The role of water films in plasticity is very important, however in addition organic matter both animate and inanimate contributes to plasticity. Bacterias found in water and soil live in clays especially in clay bodies, though they appear to be present only on the surface of freshly mined clay. Clay that has been sterilized does not become as plastic as natural clay on ageing from which it is deduced that bacteria favourably affect ageing. The former practise of adding urine, tannin etc to clay increased bacterial activity and this explains their favourable effect.
The algae that can grow in most clay bodies must also be included among the organic colloids capable of influencing plasticity. The algae alter the composition of air in the pores as the result of their metabolisim and convert some of the alumina into a form soluble in alkalies.
To be contd……….
