In China, the principal materials of which porcelain is formed, are two kinds of stone or earth, denominated kaolin and petuntse, with two kinds of oil or varnish, one of which is extracted from the hard stone of which the pe tuntses are formed, while the other is the oil of lime, ob tained, as shall be shown, by a considerably tedious pre paration. Of the nature of kaolin and petuntse, there have been many various conjectures. This question, however, may be said now to be about decided. These substances have been analysed by Reaumur, Scheffer, and others, whose opinions and inferences have very nearly coincided. The analysis of Vauquelin, which has obtain ed the authority of the late Dr. John Murray, of Edin burgh, and which is not essentially different from that of his predecessors, may be regarded as perfectly satisfactory and unexceptionable. The result of the analysis is as fol lows; the kaolin consists of silex 74, alumina 16.5, lime 2, and water 7; the petuntse of silex 74, alumina 14.5, lime 3.5. The two principal ingredients, therefore, of which porcelain is made, are of a siliceous and argillaceous cha racter, in w hich the former predominates. Though sili ceous earth is the ingredient in largest proportion in these compounds, yet it is the argillaccous substance which gives them then/character, as it communicates ductility to the mixture when soft, and renders it capable of being turned to any shape on the lathe, and of being baked, a process afterwards to be described. The perfection of porcelain consists in the purity of these ingredients ; and hence, in Europe, the purest natural clays, or those which consist of silicia and alumnia alone, are always preferred. From the above analysis, it is evident that there is rather less than five per cent. of lime in the two substances in question. Lime in that proportion does not injure, or rather it improves the character of the porcelain ; but, in greater proportions, it renders the mixtures too fusible; in which case the purity of the other ingredients, and th% greatest care or ingenuity on the part of the workmen, can be of no avail. Pettintse and kaolin are found in quarries of great depth, and of inexhaustible extent, about twenty or thirty miles from King-te-tching, and in other parts of the east. Identical substances are not to be found in any part of the western world, though analogous ones are suf ficiently abundant. "It is difficult," to quote the words of a celebrated chemist, "to procure in Europe natural clays equally pure ; and hence, in part, the difficulty of imitating the porcelain of the east. Such clays, however, have now been discovered in different countries, and the European porcelatn has attained considerable perfection. The fine Dresden porcelain, that of Berlin, the French porcelain, and the finer kinds which are formed in this country, are manufactured of the clay which has received the name of porcelain earth, and which appears in general to be derived from the decomposition of the feldspar of granite," of the nature of which, it may be remarked, pe tuntse and kaolin, according to 1I. Bomare and others, partake in an eminent degree. " The clay of Cornwall, from which the finer kinds of English porcelain are made, has this origin. Earthy mixtures, containing magnesia, are also used in the manufacture. Giobcrt analysed an earth which hadlong been employed for this purpose, and considered as a clay of great purity, and found that it con sisted almost entirely of carbonate of magnesia and silex. The proportion of the earths to each other is likewise of importance ; and from differences in this respect arises the necessity frequently of employing mixtures of clays. The proportion of silex in porcelain of a good quality, is, according to Vauquelin, at least two-thirds of Inc com position; and of alumina from a fifth to a third; magnesia is of utility by lessening the tendency which the composi tion of the other earths alone has to contract in baking. From what we know of the fusibility of mixtures of these earths, too large a proportion of magnesia will render the composition too fusible." In addition to kaolin and pctuntse, the Chinese, as men tioned above, use also two oils or varnishes in the manu facture of porcelain. Of these oils one is extracted from the stone of which the petuntses are formed, the kind which is the whitest, and whose spots are the greenest, being cho sen for this purpose. To 100 lbs. of this oil they put a mi
neral stone called shekau or kekao, resembling an alum, and which, when reduced to an impalpable powder, serves to give the oil a consistence, though it must always be kept in a liquid state. The other oil is the oil of lime, the pre paration of which, as stated above, is extremely tedious.
Having dissolved and reduced to powder large pieces of quicklime, they sprinkle water on it. On this powder they lay alternately couches of dry fern and slacked lime, till they have erected a considerably large pile. They then set fire to it ; and with the ashes that remain, and with dry fern, alternately, as before, they repeat the same process five or six times successively ; and the oil, thus prepared, is regarded as an important ingredient, (though consider able skill is required to prevent too much of it being used) in the manufacture of porcelain, as imparting to it all its lustre and transparency. It may not he improper to men tion that the term oil is used by the Chinese in a very pe culiar and vague sense. It seems with them to signify ge nerally any thing in a state of liquidity; and they call their varnishes oils, though made of. the powders of earths and stones, mixed with water.
There is still another ingredient made use of in the ma nufacture of porcelain, namely, hoache, a substance of a chalky or siliceous nature. This may be used either instead. of kaolin, by undergoing a similar process of preparation, or as a varnish, the vessel when made being plunged into it, by which means it derives the greatest splendour and whiteness. The porcelain made of hoache is extremely light and brittle, and considerably more expensive than that formed of kaolin.
In preparing the petuntse and kaolin, the first object is to break and pound them in a mortar, till they arc reduced to almost impalpable powder. In this state a quantity of water is applied, and after they have been completely amalgamated by being stirred with an iron instrument, they skim off from the surface a white substance of three or four inches in depth, which they put into another vessel of water. This process is repeated till nothing is left but the coarse residuum of the powder, which is carefully pre served, and, after being pounded again, is used as a new powder.
With regard to the second vessel, in which the skim mings of the first were put, the water being soon separated from the ingredients with which it had been blended, is poured out ; and the sediment which remains at the bottom of the vessel, is then put into a mould of a square shape, and after being dried, requires only to be mixed with the proper materials, for being fashioned into porcelain. The two substances, petuntse and kaolin, undergo a similar pre paration; though the latter, being naturally soft and more dissoluble, requires not to be broken, but merely to be im mersed in water.
The just admixture of the different ingredients is the next step in the process of the porcelain manufacture; and this roust be determined by the quality of the porcelain to For the finest porcelains they use an equal quan tity of petuntse and kaolin; and the proportion of petuntse to increase as six to four, three to one, according to the de gree of coarseness which the porcelain is meant to assume, the vessel being coarse in proportion as the quantity of petuntse exceeds that of kaolin in the nanufacture of it. The two ingredients are, when thus combined, put into a large pit or basin, well paved and cemented, and are trod den by the workmen, and hardened, till they obtain a pro per degree of consistence. They are then removed from the basin, and rolled and kneaded a second time on a slate : a process which requires the greatest care and niceness, as the smallest vacuum, or the least admixture of any thing extraneous, even a hair or a grain of sand, would render the operation a complete and total failure, and the mate rials thus adulterated of no use at any future period. The oils, or varnishes are next to he applied ; the oil of lime being generally in the ratio of one to ten of the other oil, which, as previously described, is extracted from the stone from which pentuntse is obtained. The proportion which these oils must bear to the other ingredients depends en tirely on the quality of the wdrk to which they are applied. The mode of applying these oils will be mentioned in a* subsequent part of this article.