Pour the 28 pounds of water into a 5-gallon bucket. Add the 14 pounds dry clay (OM 4 ball clay works well for a white base). Add enough sodium silicate to deflocculate (a few tablespoons). Allow to settle for 2 hours, then remix and let settle again for 24 hours. Siphon out the middle of the bucket (this is the terra sigillata) without disturbing the sludgy mixture below or the water on top. I usually get about 2 gallons of terra sigillata per batch. Throw the rest away; do not reclaim. Terra sigillata is best when the specific gravity is about 1.15, and the useful range is between 1.1–1.2.
Specific gravity is measured by weighing out 100 grams of water, marking the volume on the container, pouring out the water, and then weighing the same volume of the terra sigillata. Divide the weight of the terra sigillata by 100 to get the specific gravity. If it is too thin, evaporate some of the water. If it is too thick, allow it to settle longer. To use, apply 2–3 thin coats of terra sigillata to bone-dry greenware and then buff it.
Suggestions for adding colorants:
Combine 1 cup of terra sigillata to 1–3 tsp of Mason stains (MS). Add a few sprinkles of Epsom salts to help prevent the stain from settling.
This recipe was shared by Mark Arnold in the September 2022 issue of Ceramics Monthly.