_Exposure_ to the action of air and frost has a marked effect on many clays. When freshly dug these may be hard and difficult to crush, but after exposure they break up readily into small fragments. Clays differ greatly in the extent to which they are affected by exposure; some are completely disintegrated by standing 48 hours in the open air, whilst others are scarcely affected by exposure in bleak places through several years of storm, sunshine and frost. Usually, however, the effect of a couple of nights exposure to hard frost will produce a marked disintegration of the material. This process of exposure is known as 'weathering' and its effects are so important that it is employed whenever possible for clays requiring to be crushed before use. All clays are rendered more workable by exposure, but some of them are damaged by the oxidation of some impurities (_e.g._ pyrites) in them, though in other clays this very oxidation, if followed by the leaching action of rain, effects an important purification of the material. Weathering appears to have no effect on the chemical composition of the particles of true clay in the material, though it may decompose the impurities present. On the clay itself its action is largely physical and consists chiefly in separating the particles slightly from each other, thereby enabling water to penetrate the material more readily and facilitating the production of a plastic paste. The disintegrating action of the weather on some 'clays' is so complete that they require no crushing but can be converted into a homogeneous paste by simply kneading them with a suitable proportion of water. It is possible that on exposure to the heat of the sun's rays--particularly in tropical climates--some chemical decomposition of the clay may occur, but compared with the purely physical action of weathering the amount of such chemical decomposition must be relatively unimportant in most cases. It may, however, account for the presence of free silica and free alumina in some clays. The action of the weather on rocks, resulting in the formation of clays, is described in Chapter III.