Alum Mordant
Potassium Aluminium Sulphate Dodecahydrate
Potassium Alum is the most commonly used mordant for wool, silk and many natural dyes. It helps suitable plant dyes bond with the fibre and generally produces clear, bright colours without darkening them.
For protein fibres, use 7.5–15% WOF - weight of dry fibre.
Cotton, linen and other cellulose fibres require a different mordanting method. Alum should not be used alone on cellulose.
At a glance
| Feature | Details |
|---|---|
| Product | Potassium alum |
| Chemical name | Potassium aluminium sulphate dodecahydrate |
| Formula | KAl(SO₄)₂·12H₂O |
| CAS number | 7784-24-9 |
| Form | White crystalline powder |
| Protein fibres | Wool, silk and other animal fibres |
| Suggested quantity | 7.5–15% WOF |
| Cellulose fibres | Use with tannin and soda ash, or prepare aluminium acetate |
| Colour effect | Generally clear and bright |
| Not required for | Indigo and woad vats |
How much to use
For wool and other protein fibres, begin with:
- 7.5% WOF for paler shades or minimum mordant residue
- 10% WOF as a general starting point
- 12–15% WOF for deeper shades
WOF means weight of dry fibre. For example, 10% WOF is 10g Alum for 100g dry fibre.
More Alum does not automatically produce stronger or more colourfast results. Use the lowest amount that works well for your fibre and dye.
Cold mordanting wool
Cold mordanting is our preferred method for wool.
- Weigh the dry fibre and calculate 7.5–15% WOF Alum.
- Soak the fibre thoroughly in water.
- Dissolve Alum in a small amount of hot water.
- Add it to a container of cool water and stir.
- Add the wetted fibre and ensure it is fully immersed.
- Leave for 24–72+ hours, moving the fibre occasionally.
- Remove, rinse and dye immediately, or dry for future use.
Cream of tartar is not required for cold mordanting.
Hot mordanting wool
- Weigh the dry fibre and calculate 7.5–15% WOF Alum.
- Soak the fibre thoroughly.
- Dissolve the Alum in hot water and add it to the mordanting pot.
- Optional: add up to 5% WOF cream of tartar.
- Add the wetted fibre.
- Slowly raise the bath to approximately 80°C.
- Hold for one hour, moving the fibre gently.
- Allow it to cool before rinsing.
Avoid boiling or excessive movement, as this may felt wool.
Silk
For silk, begin with 7.5–10% WOF Alum.
Cold mordanting is particularly suitable. Handle delicate silk gently and sample first.
Cotton, linen and other cellulose fibres
Alum alone is not a complete mordant for cellulose fibres.
For cotton, linen and hemp, use one of our complete cellulose methods:
Do not use the wool recipe directly on cotton or linen.
Good to know
- Dissolve Alum completely before adding the fibre.
- Wet the fibre thoroughly for even uptake.
- Alum is not required for indigo or woad vats.
- When layering indigo with mordanted colours, apply indigo first.
- Alum supports many natural dyes but cannot make a naturally fugitive colour permanent.
- Used mordant baths may retain Alum and can often be saved for reuse.
Storage and safe handling
- Store sealed in a cool, dry place.
- Avoid creating or breathing dust.
- Wear eye protection when handling powder or concentrated solutions.
- Wear gloves where appropriate.
- For textile dyeing and craft use only.
- Not for consumption or cosmetic use.
- Use dedicated dyeing equipment.
- Keep away from food, children and pets.
- Do not release concentrated solutions into drains or natural waterways.
What you receive
Your chosen net weight of potassium aluminium sulphate dodecahydrate, supplied in a sealed pouch.
Cream of tartar, soda ash, tannins and other dyeing materials are sold separately.
Display jars, dyed fibres and photographic props are not included.
Learn with us
Join The Natural Dyers & Growers Academy to share your results and explore natural colour alongside fellow dyers and growers.
