Traditionally Indigo came from the leaves of the plant “IndigFera Tintoria” which was cultivated throughout the tropics. Towards the end of he 19th Century however, increasing demand for the dye led to the development of a synthetic version, with the only difference between the two being the absence of impurities in the latter.
A typical Indigo vat is like a bulging narrow-mouthed pot, about 10-15 feet deep and sunk into the floor of a covered area. The contents are like a living organism, and must be continuously nurtured.
When a new vat is started it is filled to about ¼ of its capacity with a thick sandy dye-liquor that has been retained from a previous vat. Indigo, slaked lime, and molasses are added and the whole thing is topped up with water. For the next two weeks the vat is fed daily with these ingredients, until it begins to look and feel ready. Finally, about 20 days after starting, when it is judged perfect, dyeing can begin.
In order to create a pattern, areas of cloth have to be prepared to resist the dye. This is usually done by block printing with a paste that prevents the dye from penetrating the fabric, but other methods such as tie-dye are also used.
The resistant paste is made by mixing earth, slaked lime, gum, a fine powder obtained from the action of insects on stored wheat, and water. This mixture is pressed through the cloth to create a smooth viscous adhesive paste.
As each length is printed it is dusted with sawdust to stop it from smudging before it is totally dry. The printed cloth is then dried in the sun before dipping in the vat.
As the cloth is drawn out it looks greenish, but on exposure to the air the indigo oxidizes and regains its original blue state.
Each time the cloth is dipped and exposed to the air a darker shade of blue is achieved. Often the cloth is reprinted with a different block to reserve patterns in a lighter blue before dipping again.
When the cloth is the desired shade of blue, it is washed to remove the resistant paste and any excess Indigo that has not adhered to the cloth.
Finally, after about 6 months of use the vat has to be emptied and clean due to the accumulation of sand and dirt, and the whole process starts again.