PHYSICAL PROPERTIES OF CLAYS. 25 



PLASTICITY. 



This is the property by virtue of which a clay can be 

 moulded into any desired form when wet, which shape 

 is retained by it when dry. 



. Just what the cause of plasticity is still remains to 

 be definitely proven, although several theories, some 

 of them very reasonable ones, have been advanced. It 

 is an exceedingly variable property and we can find all 

 stages in the transition from the highly plastic clay 

 to the slightly coherent sand. Clays, which possies 

 little plasticity are said to be lean, while highly plastic 

 ones are called fat. 



Pure or nearly pure kaolins are nearly always lean, 

 while clays low in kaolinite may be highly plastic, thus 

 for instance the clay from Chalk Bluff, and the stone- 

 ware from Prattville, containing respectively 36.50 

 and 26.98 per cent, of alumina are both lean, while the 

 clays from Fayette Court House and Fernbank con- 

 taining only 19.68 and 13 per cent, of alumina respec- 

 tively are both highly plastic. 



Cook has shown that the plasticity of some kaolins 

 may be increased by grinding them, the result being 

 to tear apart the little particles of clay which were 

 bunched or clustered together and thus permit a great- 

 er mobility of the grains or scales of clay over each 

 other. 



Mica decreases the plasticity of clay, and if, in a 

 finely divided condition, tends to make it flaky when 

 wet. 



Plasticity, whatever its cause, is an important pro- 

 perty from a commercial standpoint and highly inter- 

 esting from a scientific one. The amount of water re- 

 quired to. develop the maximum plasticity varies. If 

 too little is added the clay cracks in moulding and is 

 stiff and hard to work; if too much is mixed in with 

 the clay it becomes very soft and retains its shape with 

 difficulty. Lean clays usually require less water to 

 produce a workable mass than plastic, ones. 



The Alabama clays require from 25 to 30 per cent, 

 of water to develop their maximum plasticity. 



