510 , KEW YORK STATE MUSEUM 



PROPERTIES OF CLAY 



Pure clay would be composed entirely of tlie mineral kaolinite> 

 the liydrated silicate of alumina. A mass of it would be called 

 kaolin. "^I'lie latter is the name of the rock, the former the name 

 of the mineral composing it. 



Pure kaolin has not been found thus far, though deposits con- 

 taining as much as YO^ of kaolinite are known, and these when 

 washed yield in some instances a mass containing as much as 9 8. -5^ 

 of kaolinite. Kaolin therefor© contains a yariable amount of 

 foreign minerals, mixed with the kaolinite, or clay substance, as it 

 is sometimes called. These impurities affect the properties of 

 the kaolin materially, either as regards its shrinkage, fusibility, 

 or color in burning. The last named effect is caused by the 

 presence of feriniginous impurities. Their presence in an effective 

 amount would necessitate classing the material with residual clay. 



Kaolinite is supposed to form the base of all clays, or kaolinite 

 together with other hydrated silicates of alumina. This clay sub- 

 stance forms a variable propO'rtion of the clay mass, and stands in 

 no direct relation to the plasticity, except that plasticity is lost 

 with the expulsion of the coimbined water. The amount of clay 

 substance ranges in known clays from 6fo or lO'fo to 98.5^. The 

 former mighti be a, clay sand, the latter a nearly pure kaolin. In 

 kaolins the chief impurities are quartz, feldspar and mica, hat 

 in other clays the number of mineral impurities may be very large. 

 {See chapter on " Mineralogy of clays " p. 503) 



The properties of clay fall generally under two heads, chemical 

 and physical. The latter includes plasticity, fusibility, shrink- 

 age, tensile strength, slaking, absorption, density. The former em- 

 braces the chemical composition, which exerts an influence on the 

 physical behavior of the clay and should therefore be discussed 

 first * 



