Abstract For over two millennia, clays with perceived medicinal or alexipharmic properties have been recovered in bulk, processed into small troches or pastilles and stamped with a device or ‘seal’ as an indicator of their origin; this practice lent them their commonly applied name – terra sigillata or sealed earth. The first records are confined to the Mediterranean and Aegean regions, but early in the post-medieval period other sources in central and northern Europe came to be exploited. The history of this process of expansion is traced, the principal products of the major sources are identified by their respective seals, and some assessment is made of the validity of claims made for the effectiveness of such clays.