Terra sigillata recipe:
400 gram clay
2 gr sodium silicate
2gram soda ash
1 liter distilled, rain or ac condenser water.
Mix well with blender. Put in a transparent plastic bottle for 7 days, without moving, puncture a hole 1 cm under water separation line, let water out, pour the middle into a container, discard the sediment. Measure specific gravity to be 1.15 to 1.2, add water or leave to evaporate if needed.
Typical clay to use:
Ball Clay English or OM-4
any earthenware clay. Dry make powder and use