=Commercial china clays and kaolins= in the United Kingdom are not simple natural products but, in the state in which they are sold commercially, have all been subjected to a careful treatment with water, followed by a process of sedimentation whereby the bulk of the impurities have been removed. According to the extent to which this treatment has been carried out, they will contain 10 per cent. or more mica and quartz, with little or no tourmaline, felspar and undecomposed granite. In some parts of Europe and America, kaolins are found in a state of sufficient purity to need no treatment of this kind unless they are to be used for the very highest class of wares. [Illustration: _Magnified 220 Diameters_ _Magnified 920 Diameters_ _Magnified 220 Diameters_ _Magnified 220 Diameters_ Crystals of Kaolinite _Magnified 920 Diameters_ Crystals of Secondary Muscovite. Fig. 17. Kaolinite and Mica. (_After G. Hickling_ (36).)] Mica is usually the chief impurity as its particles are so small and their density resembles that of the purified china clay more closely than do the other minerals. In commerce the term _china clay_ is almost invariably used to denote the washed material obtained from the 'china clay rock,' but at the pits the word 'clay' is used indiscriminately for the carclazite (p. 78) and for the material obtained from it. As the term 'kaolin' is used indifferently abroad for the crude 'deposit' and for the purified commercial article, it should be understood that the following information relates solely to the substance as usually sold and not to the crude material. Commercial china clay or kaolin is a soft white or faintly yellowish substance, easily reduced to an extremely fine powder, which when mixed with twice its weight of water will pass completely through a No. 200 sieve. Its specific gravity is 2·65, but the minuteness and nature of its smallest particles and their character are such that it will remain in suspension in water for several days; it thus appears to possess colloidal properties, at any rate so far as the smaller particles are concerned. It is almost infusible, but shows signs of softening at 1880° C. (Seger Cone 39) or at a somewhat lower temperature, according to the proportion of impurities present. When heated with silica or with various metallic oxides it fuses more readily owing to the formation of silicates. China clays and kaolins are not appreciably affected by dilute acids, but some specimens are partially decomposed by boiling concentrated hydrochloric acid (26) and all are decomposed by boiling sulphuric acid, the alumina being dissolved and the silica liberated in a form easily soluble in solutions of caustic soda or potash. This has led to the conclusion that some kaolins may have been produced by weathering, as the bulk of true kaolinitic clays (such as Cornish china clay) is not affected by boiling hydrochloric acid (p. 81).