Mordanting wool and protein fibres with alum

Suzie Blackman
Sunday, 1 September 2013

If you want to dye wool, silk or other animal fibres using natural dyes, such as onion skins, you will need to use a mordant in order for the dye to bond with your yarn of fibre. Without this, the dye will not be colour-fast. You can find information about the chemical process on Wikipedia.

Alum, or potassium aluminium sulphate is the best choice of mordant for beginners as it is safe to use in your kitchen and to dispose of, and does not modify the dye colour. Despite the chemical-sounding name, alum is a naturally occurring rock mineral and can be purchased from local pharmacies. Copper sulphate (toxic, requires specialist disposal) and ferrous sulphate (irritant) are other common mordants which are used in place of, or in addition to alum to modify the dye colour. These mordants are for protein fibres only; for cotton and other cellulose fibres, the compounds used and dying process are different.

What you need

If you’re new to dyeing I recommend using superwash yarn; it absorbs dye more readily and there is no risk of it felting.

For each 100g yarn or fibre:

  • 25g alum
  • 2 tsp cream of tartar (optional, an acid modifier used to brighten colour or some natural dyes)
  • 1 litre water, or enough to cover


  • A large bowl made of a non-reactive material; stainless steel, enamel or glass
  • A large pan made of a non-reactive material
  • A heat-proof, non-reactive stirrer (I use a disposable chopstick or wooden kebab skewer)


Prepare the yarn

Make sure your yarn of fibre is clean. Tie your skeins in at least three places to prevent tangling, snug but not tight.

Fill the bowl with water, add a drop of washing up liquid. Add the yarn and push under, ensuring it is well covered. Leave to soak for 30 minutes.

Prepare the mordant solution

Place the alum and cream of tartar into the pan with a little boiling water – the heat helps the compounds dissolve. Stir until completely dissolved. Add enough water to cover the yarn and stir thoroughly.

Add the yarn

Squeeze the excess liquid from the yarn and transfer to the pan. Heat the liquid slowly and simmer gently for 1 hour.

Leave the yarn in the pan to cool slowly. If you’re using superwash-treated yarn, you can safely cool it quickly b adding warm water, then cool water. With untreated yarn, it’s best to leave it alone until it reaches room temperature.

Squeeze the excess water from the yarn. I can either be transferred to the dye-bath straight away, or left to dry and stored for later use. If the latter, tie the skeins with a coloured thread to mark them as mordanted – it’s easy to forget!

The author

Suzie Blackman

The dyer, designer, photographer, creative technologist and maker-of-things behind It's a Stitch Up. She lives in East London in a home filled with colour, fluff and house plants.


  1. Tessa says:

    Thank you, good useful info when I needed it.

  2. Jane says:

    This is very helpful – thanks for posting and love your tip about coloured thread to show mordanted skeins :-)

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