Terra sigillata is a very fine slip made from only the smallest clay particles, separated by letting a thin slip settle and siphoning off the top layer. Applied thinly and polished, it gives a silky, low-sheen surface without glaze.
What Is Terra Sigillata?
The name means sealed earth in Latin and comes from Roman red-gloss ware, which had a similar fine surface. To make it, clay is mixed with plenty of water and a deflocculant, then left to settle for a day or more. The coarse particles sink, and the fine, milky top layer is siphoned off and thickened.
Applied to bone-dry or leather-hard work in thin coats and buffed with a soft cloth or plastic bag, terra sigillata gives a soft sheen that becomes almost glossy with polishing. It can be coloured with stains or made from coloured clays.
It works best at low temperatures, because firing too hot causes the sheen to fade as the surface vitrifies. That makes it a favourite for pit, saggar, smoke and raku firing, where its smooth surface takes on carbon and colour beautifully.
Like burnished surfaces, terra sigillata isn’t watertight, so it suits decorative rather than food-contact pieces.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.