139 



The effects of age on a clay paste are sinailar to those on colloidal 

 gels, provided the conditions of storage (including the low temperature 

 and a sufficiently humid atmosphere) are favourable. The plasticity 

 of the clay is slightly increased and the colloidal properties are more 

 marked. 



From the foregoing, there appears to be a close parallelism between 

 the more important properties of plastic clays and those of other 

 colloids, but the question still remains as to Avhether these colloidal 

 properties are due to the nature of an essential constituent of clays 

 (clay substance) or to other colloidal substances which may be present. 



Other Colloids in Clay. 



The most important colloidal substances which are known definitely 

 to exist in some clays are : — • 



Colloidal silica, which may exist in the form of a siUca hydrogel, 

 with or without occluded silica hydrosol, the latter being confined 

 to any liquid portions of the clay paste, but distributed more or less 

 uniformly throughout the clay slip. Various mineral forms of silica 

 which are hydrogels are known ; they possess the anticipated 

 properties of inorganic gels and their nature is fairly well known. 

 A small percentage of colloidal silica may be extracted by boiling 

 some clays with water, with or without the addition of a little sodium 

 carbonate, but in no clay of commercial importance is the amount 

 of silica obtainable so large as to account for the whole of the 

 plasticity of such clays, though it may partly do so. 



Wolfgang Ostwald" holds the view that silicic acid sols are hydrated 

 emulsoids, i.e., the dispersed silicic acid is a liquid and not a solid. 

 Like other known emulsoids, its viscosity is high and rises very 

 rapidly after a given concentration whilst in suspensoid sols the 

 increase of viscosity is steady throughout and the viscosity is very 

 low. He also states that the silicic acid gel differs from the better 

 known organic emulsoids in possessing little elasticity. 



Attempts to increase the plasticity of sand by mixing it with silica 

 hydrosol, and coagulating the latter, do not produce a material at 

 all closely resembling plastic clay. Such a mixture, when dry, is 

 deficient in strength and even in its most plastic state it is inferior 

 to clay for modelling purposes. A still more striking difference 

 between such a mixture and a clay is that when both are dried at 

 105° C. and afterwards mixed \\dth water, the clay forms a plastic 

 paste with all its original modelling power restored, but the silica- 

 sand mixture is not " workable " or truly plastic. 



Silica gels have a pecuhar property of varying according to their 

 age as well as according to the mode of formation. When a clay 

 is soured, the proportion of silica gel tends to increase, but if soured 

 too long the silica gels grow together forming larger particles and 

 cause a diminution in the plasticity of the mass, the vapour pressure 

 in the old gel being greater than in the new one. 



A. Cushmann** found that the addition of dried colloidal silica 

 gel to a dry clay, the mixture being afterwards made into a paste 



