94 Beginnings of Porcelain 



It is quite probable that the Chinese glaze was originally made by 

 mixing together three ingredients, — a lead compound, a copper com- 

 pound, and a form of silica. The iron and lime present were probably 

 impurities existing in the raw materials used in making the glaze. 

 A glaze of this type (which is in reality a glass, since glazes generally 

 contain alumina) fuses at a very low temperature, is very brilliant, 

 has a high specific gravity, high index of refraction, and high coefficient 

 of expansion; and is easily dissolved by chemical agents (comparatively 

 so). Owing to the high coefficient of expansion, the glaze is very 

 susceptible to crazing. The glaze could be improved by the addition 

 of alumina in the form of clay, which would lower the coefficient of 

 expansion, thus reducing crazing, and would make the glaze more 

 resistant to the weathering action or to chemical agents. In good 

 glaze practice, it is customary to introduce an alkali in some form, 

 although good glazes can be produced without the use of alkali. One 

 glaze being used for glazing roofing tile has the formula: 



.oPbO 



.15 A1 2 3 1.6 SiOj, 



.i CuO 

 which is very similar to the Chinese glaze plus Al 2 Os. A mixture which 

 will produce this glaze is: 



Red lead 205 

 Copper oxide 8 

 Ball clay 39 



Potter's flint 78 

 If the Chinese glaze has been disintegrated by long exposure, the 

 alkalis would naturally be leached out partially, if not entirely." 1 



1 The material for analysis was carefully-picked, unaltered fragments [h.w.n.]. 



