Alum Mordant

Alum (potassium aluminium sulphate) is a classic mordant used in natural dyeing to help dyes bond to fibers and improve wash and light fastness. It suits most natural fibers and is especially popular for bright, clear colors.

Best for

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

How to use (by weight of fiber, WOF)

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

Powder or 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 fiber type, dye type and water chemistry. Store dry and tightly closed.

Learn more

• How to prepare fiber and fabric for natural dyeing
• Recommended tannins for cellulose: green oak gall powder