True Colors: A Century of Synthetic Dyes
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True Colore: A Century of Synthetic Dyes For more than 5,000 years, mankind has brought color into the world by extracting various pigments and using them to dye textiles, leather, paper, wood, and many other things. Up until the last century, though, ail such pigments were obtained either directly or indirectly from natural sources. Most natural dyes corne from plant parts—the berries, bark, leaves, flowers, or roots. The madder plant native to Asia and Europe provided bright red color for linen and silk. The crocus plant gave a yellow saffron dye. The little Indigofera Tinctoria bush, mainly from India, supplied a muchprized dark-blue color. The logwood tree from Mexico, Central America, and the West Indies yielded black and brown dyes. The henna shrub of North Africa and the Middle East gave a reddish-brown color, chiefly used to dye leather. Some dyes were derived from animal sources. Best known is carminé, a bright red extracted from the dried bodies of the female cochineal insect of Mexico and Central America. "Tyrian purple" of ancierit times, reserved for royalty because of its scarcity, was obtained only from some Mediterranean shellfish. Unfortunately, most natural dye substances do not bond firmly to textiles—they fade with sunlight or wash out with water. Not until 1856 was the first synthetic dye discovered. English chemist William H. Perkin stumbled on the pale purple color, mauve, while searching for something completely unrelated. In 1855 Perkin, the son of a carpenter who did not approve of young William's interest in chemistry, became the assistant of the German chemist A.W. von Hofmann. Hofmann had speculated aloud on the possibility of synthesizing quinine in the laboratory. Quinine, the valuable alkaloid used to fight against malaria, was obtained from the bark of the tropical cinchona tree. If it could instead be made from cheap coal-tar chemicals, it would be a boon to medicine and amass a fortune for the inventor of the process because it would eliminate European dependence on distant imports. Excited by the idea, Perkin went home during his Easter vacation in 1856 to try on his own. He was enthusiastic—and only 18 at the time—but his attempt was not successful for many reasons. The chemical structure of quinine was not known at the 54
time, and even if it had been the primitive synthetic methods available were not sufficient to the task. But in one of his attempts, Perkin mixed the coal-tar derivative aniline with potassium dichromate. Before pouring out the failed mixture, he noticed a purplish tint to the liquid. By some instinct, he decided to add alcohol to the compound, which dissolved out a beautiful purple precipitate. Perkin immediately wondered if the substance might be used as a dye. Earlier, he and Arthur H. Church had patented a process for obtaining "nitrosonaphthalene" by reducing dinitrobenzene and dinitronaphthalene, which turned out to be the first of the azo-dyes used in later manufacturing. Perkin sent a sample of his new purple compound to a dyeing company in Perth, Sco
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