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Scouring removes the dirt and grease that fibres accumulate during manufacturing and transit.
You really need to use soda ash to scour cotton properly; washing soda is not aggressive enough (buy Soda Ash here). Scour at most 100 grams 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.
Weigh out 35 grams 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.
When bubbles have subsided place the wet fabric into the water and gently stir using a long 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. You will probably be surprised at how dirty the water looks.
Finally rinse the fabric. If the scouring water was brown or grey (i.e. more than mildly dirty), repeat the process with new water and soda ash.
Try weighing your dry fabric before you scour, and then weigh it again afterwards, when it has dried. You will probably find that, as well as shrinking, your fabric has lost about 10% in weight.
I washed and scoured 2.50 metres of cotton, which I wanted to dye with indigo and then make into a shirt. Before scouring, the fabric weighed 350 grams, afterwards it weighed 315 grams. The 35 grams it lost was due to all the oils and waxes used in the manufacturing process and the sizing (starches) added to make the fabric look good when you buy it. By checking the weight loss you can see how important it is to remove all those additives by scouring your fabric well before you mordant or dye.
Note: You need to scour cotton even if you plan to dye it with indigo or woad.
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3. How to mordant cotton (opens new page)