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Natural dyes Until the s virtually all dyes were obtained from natural sources, most commonly from vegetablessuch as plantstreesand lichenswith a few from insects. Solid evidence that dyeing methods are more than 4, years old has been provided by dyed fabrics found in Egyptian tombs.

Ancient hieroglyphs describe extraction and application of natural dyes.

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Countless attempts have been made to extract dyes from brightly coloured plants and flowers; yet only a dozen or so natural dyes found widespread use. Undoubtedly most attempts failed because most natural dyes are not highly stable and occur as components of complex mixtures, the successful separation of which would be unlikely by the crude methods employed in ancient times.

Nevertheless, studies of these dyes in the s provided a base for development of synthetic dyes, which dominated the market by The heterocyclic dye known as indigo is a naturally occurring compound that can be obtained from plants in the genus Indigofera.

Two natural dyes, alizarin and indigohave major significance. Alizarin is a red dye extracted from the roots of the madder plantRubia tinctorium. Two other red dyes were obtained from scale insects. These include kermesobtained from Coccus ilicis or Kermes iliciswhich infects the Kermes oak, and cochinealobtained from Dactylopius coccus, which lives on prickly pear cactus in Mexico.

The principal coloured components in these dyes are kermesic and carminic acids, respectively, whose similarity was established by In their natural state many colorants are rendered water-soluble through the presence of sugar residues. These sugars, however, are often lost during dye isolation procedures.

Probably the oldest known dye is the blue dye indigo, obtained in Europe from the leaves of the dyerswoad herb, Isatis tinctoria, and in Asia from the indigo plant, Indigofera tinctoria.

Even by modern standards, both alizarin and indigo have very good dyeing properties, and indigo remains a favoured dye for denim, although synthetic indigo has replaced the natural material.

Dyerswoad, Isatis tinctoria, is a source of indigo, considered the oldest known dye. With a process developed by the Phoenicians, a derivative of indigo, Tyrian purplewas extracted in very small amounts from the glands of a snail, Murex brandaris, indigenous to the Mediterranean Sea.

Experiments in yielded 1. Historically, this dye was also called royal purple because kings, emperors, and high priests had the exclusive right to wear garments dyed with it, as is well documented in the Hebrew Bible and illustrated for Roman emperors on mosaics in Ravenna, Italy.

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By the s, with the decline of the Eastern Roman Empirethe Mediterranean purple industry died out. Natural yellow dyes include louting, from the leaves of weld, Reseda luteola, and quercetin, from the bark of the North American oak tree, Quercus tinctoria. These are in the flavonoid family, a group of compounds occurring almost exclusively in higher plants and producing the colours of many flowers.

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In fact, these compounds can produce all the colours of the rainbow except green. Luteolin, a yellow crystalline pigmentwas used with indigo to produce Lincoln green, the colour associated with Robin Hood and his merry men. Another group of compounds, the carotenoidspresent in all green plants, produce yellow to red shades.

Lycopenefrom which all carotenoids are derived, produces the red colour of tomatoes. An ancient natural yellow dye, crocetin, was obtained from the stigmas of Crocus sativus; this dye is undoubtedly derived from lycopene in the plant.

Few of the flavonoid and carotenoid colorants would have survived ancient extraction processes. Logwood is the only natural dye used today.

Heartwood extracts of the logwood tree, Haematoxylon campechianum, yield hematoxylin, which oxidizes to hematein during isolation. The latter is red but in combination with chromium gives shades of charcoal, gray, and black; it is used mainly to dye silk and leather.

Mordants Highly skilled craftsmen with closely guarded secret formulas rendered dyeing a well-protected trade. The formation of different colours by mixing red, blue, and yellow dyes was well known in ancient times, as was the use of metal salts to aid the retention of dyes on the desired material and to vary the resultant colours.

These are adsorbed on the fibre and react with the dye to produce a less soluble form that is held to the fabric. No doubt the secret processes included other ingredients to improve the final results. Mordants also were used to vary the colours produced from a single dye.

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For example, treatment with aluminum hydroxide, Al OH 3, before dyeing with alizarin produces Turkey red, the traditional red of British and French army uniforms.

Alizarin gives violet colours with magnesium mordants, purple-red with calcium mordants, blue with barium mordants, and black-violet with ferrous salts.

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Aroundchromium salts, used as mordants, were found to provide superior dye retention and, in time, largely displaced the others; chromium mordants are still widely used for wool and, to some extent, for silk and nylon.

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