Introduction
Indigo dye is an important dyestuff with a unique shade of blue color. The natural dye comes from several species of plant .The dye gives a brilliant and eye-catching blue color to the fabric. This color partially penetrates the fabric but then also imparts surface blue color to the fabric. The word indigo is derived from the Latin Indicum and the Greek indikon meaning 'blue dye from India' or being more specific-'Indian substance'.

Natural Indigo is the oldest known dye to mankind. When the synthetic substitute of the dye was not invented, all blue textiles were used to be dyed with 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'. The natural production of indigo is extracted from the leaves from a variety of plant species including indigo, woad, and polygonum. Only the leaves are used since they contain the greatest concentration of the dye molecules.

Structure
Indigo is a dark blue crystalline powder that melts at 390-392C. It is soluble in chloroform, nitrobenzene, or concentrated sulfuric acid, but not soluble in water, alcohol and ether. The chemical structure/formula of indigo is C16H10N2O2. Indican is colorless and soluble in water which is a by-product of indigo. Indican is hydrolyzed to glucose and indoxyl .Indigo is converted to indoxyl by mild oxidation. When indigo is decomposed simple compounds like aniline and picric acid are produced . Reduction of urea to indigo white is of great importance The indigo white after it is applied to the fabric is reoxidized to indigo .

History

Indigo is one of the oldest dyes that was used for textile dyeing and printing. Asian countries like India, China and Japan had used indigo as a dye for centuries. The ancient civilizations in Mesopotamia, Egypt, Greece, Rome, Britain, Peru, Iran, and Africa had the knowledge about the dye. Since Neolithic times in Europe this dye is being used It was highly prized for its colour and light fastness .Until the end of the 19th century; the sole source was from plants, woad (Isatis tinctoria) and Dyer's Knotweed (Polygonum tinctorum) in temperate climates and Indigofera species in the tropics. Woad was widely grown in Europe, making some regions, especially Toulouse (France) and Erfurt (Germany) making them economically very prosperous. It was used to make a woad vat for dyeing with indigo from India after that period It belongs to the legume family and its three hundred plus species have been identified. In ancient times, indigo was considered a precious commodity because plant leaves contain only about small amount of the dye (about 2-4%). Indigo remained a rare commodity in Europe throughout the Middle Ages; woad, a dye derived from a related plant species, was used instead. Indigofera tinctoria was originally domesticated in India; it is mentioned in manuscripts dating from the 4th century BC about its origin in India.

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 were dyed by indigo dye, the term blue collar worker has been inspired from this aspect .It was a luxury item imported to the Mediterranean from India by Arab merchants. The Portuguese explorer Vasco da Gama in late 15th century discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands, China, and Japan .It resulted in avoidance of the heavy duties imposed by Persian, Levantine, and Greek middlemen on the importers and also the lengthy and dangerous land routes which had previously been used for transportation . As a result, the importation and use of indigo in Europe rose significantly. In the 19th century, the British obtained much indigo from India .When the synthetic substitute of indigo was invented the demand for natural indigo dropped, as a consequence for many indigo farmers the raising of indigo became loss making business.

Cultivation
Since synthetic indigo dye is being used ,use of natural indigo dye has almost became extinct , 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. In present time, indigo is still cultivated for dye on a small scale basis in India (particularly in the northern part of Karnataka) and in some parts of Africa and Central America. It is frequently grown as a secondary crop. Dissemination is usually by seeds which are sown at first in seed beds or directly into the field. Germination takes about 4 to 5 days. When the plants are about 4 to 5 months old and starts flowering then the branches are harvested. At 3 to 4 months intervals three times a year the plants could be continued to be harvested. The total life span of the plants could range from time period 2 to 3 years.

Processing

A. For Natural Indigo Dye

. The leaves of indigo go through a process of fermentation and then oxidized to yield the blue dye. Traditionally fermentation was carried out by bacteria. The harvested plants first of all, packed into tanks and is covered with water. After a few hours of soaking the plants in water, the leaves become saturated and fermentation begins. A thick layer of bubbles and foam forms at the top of the tank. The process can be so vigorous that planks are placed on top of the tub to keep the plants in. This process can take up to a day and a half to complete, but the timing has to be perfect .The indigo makers will smell and taste the fluid to check whether the dye is ready or not. Even if an hour is taken extra it could ruin it. As soon as the liquid tastes sweet and is a dark blue colour, it is drained off into another vessel at a lower level, leaving the plants behind. The liquid now formed contains indoxyl.

