An organic colorant derived from coal tar– and petroleum-based intermediates and applied by a variety of methods to impart bright, permanent colors to textile fibers. Organic dyes were first synthesized by Perkin in England (1856) and were later developed by Hofmann in Germany. Some, called fugitive, are unstable to sunlight, heat, and acids or bases; others, called fast, are not. Direct (or substantive) dyes can be used effectively without “assistants”; indirect dyes require either chemical reduction (vat type) or a third substance (mordant), usually a metal salt or tannic acid, to bind the dye to the fiber. A noteworthy development is the fiber-reactive group, wherein the dye reacts chemically with cellulose. Dyes may be either acidic or basic, and their effectiveness on a given fiber depends on this factor. Some types are soluble in water; others are not, but can be made so by specific chemical treatment. The central problem is the affinity of a dye for a fiber, and this involves both the chemical nature and the physical state of the dye, i.e., whether acidic or basic and whether colloidal, molecular, or ionic. Neither colloidal particles nor dissociated ions are accepted by a fiber; the dye must be in molecular dispersion to be effective. Further details will be found in the following entries: acetate dye, anthraquinone dye, acid dye, azo dye, alizarin, aniline, fiber-reactive dye, eosin, intermediate, resist, stilbene dye, sulfide dye. The chemical classes of coloring matters and their arrangement according to chemical structures have been designated numerically in the Colour Index, a British publication.See Colorant; Pigment.
