Use XX Sagar Clay for a white base, and Red Art and Neuman Red as bases. To make the Sig mix 20 lbs of water with 10 lbs of clay and a tbsp or so sodium silicate. Mix well with a drill then let it sit for at least 1 day until it separates. The heavy particles will sink to the bottom, water will separate out on top, and the thin stuff in the middle is the Sig. siphon off the sig.
Alternatively: here is a description and alternative but similar recipe!
There is lots and lots of info on the internet about terra sig! Do a search.
I found two places that you can buy it....