CLAYS OF NEW YOKK 539 



Plasticity 



This property permits the clay to be molded into any desired 

 form when wet, which shape it retains when dry. 



Just what the cause of plasticity is, remains to be proven mth 

 certainty. We find this property best developed in the pasty clay, 

 but even here it is exceedingly variable, and it is possible to col- 

 lect a series of samples showing all grades of transition from a 

 very plastic clay to a mass of sand, which would not ordinarily be 

 looked on as plastic, and yet does possess a slight amount of pasti- 

 ness resembling plasticity, if ground very fine. We can not say, 

 therefore, that plasticity is confined to clay, but simply that the 

 physical conditions existing within a mass of clay are such as tend 

 to produce the maximum degree of tenacity, the highest grade of 

 plasticity. 



Many theories have been advanced to explain this remarkable 

 property. For a long time plasticity was supposed to be directly 

 connected with the hydrated silicate of alumina, or kaolinite; clays 

 high in kaoKnite were said to be very plastic. This is plainly not 

 true, as any series of clays tested will demonstrate. Pure or nearly 

 pure kaolins are very lean, while clays low in kaolinite may be 

 highly plastic. 



Prof. G. H. Cook-^ considered plasticity to be due to a plate 

 structure present in the clay, the plates sliding over each other 

 and thus permitting mobility in the mass without cracking. He 

 farther found that in the kstolins the plates of kaolinite were fre- 

 quently collected in little bunches, and that, after these clays were 

 rubbed in a mortar, in order to tear apart the plates, they showed 

 increased plasticity. 



There seems to be much to commend this theory as far as it 

 goes. Compare for example the white washed kaolin from Dills- 

 boro, N. C, with the washed, white plastic clay from Edgar, Pla., 

 the one a residual clay, found just where it was formed, the other 



' N. J. Geol. sur. 1878. Clays of New Jersey. 



