AbstractThe mechanical properties of plastic clay masses were investigated in terms of the fundamental concepts, stress and strain, and the interpretation involved the general terms, plasticity and workability. Compressive forces, applied by means of newly designed apparatus, were used throughout. A description is given of the apparatus as well as of the results for twelve different bodies, five individual clays, and five clayflint mixtures. In addition to composition, the factors investigated were the effect of water content, aging, rate of stress application, and cyclic loading. It was found that “moisture equivalent,” that is, the moisture content required for an arbitrarily selected equilibrium stress value, is a satisfactory parameter for the comparison of the plastic properties of clays and of similar body compositions. Aging increased the plasticity of ball clays progressively, whereas the china clays and a porcelain body became more workable. Increasing the rate of stress application caused progressive increases in the equilibrium stress. On the basis of the results obtained, a practical method of distinguishing between plasticity and workability is presen