Mordants for Natural Dyeing: The Complete Guide

Most natural dyes need a mordant to give colors that last. A mordant is a metal salt, usually aluminum or iron, that binds the dye to the fiber and improves light and wash fastness. This guide explains which mordants to use, how much, and how the process differs for wool and silk versus cotton and linen.

Tools: get exact grams of alum, tannin, iron and soda ash with our free natural dye calculator. For more detail, read our guides to natural tannins, iron mordant and soda ash, chalk and gum arabic.

What is a mordant?

The word comes from the Latin mordere, "to bite". A mordant forms a bridge between the fiber and the dye molecule, so the color holds on through washing and sunlight. Without one, many dyes simply wash out or fade quickly. A few dyes, such as indigo and some tannin-rich dyes, do not need a mordant; see 4 natural dyes that don't need a mordant.

All mordant amounts are given as a percentage of WOF (weight of fiber): the dry weight of your fiber or fabric. For 100 g of wool, 15% WOF alum means 15 g.

Protein fibers vs. cellulose fibers

Mordanting methods for the two fiber groups are not interchangeable:

• Protein (animal) fibers such as wool and silk take up alum easily. They are usually mordanted with alum and cream of tartar in a warm bath. Read how to mordant wool and silk.
• Cellulose (plant) fibers such as cotton, linen and hemp bind alum poorly. They need a tannin first, then alum (or aluminum acetate). Read how to mordant cotton and linen.

The main mordants and helpers

Alum (potassium aluminum sulfate)

The most widely used mordant for all fibers. It is inexpensive, safe to use with basic care, keeps colors clear and bright, and improves fastness for nearly every natural dye. Use 10–15% WOF for wool and silk and about 15% WOF for tannin-treated cotton and linen. Our alum mordant is refined and free of iron impurities. Full recipes are in our alum mordant guide.

Other aluminum mordants

• Aluminum acetate: often preferred for cotton and linen, and the standard mordant for printing with natural dyes. Use 5–8% WOF. Follow it with a chalk (calcium carbonate) bath to fix the mordant and remove any unbound aluminum. You can make a substitute at home: for 1 kg of fabric, dissolve 150 g sodium acetate (or calcium acetate) and 150 g alum in 3 liters of hot water and add this to your mordant bath.
• Aluminum sulfate: contains more aluminum by weight than alum, so you need less of it.
• Aluminum formate: a gentle, cold-process mordant that can be used on wool, silk, cotton and linen.

Iron (ferrous sulfate)

Iron is used in very small amounts: 0.25–2% WOF as an afterbath, or 2–4% WOF as a mordant on cotton and linen (never more than 4%). It "saddens" colors, shifting them toward grays, greens, purples and blacks, and turns tannins gray to black. Too much iron makes fibers brittle, and traces of it on equipment darken later dye baths, so keep separate tools for iron. See madder purples with iron.

Tannins

Tannins are not metal salts, so they are not true mordants, but they are an essential part of mordanting cellulose fibers. They also deepen colors and add fastness. Common tannins are oak gall, sumac, pomegranate and myrobalan, typically at 8–10% WOF. Learn more in our oak gall tannin guide or shop natural tannins.

Mordant assists

• Cream of tartar: added to alum baths for wool and silk (about 5–7% WOF) to keep fibers soft and colors bright. See our cream of tartar guide.
• Soda ash: used to scour cotton and linen, and in small amounts (about 2% WOF) with alum for cellulose fibers.
• Calcium carbonate (chalk): used after aluminum acetate, and to harden soft water for madder and weld.

Mordants to avoid

Some old recipes call for heavy-metal mordants such as chrome, copper or tin. We do not recommend them: they can be toxic and are difficult to use and dispose of safely. Most of the colors they give can be achieved with alum, iron, overdyeing or a change in method. For a plant-based option, some dyers use Symplocos, a plant that naturally accumulates aluminum.

Which mordant gives which result?

The same dye can look very different with different mordants. Our comparison of mordant methods with buckthorn dye shows the effect side by side.

Safety

Wear gloves and a dust mask when handling mordant powders, use pots and tools only for dyeing, keep everything away from children and pets, and dispose of spent baths according to your local rules.

Frequently asked questions

Do all natural dyes need a mordant?

Most do. Indigo and some tannin-rich dyes are exceptions.

Which mordant is best for beginners?

Alum. It works on all fibers, is affordable and is easy to use. Our cellulose mordant studio kit has everything you need for cotton and linen.

Can I reuse a mordant bath?

Yes. An alum bath still contains some mordant after use. Top it up with about half the original amount of alum for the next batch.

Shop: Alum & iron mordants · Dye assists & modifiers · All natural dyes & mordants