The results of recent studies of the gloss coating on Samian and Attic pottery are reviewed. The application of the gloss as a slip or suspension of predominantly illitic clay particles offered the advantages of plabey particle morphology, to produce a smooth surface, and high K₂O content, inducing vitrification. High quality glosses have up to 10% or more K₂O, indicating they are almost entire illite; iron oxide contents are 9-15 wt%. Poor quality glosses have low K₂O, but high CaO.