130 



(ii) The extremely small size of the particles, their lamellar 

 shape, large surface (due to their porosity), and (possibly) their 

 fissile character. In such small particles, the phenomena of 

 cohesion are quite different from those in larger jDarticles. 



(iii) The hydrolysing action of water on the particles and the 

 probable production of inorganic colloid matter. If this is absent 

 or neutrahzed by hydrogen-ions added purposely or occurring 

 naturally or through fermentation of the organic matter in the 

 clay, the plasticity will continue to increase until an excess 

 of hydroxyl-ions is again produced; when the concentrations 

 of hydroxyl-ions is large, the negatively charged clay particles 

 will go into suspension. As the extent to which water can be 

 dissociated is very limited, the plasticity of the clay can only 

 be increased at so slow a rate that it is unlikely that sUghtly 

 plastic clays (kaolin) can ever be made highly plastic by artificial 

 means, though the increase in plasticity may be sufficient to 

 show the nature of the reactions which take place. 



(iv) The presence of organic colloid matter due to impurities 

 in the clay, or added purposely, may still further increase the 

 plasticity. 



(v) The presence of minute quantities of soluble salts may 

 exercise a pronounced effect on the plasticity. Their action has been 

 mentioned under Viscosity (p. 121). Plasticity appears to be a 

 resultant of several properties (see also Cohesion, Adsorption, Tensile 

 Strength, Binding Power, &c.). 



To increase plasticity. — The Hmits mthin which the plasticity of 

 clay may be mcreased by the addition of soluble salts are very small, 

 but there is such an abundance of naturally plastic clays that it is 

 only where materials of exceptional purity are required that an 

 increase in plasticity is desirable. 



A small increase in plasticity may be obtained : — 



(1) By increasing the hj^drogen-ions in the material, by 

 alloA\ing the organic matter in the clay to decompose (ferment) 

 and become acid, by adding weak acid, or by keeping the clay 

 in intimate contact with fresh water by stirring the two together. 

 This appears to hydrolyse the clay and forms colloid matter on 

 the particles. It is important to have the particles of clay 

 as small as possible in order to faciUtate the hydrolysis. If 

 water alone is used for this purpose, the clay must be allowed 

 to stand until fermentation of the organic matter begins, and 

 the mass reacts faintly acid. In any case, the time required 

 for an appreciable increase in the plasticity may be several 

 years. No addition of any electrolytes or substance other than 

 plastic clay can increase the true plasticity of a paste chiefly 

 composed of non-j)lastic materials. Many of the so-called " lean 

 clays " are of this nature; they are rich in inert matter, but 

 the proportion of coUoidal matter in them is very small. Such 

 clays can only be made more plastic by removing a large 

 proportion of the inert matter naturally present in them or by 



