Alum (Potassium Aluminium Sulphate) — Mordant & Dye Assistant

Alum (Potassium Aluminium Sulphate) — Mordant & Dye Assistant

Alum is a classic mordant used in natural dyeing to help dyes bond to fibres and improve wash and light fastness. It’s suitable for most natural fibres and is especially popular for bright, clear colours.

Best for

• Protein fibres: wool, silk
• Cellulose fibres: cotton, linen, hemp, bamboo (tannin pre-treatment recommended)

How to use (by weight of fibre / WOF)

• Wool & silk: use 10–15% WOF alum. Dissolve fully in hot water, add to a mordant bath, then add pre-wetted fibre and hold warm for 1–2 hours (or soak longer for convenience).
• Cotton & linen: first treat with tannin (8–10% WOF), then mordant with alum (15% WOF) + soda ash (2% WOF) in a warm bath.

Powder vs Crystals

• Powder: dissolves faster and is ideal for quick prep or small batches.
• Crystals: easy to handle and measure for larger batches.

       (Results are the same when measured by weight.)

Notes

Results vary with fibre type, dye type and water chemistry. Store dry and tightly closed.

Learn more

• Scouring & Mordanting Guide (blog link)
• Recommended tannins for cellulose