Post-harvest

Most ancient dyes needed a mordant to fix them to the fibres being dyed. alum and various iron salts, earths and organic matter were used. Many dyes were prepared involving a method of fermentation, in some cases plant materials were left for many years. It is recorded that in the making of the famous Bulgarian Gobelin tapestries walnut husk were covered with water and left to ferment for two years before being used to dye the wool. In some cases various different plants were used to produce the necessary acids for dissolving some dyes. Dilute acids have the same effect as fermentation, and an earlier process involved the use of urine to dissolve various dyes. Conservationists repairing and cleaning old woollen tapestries can become well aware that urine was used as a mordant.

The chemical substances responsible for the blue colours in indigo and woad are exactly the same, but arise from slightly different precursors. In indigo, the naturally occurring precursor is a colourless water-soluble compound of indoxyl. Indoxyl is oxidized with oxygen from the air to produce indigotin, which is blue and insoluble. The blue of woad is also a result of indigotin, but there a precursor isatan-B hydrolyses to form the indoxyl leuco-indigo, two molecules of that combine to form indigotin. Both plants also contain indirubin and flavonols derived from kaempferol as well. Both woad and indigo are substantive dyes and do not need mordants to effect chemical binding. In 1883 the German chemist Adolf Bayer discovered the chemical structure of indigo which then made it possible to produce the dye synthetically in the 1890s. His method is still used - this synthesises the indoxyl by fusion with sodium phenylglycinate in a mixture of caustic soda and sodamide. Indigo can be converted into other similar compounds but the only commercially important chemical reaction is the reduction of the soluble yellow indoxyl leuco-indigo which is the form in which it is applied to textile fibres from where it then reoxidizes to indigo and turns blue.

Woad does not produce as much indigotin as indigo, though the chemical composition is the same. In ancient times the leaves of woad were pulped between immense wooden rollers turned by horses each led by a man. The woad was heaped, drained, kneaded and rolled into balls, two handfuls of the pulp making a ball. These balls were then dried, powdered, wetted again and fermented and left for 9 weeks during which time they were turned and sprinkled with water. The result was a dark clayey substance which as the dye. It was used as the basis for many 'sad colours' as the dark blue could be reduced to different shades and used as a fixer of other colours.

The chemical compounds responsible for the reds of madder and munjeet are the three-ring compounds the anthraquinones, the most active of which is alizarin. The pigments in madder are alizarin, rubiadin, purpurin, pseudopurpurin and mungistin, while munjeet contains many of these but also a unique pigment known as munjistin. Madder and munjeet both require a mordant to produce the deep red, common mordants used are alum (potassium or ammonium sulphate), chrome (chromium dichromate) and copper (copper sulphate). Depending upon the mordant used, the colour produced from madder can vary from apricot orange to deep red.

Lawsone (a napthaquinone substantive dye) in the leaves of henna reacts with the keratin in skin and human hair allowing it to hold an orange-red pigment. Other chemicals active in henna include the napthaquinone juglone (also found in walnuts) several flavonoids and tannins. The leaves and young twigs are pulverized into a fine powder, sometimes with various other plan additives. This is then made into a paste with hot water and then spread or painted into designs on the area of skin to be dyed, usually be left overnight. The action of the active components when mixed with hot water and applied to hair seals the oils and tightens the hair cuticle giving a rich, healthy shine. Modern henna hair colorants are applied for 1-2 hours, and wash out within 2-3 months. Distilled water from the sweet-smelling flowers can be made into a perfume, or used as a cosmetic. Henna used as a substantive dye gives a light tan, while the use of mordants such as alum, chrome or copper gives different shades of brown. Tin (stannous chloride) and iron (ferrous sulphate) used as mordants with henna give more orangish shades.

Medically henna has been used internally and externally for the treatment of jaundice, leprosy, smallpox and the fruit is thought to have properties that regulate menstruation.

The yellows of both weld and saffron are due to long-chain chemicals called carotenoids; Vitamin A is also a carotenoid. The main carotenoids in weld are luteolin and apigenin, luteolin is not found in other species of the genus Reseda. The principal pigment in saffron is crocin. Both these yellow dyes are adjective and are used principally with mordants, raw wool takes up very little colour. Weld used with alum or tin produces the brightest yellows, other mordants tend to produce greens or olive-greens. Use of tin (stannous chloride) as a mordant with saffron also produces a brilliant yellow, but in contrast to weld, the dye is readily taken up by unmordanted wool. Turmeric contains the carotenoid curcumin, also known in the food industry as the permitted food colouring E100.

The principal component in Juglans species is the napthaquinone juglone, whose action and ultimate colour is modified by the presence of acidic tannins also found in the leaves and husks. Mordants used with walnuts include copper, iron and alum – often in combination.

Since dyes were synthesized more than a century ago, vegetable dyes are rarely used commercially. But plant dyes produce more mellow tones as can be seen in 'village' or 'country' Oriental carpets, pre-industrial tapestries or ancient leather used in, say, the bindings of books. It is difficult to replicate these effects with synthetic dyes and the use of natural dyes is enjoying a renaissance. That said, however, it must be pointed out that the chemicals in these plant products are every bit as chemical as those of synthetic dyes and can have serious environmental effects if used without due care.


From: Seeds of trade (http://www.nhm.ac.uk/seeds)

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