This cup was made from a special clay from the island of Lemnos in the Aegean, thought to have health benefits including offering protection against poison. The clay was used to make drinking vessels in prehistoric times, and people even ate the clay itself.The use of clay to counteract possible poison actually has a very sound scientific basis (although prior to the 20th century the mechanism was not known). Many clays throughout the world are able to act as a de facto cation exchange resin, absorbing metallic ions. Having the clay in one's stomach prior to ingesting poisoned food would in fact offer a measure of protection - at least against arsenic and other heavy metal poisons. I should think it would be less effective when fashioned into a vessel.
The Ottomans conquered Lemnos in 1453, and the Ottoman governor of the island presided over an annual ceremony to dig up the clay on 6 August each year. This may have been a revival of the tradition from Antiquity, or the clay may have been in continuous use on the island. Because it was only excavated for 6 hours per year, the clay was very rare, and so vessels made from it were marked with a special seal to prove that they were genuine. The seal can be seen at the base of the handle of this cup. The Latin word for a seal - sigillum - gave these vessels the name terra sigillata or 'sealed earth'. Wares made from this clay are also known as Terra Lemnia after the island of Lemnos.
Red clay from Lemnos was particularly prized, and was used at the Ottoman court and even shaved into the Sultan's food. Whiter clay was used to make vessels like this for sale in the Istanbul bazaar.
The modern relevance is that persons whose cultural background results in their developing the habit of eating clay can put themselves at substantial risk of zinc deficiency. If I remember correctly, the first clinical reports of zinc deficiency came from Turkey, not far from the region where this vessel was fashioned, but clay-eating is a form of pica which also used to be common in the rural southern United States.
Via A London Salmagundi.