CHEMICAL PROPERTIES OF CLAYS. 9 



widely in their physical characters, and therefore act 

 entirely opposite when used for the manufacture of 

 clay products. 



Pure clay or kaolin would be composed entirely of 

 kaolinite, the hydrated cilicate of alumina. These 

 two terms are often confounded and it is well to em- 

 pahasize the fact that kaolinite refers to the mineral 

 species, while the term kaolin is applied ito the mass. 

 Pure kaolin has net thus far been found, although 

 deposits containing as much as 98 per cent, of it are 

 known, and the othe" two per cent, consists of foreign 

 matter. The kaolin therefore contains a variable 

 amount of mineral 'mpurities mixed in with the kao- 

 linite or the clay substance, as it is some times called, 

 and these impurities may affect both the chemical and 

 the physical properties to a variable extent, depend- 

 ing upon the quantity and the kind of them present. 

 The clay substance is always present but in a 

 variable amount, and it stands in no direct relation to 

 the plasticity, except in so far that the latter is lost 

 when the combined water is driven off. 



The amount of clay substance in clays ranges from 

 5 or 10 per cent, to 98.5 per cent. 



The chief impurities in kaolin* are quartz, feldspar 

 and mica, but in other clays the number of mineral 

 species present may indeed be large. 



CHEMICAL PROPERTIES. 



The chemical composition of a clay directly influ- 

 ences its fusibility, and the color to which it burns. 



The compounds which may be found in clay are 

 silica, alumina, iron oxide, lime, magnesia, potash, 

 soda, sulphuric acid, phosphoric acid, manganese 

 oxide and organic matter. Compounds of chromium 

 and vanadium may also be present at times in small 

 amounts. All of these substances are not present in 

 every clay, but most of them are. 



Pure clay would contain silica, alumina and com- 

 bined water, but the purest clay known commonly 

 contain at least traces of iron oxide, lime and alkalies. 



