up 1976 download from fileplanet

can be estimated by their adsorption of dyes, such as malachite green, and that when this is done the plasticity of a clay varies with its relative colloidal content. The increase of plasticity caused by the addition of caustic soda is explained by saying that enough of the gel is con- verted into sol to make the slip appreciably thinner, so that it can be poured freely. If the alkali be neutralised by acid, the mass again stiffens by gel formation. The soluble salt theory was advanced by Purdy and Moore in 1907, but Seger had several times noted that there seems to be a close relationship between the amount of soluble salts in a clay and its plasticity. He found, for example, that bricks made of plastic clays are peculiarly liable to efflorescence of salts on their surfaces. According to this theory, the adsorbed or otherwise held salts form, on addition of water, a viscous solution round the grains of clay, resulting in an enveloping liquid medium of high surface tension. To account for the fact that flint or other minerals of surface factor equal to that of clay do not develop plasticity when soluble salts are added, it is supposed that clay particles have an adsorptive power far in excess of that of any other mineral substance. A third theory of the cause of plasticity is that of Grout, which involves the notion of a molecular attraction between 16 CERAMIC CHEMISTRY. the clay particles and the water. When clay is made into a slip, each grain is enveloped in a film of water. If now