The physical structure of translucent wares may be described as that of a transparent glass, holding opaque infusible particles of kaolin or other substances in suspension. Microscopic examination of a thin plate of hard porcelain reveals opaque rods, granules, and fragments of quartz, and spherical bubbles, bound together by a vitreous cement. The want of transparency is caused by the repeated reflection and refraction of light by these obstructive particles. The chemical nature of the different translucent wares is best illustrated by reference to the table of analyses (p. 1558). The proportions of alkalies, and of calcic, ferric, and magussic oxides reckoned together are, approximately, in English china, 12 ; in Parian, 8 ; in Sevres, 8 ; in Chinese, 7 ; and in Berlin and Japanese, 4. Of the alkaline constituents of Japanese porcelain, the sodic oxide is in excess ; whereas in the case of Chinese porcelain, potassie oxide preponderates.
The raw materials used in the manufacture of hard porcelains are kaolin and felspar, or a felspathic mineral. The felspathic constituent or petuntze of the Chinese potters is potaesium felspar, the orthoclase or adularia, of mineralogists. The general processes of manufacture and manipulation in China and Japan are similar to those employed in Europe, although of a roug-her character. Homogeneity is procured in the prepared mixture by repeated treading, and the mixture is always stored fur a considerable time before being worked into ware. Hand-driven throwing-wheels are employed, and dried clay is used for moulds in the place of plaster of Parie. The glaze, which is made of felspar aud lime, requires an intense heat for its fusion. There are few metallic oxides, except that of cobalt, which can withstand the requisite temperature, and it is therefore necessary to apply the less stable colours in the form of enamels upon the glaze, and to fuse them at a lower heat. The wares are generally subjected to a moderate fire before the applic,ation of the under-glaze colour and the glaze, althongh it is possible to fire at one time for biscuit, under-glaze colour, and glaze.
avres hard Porcelain.—The raw materials uaed at Sevres are kaolin from St. Yrieix, and chalk. The kaolin in its natural condition contains a considerable but varying proportion of felspathic sand. If the raw kaolin be exposed to an intense heat, it fuses to an opaline glass. In order to regulate the fusibility of the resultant ware, it is necessary first t,o separate the felspathic sand from the kaolin, and then to remix it in proper proportions. Separation is effected by agitating
the raw kaolin with water in a suitable receptacle, allowing the heavy felspathic material to deposit, and drawing off the suspended kaolin into a second or series of receptacles, where it is gradually deposited, and from which the water, t,ogether with the floating and soluble impurities, may be run off from above. The felspathic sand is collected and ground with water in a mill with atone runners. The chalk is also ground, and the kaolin, felspar, and chalk, after passing a suc cession of sieves, are mixed together by measure in a liquid condition. The liquid mixture is ronsonlated partly by the expression of the water through a filtering medium of prepared linen, and partly by absorption of the water by plaster of Paris. The paste so formed is kneaded and beaten upon slabs of plaster of Paris, moulded into lumps, and stored in cellars for future use. The average proportion in which the three ingredients are mixed is : kaolin, 48 ; felspathic sand, 48 ; and chalk, 40. The comparatively large proportion of felspar and lime renders the mass deficient in plasticity. To partially counteract this defect, parings of nnburnt ware are mixed with the dried and pulverized paste. This mixture is again carefully kneaded, and incorporated with sufficient water to produce the amount of plasticity requisite for manipulation. The processes of throwing, pressing, moulding, and casting, are similar to those already described, although modified in certain respects to suit the nature of the mixture. The mixture is less plastic, and more liable to defects from deficient homogeneity, than is the case with the mixture for English earthen-ware and china. In throwing, it is customary to subject the mass of paste, after being kneaded, to a preliminary moulding on the wheel, in order to ensure the regular aggregation of the constituent particles ; it is also usual to allow the substance uf the article in the rough greatly to exceed that of the finished pattern, provided the internal diameter remains the same, so that the exterior may be pared away by turning, and signs of unequal pressure be removed. The paste is prepared for pressing and moulding, by the passage over it of a wooden roller running upon guides ; and the bat of paste is raised from the bed of plaster of Paris upon which it has been rolled, by means of an underlying film of skin or other material. In casting, the liquid paste is allowed to remain in the mould for a considerable time.