An extensive study of a number of clays from Greece and England has been undertaken. Mössbauer and magnetic susceptibility measurements have been performed for the raw clays and samples of them, fired at 700°C, 900°C, and 1080°C. The effort of this work is concentrated on studying the behavior of the clays during firing in relation to their chemical composition. A preliminary classification from the Mössbauer parameters allows a characterization of two groups according to the calcium content of the clays. The results of this study are expected to allow a provenance characterization of ancient pottery and to give information on the firing temperature and atmosphere employed in their manufacture. Comparative data between clays fired in the laboratory and ancient pottery are presented. -- AATA