Plasticity is diminished by heating clays, and whilst much of it may be recovered if the temperature has not risen above 400° C. it cannot be completely restored. Moreover, a clay which has once been heated to a temperature above 100° C. dries in a somewhat different manner to a raw clay. This is well shown in fig. 5 in which are summarized the results obtained by A. V. Bleininger on a sample of ball clay from Dorset before heating and after portions of it had been heated for 16 hours to 200°, 250°, 300°, 350° and 400° C. respectively. It is not impossible that if subjected to the influence of water for a sufficiently long time the whole of the plasticity of a heated clay may be restored, providing that the temperature has not been sufficient to cause a destruction of the clay molecule, but as this resumption requires a certain amount of time, Bleininger has proposed to use the reduction in plasticity effected by the heating to enable excessively plastic clays to be worked without the necessity of adding non-plastic material to them. If any destruction of the clay-molecules has occurred, the plasticity of that portion of the clay can never be restored. The _binding power_ of clays is a characteristic closely connected with plasticity and occasionally confused with it. All plastic clays have the power of remaining plastic when mixed with materials such as sand, brick-dust ('grog') and other materials which are quite devoid of plasticity. The extent to which a clay can thus bind other materials together into a plastic mass depends, apparently, on the plasticity of the clay itself and on the size and nature of the particles of the added material; the more plastic the clay the larger will be the amount of material it can thus 'bind,' and the finer the latter the more easily will it form a strong material when mixed with a plastic clay. Rohland (5) has shown that the binding power of clay is not alone due to its cohesion, but that it is closely associated with the colloidal nature of plastic clays: 'fat' clays being those which are highly colloidal, highly plastic and possessing great binding power, whilst 'lean' clays are those deficient in these characteristics. The fact that, as a general rule, the dark coloured clays possess the most binding power, confirms this suggestion, as the dark colour is largely due to organic materials, probably in a colloidal state.