How to Mordant Wool and Silk (Protein Fibers) with Alum

Wool and silk are protein fibers, and they take natural dyes beautifully once they are mordanted. Mordanting means treating the fiber with a metal salt, usually alum, so the dye bonds to it and the color lasts through washing and light. This guide shows you how to mordant wool and silk with alum and cream of tartar, step by step.

What you need

• Alum (potassium aluminum sulfate): 10–15% WOF
• Cream of tartar: 5–7% WOF (optional, but it keeps wool soft and colors bright)
• A stainless steel pot used only for dyeing, a thermometer, a glass jar and a wooden spoon
• Gloves and a dust mask

WOF means the dry weight of fiber. For 100 g of wool, use 10–15 g of alum and 5–7 g of cream of tartar.

Step 1: Wash and weigh the fiber

Wash the fiber gently in warm water with a little pH-neutral soap to remove dirt and oils, then rinse. Weigh it dry first so you can calculate the mordant. Keep the fiber wet until you mordant it.

How to mordant wool

1. Dissolve the alum and cream of tartar in hot water in a glass jar.
2. Fill the pot with enough warm water to cover the wool, and stir in the dissolved mordant.
3. Add the wet wool and press it gently under the water.
4. Heat slowly to about 80 °C (180 °F). Avoid sudden temperature changes and too much stirring, which can felt the wool.
5. Hold at this temperature for about an hour, then turn off the heat and let the wool cool in the bath for at least 12 hours, or overnight.
6. Lift out the wool, rinse it in water of the same temperature, and gently squeeze out the water. Do not wring it.
7. Dye straight away, or dry it away from direct sunlight and wet it again before dyeing.

How to mordant silk

Silk is mordanted the same way, but at a lower temperature, because heat can dull its natural shine.

1. Dissolve the alum and cream of tartar in hot water and add it to a pot of warm water.
2. Add the wet silk and stir gently.
3. Heat slowly to about 70 °C (160 °F), then turn off the heat.
4. Leave the silk in the bath for at least 12 hours, or overnight.
5. Rinse, gently squeeze out the water without wringing or twisting, and dye or dry.

Cold mordanting

You can also mordant wool and silk without heat: dissolve the alum and cream of tartar, add them to a bucket of room-temperature water with the fiber, and leave it for one to three days. This is gentle on the fibers and saves energy.

Next step: dyeing

Alum-mordanted wool and silk give strong colors with almost every natural dye. Try madder for reds, weld for yellows, or pomegranate for golden tones. For silk tips, read how to dye silk with natural dyes.

Frequently asked questions

Why add cream of tartar?

It helps the alum bind evenly, keeps wool soft and gives brighter colors. Read our cream of tartar guide.

Can I use too much alum?

Yes. Too much alum can make wool feel sticky or harsh. Stay within 10–15% WOF.

Can I reuse the mordant bath?

Yes. Top it up with about half the original amount of alum for the next batch.

Related: Guide to mordants · Alum mordant guide · Mordanting cotton and linen · Shop mordants