New life in an old dye

Five hundred species of plants have a characteristic that the fashionable and the chic find irresistable. The substance extracted from their leaves yields the only natural blue dye known to man - indigo. It made fortunes for farmers in mediaeval times. Since then, synthetic dyes have taken the time and toil out of colouring textiles and natural dyes have faded out of use. But natural fibres are making a comeback in the fashion industry and natural fibres demand a natural dye. There will never be enough land to spare to grow sufficient indigo-bearing plants to dye the world's two billion pairs of jeans but could indigo help take some farming businesses out of the red?

credit: Jenny Balfour-Paul

There are indigo plants in both temperate and tropical regions. The most important is Indigofera tinctoria which was grown in colonial times in India, the West Indies, Central and South America. In Japan there is Polygonum tinctorum and, in Europe, woad (Isatis tinctoria). The substance that they all contain is indican, a by-product of the plant's own metabolic process that helps the plant to resist pest attack and discourages grazing animals. In humans, indican is reportedly anti-bacterial, and will reduce fevers and swellings, and fight skin infections. But the market as traditional medicine is modest in comparison to that for dyeing cloth.

The great drawback to indigo has always been the length and complexity of the processing required. In simple terms, the plant has to be picked, without bruising the leaves, and then soaked, while fresh, in warm, alkali water to release the indican which must then be oxygenated to produce blue indigo. In mediaeval times the whole process required twelve weeks or more and workers had to stand in huge vats of noxious liquid, whisking it with staves or bare hands. Even today, the traditional technique requires at least seven days but, in Guinea, indigo is now being produced in powdered form after a few hours of anaerobic fermentation of indigofera followed by filtration, decanting and drying in the sun. The techniques have been developed by PERTEGUI (Study and Research Project on Guinea's Indigenous Technologies) which is an IDRC-supported project. The work of Guinean dyers is now well known throughout West Africa. Dyers can buy powdered indigo on the market, thus avoiding the extraction stage and saving considerable time. Moreover, there has been an eight to ten-fold decrease in solid and liquid wastes from exhausted dye baths - a clear environmental improvement - and the nauseating fumes produced by conventional dye baths have also been eliminated, improving working conditions.

El Salvador was once a major producer of indigo under Spanish rule and elderly people remember the industry, which continued after the colonists left. Now, under a German backed project, the old Spanish dye tanks have been cleaned out and are being used to produce indigo again, but by more efficient methods.

In the East of England, traditional home of the indigo-bearing brassica, woad, extraction is also more efficient now than when it was used to paint the faces of local warriors to terrify invaders. Following major investment which, in itself, indicates the expected market, a mobile processing unit, taken to the field, turns green leaf to blue dye in 30 minutes. British indigo growers are delighted that the biggest dyer in the country is expressing great interest in reverting to natural indigo.

Currently 8,000 tonnes of indigo are imported to Europe, 400 tonnes of which is natural indigo mainly from India. That share is expected to treble in the next ten years and new markets are being sought all the time. Inkjet printers, for example, could use indigo pigment to complement environmentally friendly paper. Farmers and dye makers in tropical countries have more promising indigo-bearing plant species and should be able to compete. In music and mood, the expression 'the blues' means mournful and depressed. There is nothing depressed about the market for the natural colour blue!

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