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In industry, artificial dyes have been employed to the virtual exclusion of natural colouring agents for well over a century. Until the1850s, only a small number of artificial colouring agents were known. The great majority was of natural origin. Their preparation was difficult and time-consuming, so that colourful clothes and draperies became symbols of wealth and prestige.
Natural Dyestuffs
The most important dyes extracted from animal sources are natural sepia (from the ink sac of the cuttlefish), crimson (from the kermes louse) and Tyrian purple (the Imperial purple of the ancient world, from the murex shellfish). Very many dyes have been extracted traditionally from roots, berries, flowerheads, barks and leaves. Red dyes include madder (from 'dyer's root', the madder plant), brazilwood, beetroot, cranberry, safflower ('dyer's thistle'), and orchil ('dyer's moss'). Orange dye is obtained from stigmas of the saffron flower, yellows from camomile and milkwort flowers, plus weld ('dyer's herb'). Greens are obtainable from ripe buckthorn berries and ragweed; and blue from the woad plant (also called 'dyer's weed') and indigo.
As a rule, vegetable dyes are extracted by pounding or cutting up the colouring material. This is immersed in water, heated to just below boiling point and simmered until the colour has been transferred from the dye solution. For most natural dyestuffs to be effective, the article to be coloured must first be saturated with a fixer or mordant, a mineral compound which 'bites' the fibres of the cloth in order to permit the insoluble adhesion of the colouring agent. (A typical mordant consists of a 4:1 mixture of alum and cream of tartar.) The use of other metallic salts, as those containing chromium, copper, tin or iron, make it possible to obtain a considerable range of colours from a single dye source.
Artificial Dyestuffs
Before the nineteenth century, almost all dyestuffs were of natural origin, and their chemical constitutions unknown. The breakthrough came in 1856 when the English chemist William Perkin (1838-1907) accidentally discovered a vivid purple solution on mixing aniline (from coal tar) with potassium bichromate and alcohol. The commercial success of the resulting dye, commonly known as mauve, prompted the successful analysis of other natural dyestuffs, including those of alizarin (originally from the madder plant) in 1868, and artificial indigo in 1880.
A wide range of natural organisms is preserved in coal deposits and it is possible, by modifying the chemical constituents of coal, to synthesise a wide variety of colouring agents which form the basis of the modern dyestuffs industry. Derivative paints and inks are the artificial lakes (essentially 'dyed white pigments' prepared by depositing or precipitating a dye onto an inert inorganic pigment, such as blanc fixe or alumina hydrate.
In 1936 the commercial production of copper phthalocyanine (introduced originally as Monastral blue) from coal tar, made available an important new range of blue lakes highly fast to light and of exceptional staining power. Other synthetic dyes useful as artists' colours followed, most importantly naphthol red, quinacridone purple and dioxazine violet.
Such artificial dyes have long since overcome the impermanence of their 'aniline' period, and offer inexpensive substitutes for many traditional artists' paints. Natural dyes, such as henna or annatto, are still preferred for some traditional methods of hand-dyeing and for cosmetics or foodstuffs, since they are generally non-toxic and may blend together with age.
R.O.
Bibliography
Alison Cole (1993), Colour. London & New York: Dorling Kindersley.
Keith McLaren (1983), The Colour Science of Dyes and Pigments. Bristol: Adam Hilger.
Hazel Rossotti (1983), Colour: Why the World Isn't Grey. Harmondsworth: Penguin Books.
Copyright © 1998 Roy Osborne. All Rights Reserved.
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