Terra sigillata

Back in 2021 I had been making burnished earthenware teapots for a while, and I wanted a more efficient way to create a smooth & simple surface for my pots.

Some burnished pots

I found the recipe for “Super-Refined Terra Sigillata” by (my hero) Tony Hansen, followed it, and liked the results.

Terra sigillata (“sealed earth”) is a clay slip consisting of very fine particles. Because the particles are so small, the slip layer dries to a smooth surface that is nice to touch and reasonably water-repellent. Coarse clay particles are removed through sedimentation.

Fine clay slips were first used in the Middle East ca. 1000 BC, and later on Greek and Roman pottery. Knowledge about the terra sigillata technique was lost during the middle ages, but rediscovered in the 19th century by the German chemist Henrich Schumann, who was seeking low-cost coatings for sewer pipes. (My favourite ceramics story.)

Slipware bowl (vase of Susa)

Made in the fifth millenium BC, in present-day Iran.

Currently in the collection of the Musée d’Archéologie nationale in Saint-Germain-en-Laye, France.

I am still using the sig batch I made in the spring of 2022, but I keep experimenting with it. My latest project revolves around reduction firing (in my electric kiln, using a saggar), which turns red earthenware clay black.

Test tiles

Same clay, same sig, same firing temperature (1125ºC). The black tile is fired in reduction (= a lack of oxygen in the kiln), the red tile in oxidation (= plenty of oxygen in the kiln).

To polish or not to polish?

Before firing, the difference is astonishing. After firing (1125ºC) it is visible to (the attentive) eye, but too subtle for the camera.