dye, any substance, natural or synthetic, used to color various materials, especially textiles, leather, and food. Natural dyes are so called because they are obtained from plants (e.g., alizarin, catechu, indigo, and logwood), from animals (e.g., cochineal, kermes, and Tyrian purple), and from certain naturally occurring minerals (e.g., ocher and Prussian blue). They have been almost entirely replaced in modern dyeing by synthetic dyes. Most of these are prepared from coal tar, being formed from an aromatic hydrocarbon such as benzene, from which indigo is derived (see also aniline), or anthracene, which yields alizarin. Although some materials, e.g., silk and wool, can be colored simply by being dipped in the dye (the dyes so used are consequently called direct dyes), others, including cotton, commonly require the use of a mordant (see also lake). Alizarin is a mordant dye and the color it gives depends upon the mordant used. Dyes are classified also as acidic or basic according to the medium required in the dyeing process. A vat dye, e.g., indigo, is so called from the method of its application; it is first treated chemically so that it becomes soluble and is then used for coloring materials bathed in a vat. When the materials become impregnated with the dye, they are removed and dried in air, the indigo reverting to its original, insoluble form. The process by which a dye becomes "attached" to the material it colors is not definitely known. One theory holds that a chemical reaction takes place between the dye and the treated fiber; another proposes that the dye is absorbed by the fiber. Dyeing is an ancient industry. The Chinese, Persians, and Indians used natural dyes many centuries ago, including indigo, probably the oldest dye in use, and Tyrian purple, derived from a species of snail. The Egyptians prepared some brilliant colors. In the 13th and 14th cent. dyeing assumed importance in Italy; the methods employed were carried to other parts of Europe and, as new dyes became known, the dyeing industry flourished and grew. Cochineal was introduced from Mexico. Finally, in the 19th cent. the work of W. H. Perkin and Adolf von Baeyer produced the first synthetic dyes.

Any of a class of intensely coloured complex organic compounds used to colour textiles, leather, paper, and other materials. Dyes known to the ancients came from plants such as indigo and madder (see madder family) or from the shells of mollusks; today most dyes are made from coal tar and petrochemicals. The chemical structure of dyes is relatively easy to modify, so many new colours and types of dyes have been synthesized. Dye molecules are deposited from solution onto materials in such a way that they cannot be removed by the original solvent. Fibre-reactive dyes form a covalent bond with the fibre. Other dyes require prior application of a mordant, an inorganic material that causes the dye to precipitate as an insoluble salt. Another technique is vat dyeing, in which a soluble colourless compound is absorbed by the fibres, then oxidized (see oxidation-reduction) to the insoluble coloured compound, making it remarkably resistant to the fading effects of washing, light, and chemicals. Seealso azo dye.

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Any of a large class of synthetic organic compounds, dyes containing two nitrogen atoms joined by a double bond (see bonding, covalent bond) in the form singlehorzbondNdoublehorzbondNsinglehorzbond as part of their molecular structure. More than half the dyes in commercial use are azo dyes. They are usually classed according to the fibres for which they are useful or the methods by which they are applied; direct dyes are absorbed from solution by the fibres, but others require a second solution (a mordant) or a second step before the colour is fast.

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