The word mordant comes from the French "to bite". It is a process by which the dye can bite onto your fiber - instead of washing out. A mordant is a metal compound, the most common being alum, but tin, chrome, copper and iron also work. Some dye materials will give quite different colors depending on the mordant used.

It is recommended to pre-mordant yarn, keeping it wet until it is used for dyeing.

For rovings and clean wool, the mordant can be added to the dye bath to reduce the amount of handling. The recipes for wool and other protein fibers is given here.

In all cases (for yarn/fabric):
1. Weigh the dry fiber.
2. Soak the fiber to get it thoroughly wet.
3. Dissolve the required amount of mordant in hot water.
4. Fill a large (non-food) pot with water and stir in the mordant.
5. Add the fiber and simmer for an hour.
6. Rinse the fiber.

For alum, Rita Buchanan gives the ratios for three amounts:
1 One pound of clean dry yarn to 4 Tbsp alum and 4 tsp cream of tartar (tartaric acid).
2. For four ounces, 1 Tbsp alum and 1 tsp cream of tartar.
3. For any amount, 14% alum, 4% cream of tartar by weight.

Judy van Stralen likes to use the minimum amount of mordant, in this case 10% alum and 5% cream of tartar by weight.

For tin, Rita uses 2% by weight or 2 tsp/pound, while Judy uses 0.5% by weight. For chrome, use 3-4% by weight, or 3 tsp/pound. Handle with care.

Copper and iron can make the fiber feel harsh and are best used as postmordants to change the color (adding green and brown, respectively), rather than as the main mordant.

For cotton and other cellulose fibers, recipes are in the books. Make sure the fiber is very clean first by simmering for an hour with detergent and washing soda.