Alum Mordant Guide: WOF Ratios and Recipes for Wool, Silk, Cotton and Linen

Alum (potassium aluminum sulfate) is the most widely used mordant in natural dyeing. It helps dyes bond to fibers, improves wash and light fastness, and gives clean, bright colors, especially on wool and silk. This guide explains what alum does, how to measure it by WOF, and gives practical recipes for each fiber type.

Shop: Alum mordant (potassium aluminum sulfate)

What does alum do?

A mordant works like a bridge between the dye and the fiber. Alum binds to the fiber, and the dye then binds to the alum, so the color holds through washing and sunlight. Alum does not add color itself. It has been used this way for more than 2,000 years. For an overview of all mordants, read our complete guide to mordants.

Powder or crystals?

Alum powder and crystals give the same results when you measure them by weight. Powder dissolves faster, which is handy for small batches; crystals are easy to handle in larger batches.

What is WOF?

WOF means weight of fiber: the dry weight of your fiber or fabric. All amounts in this guide are based on it. For example, if you dye 100 g of fabric, 15% WOF alum is 15 g.

Scour first

A common mistake is to assume new fabric is clean. Oils, starch, finishing agents and softeners stop the mordant and dye from bonding evenly, which leads to patchy or dull color. Always scour first, especially cotton and linen. See how to prepare fabric for natural dyeing.

Alum recipes by fiber

Wool and silk (protein fibers)

• Alum: 10–15% WOF (10–15 g for 100 g of wool)
• Optional: cream of tartar 5–7% WOF to keep wool soft and colors bright

1. Soak the fiber in water until it is fully wet.
2. Dissolve the alum (and cream of tartar) completely in hot water in a separate container.
3. Add the solution to a pot of warm water large enough for the fiber to move freely.
4. Add the fiber and heat slowly to about 80 °C (175 °F). Do not boil.
5. Hold for 1 hour, moving the fiber gently now and then, then let it cool in the bath. You can also leave it overnight.
6. Rinse lightly and dye, or dry it and wet it again when you are ready.

Cold method: wool and silk can also be mordanted cold by soaking them in the alum solution in a bucket for one to three days. This is gentler on the fibers, but do not leave them much longer or they can feel slimy. More detail is in our guide to mordanting wool and silk.

Cotton and linen (cellulose fibers)

Cellulose fibers do not bind alum as well as protein fibers, so they are usually treated with tannin first, then alum.

• Step 1, tannin: 8–10% WOF, for example oak gall powder
• Step 2, alum: 15% WOF
• Helper: soda ash 2% WOF

Example for 100 g of cotton: 8–10 g tannin, 15 g alum and 2 g soda ash.

1. Soak the scoured fabric in a warm tannin bath for several hours or overnight, then wring it out.
2. Dissolve the alum and the soda ash separately in hot water, then add both to a bucket of warm water. It will fizz.
3. Add the fabric and leave it for several hours or overnight, stirring now and then.
4. Rinse lightly and dye.

Prefer everything in one box? Our cellulose mordant studio kit contains tannin, alum and soda ash. For other options such as aluminum acetate, read mordanting cotton and linen.

Where alum fits in the process

Scour → tannin (cotton and linen only) → alum mordant → dye bath

Explore our natural dyes and mordants to choose your colors.

Troubleshooting

• Patchy color: the fabric was not fully wet, or was not moved enough in the bath.
• Dull or weak color: the tannin step was skipped on cotton, the water is very hard, or the fabric was not scoured well.
• Undissolved alum: always dissolve alum completely before adding the fiber.
• Different results each time: measure by WOF and note your times, temperatures, water and amounts.

Storage and safety

Store alum dry, tightly closed and clearly labeled. Avoid breathing in the dust, wear gloves, and keep it away from children and pets.

Frequently asked questions

Do alum powder and crystals give different results?

No. The results are the same when measured by weight.

Is tannin required for cotton?

It is not strictly required, but it gives much stronger and more even color, so we recommend it.

How much alum should I start with?

10–15% WOF for wool and silk, and about 15% WOF after tannin for cotton and linen.

Related: Alum & iron mordants · Cream of tartar guide · Oak gall tannin guide