Uses:

Colour modifying

Calcium carbonate (chalk) – is mild, mineral alkali mainly used for modifying water pH in natural dyeing.

It hardens water to facilitate  richer reds with madder. It also brightens  and enhances yellow dyestuff like weld and goldenrod.

Use as an afterbath or directly to the dyebath but take care with protein fibres like wool which can be damaged by high pH values.

Printing

Calcium carbonate is also used with wheatbran to remove gums when printing.