AbstractA Kentucky ball clay and a Georgia kaolin were resolved into seven practically monodisperse fractions, ranging from 0.2 to 20.0 microns in mean size. The distribution of sizes and the physical characteristics of the fractions were determined to place the fractions on a comparable basis. Plasticity, drying shrinkage, and dry strength measurements were made on the fractions. There was no definite relationship between drying shrinkage and particle size, but, in general, the drying shrinkage increased with decreasing particle size. The drying shrinkage measurements, however, afforded a means for computing the thickness of water films at the maximum plasticity developed. This computed thickness was between 90 and 160 Angstrom units at maximum plasticity. The dry strength increased up to a certain limit in direct relation with increased area of surface or decreased particle size. The optimum plasticity value, taken as the product of yield and strain, increased with increasing surface area according to the relation.LogS = AP + B.WhereS= surface available per 100 g. clay.P= plasticity.AandB= constants.The constant,B, is equal to 180 × 104for the kaolinite used in this investigation. This means that there must be more than 180 × 104square centimeters of total surface per 100 grams of clay on the basis of platelike particles before the material will exhibit plasticity. This surface represents a monodisperse system of particles approximately 2 microns in diameter; systems containing particles more than 2 microns in diameter exhibit the Osborne‐Reynolds close‐packing e