AbstractA large number of refractory clays show secondary or after‐expansion with a marked increase in volume, usually occurring in the range from cones 6 to 14. Data presented in this paper were secured in an investigation made by the Ohio State University Engineering Experiment Station on twenty‐one representative clays from Missouri, Ohio, Kentucky, and Pennsylvania. The findings indicate that the expansion is caused by opening up the natural lamination or bedding planes in the clay grains. Two methods for reducing or eliminating the expansion are given. Data concerning chemical analyses, petrographic examinations, and X‐ray studies are presented.