. To stimulate oxidation of the indoxyl the liquid is stirred continously for many hours for it requires oxygen for oxidation. On alternative basis people will get into the vats to tread up and down to stir it up. After sometime 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 bluish sludge. To remove impurities and to stop the enzyme reaction which made the indigo the water is drained and filtered. The sludge is then dried to produce indigo 'cake' which is cut into cubes or made into balls.

. The Japanese used another method of extracting indigo from the polygonum plant, the plant is mixed with wheat husk powder, limestone powder, lye ash, and sake in this method of extracting indigo. The mixture is allowed to ferment for about one week to form A dye pigment which is called sukumo is formed from the fermented mixture.

. The indigo was used to be dissolved in stale urine in Europe as a pre-industrial process for dyeing in earlier times .The water-insoluble indigo is converted to soluble substance known as indigo white or leucoindigo by urine, which produces a yellow-green solution. After the indigo white oxidizes the fabric turns into blue color fabric.

. Another preindustrial method, used in Japan, was to dissolve the indigo in a heated tub in which a culture of thermophilic, anaerobic bacteria was maintained .Insoluble indigo could be converted into soluble indigo white by certain specific species of bacteria when they generate hydrogen as a metabolic product. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki.

B. For Synthetic Production of Indigo Dye
. The synthesis reactions are conducted in large stainless steel or glass reaction containers. These containers hold jackets to permit steam or cold water to flow around the batch, as the reactions progress. On account of complication of these chemical processes, the dye is usually made in batch quantities. The Germans have invented methods for continuous process manufacturing.

. The first commercial method of producing indigo was based on Heumann's work. In this process, N-phenylglycine is treated with alkali to produce indoxyl, which could be then converted to indigotin by contact with air. But the amount of dye produced by this process is very less. More competent method, synthesis utilizes anthranilic acid. A alternate of this method (which is widely used) involves the reaction of aniline, formaldehyde, and hydrogen cyanide to form phenylglycinonitrile. This material is thereafter hydrolyzed to yield phenylglycine which is then converted to indigotin. Currently a method which uses sodamide with alkali to convert phenylglycine to indoxyl is used. Sodamide reacts with excess water, thus lowering the overall reaction temperature from almost 570F (300C) to 392F (200C). Efficient reaction process is achieved through it.

. After completion of the chemical reaction processes, the finished dye has to be washed to remove impurities and then dried. The dried powder then can be packed in drums or reconstituted with water to form a 20% solution and filled in containers.

Usage
. Indigo blue is the primary color of blue jeans in the west, its importance as result is increased in present time. In the United States indigo is primarily used to dye cotton clothes and blue jeans .Over one billion pairs of jeans around the world are dyed blue with indigo. Deep navy blue colors on wool are produced by indigo dye.

. The Romans in earlier times, used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item for Arabs which was imported to the Mediterranean from India by Arab merchants.
. Indigotinesulfonate is used as a dye in renal function testing and as a reagent for the detection of nitrates and chlorates .It is also used in the testing of milk.

. Indigo is also used as a food coloring under the name FD&C Blue No. 2.

. It was the original dye of the "Levi's" blue jeans, a trademark color for durability. It is the only natural blue dye that is permanent in nature.

. On many instances from Europe to the Arab world, historical use of blue to stain skin for war, religious and social rites were found.

. Indigo in combination with resist techniques create decorative wooden items.

. In certain cultures indigo dye is also used for craft purpose such as on paper, leather and on silk.

The Future

Scientists all over the world are carrying out research programmes on inventing a method of producing indigo dye which is less hazardous to health .Using biocatalysts in the dye reaction process might work out for producing environment friendly dye. By making through a biological route, indigo dye would become one of the first high-volume chemicals by the biological route. Genencor International, of Rochester New York, is experimenting on a process to produce indigo using biotechnology. However, at this stage the technology is expensive and production costs might be prohibitive. The natural dye comes from several species of plant, but nearly all indigo produced today is synthetic in nature.

Reference:
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