128 



but this does not really affect the cause of plasticity. Grinding is not a 

 cause of plasticity, though Johnson and Blake claim to have made a 

 non-plastic china clay plastic by fine grinding. 



It has been suggested by Olschewsky, wlio based his experiments 

 on those of Daubee, that the water used has a chemical action, and 

 that plasticity is due to the formation of a system of capillaries in 

 the clay, a felt-Uke or spongy material being formed, and in this way, 

 the clay particles are able to come into closer contact, owing to the 

 production of a kind of gelatinous or colloidal film, but the presence 

 of an alkaU appears to be essential for this alteration to take place. 

 Thus, Mellof found ground pottery, felspar, and Cornish stone become 

 plastic on heating -with water under pressure to a temperature of 

 300° C. for several days, but china clay and flmt are scarcely affected. 

 The finer a substance is gromid the more complete is its reaction with 

 water, because a small particle has a greater surface in proportion 

 to the water than a coarse one. If the particles are sufficiently fine, 

 water may, indeed, act in a similar mamier to a caustic alkah ; thus, 

 very finely divided siUca becomes colloidal when brought into contact 

 with boiling water, just as coarser particles do when brought into 

 contact with a boiling solution of caustic potash. 



Koerner found that other substances (as alumina) become 

 sufficiently finely divided in water, but their power of cohesion is 

 lost on drying, and suggested that the plasticity may be brought 

 about in a similar manner. This would explain why it is impossible 

 to produce highly plastic clays from kaolin. 



As many organic substances possess certain characteristics of 

 plasticity, several suggestions have been made that these may be the 

 cause of plasticity in clay. It is found, however, that there is no 

 definite relation between the plasticity and the proportion of carbon 

 in the clay, dark coloured clays, rich in carbonaceous matter, being 

 no more plastic than Ughter ones almost free from this material. 



Several observers have suggested that bacteria produce plasticity, 

 but Hecht and Gosmann have not found sufficient data to warrant 

 this suggestion, especially as it has not been found possible to increase 

 the plasticity of clay by inoculation. 



Whenever plastic clay is subjected to pressure it tends to obey the 

 laws of fluids, transmitting its pressure to all parts of its mass 

 and flowing through an orifice through which it can escape, though 

 it is far from being a perfect fluid. From this arises the modern 

 conception of clay as a very ^^iscous liquid in which every particle 

 of solid matter is surrounded by a film of Uquid, so that the particles 

 are virtually m a state of suspension, and hence, that a plastic clay is, 

 at anj" rate in part, in a colloidal condition. 



As far back as 1872 Scldoesing* suggested that the plasticity of 

 clay was due to its colloidal nature, and claimed to have found an 

 amorphous material of the same composition as kaoUn which had 

 all the characteristics of a colloid, and was termed by him argile 

 colloidale. Very Uttle notice was taken of this suggestion or of the 



* This has more recently been confirmed by Cohn and Atterbm-g, who found 

 that precipitated barium sulphate and calcium fluoride are both plastic when 

 fresh. 



