Indigo dye is a natural dye that is extracted from plants and animals. Though the dye was and is still produced organically, most of the indigo dye available in the market today is mass produced through chemical processes to satisfy increased demand for blue dye.
Indigo is thus named due to its distinct blue colour. It is one of the oldest dyes in existence and has been used for many centuries in ancient India, China and Japan, for dying and printing textiles. India, which was the main producer of Indigo, exported the dye to Europe and the Mediterranean region through Portuguese and Arab traders. The Greeks and Romans also bought their indigo dye from India and used it as a luxury commodity. Other ancient civilisations from Africa, Mesopotamia and Egypt have also used indigo for centuries. In most west African cultures, indigo was used to dye garments worn by wealthy people to symbolise their status.
Indigo is primarily used for dying cotton yarn which is used for making blue jeans. It takes around 3 to 12 grams of indigo dye to dye enough yarn to produce one pair of jeans. Though rare, indigo is also used for dying other natural fibres such as wool and silk.
Plants and animals that produce natural indigo
Indigo can be produced from many plant species but as most plants contain the dye in small concentrations, it makes the dye difficult to extract. Most plants also produce a lower quality of indigo, which loses its blue colour to assume a greenish tinge when it comes into contact with other dyes.
Plants of the genus Indigofera are favourite for Indigo production, because they yield a high quantity of the blue dye. In Asian countries, Indigofera tinctoria (known as true Indigo) is the main plant grown for indigo production.
Añil and Dyers Knotweed, grown in Central and Southern American countries, are popular alternatives for Indigofera in those regions. Indigo dye is produced from the woad plant in European countries.
Indigo can also be produced from animals. Sea molluscs, known as Murex, are a source of blue and red Indigo. When the two dyes are mixed, an array of purple hues, called tyrian purple, is produced. Exposing the Red Indigo (also known as Dibromo Indigo) to light will yield royal blue or hyacinth purple, which are both types of blue indigo.
Natural extraction of Indigo
Indigo is produced from Indican, a colourless substance contained in leaves of indigo-producing plants. The leaves are soaked in water and left to ferment. During fermentation, Indican breaks down to produce indoxyl and glucose. The fermented indoxyl-rich liquid is drained into another tank where it’s stirred to mix with oxygen, a process that turns the liquid blue.
After draining excess fluid, lye is then mixed into the solution to form cakes and once the cakes dry, they are ground into powder.
Synthetic production of indigo
Due to the need for mass production, the necessity to increase production efficiency and the advancement of chemical knowledge, production of indigo has gradually moved from natural to synthetic.
Synthetic indigo was first commercially produced in 1897 and between the same year and 1914, natural production dropped from 19,000 tons to 1000 tons.
The most widely used method of synthetic indigo production today involves forming phenylglycinonitrile by combining anilinine, hydrogen cyanide and formaldehyde. The phenylglycinonitrile is broken down into phenylglycine, which is then treated with sodamide, to yield indigo. This method is preferred because sodamide lowers the amount of energy required to process the mixture, thus making the process more efficient.
Differences between synthetic indigo and natural indigo
Using natural indigo
Minimises the amount of pollutants to dispose off
The waste produced during production is mostly plant polymers, which are biodegradable.
Tests have shown that insects and other animals can survive in a fermenting mixture of natural indigo
Growing indigo-producing plants provides a source of income for farmers and workers
Plants provide green cover which is good for the environment
Using synthetic indigo
Involves petrochemicals which yield more pollutants
The pollutants and impurities produced during manufacture, such as anilinine, are not biodegradable
When these pollutants are dumped into rivers and lakes, they kill aquatic life and make the water poisonous to humans and other terrestrial animals.