THE CONDITIONS GOVERNING THE DIFFUSION OF WATER IN CLAYS*

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Abstract
Drying tests were made on Florida clay under various conditions of temperature, relative humidity, and thickness of the specimen and on packed grains of Indiana shale of various grain sizes and water contents. The results show the effects of these variables on the rate of drying and indicate that the rates of evaporation are less than would be expected for free liquid evaporation; that no initial constant rate of drying, as proposed by Sherwood and his associates, exists in the drying of clays; that the diffusion coefficient in the diffusion equation is not a constant, as assumed; and that the equation, as used by Sherwood, Newman, and others, is not applicable to the drying of clays. In general, the drying of clays may be said to take place in two stages, an initial stage during which the rate of drying decreases slowly and approximately linearly until shrinkage is completed at the surface, and a second stage during which the rate of drying falls off rapidly because of increased resistance to diffusion of liquid and vapor from the interior of the ware. The rather sharp discontinuity separating the two branches of the rate curve has been called the critical point or the surface shrinkage limit.

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