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pottery is subjected in order to achieve its final character of permanence and rigidity in the body and of vitrification of the glaze with the development of colour. The tem- perature range in which chemical and physical changes may proceed in a clay body extends from atmospheric temperature to 1,850 C., at which point the most re- fractory clays are affected. At ordinary temperatures we have colloidal action, at 200 the changes induced by the so-called pre-heating treatment, at 440 to 500 dehydration has almost ceased, and plasticity is lost beyond recovery. According to Witt and Knote, A1 2 3 . 2Si0 2 . 2H 2 decomposes on dehydration into water, and the silicates A1 2 3 . Si0 2 and A1 2 3 . 3Si0 2 or some higher silicate, and not into the monosilicate and free silica as formerly sup- posed. Mellor states, however, that the endothermal change shown at 500 corresponds with the decomposition of kaolinite into free silica, free alumina and water. Pro- ceeding to higher temperatures the density decreases, but suddenly increases at 950, where an exothermic re- action obtains. Thence changes occur in clay bodies which are much the same in all, but take place at different tem- peratures according to the refractoriness of the body. But under sufficiently severe heat every body assumes a crystalline structure similar to the sillimanite in porce- lain. At still higher temperatures vitrification proceeds to a finish, accompanied by an increase in specific volume. The melting point of a body or glaze depends chiefly