The ancient Greeks called the plant dye indigo by a name which is pronounced as indikón and means Indian. The Latin name, used by the Romans, was indicum, from where it mutated to indigo in English. The dye was highly valued, from royalty to militaries (as in navy blue) and came from a tropical genus of plants, Indigofera, some of which were native to the Indian subcontinent.
Leaves of these plants contain upto 0.5% of Indican which when exposed to oxygen produces the blue substance Indigotin. Cakes of this were a major item of trade from India. Crossing the sea to the Middle East, Arab merchants would carry it across deserts to the Mediterranean and into Europe where it was a highly prized item, used in the colouring of silk, in paintings and murals, and in cosmetics.
The establishing of sea routes to India from Europe led to a rapid rise in indigo export, as it was one of the few reliable dyes for cotton — a fabric of the masses.
The dye named Suranji got from Indian mulberry ( Morinda tinctoria, aal in Hindi; manjanatti or manjanunnai in Tamil), gives a bright yellow-red, or chocolate, or even black colour to cotton, depending on the method of ‘fixing’ it to cloth. The roots of Manjistha, the Indian madder ( Rubia cordifolia –manjith in Hindi; manditta in Tamil) yield a red pigment called purpurin, that in 1869, became the first dye to be synthesised in a laboratory, as alizarin.
Safflower ( Carthamus tinctorius – Kusum in Hindi; Kusumba in Tamil), of which India is today a major producer on account of the oil, was the source of carthamin and carthamidin, which lend a red colour to cotton and a distinctive orange-red to silk.
Over 160 tonnes of the dried flowers of this plant were exported from India every year until synthetic aniline dyes overtook the natural variant.
Colour of jeans
By the beginning of the twentieth century, relentless progress in chemistry had led to cheap methods of synthesising indigo, which replaced the plant dye in most applications. A typical pair of blue jeans is loaded with 3-5 grams of synthetic indigo. Over 40,000 tonnes of indigo are produced every year.
The environmental impact of using synthetic dyes is considerable, and the dyeing of textiles is a major source of water pollution. Most synthetic dyes are made from petrochemical derivatives.
Indigo is by itself non-toxic, but it is insoluble in water and strongly alkaline lyes are needed to dissolve and fix it to fabrics.
That same pair of jeans requires over 100 litres of water during the dyeing process alone, and along with the lye, about 15% of the dye escapes into the waterways as pollution. Jeans-wearers, think about it!
Luckily, this means that the use of natural, plant-origin dyes has not been phased out. Indigo continues to be cultivated. The environmental impact of natural dyes is much lesser than that of synthetic equivalents. The search for better and better techniques and procedures for using natural dyes from other commonly available herbs, shrubs and trees continues.
The work of Padma Shri Vankar (formerly of IIT Kanpur), her colleagues and other Indian groups is worthy of note. Collectively, they have identified and sometimes successfully recreated dye-extraction and dyeing methods for several plant species.
The list includes: (a) Nepal barberry ( Mahonia napaulensis, Daaruhaldi in Hindi; Mullumanjanathi in Tamil): The Apatani tribe of Arunachal Pradesh has for long used this plant for colouring their weaves. (b) Wild canna ( Canna indica – Sarvajjaya in Hindi; Kalvazhai in Tamil) The flowers of this ornamental plant are bright red, from an alcohol-soluble dye that can be easily fixed on to cotton and stands fast. (c) Flame of the forest ( Butea monosperma, Palash in Hindi) is a native of our subcontinent and has an eye-catching flower from which is derived a traditional colour of the Holi festival. Sun-dried petals of this flower are rich in dyes that can be extracted by water.
Meanwhile, biotechnologists look to bypass the environmental costs of chemical methods. This has been proved in concept by engineering bacteria to produce the indigo precursor, indican.
The conversion of this to the dye is performed by enzymes on the surface of wet denim, thus eliminating several toxic effluents (Hsu et al., Nature Chemical Biology 2018, doi:10.1038/nchembio.2552).
(This article was written in collaboration with Sushil Chandani who works in molecular modelling, [email protected])