Let clay slake down in water, mix well for 5–10 minutes; you cannot mix it too much. Add sodium silicate (0.5% of dry weight of clay. The amount of deflocculant may vary depending on the clay). Mix well again. Set bucket off the floor about 6 inches. Let sit undisturbed, covered for 8 hours (the amount of settling time may vary depending on the clay). Siphon off center layer. Note: I find that this sigillata mostly settles into just two layers rather than the more typical three layers. If that is the case for you, too, use the top layer.
For an OM 4 Base Terra Sigillata, use 1 part OM 4 Ball Clay and 2 parts water. I use 20 pounds of water and 10 pounds of OM 4, then add 22.7 grams of sodium silicate. This yields about 2 gallons of terra sigillata. Let base settle for exactly 8 hours, then siphon off center layer. Brush onto bone-dry clay. This recipe was originally sourced from Kari Radasch.
This recipe was shared by Emmett Freeman in the January 2020 issue of Ceramics Monthly.