CHAPTER XIII 
CLAY 
Essextiar ProperTIES OF CLAY—Clay is the typical member 
of the argillaceous rocks (Latin, argilla, clay). The best 
known property of clay is its plasticity when moist, and the 
name appears to come from the same root as clog. The 
essential characteristic of clay (cf. p. 191) is the size of its 
particles, which are not more than one five-thousandth of an 
inch (-005 mm.) in diameter. As explained on page 191 the 
distinction between sand! and clay is physical and not 
chemical. 
The plasticity of clay is not fully understood. It has been 
attributed to chemical composition, though a sand may be 
made plastic by being finely ground without any change 
in composition; to water, but materials that are identical 
in water content vary in plasticity; to the shape of the 
particles, but either the elongated or globular form are re- 
garded as favourable to it (Ries, Clays, 1906, pp. 97, 99); 
and to the presence of a colloid (H. E. Ashley, U.S.G.S., 
Bull. 388, 1909, pp. 42, 59). The plasticity of clay has no 
single cause, but is due to various factors, such as a colloid 
or very fine particles in the interstices, which allow move- 
ment between the particles, and act like water in quicksand. 
The shape of the particles naturally has some influence, 
Common clay is made by the breaking up of rocks and the 
deposition of the finer particles as a bed. Clay usually con- 
tains grains of silica in a base of silicate of alumina, with 
small amounts of carbonate of lime and an iron compound. 
The main industrial value of clay depends upon the fact 
! For British and important foreign sands and their economic factors, 
see P. G. H. Boswell, British Resources of Sands used in Glass Manu- 
facture, 1918, which includes an account of sands used for most other 
purposes. 
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