Terra Sigliata, meaning sealed earth, is a pot finish as old as clay. It's really clay on clay, a slip made with a fine-grained mix of earth and water. The mix is allowed to settle out until only the top layer with the finest grains of clay remains.
I've only done a couple of terra-sig pieces and that was in a workshop using gas reduction. It was interesting,
The blue flashes on this tile were the result of an application of a final layer made up of a mix of chicken feed and chicken poop. Who knew?
After the tile came out of the kiln, I applied brush strokes of black paint.