Hard Odin is made of silex, generally a fine sea-sand, kaolin or chili-clay, and felspar ; sometimes the felspar is omitted, and a small quantity of selcnite, or carbonate of barytes, or atroutian, in used in its place.
Hard China body is made of kaolin 70, felspar 14, sand 12, aelenito 4. The mixture is sometimes made in lime-water, and must be ground very fine at a mill. When the paste in moulded into the desired forms, it is fired in the biscuit-oven at a moderate heat ; when taken out, it is very bibulous, and opaque. In this state it is dipped in the glaze, which is felspar, ground fine, with the addition of a little alkali. Some prefer a mixture of felspar and broken china ground together. It is then submitted to a second fire of great intensity, which not only melts the glaze on the surface, but vitrifies the entire mass, which thus unites with the softer matter on the sur face so completely, that the glaze does not form a coating of glass upon the body, but appears like the surface of a stone polished by a lapidary. This homogeneity prevents any crazing ; but the process is one of much hazard, for if the fire is prolonged beyond the critical moment when the vitrification is effected, and the surface has become bright, the goods will molt, and sink in the Baggers. Oriental, French, and German china are made on this principle ; but the English have followed the safer practice which we shall next describe, and which the French manufacturers, aware of its advantages, now adopt. Soft or Tender China is made by firing the biscuit to its full vitrescence in the first oven, the shape of the articles being preserved, during their vitres cence, by being imbedded in flint-powder, &c. ; and then glazing in the second oven at a lower degree of heat, so as not to endanger the melting of the goods. The union between the body and the glaze is however more complete than in earthenware, the glazing-fire being much more intense. Bones calcined and ground are Largely used in the manufacture of English china, combined with alnminous and silicious earths in such proportions that they will vitrify together. This effect is promoted by the phosphoric acid of the bones, which at a high heat diffuses itself through all the materials, and unites them into a translucent enamel, less apt to sink and lose its form than the hard porcelain ; it therefore may be Made in larger ovens, and with less risk of loss to the potter. English China body consists of bone 46, china clay 31, Cornish granite 23. For largo pieces, such as dishes, &c., a little Dorset clay is added, to give more ductility in working and more stability in the fire; but when great purity of whiteness and transparency are wanted, a higher degree of vitrescence is obtained by fritting Biles and bone together as the basis of the body.
Frilled China body comprises bone 55, Lynn sand 27, potash 2, cal cined together ; then ground fine, with China clay 18. Glaze for China is harder than for earthenware. The following is excellent Cornish granite 25, soda 6, borax 3, nitre 1 ; mix, and frit in gloss oven ; then take frit 26, Cornish granite 26, white-lead 31, flint 7, carbonate of lime 7, oxide of tin 3. The whiteness is increased by the addition of a little oxide of cobalt. Another China glaze consists of felspar 38, Lynn sand 24, carbonate of lime 11, borax 27 : to be fritted ; then take frit 60, Cornish granite 20, white-lead 20.
The application of the glaze and mode of firing are the same 33 already described.
The decoration of china by enamel colours and gold affords employment to a great number of persons, some of whom attain great excellence in their art. The colours used are all prepared from metallic oxides, which aro ground with fluxes, or fusible glasses, of various degrees of softness, suited to the peculiar colours with which they are used. The flux of most general application is made of red-lead 6, borax 4, flint 2. When painted, the goods are placed in the enamel-kiln, where the fluxed colours melt, and fasten to the glazed surface. The judicious management of these mixtures requires much study, and is still susceptible of improvement by the aid of chemical science. Blues are made from cobalt, varied by the addition of the oxides of zinc or tin. Phosphoric acid gives it a rich purple tint ; alumina also varies its line. Green is from oxide of copper, melted with a soft flux, and ground ; its tints are varied by adding blue or yellow or white enamel Fine greens, which bear a fire that destroys a copper-green, are made from pro toxide of chrome, varied with the addition of cobalt, lead, tin, or alumina. Red consists of nitrate of iron, dichromate of lead, or muriate of manganese. Pink, of eubchromato of tin. Rose-colour, of gold and tin (precipitate of oafish* with a little silver. Brown, of chromate of iron or antimony, lead, and manganese. Orange, of anti mony, tin, and iron. Yellow, of antimony, tin, and lead ; also chromate of lead. Black, of oxide of platinum or iron, cobalt, nickel, and antimony. White, of arsenic and tin. An infinite variety of tints may be obtained by a skilful combination of the above with suitable fluxes, the description of which in detail would exceed our limits. Few potters prepare their own colours ; they are supplied by persons who devote themselves to that art.