This is finely powdered, fully dissolvable, pure Calcium Carbonate.
Chalk is a very important assist in the production of a good bright yellow with weld on silk and wool. It also helps develop a good red with Madder on Alum mordant. In both cases, just a teaspoon per pound fabric is usually enough to appreciably improve the color. Do not heat silk with chalk in the bath, but add the chalk at the end of heating, then let the fabric sit in the dye bath overnight.