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

Alum (Potassium Aluminium Sulphate) is one of the most widely used mordants in natural dyeing. It helps dyes bond to fibres, can improve wash and light fastness, and often supports cleaner, brighter results—especially on protein fibres like wool and silk.

In this guide, you’ll learn what alum is, how to use it with WOF (Weight of Fibre) ratios, and practical recipes for different fibre types.

Alum product page: Alum (Potassium Aluminium Sulphate) — Mordant & Dye Assistant

1) What is alum? What does a “mordant” do?

A mordant acts like a bridge between dye molecules and fibres. When used correctly, it helps colour bind more permanently and makes results more consistent. Alum is a mineral-based salt and is commonly used as a “foundation mordant” in many natural dyeing processes.

2) Powder or crystals? (Short)

Powder and crystals give the same results when measured by weight. The difference is handling:

• Powder: dissolves faster (great for small batches / quick prep)
• Crystals: easy to measure and handle (convenient for larger batches)

3) Which fibres work best with alum?

Alum can be used on many natural fibres, but the approach differs by fibre type:

• Protein fibres: wool, silk (alum performs especially well here)
• Cellulose fibres: cotton, linen, hemp, bamboo (a tannin pre-treatment is commonly recommended)

You can dye cellulose without tannin, but in many processes a tannin step improves durability and consistency.

Tannin product (example):  Oak Gall Powder (Tannin)

4) What is WOF?

WOF means Weight Of Fibre (or fabric) when dry. All ratios in this guide are calculated using the dry weight.

Example: if you are dyeing 100 g of fabric, all calculations are based on 100 g.

5) Why scouring (pre-washing) matters

One of the most common natural dyeing mistakes is assuming fabric is “clean” and going straight into dyeing. If fabric contains residues from production—such as finishing agents, oils, dirt, or softeners—dye may not bond evenly, leading to patchy or dull results. A good scour / pre-wash (especially for cotton and linen) can dramatically improve the final outcome.

6) Practical Recipes (WOF): How to use alum

A) Wool & Silk (Protein fibres)

Typical alum amount: 10–15% WOF

Example: for 100 g wool → use 10–15 g alum

Steps

1. Pre-wet the fibre/fabric (for even uptake).
2. Dissolve alum fully in hot water in a separate container.
3. Add the dissolved alum to a warm mordant bath (avoid a rolling boil).
4. Add fibre/fabric and move gently for even penetration.
5. Keep warm for 1–2 hours. If convenient, soak longer/overnight, then rinse lightly and proceed to dyeing.

Optional support: Some wool/silk recipes use Cream of Tartar as a helper (it may affect brightness and handle).

You can find Cream of Tartar here:  Cream of Tartar

 

 

B) Cotton & Linen (Cellulose fibres)

For cellulose fibres, many processes recommend the sequence tannin → alum.

Step 1 — Tannin pre-treatment: 8–10% WOF

Step 2 — Alum mordant: 15% WOF

Helper (recommended / optional): Soda ash 2% WOF

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

Steps

1. Prepare a tannin bath; treat the fabric; then rinse lightly.
2. In the alum bath, dissolve alum fully, add fabric, and move occasionally.
3. Rinse and proceed to dyeing.

Why tannin first?

Cellulose fibres don’t bind alum as readily as protein fibres. Tannin helps create a better base for alum and dye to attach.

You can find soda ash here:  Sodium Carbonate (Soda Ash)

7) How alum connects to natural dyes (Short)

Alum doesn’t create colour on its own—it supports the dyeing process. Think of it as a step that helps dye bond to fibre more effectively. A simple workflow looks like this:

• Scour / pre-wash → tannin (for cellulose) → alum mordant → dye bath

Explore our dyes and mordants collection: Natural Dyes & Mordants

8) Troubleshooting (common issues)

• Patchy results: fabric not fully pre-wetted, or not moved enough in the bath.
• Dull/weak colour: missing tannin step on cellulose, hard water, or insufficient scouring.
• Powder clumping: always dissolve alum fully before adding fibre/fabric.
• Inconsistent batches: measure by WOF and keep notes (time, temperature, water, quantities).

9) Storage & safety

Store alum dry, tightly closed, and clearly labelled. Avoid inhaling dust (especially powder). Keep away from children and pets.

10) FAQ

Do powder and crystals give different results?

No—results are the same when measured by weight.

Is tannin required for cotton?

Not strictly, but it’s recommended in many processes for stronger, more consistent results.

What ratio should I start with?

For wool/silk, 10–15% WOF is a solid starting point. For cotton/linen, after tannin, alum is commonly used around 15% WOF.