A chemical substance, usually containing a metallic ion, used to facilitate the fixing of a dye to a fiber. From the earliest days of dyeing, it was observed that for many natural dyes the color imparted to a fabric was enhanced and the fastness (resistance to fading or running) improved if the fabric was first treated with a solution containing a soluble metallic salt of chromium, aluminum, or copper. In the simplest process model, metal ions are incorporated on the surface and within the fibers of the fabric and, upon exposure to dye molecules at the appropriate pH and temperature, form colored metal-dye complexes. The metal ion is called a mordant. The complex formation process is called chelation, and can result in a fast and deeply colored fabric. One of the oldest mordant dyes is alizarin, derived from the root of the madder plant. Chemically, it is an anthraquinone derivative, and the dye color depends upon the metal with which it is complexed; with barium, a blue color is obtained, and with aluminum a rose-red.