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

Red Art

Gold Art

German GRT

any earthenware clay. Dry make powder and use

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