A mordant is a substance used in fabric and tissue dyeing that allows the dye to stick to the substrate. Clothing and tissue samples can be dyed without a mordant, and the dye may appear quite vivid. If the clothing or tissue is exposed to water, though, the dye will wash right off. Mordants work by creating a coordination complex, or a metallic bonding, with the dyeing substance. Many substances are called mordants, and they fulfill the same role, but they are not technically mordants; true mordants are metal molecules with a valency of two or higher and most commonly three.
When a piece of fabric or a tissue sample is dyed, it is treated with a staining substance. This substance is able to change the color of the fabric or tissue, making the fabric more appealing to wear and the tissue easier to see under a microscope. While the dye will hold tightly to the fabric or tissue, water is able to quickly rinse the dye away, making the dye temporary.
Mordants are used to ensure the permanence of a dye. A mordant is a substance that is used in conjunction with the dye to chemically bond the dye to the surface. This is done through a coordination complex. In a coordination complex, metallic atoms from the mordant bond with the nearby dye, creating a powerful bond that water cannot easily disintegrate.
There are three ways to apply a mordant. In pre-mordanting, the substrate encounters the mordant and is dyed afterward. Meta-mordanting combines mordants and dyes, so both are mixed together in the same dye bath. Post-mordanting is the process of introducing the mordant after the substrate is dyed.
Many substances used during the dyeing process are called mordants but are not truly mordants. These are trapping agents, which inhibit the removal of dye, while mordants are able to entirely stop the removal. Trapping agents are often non-metals, and they can be used with a larger variety of dyes.
True mordants must be metallic. The metal's valency, or the ability to make chemical bonds, must be two or higher. Commonly, metals such as iron and aluminum are used as mordants, and both of these metals have a valency of three. Metal is needed to create the intense bond between the mordant and dye because, otherwise, the coordination complex cannot be completed.