Calcium Carbonate

Calcium carbonate (chalk) is a dye assist, not a mordant. Natural dyers use it in two ways: to add calcium to soft water so that madder and weld give deeper, truer colors, and as a chalk bath that fixes aluminum acetate and printed mordants on cotton and linen.

Sizes: 50 g and 400 g.

How much calcium carbonate to use

• In a madder dye bath: 2–6% WOF (2–6 g per 100 g of fiber) if your water is soft. It gives deeper, redder madder.
• In a weld dye bath: 4–8% WOF, added with the weld from the start, for stronger, brighter yellows.
• Chalk bath (fixing): 10 g per liter of warm water. Soak fabric for about 10 minutes after mordanting with aluminum acetate or after printing a mordant paste.

How to use it

1. Mix the weighed powder with a little water into a smooth slurry; it does not fully dissolve.
2. For dye baths, stir it into the water before adding the dye and fiber.
3. For a chalk bath, stir it into warm water, add the mordanted fabric, move it for about 10 minutes, then rinse and dye.

When not to use it

• If your tap water is already hard (lots of limescale), you usually do not need it.
• Do not use it as a mordant: it does not fix dyes on its own. For that, use alum.
• Blackboard chalk, climbing chalk and garden lime are different compounds and are not substitutes.

Related: How to dye with madder · Weld dye guide · Mordanting cotton and linen

Frequently asked questions

Is calcium carbonate a mordant?

No. It is a dye assist. It improves madder and weld colors in soft water and fixes aluminum acetate mordant, but it does not fix dyes by itself.

Do I need chalk for madder?

Only if your water is soft. In soft water, 2–6% WOF calcium carbonate gives deeper, redder madder.

What is a chalk bath?

A bath of about 10 g calcium carbonate per liter of warm water, used for 10 minutes after aluminum acetate mordanting or mordant printing to fix the mordant.

Read the guide: Soda ash, chalk and gum arabic guide

Work out exact grams for your fabric with our free natural dye calculator and WOF chart.