Indigo is one of the oldest Natural Dyes used in textile printing. It imparts a brilliant blue hue to fabrics. Indigo belongs to a class of dyes called vat dyes, which are among the oldest natural coloring substances used for textiles.
Indigo was known throughout the ancient world for its ability to color fabrics a deep blue. Egyptian artifacts suggest that indigo was employed as early as 1600 BC and it has been found in Africa, India, Indonesia, and China. India is considered to be the oldest centre of indigo dyeing in the world.
The manufacture of natural indigo lasted well into the early 1900s. Demand for indigo dramatically increased during the industrial revolution, in part due to the popularity of Levi Strauss's blue denim jeans. The natural extraction process was expensive and could not produce the mass quantities required for the burgeoning garment industry. So chemists began searching for synthetic methods of producing the dye.
In 1905 Adolf von Baeyer (the scientist who also formulated aspirin) was awarded the Nobel Prize for discovering the molecular structure of indigo, and developing a process to produce it synthetically. The natural dye was quickly replaced by the new synthetic, ending an ancient and honored botanical history.
 Sources of Natural Indigo
The dye may be extracted from several plants, but historically the indigo plant was the most commonly used as it was widely available. It belongs to the legume family and over three hundred species have been identified. Indigo tinctoria and Indigo suifruticosa are the most common. Here is a list of sources of natural indigo:
- Indigofera tinctoria from India
- Indigofera suffruticosa from Mexico, the Caribbean and South America
- Isatis tinctoria (woad) from Europe and Egypt
- Lonchocarpus cyanescens from West Africa
- Marsdenia (milkweed) from Sumatra
- Nerium tinctorium (oleander) from India and the Far East
- Polygonum tinctorium (Japanese indigo or buckwheat)
 The Process of Indigo Dyeing
The process of Indigo Dyeing is based on a reduction and oxidation process. Indigo isn't water-soluble. Therefore, in order to dye cloth, Indigo has to be changed into a water soluble substance, Indigo white, using a reduction agent Over the years, different reducing agents have been used in Indigo Dyeing. In the olden days, the most commonly used reducing agent was stale urine. Anecdotal evidence from the early 1800s suggests that in Britain, indigo dyers specially used the urine of persons who had over indulged in alcohol the night before. Synthetic urea replaced urine in the 1800s. Commonly used reducing agents today include thiourea dioxide or thiox, sodium hydrosulfite and Zinc.
Indigo white imparts a yellowish hue to the cloth. When this (yellow) dyed wool is taken out and exposed to the oxygen in the air, the Indigo white oxidizes back to indigo, changing the colour of the cloth to blue. The process of dyeing with fresh Indigo leaves is as follows:
Put fresh indigo leaves in a bucket, adding just enough hot tap water to cover them. Heat the solution slowly to 160 degrees Fahrenheit for two hours.
Strain the liquor, squeezing the indigo leaves well. Add baking soda to it and stir.
Pour the liquid from one bucket to another until the solution turns dark green/blue. This oxidizes the dissolved indoxyl, changing it to indigo.
Dissolve Thiox (the reduction agent) in warm water, pour it into the dyebath, cover and set the pot in a larger container of water that is just hot enough to keep the dyebath at 100 to 120 degrees Fahrenheit for about an hour.
Meanwhile, soak yarn/fabric in hot water.
When the dyebath turns yellow, soak wet yarn/fabric in it for 20 minutes. Remove it gently and let it oxidize by hanging it on a wooden rack. The yarn will turn blue as it reacts with oxygen in the air.
For intense blue color, soak the dried yarn/fabric again in the dyebath.
To try Indigo Dyeing at home, go to Treating with alum as mordant
 Dyeing Methods that use Indigo
One method of dyeing using Indigo is Resist Dyeing. In this, the area in the pattern that does not need to be dyed, is painted with wax (which resists the dye). The fabric is then dyed with Indigo, and the wax is removed by boiling. Ajrakh Printing in Kutch and Sindh, and Kalamkari textiles from South India use this dyeing technique.
Another method of creating patterns using Indigo dye is to dye fabric all over with indigo first and then remove dye from some areas by printing with bleach. This is called discharge dyeing and was not used much until the 19th century.
Indigo cannot be used for Block Printing on Textiles where the dye is painted onto patterned blocks and pressed onto the fabric. This is because to work properly the dye needs to oxidise from indigo white to indigo blue on the fabric, not on the blocks which are exposed to the air.
To buy Indigo-dyed products, go to Anokhi Home and Fabindia
 Other uses of Indigo
Blue indigotin has been traditionally ground and mixed with water or oil to make ink and paint. Indigo is used like this in many Asian paintings such as the Patachitras of Orissa. The frescoes of the classical world also used indigo-based paints.
Ancient Egyptians, Greeks and Romans also used Indigo as eye shadow and in crayons.
- Indigo Dyeing