Terra sigillata red and Greek black are fundamentally the same; in both cases the gloss is obtained by painting or 'slipping' the partly dried pot with a fine suspension of illite clay containing a certain amount of iron. The ware may then be fired either to obtain terra sigillata red, when it is kept from contact with reducing gases, or to obtain Greek black if exposed to reducing gases; in each case the colour is due to an oxide of iron. The important points are, that particular mineralogical species of clay is essential and technologically the process is one of extreme economy and simplicity.