A coloring material used in printing inks, distinguished from a pigment by its vehicle-solubility, which pigments lack. Dyes, though more commonly used in the coloring of textiles, are frequently used in inks to impart a set of desired optical properties—such as transparency, purity, and color strength—not achievable with pigments. Dyes are most commonly used in liquid inks (such as those used in gravure and flexography), but special formulations allow some dyestuffs to be added to paste inks used in lithography and screen process printing. However, the classification of a dye often overlaps with some pigments. Dyes are frequently used as toners. Some common dyes include Eosine, Methyl Violet, Victoria Blue, Rhodamine B, and many others. (See also Pigment.)
Historically, most dyes were obtained from natural sources, such as plants, notably the woad plant ('Isatis tinctoria'), a source of blue dye; the indigo plant ('Indigofera tinctoria'), a source of indigo dye; and some species of madderwort (several species of the genus Rubia, most notably R. tinctorum), a source of a deep red dye (also called madder). Many dyes were also obtained from animals, such as mollusks and insects. However, it took 20,000 mollusks to dye one square yard of fabric, so dyed materials tended to be rare and expensive.
The first synthetic colorants were developed in the eighteenth and nineteenth centuries, most notably Prussian Blue, the first important synthetic pigment, invented in 1704. The first important synthetic dye, aniline dye, was created by accident in 1856 by William Perkin, who was actually trying synthesize quinine. (See Aniline Dye.)