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Traditional Indigo Dyeing before visiting the dyer we first have a bath

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The process follows closely the reported methods in the literature; i.e., a fermentation process by the natural microflora, followed by oxidation in air to form the insoluble indigo dye, reduction of this to form the soluble white indigo, and finally followed by reoxidation to form the blue dye again on the fabric. It must be remembered that the indigo plant itself does not contain the indigo dye, but the precursor molecule indican, a glucoside, which is hydrolyzed to the soluble indoxyl and a molecule of glucose by bacteria. It is when this indoxyl reacts with air that the in soluble indigotin (the blue indigo dye) is formed. Thus the balls of dyes sold are actually balls of solid indigo.

Because this indigo is insoluble in water it cannot be used as such, and so has to be reduced to the white soluble indigo by bacteria in the natural microflora or with sodium hydrosulfite, a reducing agent. The white indigo is soluble in alkaline water, hence the function of the sodium hydroxide is to provide the crucial alkaline environment for the enzymatic catalysis of the reaction which hydrolyzes the gluco side indican to indoxyl and D-glucose.
A pH of about 8.0 is maintained in the dyebath, which is optimal for the production of the dye. When the fiber is immersed into the dyebath, it takes up the white soluble leuco dye, and when dried in air, oxygen reoxidizes the soluble form back to the insoluble blue indigo on the fiber.

It is quite amazing that even though these traditional dyers are not chemists, they kept their vats empirically- with the aid of smell mostly between pH 8.5 and 9. This is the best pH range for the recovery of indigo; a bit below 8.5 or above 9 and the yield drops precipitously. Their expertise involved a very finely controlled balancing of all the variables empirically, such as the concentration of the dye, temperature of the water, alkalinity, amount of reduction, and length and number of immersions. They use taste, smell, feel, appearance, and their many years of experience, yet as with dyeing even in the west, one bath is different from the next.

Of importance also, is the fact that rusty nails and cups are added to the dyebath to darken the color. This is clearly the ferric ions on these rusty materials acting as a mordant, much in the same way as ferric sulfate does. Other mordants used in this indigenous process include the ash of Sterculia tragacantha, which acts as a kind of local alum, and is used by some dyers instead of caustic soda. This mordant property of the ash was also reported by Abbiw. The leaves of Aichornia cordifolia also darkens the hue. Not surprisingly, it has been reported to fix colors; i.e., it is a mordant and contains tannin, in addition to being a dye itself. Also, both Jatropa and Mangifera produce dyes and contain tannins, whereas the black powder called colmet is also used to darken the color of the indigo dye, and the scented soap used to wash the dyed fabric reduces the unique odor produced in the fermentation process

One cannot help appreciate the soundness of some of the practices discussed and the scientific basis of a number of them. The indigenous knowledge and practices of these local dyers may reveal ideas that contain seeds of adaptive value. One can therefore draw enlightenment and insight from these experienced craftswomen and men.

In view of the perceived deleterious environmental consequences of the use of synthetic dyes, there is a trend to go back to the original natural dyes. In addition, there is a large market in Western countries for carpets made of natural wool and dyes, amongst others.

The current focus will therefore take us full circle, back to the natural production of indigo from plants and bacteria. In addition, several other scientists have been studying various aspects of the indigo-dyeing process, including the microbiology and genetic engineering, with a view to improving their efficiency. The European Union project on sustainable production of plant-derived indigo, which aims to satisfy 5% of the European indigo market with indigo producing crops by 2005, is investigating the agronomy, dye extraction, microbiology, biochemistry, standardization, quality control, and environmental impact.
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