Mordants are used to prepare fabric for natural dyes so that they will "stick" to it. The colors you get with natural dyes vary depending on what mordant you use and also what fiber you are dyeing. You can often get a large range from each natural dye color by varying the mordants and how you use them. It is a complex subject and we highly recommend the book below as an introduction. Dharma has chosen to only carry the mordants that are the safest to use. You will read about others, but they can be somewhat to very toxic if not handled properly and we did not feel comfortable carrying them. The Cream of Tarter is a ph buffer and softens the water for improved results. Tannic acid can be used by itself as a mordant, or to darken the colors acheived when using Alum as a mordant. Alum is the least toxic of the metallic salt Mordants.