CHINA CLAYS. 117 



clay as sulphates they may cause blisters, especially if the 

 clay is heated too rapidly, and the same holds true of sul- 

 phate of lime or gypsum. Many washed kaolins ap- 

 proach very closely to the theoretical composition of kao- 

 linite, while others 'even when washed may contain a 

 high percentage of total silica due to -the presence of much 

 quartz and perhaps feldspar. If these two accessory min- 

 erals contain no iron they are harmless, especially if 

 finely divided, and the rational analysis of clay is known. 

 ( See method of clay analysis. ) The term kaolin is usu- 

 ally, and always should be restricted to white burning 

 clays of residual origin. They are in most instances 

 highly refractory, but they might also be of such compo- 

 sition as to bring about fusion at a low temperature, and 

 at the same time burn white. It is the absence of plastici- 

 ty in kaolins that necessitates the addition of ball clay, 

 but some manufacturers use only the ball clay, mixed 

 with quartz and feldspar for making porcelain. The 

 last two minerals are indispensable ingredients of white- 

 ware mixture, quartz being added for the purpose of pre- 

 venting excessive shrinkage, and feldspar on account of 

 its easy fusibility binding the mass together. 



China clays should contain a low percentage of iron 

 oxide, in fact the less the better, for in burning this com- 

 pound tends to color the clay yellow or red. While the 

 percentage of iron oxide should be under 1 per cent., 

 nevertheless many of the best china clays used contain 

 1.25 to 1.35 per cent, of iron oxide. This production of 

 a yellowish tint from such a quantity is prevented in two 

 ways, first by adding a small amount of cobalt oxide to 

 the white- ware mixture, or secondly by taking advantage 

 of the fact that when the kiln, in which the ware is 

 burned, is heated to a high temperature the fire tends to 

 act reducing, thereby changing the iron coloration from 

 yellow to bluish or bluish gray, and making it less no- 

 ticeable. 



