Now that you have scoured your cotton, you are ready to start mordanting it.
It is very important to work the fibres in the pot with gloved hands but check that the water is not too hot for your hands first. Squeeze the fabric inside the pot with your hands, then unfold the fabric a few times, making sure the mordants penetrate in every nook and cranny. If you just plonk your fabric in the saucepan and leave it there you will get a mottled effect when you dye (which may be just what you want).
Use no more than 100 grams of fibre in a ten litre saucepan. By fibre, I mean fabric, yarn, or cotton sliver from a specialist shop. If you use more than 10 grams of fibre per litre, you will find it difficult to stir the fibre in the dye pot, and the mordant will not penetrate so well. A ten litre saucepan is quite heavy to lift, and I would not want to use a pot any larger than that.
The basic aluminium acetate method is quicker, as you only need to make one mordant bath which means you will also use less energy. Aluminium acetate is also cheaper, even though it costs more than alum, as you will be using only 7 to 10 grams of aluminium acetate instead of 50 grams of alum.
The 3 step tannin and alum method is a useful alternative, as alum is slightly safer chemical to use than aluminium acetate as it is not such a fine powder; also aluminium acetate is not so widely available.
Aluminium acetate is recommended by Liles for plant fibres. This method seems to yield deeper and clearer colours, as it uses no tannin which can slightly yellow the fibres. It also has the advantage of using only one ingredient and one mordant bath, being therefore both quicker and cheaper.
Note: aluminium acetate is a very fine powder, weigh it carefully and use a mask when weighing this and other fine powders.
You will need;
7 to 10 grams of aluminium acetate
100 grams of scoured cotton fabric or yarn
Soak 100 grams of cotton fibres in warm water for at least two hours.
Half fill the dye pot with hot tap water
Dissolve the aluminium acetate in a small container with boiling water.
Add the dissolved aluminium acetate to the dye pot and stir. There is no need for extra heat as the warmth from the hot tap water should be enough.
Add the wet cotton and squeeze it a few times wearing gloves. If you are mordanting yarn, especially fine weaving yarn, work the fibre in the pot very carefully, otherwise the yarn will get tangled. Leave overnight. Wring well and dry. Liles recommends waiting until the vinegar smell has disappeared, which he says can take up to 4 days.
When you are ready to dye, rinse the fibre carefully to remove any unattached mordant.