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Mordanting Cotton

Cotton is very time consuming to mordant. First it needs scouring and then it needs a three-step process, with consecutive mordant baths of alum, tannin and then alum again.

Scouring Cotton
It is necessary to scour the cotton, even if you plan to dye it with indigo. Scouring removes the dirt and grease that fibres accumulate during manufacturing and transit. You really need to use soda ash for scouring cotton properly; washing soda is not aggressive enough. I found I need to use a proper hob to do the scouring, as a two ring electric plate cannot cope with either large saucepans or the long boiling. Scour at most 100grams of fibre in a 10 litre saucepan.

• First wash the cotton in the washing machine using a long programme and very hot water. Use a non-bleach washing powder, and do not use fabric softeners.
• Half fill a 10 litre stainless steel pot with water. Do not fill it more than that because it is likely to boil over and it may even kill the flame on a gas ring. Carefully add 35g soda ash (about ¼ cup) into the pot.
• Place the wet fabric into the water and swish it around using a stainless steel spoon. Leave the spoon inside the pot, to prop the lid slightly open which prevents the liquid from boiling over. Bring the water to the boil.
• Adjust the heat to low boil/hard simmer and allow the fibre to boil half covered for two hours. Stir the fabric every 15 minutes to make sure it is being adequately scoured. Put the lid down when stirring and hold the pot with one gloved hand to prevent it from toppling over.
• After two hours remove the saucepan from the heat source and allow the fabric to cool down until it can be safely removed from the water. I am always surprised at how dirty the water looks.
• Finally rinse the fabric. If the scouring water was brown or grey (that is to say more than mildly dirty), repeat the process with new water and soda ash.

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Mordanting Cotton with Alum, Tannin & Alum
Use no more than 100g of fibre in a ten litre saucepan. By fibre I mean fabric, yarn, cotton balls sold at a chemist or pharmacist, or cotton sliver from a specialist shop. If you use more than 10g of fibre per litre, you will find it difficult to stir the fibres in the dye pot, and the mordant will not penetrate well. A ten litre saucepan is quite heavy to lift, and I would not want to use a pot any bigger than that. Buy alum mordant here.

Total weights to mordant 100g of fibre:-
Alum:            50g (25g in each of Baths 1 & 3)
Soda Ash:      12g (6g in each of Baths 1 & 3)
Tannic Acid:   6g (for Bath 2 only)

• Pre-wet 100g of cotton fibres in warm water for at least two hours.
• Bath 1: Fill half of dye pot with water. Bring nearly to boil. Dissolve 25g of alum in a small container with boiling water. Slowly add alum to pot and stir well. Weigh out 6g of soda ash but do NOT add water to the soda ash! Instead add the soda ash slowly and carefully to the water in the pot. Soda ash causes the water to bubble vigorously; when bubbles have subsided add more hot water to the pot. Add pre-wetted fibres. Heat to simmering point. Simmer for an hour. Stir fabric every fifteen minutes. Leave fibres in the pot for 24 hours. It is not a problem to leave fibre in the pot at any of the stages for a few days. Wring well and dry. Leave the fibre to age for at least a week. When you are ready for the next step, wash the fibre well.
• Bath 2: (Tannin): Fill pot 2/3 with hot water. Dissolve 6g of tannic acid in a small container with boiling water, (otherwise particles may stick to bottom of saucepan). Add tannic acid to pot and dissolve well. Add pre-wetted cotton. Simmer for one hour, leave for 24 hours. Wring well and dry. Leave the fibre to age for at least a week. When you are ready for the next step, wash the fibre well.
• Bath 3: (Second Alum Bath): repeat alum mordanting as described for Bath 1.

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Pure white areas: mordanting gives the fibres a pale yellow tinge. If you want areas of pure white, tie or mask the area before mordanting.
Dark colours: To get dark colours dye the fibres, leave them to age for a week, wash fibres, re-mordant with alum and soda ash and dye again. For really dark colours re-mordant using the three step process.

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Mordanting Silk, or
Mordanting Wool

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Website and photos by Mike Roberts                 © 2006-08 WildColours
Page last updated 30 September 2008