[14] From the Greek πἡλινος = made of clay.
The substances most resembling this 'ideal clay' which have, up to the present been isolated, are:
(a) Kaolinite. Found in a crystalline form in china clays and kaolins ([p. 107]).
(b) Clayite. A material of the same chemical composition as kaolinite, but whose crystalline nature (if it be crystalline) has not been identified—chiefly obtained from china clays and kaolins.
(c) Pelinite. A material similar to clayite, but differing from it in being highly plastic and, to some extent, of a colloidal nature—obtained from plastic clays.
(d) Laterite. A material resembling clayite in physical appearance, but containing free alumina and free silica ([p. 80]).
(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.