Iron has been used for centuries to fix color to wool and to darken, or “sadden” it. It also tends to bring out the green in natural dyes such as
marigold, fern, and comfrey. It is not always necessary to purchase prepared mordants. Most of the time, I simply use an iron kettle to dye wool that has been mordanted with alum or not mordanted at all. Iron nails, old flat irons, and other small iron household hardware can also be used. The drawback is that you cannot accurately measure the actual amount of the mordant, but I’ve found, over and over, that simply keeping an eye on the dyepot, until the color I’m looking for has been attained, is sufficient. The advantages are that the mordant is free, and safer than chemical compounds, but it is still a good idea to limit skin contact. When the dye process is complete, I rinse the wool in dish detergent or shampoo in water into which about 1 teaspoon of salt has been added.