is a decorating medium used on ancient Middle Eastern and Mediterranean ceramics, and has experienced a resurgence in popularity in contemporary studio ceramics. "Terra sig" can be made from any kind of naturally occurring clay, mixed as a very thin liquid slip and settled to separate out only the finest particles to be used as terra sig. When applied to greenware (unfired) clay surfaces, terra sig can be polished with a soft cloth or brush to achieve a shine ranging from a smooth silky luster to a high gloss. The polished surface can only be retained if fired within the low-fire range and will lose its shine if fired higher, but can still display an appealing silky quality. Traditionally, terra sig was only used on low-fired earthenware.
Terra sigillata (or "sealed earth") is a name for two distinct items in antiquity. First, it is one term by which Samian ware was known. Secondly, it was a term for a medicinal soil from the island of Lemnos.
The latter was called "sealed" because cakes of it were pressed together and stamped with the head of Artemis. Later, it bore the seal of the Grand Signior. This soil's particular mineralic content was such that, in the Renaissance, it was seen as a proof against poisoning, as well as a general cure for any bodily impurities, and it was highly prized as a medicine and medicinal component.