Samples of 5th- and 4th-century BC black gloss, and for comparison a 5th-millennium BC black glaze, have been analyzed with an electron microprobe. The compositions of both the glaze and the underlying body were determined. The resulting data and the few other glaze/body data in the literature are compared with results of separating fine glaze fractions from Attic and Italian clays and replicating the black gloss process. Results indicate that the Greeks and Romans did more than merely separate a fine clay fraction for the gloss. They may have added potash and iron oxide, or selected special glaze clays for the purpose. The chemical technology of glaze paint preparation during the 5th and 4th centuries BC must have undergone considerable empirical development.