Dyes

Dyes are colored substances used to impart color to materials, most commonly textiles, foods, cosmetics, and inks. They are typically soluble in the dye bath and interact chemically with the substrate, forming bonds that fix the color. This contrasts with pigments, which are insoluble particles that color a medium by dispersion rather than chemical binding.

Dyes are categorized by origin (natural from plants, insects, minerals, and synthetic) and by chemistry (reactive, direct, acid, basic, vat, disperse, and mordant). Reactive dyes form covalent bonds with cellulose; direct dyes rely on ionic and hydrogen bonding with cellulose. Acid dyes color protein fibers and nylon; basic dyes suit acrylics. Vat dyes, including indigo, are applied in reduced form and oxidized to insoluble colors. Disperse dyes are finely milled, water-insoluble dyes used for polyester. Mordant dyes require metal salts to fix to natural fibers.

Dyeing processes depend on substrate and dye class. Cotton and viscose are colored with reactive or direct dyes, wool and silk with acid dyes, and polyester with disperse dyes. Mordants enable natural dyes to bond by forming coordination complexes. The depth and fastness of color depend on molecular structure, fiber chemistry, and dyeing conditions such as temperature, pH, and time.

The history of dyes spans ancient natural products such as madder, cochineal, indigo, and saffron to the chemical revolution of the 19th century. In 1856, William Henry Perkin discovered the first synthetic dye, mauveine, inaugurating the synthetic-dye era that expanded color variety and cost efficiency while shaping the textile industry.

Today, dyes are central to textiles, food flavoring and coloration, cosmetics, and printing inks. Environmental concerns arise from dye effluents releasing color and chemicals into watercourses; modern practice emphasizes wastewater treatment, selective dyeing, and environmentally sustainable or bio-based dyes to reduce ecological impact.