CLAY WORKING INDUSTRIES. ]23 



connection with the pure clays, would make so refractory a mixture that it 

 would be impossible to vitrify or make solid by heat; therefore it is neces- 

 sary to add felspar, which takes the place of the impurities of the stoneware 

 clays. This felspar is ground to a powder like the flint. 



The mixture of these ingredients then is the body of white pottery; 

 kaolins for the body, ball clays for plasticity, silica to counteract 

 shrinkage and felspar to make the mixture fusible. The proportions of 

 these ingredients each potter keeps to himself. Each one has his own 

 recipes, which he has worked over for 3iears perhaps, and which he knows 

 and understands fully. These recipes are his secret, his stock in trade and 

 recipes for body and glaze are to potters a merchantable article. 



The variations in this body which are made to produce the three 

 grades of wares, CC, White, Granite and China, are not important in 

 theory, they are mostly changes in the quality of the ingredients. CC, 

 was a term originally for " common clay" wares and means a low grade of 

 white pottery. The ingredients are the same kinds as those used in the 

 best grades, but the off color varieties and cheapest goods are used alto- 

 gether, and the slight cast of color, due to iron, which will creep into 

 white ware bodies is not counteracted by the necessary chemicals, but 

 instead, a glaze is used which hides the color of the body from view and 

 makes the surface appear nearly as well as if the body were of a good 

 color.. 



In white granite, the best materials of each class are bought; the 

 color of the body due to iron, is corrected by adding a minute quantity 

 of cobalt oxide, the yellow of the iron is thus neutralized by the blue of 

 the cobalt, and a green color is produced. When the ware is seen in 

 bulk the green is easily detected, but in a single article it is much less 

 apparent than either the yellow or blue cast would be. 



The aim in white granite is to produce strong white pottery, able to 

 resist a sharp blow, and able to bear scrutiny as to its tint. It is not 

 supposed to be thoroughly vitrified or to have any approach to translu- 

 cency. 



In china the aim is to produce a body which shall become more 

 thoroughly vitrified or fused in its nature and which shows a power to 

 transmit light like an opaque glass. 



This can only be attained by giving it such a composition that it can 

 be vitrified at the ordinary heats employed by the potter. This increased 

 fusibility is of course brought about by the increased use of fluxes. 

 China which is fluxed by felspar is called technically spar china. It 

 shows a yellowish color to transmitted light. China in which a part of 

 this spar is replaced by calcined bones or phosphate of lime, is called 

 bone china; it shows a bluish white color to transmitted light and a 

 much more delicate beauty under all conditions. 



China bears the same relation to white granite, that stoneware bears 

 to earthenware; its burning is accomplished only by use of all the care, 



