How to Mordant Cotton and Linen (Cellulose Fibers)

Cotton, linen and hemp are cellulose (plant) fibers. They do not bind natural dyes or alum as easily as wool and silk, so they need a little more preparation. This guide explains the two main ways to mordant cotton and linen: tannin and alum, and aluminum acetate.

Why cotton and linen need special treatment

A mordant is a metal salt that binds the dye to the fiber and improves wash and light fastness. Protein fibers grab alum readily, but cellulose fibers hold on to very little of it. A tannin treatment first gives the alum something to bond to, and the result is deeper, more even and longer-lasting color. For an overview of all mordants, see our guide to mordants.

Step 1: Scour the fabric

Scouring removes the natural waxes, starch and finishing residues in cotton and linen, which otherwise cause patchy dyeing. Simmer the fabric for 1–2 hours with soda ash (about 2–5% WOF) and a little pH-neutral soap, then rinse well. Full instructions are in how to prepare fabric for natural dyeing.

Method 1: Tannin and alum

This is the most common method and works with simple, easy-to-find materials.

Tannin bath

• Tannin: 8–10% WOF, for example oak gall powder or sumac leaf powder

1. Dissolve the tannin in boiling water, then add it to a bucket of hot water big enough for the fabric to move freely.
2. Add the wet, scoured fabric and leave it for several hours or overnight, stirring now and then.
3. Wring the fabric out well. Do not rinse heavily.

Alum bath

• Alum: 15% WOF
• Soda ash: 2% WOF

1. Dissolve the alum and the soda ash separately in hot water, then add both to a bucket of warm water. The mixture will fizz.
2. Add the tannin-treated fabric and leave it for several hours or overnight, stirring now and then.
3. Rinse lightly and dye, or dry the fabric and wet it again before dyeing.

For the deepest colors, some dyers repeat the steps: alum, tannin, then alum again. Our cellulose mordant studio kit contains the tannin, alum and soda ash you need.

Method 2: Aluminum acetate

Aluminum acetate is often preferred for cellulose fibers and is the standard mordant for printing with natural dyes. It can give richer shades without a tannin step.

1. Dissolve 5–8% WOF aluminum acetate in hot water and add it to a bucket of warm water.
2. Add the wet, scoured fabric and leave it for several hours or overnight.
3. Move the fabric to a bath of calcium carbonate (chalk), about 10 g per liter of warm water, for 10 minutes to fix the mordant.
4. Rinse and dye.

Tips for success

• Use stainless steel or enamel pots; aluminum, copper or iron pots can change the color.
• Do not use too much mordant: excess can make fibers stiff and weaker.
• Keep the fabric moving in the first minutes of each bath for even results.
• Wear gloves and a dust mask when handling powders.

Frequently asked questions

Can I mordant cotton with alum only?

You can, but colors will be paler and less fast. A tannin step makes a big difference.

Which tannin should I use?

Clear tannins such as white oak gall or sumac keep colors bright. Darker tannins such as pomegranate add a yellow or beige tone. See our oak gall tannin guide.

Which fabrics work best?

Undyed, untreated cotton and linen. Browse our undyed fabric by the yard and how to dye wide cotton-linen fabric.

Related: Alum mordant guide · Mordanting wool and silk · Shop natural tannins