Page:The American Cyclopædia (1879) Volume XIII.djvu/815

 POTTERY AND PORCELAIN 791 with water sufficient to form a thin slip which shall contain just enough of the solid materials FIG. 25. Grinding Mill. in a state of suspension to cause a coating of the proper thickness when the ware is dipped in it a certain length of time. The particles are kept suspended by stirring, and the thick- ness of the slip may be regulated by the fre- quency of the stirring, and varied from time to time if different thicknesses of glaze are required. The porous biscuit is dipped in the glaze slip, and quickly placed on a bench at the edge of the dipping vat, from whence the pieces are removed by an assistant. The rapid absorption of the water causes the deposit of a thin paste upon the surface, which fuses and flows over it at the second firing. Hard por- celain is composed of finer materials than fine stoneware, and the glazing is very different, and more nearly resembles the body of the ware. The throwing and moulding of porce- lain requires more care and skill than that of coarser ware. Some of the pieces are exceed- Kiln. ingly thin and delicate, and can only be formed by experienced workmen. There are difficul- ties in making large thin vases, which have been overcome at Sevres by the employment of atmospheric compression and exhaustion. A plaster mould is coated upon the inside with sufficient slip to form ware of the desired thickness, and this is held against the side of the mould until it acquires enough firmness to support itself by either covering the mouth of the mould and forcing air into the interior, or exhausting the air through the porous plas- ter by means of an exterior air-tight case. The first firing is of a much lower heat than that for stone or earthenware, converting the material into a soft instead of a hard biscuit, and is conducted in the upper story of the porcelain kiln, shown in fig. 26. ' The biscuit is then dipped into a glaze composed of ground quartz or flints, feldspar, lime, and porcelain clay, which fuses at a white heat and fluxes with the body of the ware, so as to form with it as well as upon the surface a vitrified trans- lucent mass, which will not check or " craze " upon the surface when subjected to variations of temperature. The biscuit is placed in the lower story of the kiln, when during the same firing it is converted into finished porcelain. The average composition of porcelain body may be stated as follows : silica 66, alumina 30, potash 3-4, magnesia 0*6, lime 0'5. The following is an analysis of the glaze of mod- ern Sevres china, by Malaguti : silica 73-4, alumina 15*7, potash T'4, lime 1*9, magnesia 0'3, loss by water 1-3 = 100 pints. Sometimes the glaze is composed of kaolin, gypsum, and broken porcelain. The beauty of porcelain depends upon the proper fusibility of the glaze, because when too difficult of fusion it will not form an even enough surface, while if too fusible it becomes too much absorbed by the body. Tender porcelain is of two kinds, the natural tender porcelain of England, in which kaolin is the basis, and the artificial tender porcelain (pate tendre, soft paste) of France^ or old Sevres china. The average pro- portion of ingredients in English tender por- celain are as follows: calcined bone dust 47, kaolin 34, and feldspar 19 parts in 100. The phosphate of lime in the bones diffuses itself throughout the mass, forming a translucent enamel-like substance, which shrinks less in baking than hard porcelain, but has much less strength. It is said that the principal cause of the continuance of the exclusive manufac- ture of this kind of porcelain in England is the impossibility of obtaining material suffi- ciently cheap to make seggars which are able to bear the heat necessary for the baking of hard porcelain. A frit of bone, flint, or quartz and pearlash is sometimes first made, and then mixed with the kaolin and feldspar. The ware is baked twice, at a much lower heat than for hard porcelain; the first firing lasts the longest, from 48 to 50 hours. The biscuit is then glazed with a mixture of feldspar, car- bonate of lime, borax, and white lead, and baked the second time at a lower temperature and during a shorter time.. Colored glazes may be made by adding oxides pf manganese, copper iron, chromium, or cobalt, and opaque dazes by the addition of oxide of tin or phos- phate of lime. The artificial tender porcelain