CERAMICS. 459 



A white-burning paste which is usually prepared from two kinds 

 of substances finely mixed and pulverized, namely, kaolin^ (porce- 

 lain or pipeclay), and some other mineral rich in silicic acid, the 

 so-called flux — usually felspar or pegmatite, porcelain stone,^ or 

 some other white-burning form of quartz, is used in the finer cer- 

 amics. Kaolin is distinguished by its plastic character and fire- 

 proof quality ; the other constituent of the paste is called flux 

 because it is fusible, and melts in the heat of the porcelain furnace 

 to a glassy mass. 



The relative proportions of the two ingredients of the paste, 

 and the degree of heat to which the articles must be subjected 

 in burning, depends on whether they are to have an earthy or 

 glassy fracture, and whether they are to be opaque or trans- 

 parent, and consequently whether they will be called porcelain or 

 Faience. 



Kaolin is formed by the decomposition of felspar and kindred 

 minerals. It is found as a product of the disintegration of felspar 

 rock, especially of granite, porphyry and gneiss, always in situ, and 

 frequently so mingled with the solid quartz of these rocks, that 

 one can follow all the steps of the disintegration. Common 

 potter's clay, however, for example the fatty, brown-coal clay (and 

 also the loam of our fields) is a sediment formation, which is pro- 

 duced by water carrying off the original products of the decom- 

 position of the rocks rich in alumina, and finally depositing them in 

 layers, so that their origin is no longer recognisable. 



Porcelain, like glass, is really a bi-silicate of alumina with alkali. 

 But while in glass the proportion of silicic acid is at least 95 per 

 cent., it varies in porcelain between 58 and 82 per cent. The pro- 

 portion of alumina varies between 9 and 38 per cent. The amount 

 of the alkali, in which potash usually predominates largely, amounts 

 only exceptionally to 5 or 6 per cent. The lime present is seldom 

 more than a fraction of one per cent. A higher per centage of 

 silicic acid in the porcelain paste diminishes its plastic character 

 (makes it dry), but renders the baking process easier, and furnishes 

 a fine translucent porcelain more like glass, though not so hard. 

 On the other hand, the quantity of alumina increases the plasticity, 

 hardness and infusible quality. The paste, which is rich in 

 alumina, however, requires more heat in baking, and furnishes a 

 porcelain less transparent and more difiicult to be treated by the 

 decorator. 



^ The name comes from Kao-ling, i.e. " high back." This is the name of 

 a hill, east of the Chinese porcelain city, King-te-chin, which hill, however, 

 does not yield the product of decomposition which we in Europe call kaolin, but 

 a phyllite, whose chemical composition resembles that of the Swedish Hdlle- 

 flinta {?). It approaches that of the Japanese porcelain stone and of peg- 

 matite, as is shown by a collection of analyses at the close of this chapter. 



2 We shall see further on that these porcelain stones, which are wanting in 

 our pottery industry, contribute greatly to that of China and Japan. 



