A determination of the firing temperature of ancient pottery may be of value in its thermoluminescent dating. Such determination requires an understanding of the thermal behavior of clays, which is different for the three major clay groups: kaolins, montmorillonites and micas. The first change on heating is loss of hydroxyl ions. At higher temperatures (over 1000°C) recombination of ions to form new minerals occurs. The original mineralogy of fired clays can sometimes be deduced by a study of the new minerals formed by heating to over 1000°C. -- AATA