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Mordanting Cellulose fibres

Updated: Feb 24



When working with plant-based fibres like cotton, linen, hemp and bamboo, one of the most important steps in natural dyeing is mordanting. Unlike protein fibres (such as wool and silk), cellulose fibres don’t naturally bond as easily with many plant dyes.

Mordanting prepares the fibre so the colour adheres better, deepens in tone, and lasts longer through washing and sunlight.

If you’ve ever wondered why your cotton samples look faded while your wool turns out rich and vibrant, the answer is almost always your mordanting process.


These instructions are generally for immersion dyeing but are also helpful when preparing fabric for eco printing/bundle dyeing and printing. Ferrous sulphate is also commonly used in this process but will interact very strongly with tannins so that must be taken in to consideration when mordanting. You do not want to pre treat with tannin and then Ferrous (unless you are wanting to create a particular effect).



Common Mordants for Cellulose Fibres

Tannins - See available Tannins here 

Tannins help cellulose fibres bond more effectively with alum. They add an important extra layer that improves depth of colour and fastness. Your fibres will need to be pre treated with a tannin before the Alum treatment.

Common tannins include:

  • Myrobalan

  • Gall nut

  • Pomegranate rind

  • Tara powder

  • Walnut hulls

  • Eucalyptus bark and leaves

  • Tea

  • Acacia

Expect a soft yellow, beige or grey cast after tannin treatment, depending on what plant stuff you have used, this is normal and beneficial.


Aluminium Acetate - Find here

The gold standard for cellulose mordanting. It provides bright, clear colours and excellent wash fastness.

Pre treating your fibre with a tannin is optional but will provide a better result.

You will need to also treat your fabric with a calcium carbonate or wheat bran bath after the Alum to help fix it to the fibre.


Aluminium Sulphate (Alum) or Potassim Aluminium Sulphate (Potash Alum) Find here 

More widely available and simple to use, especially for beginners. You will need to pre treat the fabric or fibre with a tannin.


Please note: Avoid Inhalation - Use gloves when touching the liquid and avoid inhaling dust or steam of Aluminium Sulphate and acetate.


Step-by-Step Guide: Mordanting Cellulose Fibres


1. Scour Your Fibre


2. Tannin Bath (optional when using Alum Acetate)

Especially important for cotton and linen.

  1. Soak your scoured fibre in water.

  2. Fill a pot with hot water (40-50C) enough for fabric to move freely.

  3. Dissolve your chosen tannin in hot water (usually 10 - 15% WOF) and add to your water.

  4. Add fibre and soak for 1–2 hours or overnight, stir to ensure even coverage.

  5. Rinse lightly or squeeze out excess and move on to Alum bath.

This step gives the alum something to bond to.


4. Alum Sulphate Bath

Use Alum Sulphate at 10 - 15% WOF

Make sure you have pre treated with a tannin.

  1. Measure out Alum Sulphate at 10 - 15% WOF and dissolve in container with hot water.

  2. Fill a pot with hot water and add the Alum.

  3. Heat gently and add pre wet fibres. Hold at 60C for 60 minutes. Stirring frequently to ensure even coverage.

  4. Rinse in warm water.


3. Alum Acetate Bath

Use Alum acetate at 10% WOF

  1. Measure out Alum acetate and dissolve in hot water

  2. Fill a pot with hot water - enough for your fabric to move around freely and add dissolved Alum Acetate.

  3. Add fibres and heat gently.

  4. Hold at 60–70°C for 1 hour, making sure to stir occasionally to distribute evenly, then cool in the bath for best uptake and go on to calcium or Bran bath. No need to rinse.

You may save and reuse your Alum Acetate bath. If so, you can add half the amount of Alum for the new bath.


4. Calcium Carbonate or Wheat Bran Bath

  1. Measure out 5% WOF calcium carbonate or wheat bran.

  2. Dissolve in a bucket of hot water. When using wheat bran it's best to make a 'tea bag' and let it sit for approx 30 minutes before adding fibre.

  3. Add fibres and soak for 15 - 20 minutes.

  4. Rinse lightly

This step helps “set” the mordant. The calcium carbonate bath can be reused for some time. The bran bath may also be reused but may go off after a day or two and should be discarded.


Dry or Use Immediately?

You can dye straight away or let your mordanted fibres dry and store them for future use. Many dyers find that letting fibres rest for 24 hours enhances colour uptake.


Troubleshooting Common Issues

Uneven colours: Often caused by inadequate scouring or overcrowding in the bath.

Muted colours: Increase tannin percentage or switch to aluminium acetate.

Stiff fibre: Reduce tannin percentage or rinse after tannin bath.

Patchiness with alum: Make sure alum is fully dissolved before adding fibre and stir frequently.


Mordanting cellulose fibres might seem like an extra step, but it’s truly the foundation of beautiful natural dyeing. Once you understand how tannins and alum work together, you’ll unlock deeper colours, better consistency, and stronger results across all your botanical dye projects.


Download these easy reference guides for the Mordant process.




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