AbstractFour Illinois pottery clays, representing two extreme types and two intermediate types, are described. Porosity and thermal expansion tests were made on the raw and fired clays. The temperature of the inversion of low to high cristobalite in these clays is much lower than is usually accepted for the lower limit of this inversion because cristobalite is formed at low temperatures. A limited amount of data was obtained on the volume changes of raw clays during firing, the organic matter content, and thermal dissociation.