|What is the chemistry involved with natural dyes adhering to surfaces? — AG, Aloha, OR
Unless a chemical reaction binds them permanently in place, dye molecules that are soluble enough to wash into fabrics are equally likely to wash back out of the fabrics later on. To remain in place, the dyes must undergo chemical reactions that attach them to the fibers of the fabric. Some dyes react spontaneously to the fabric molecules but many others need help. The traditional scheme for binding dyes to fabrics involves mordents—relatively colorless chemicals that bind to both fabric and dye, and that hold the two together. Tannic acid and various metal salts have been used as mordents for centuries. They form insoluble compounds that wedge themselves into hollow spaces in the fibers and then bind chemically to the dye molecules. These mordents hold the dye molecules in place in much the same way that technical climbing gear holds rock climbers to the face of a cliff.