(_e_) _Clay Substance._ A general term indicating any of the foregoing or a mixture of them; it is also applied (unwisely) to the material obtained when a natural clay is freed from its coarser impurities by elutriation (p. 7). The Chief Characteristics of 'True Clay' from Different Sources. In so far as it can be isolated _true clay_ appears to be an amorphous, or practically amorphous, material which may under suitable conditions crystallize into rhombic plates of kaolinite. The particles of which it is composed are extremely small, being always less than 0·0004 in. in diameter. They adsorb dyes from solutions and show other properties characteristic of colloid substances though in a very variable degree, some clays appearing to contain a much larger proportion of colloidal matter than do others. To some extent the power of adsorption of salts and colouring matters appears to be connected with the plasticity (p. 41) of the material, but this latter property varies so greatly in clayite or pelinite from different sources as to make any generalization impossible. True clay substance appears to be quite white, any colour present being almost invariably traceable to ferric compounds or to carbonaceous matter. The latter is of small importance to potters as it burns away in the kiln. The specific gravity of clay substance is 2·65 according to Hecht, the lower figures sometimes reported being too low. Its hardness is usually less than that of talc--the softest substance on Mohs' scale--but some shales are so indurated as to scratch quartz. It is quite insoluble in water and in dilute solutions of acids or alkalies, but is decomposed by hydrofluoric acid and by concentrated sulphuric acid when heated, alumina entering into solution and silica being precipitated in a colloidal condition. It absorbs water easily until a definite state of saturation has been reached, after which it becomes impervious unless the proportion of water is so large and the time of exposure so great that the material falls to an irregular mass which may be converted into a slurry of uniform consistency by gently stirring it. With a moderate amount of water, pelinite develops sufficient plasticity to enable it to be modelled with facility, but clayite and some specimens of pelinite are somewhat deficient in this respect. The pelinitic particles usually possess the capacity to retain their plasticity after being mixed with considerable proportions of sand or other non-plastic material and are then said to possess a high binding power (p. 28).