Fabric Dyeing: Calcium Carbonate (also referred to as chalk) is a white powdery mineral that is a common addition to the madder dye bath to deepen shades. It is also one of the principle minerals that create hard water.
For best reds, 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 (clear, non-sudzing) to your dye bath until the colour of the solution is reddish rather than orange.