KAOLIN IIS" WISCOKSLST. 25 



use lias always been in the ceramic arts. It is also used to make 

 fire-brick and refractory vessels, and to some extent in making 

 alum. The two former of these uses are the important ones. 



Use of kaolin in the ceramic arts. — For making the finer kinds 

 of pottery the important qualities of the kaolin are its color after 

 burning; plasticity; and capacity of hardening well under heat 

 without fusion. The plasticity is necessary for the moulding, the 

 last named property for the perfect retention of the moulded form. 

 Pure kaolinite is almost absolutely infusible under heat, simply 

 losing its water and becoming an anhydrous silicate of alumina. 

 This refractory property is lessened hy the addition of any other 

 bases; least by magnesia, more by lime, still more by iron oxyds, 

 and most by the alkalies. The table of analyses of foreign clays 

 given below, will serve to indicate how the Wisconsin clays rank 

 in this regard. 



f5|Tlie many kinds of cla^^-ware may be grouped conveniently into 

 the dense andporo^<s kinds,* according to the internal texture of 

 the mass. Certain kinds of the dense wares are the ones for which 

 kaolin is chiefly used. The ordinary " true " or " hard " porcelain 

 consists of (1) a body of previously washed kaolin, and (2) a fusi- 

 ble binding material, which by its fusion fills the pores of the baked 

 clay and thus renders the ware homogeneous and translucent. 

 This binding material, or " flux, " is composed chiefly of felspar, to 

 which are added other ingredients, such as quartz, gypsum, etc. In 

 general, the three ingredients of porcelain are kaolin, felspar and 

 quartz. True porcelain has usually no external glaze placed upon 

 it, its glaze being imparted by the flux which renders it translu- 

 cent. To give an idea of the amount of kaolin needed in making 

 porcelain, I select the following admixtures used at some of the 

 famous European manufactories :t 



* Wagner's Chemical Technology. f Knapp. Chem. app. to Arts p. 229. 



