How many types of Kaolin? Classification of Kaolin.

Ordinary washing process does not deprive nature kaolin of their accompanying impurities and according to the presence of theme impurities kaolins are graded as follows.

•       Pure kaolins do not contain more than 5% of free silica and 2% of fluxes. After firing, the best qualities form mass of milky whiteness. They are used in the manufacture of porcelain and fine earthenware, and in several non-ceramic industries.
•       Alkaline kaolins may contain about 5% of alkalis derived from feldspar and mica. The proportion of iron may be up to 2%. They can be purified by very careful washing which removes the greater part of alkalis, but are inferior to the first group. These clays are used for making earthenware and fine pottery.
•       Siliceous kaolins contain up to 25% free silica in the state of impalpable powder. These clays are not very plastic as can be judge from their composition and they are used in the making of porcelain and certain kind of faience where very plastic bodies are not required.
•       Alkali siliceous kaolins contain a considerable proportion of both alkalis and silica; they partake of the properties of the two preceding kinds but are less refractory.
•       Ferruginous kaolins prevent a white body being produced on firing. They are chiefly used for the manufacturing of refractory materials provided they do not contain too much alkali. Kaolins with high lime content are known as calcarious.