Calcium carbonate for natural dying
Calcium Carbonate is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. It is a white powdery mineral that is a common addition to the dye bath to deepen shades. It is also one of the principle minerals that create hard water.
For best reds from Madder, when dyeing cotton, Calcium Carbonate can be added to the dye bath. This binds with the Madder and enriches the colour. For wool or silk, add some ammonia to your dye bath until the colour of the solution is reddish rather than orange.
It can also be added to paint to give body and density, as well as provide a white pigment on its own. It is widely available.