To achieve bright colours with natural dyes, you need to mordant your wool, cotton or silk beforehand. With the exception of woad and indigo, most natural dyes do not adhere very well to the fibres and need the help of a mordant.
A mordant is a chemical binding agent that adheres well to both the fibres and the dye. The word comes from the Latin mordere, which means to bite.
If you want colours that are fast and bright, your fibre must be very clean before you mordant it, otherwise neither the mordant nor the dye will penetrate properly. The process of cleaning the fibre for dyeing is often called ‘scouring’. Clean knitting yarns and silk scarves often only require a gentle washing. Wool fleeces and cotton fabrics need to be properly scoured. You will find more information on how to do this on the pages on mordanting wool and mordanting cotton.
The most commonly used mordant for wool is alum powder, with cream of tartar being used as an assistant. Cream of tartar improves the consistency of the colour as well as the consistency between batches. Buy alum mordant and cream of tartar here.
When you mordant cotton and other plant fibres you need to use tannin as well as alum. Gallnut extract is a natural product and an affordable substitute for tannic acid. Aluminium acetate is an easier and quicker alternative to the alum/tannin process.
It is important not to put too much fibre in the mordanting pan. You need to leave enough space for the fibre to move. As a guide, you can mordant (or dye) 100 to 150 grams of fibre in a 10 litre saucepan. If you can, leave the fibre in the mordant overnight. Then gently rinse the fibre to remove any unattached mordant, and either dye it straightaway or dry it for further use.