Indigo - production & trade

Until the development of a synthetic substitute, all blue textiles were dyed with natural indigo. These included the blue serge uniforms worn by the British police force and hospital staff, as well as military personnel and workman's clothes worn by millions of people, inspiring the term 'blue-collar worker'.

Cultivation

A black and white photograph from 1900, showing the interior of an indigo factory in British India.
Image: The filtering table and boilers in an indigo factory in British India, 1900.

The use of plant indigo has nearly died out and has been almost completely replaced by synthetic indigo. But in recent years, the demand for natural dyes has been increasing in many countries again, because of health and pollution effects and a revival of interest in the relationship between dyes and culture.

At present, indigo is still cultivated for dye on a small scale in India (particularly in the northern part of Karnataka) and in some parts of Africa and Central America.

It is grown often as a secondary crop. Propagation is usually by seeds which are sown at first in seed beds or directly into the field. Germination takes about 4 days. Branches are harvested when the plants are about 4 to 5 months old and flowering. The plants can continue to be harvested like this at 3 to 4 months intervals three times a year. The total life span of a crop can be 2 to 3 years.

Processing

As part of their preparation, the leaves of indigo must go through a process of fermentation and then oxidation to yield the blue dye. Traditionally fermentation is carried out naturally by bacteria. The harvested plants are packed into tanks and covered with water. After a few hours, the leaves become saturated and fermentation begins. A thick layer of bubbles and scum forms at the top of the tank. The process can be so vigorous that planks are placed on top of the vat to keep the plants in. This process can take up to a day and a half to complete, but must be finely timed. The indigo makers will smell and taste the fluid to check. Even an hour too long could ruin it. As soon as the liquid tastes sweet and is a dark blue colour, it is siphoned into another vat at a lower level, leaving the plants behind. The liquid now contains indoxyl.

The liquid is then stirred continuously for several hours because it needs oxygen from the air to stimulate oxidation of the indoxyl. Alternatively people will get into the vats and tread up and down to stir it up. Eventually the liquid turns a yellow-brown colour with floating dark blue patches. The solution is left to rest and the insoluble indigo settles to the bottom of the tank as a blueish sludge. The water is drained and filtered to remove impurities and to stop the enzyme reaction which made the indigo. The sludge is dried to produce indigo 'cake' which is cut into cubes or made into balls.