; which proves that the combination of the oxide of this metal with that of gold is not necessary to the existence of the purple colour.

"Violet