_Felspar_, _mica_ and other adventitious silicates occur in many natural clays in so fine a state of division that their removal would be unremunerative. In addition to this they act as fluxes when the clays are heated in kilns, binding the less fusible particles together and forming a far stronger mass than would otherwise be produced. Consequently, they are valuable constituents in clays used for the manufacture of articles in which strength or imperviousness is important. If these minerals are present in the form of particles which are sufficiently large to be removed by elutriation in the manner described on the previous page, the purification of the clay is not difficult. Usually, however, the most careful treatment fails to remove all these minerals; their presence may then be detected by microscopical examination and by chemical analysis. For most of the purposes for which clays are used, small proportions of these silicates are unimportant, but where clays of a highly refractory nature are required; and for most of the purposes for which china clays (kaolins) are employed, they must not be present to the extent of more than 5 per cent., smaller proportions being preferable.
[Illustration: Fig. 1. Quartz crystals, natural size. (From Miers' Mineralogy _by permission of Macmillan & Co._)]
_Oxides_, _sulphides_, _sulphates_ and _carbonates_ of various metals form the third class of impurities in clays. Of these, the most important are calcium oxide (lime), calcium carbonate (chalk and limestone), calcium sulphate (gypsum and selenite), the corresponding magnesia, magnesium carbonate, and sulphate, the various iron oxides, ferrous carbonate and iron sulphides (pyrite and marcasite) (p. 13).
Potash and soda compounds are commonly present as constituents of the felspar, mica, or other silicates present, and need no further description, though small proportions of _soluble salts_--chiefly sodium, potassium, calcium and magnesium sulphates--occur in most clays and may cause a white scum on bricks and terra-cotta made from them.
_Lime and magnesia compounds_ may occur as silicates (varieties of felspar, mica, etc.), but their most important occurrence is as chalk or limestone. _Chalk_ is a constant constituent of malms[2] and of many marls, but the latter may contain limestone particles. _Limestone_ occurs in many marls and to a smaller extent in other clays. In the boulder clays it frequently forms a large portion of the stony material. If the grains are very small (as in chalk), the lime compounds act as a flux, reducing the heat-resisting power of the clay and increasing the amount of vitrification; they produce in extreme cases a slag-like mass when the clay is intensely heated. If, on the contrary, the grains are larger (as frequently occurs with limestone), they are converted into lime or magnesia when the clay is 'burned' in a kiln, and the lime, on exposure to weather, absorbs moisture (_i.e. slakes_), swells, and may disintegrate the articles made from the clay. Limestone (except when in a very finely divided state) is almost invariably objectionable in clays, but chalk is frequently a valuable constituent.