126 



ammonia, caustic soda, caustic potash, lime, sodium carbonate, 

 potassium carbonate, borax, and water glass. They ajopear to do this 

 by coagulating the colloidal portion of the clay, but their action 

 may be prevented by the addition of a sufficient quantity of weak 

 acid to neutralise the alkah in the clay. 



The addition of certain organic acids as humus, or of gum, glue 

 and starch confers a pseudo -plasticity on clay winch is, however, 

 quite different from true plasticity and makes the clay " sticky " 

 rather than plastic. 



The stickiness of certain clays (e.g., London clay) is very pronounced, 

 but must not be confused with true plasticity. Ashley' has stated 

 that if the granular constituent is removed from a plastic body it loses 

 plasticity and becomes sticky untU the granular constituent is restored. 

 This suggests that the practice of adding granular material of a non- 

 plastic nature, so common among the users of London clay is based 

 upon a sound princiijle. The stickiness of clay may be regarded as 

 due to coUoidal material which is not properly distributed throughout 

 the inert granular mass. 



Plasticity does not appear to be comiected with the chemical 

 composition, as clays which yield the same results on analysis may 

 differ widely in plasticity, yet on heating above 415°-600° C. all 

 clays lose their plasticity, and it caimot be restored. It is also a curious 

 fact that the clays which are richest in " true clay " are seldom so 

 plastic as those which are not so pure, so that any pecuUar structure 

 of the clay molecule can scarcely account for its plasticitj'^, though 

 several eminent investigators have laid stress on this suggested cause. 



Several investigators have attiibuted the plasticity to the shape 

 or size of the clay particles. Thus, Aron considered plasticity was 

 due to the particles being spherical, but Zschokke, Biedermann, and 

 Herzfeld dispute this, and attribute it to the presence of fiat and 

 laminated crystals,* a view early put forward by Johnson and Blake, 

 and held later by Bourry", who stated that plasticity becomes greater 

 in proportion as the grains diminish, and that all minerals if reduced 

 . to a sufficiently impalpable powder, will on the addition of a Uquid 

 produce bodies having a certain amount of plasticity. 



Accorduig to Le ChateUer, the lamellar structure and the weU- 

 known capillary attraction are a sufficient cause of plasticity. He has 

 shown that aU plastic masses contain a large proportion of air by 

 comparmg their density with that of clay and water, and that in each 

 plastic mass there are innumerable capillaries of not more than one 

 three-thousandth of an inch in diameter. He concludes that the 

 tension of the menisci between the water-surface and the air-surface 

 in these capillaries explains the toughness of the plastic mass, as the 

 capillary force prevents the mass from breaking up under pressure, 

 but allows the minute particles to slip over each other, and yet adhere 

 so strongly that the mass re tarns the new form when the pressure is 

 removed. In other words, clay is plastic when sufficient water is 



* The particles are so extremely minute that it is exceedingly difficult to 

 ascertain their shape. Le Chatelier has noticed tliat if the material is distiubed 

 when under tlio microscope, the crj^stalline form may be observed for a fraction 

 of a second by polarised liglit if their symmetrical axis is perpendicular to the 

 microscope axis. As soon as they are flat they are isoti-ojjic. 



