The comparison of chemical compositions of ceramic bodies of nine Byzantine plates kept in the Musée national de céramique, Sèvres, with those of reference archaeological samples enabled us to associate most of the plates to the 12th-13th century production of Chalcis. Two pieces, however, raise the question of authenticity. Geochemical perturbations related to prolonged immersion in a marine environment are also discussed and a specific methodology is proposed.