Indigo dye

Indigo dye is an organic compound with a distinctive blue color (see indigo). Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today — several thousand tons each year — is synthetic. It is the blue of blue jeans.

Uses
The primary use for indigo is as a dye for cotton yarn, which is mainly for the production of denim cloth for blue jeans. On average, a pair of blue jean trousers requires 3 – 12 g of indigo. Small amounts are used for dyeing wool and silk.

Indigo carmine, or indigotine, is an indigo derivative which is also used as a colorant. Approximately 20M kilograms are produced annually, again mainly for blue jeans. It is also used as a food colorant, and is listed in the USA as FD&C Blue No. 2, and in the European Union as E Number E132.

Plant sources
A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia (Japanese: リュウキュウアイ. Chinese: 馬藍/山藍). In Central and South America the two species Indigofera suffruticosa (Añil) and Indigofera arrecta (Natal indigo) were the most important. In temperate climates indigo can also be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye.

Extraction
The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan. Indican readily hydrolyzes to release β-D-glucose and indoxyl. Oxidation by exposure to air converts indoxyl to indigo. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2 – 0.8 % of this compound. The leaves were soaked in water and fermented in order to convert the glycoside indican present in the plant to the blue dye indigotin. The precipitate from the fermented leaf solution was mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple.

Cultivation
The demand for indigo in the 19th century is indicated by the fact that in 1897, 7000 square kilometers were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg consists of 2,586 square kilometers.

In literature, the play Nildarpan by Dinabandhu Mitra is based on the indigo slavery and forceful cultivation of indigo in India. It played an essential part in the Bengali indigo revolt of 1858 called Nilbidraha.

History of natural indigo
Indigo was used in India, which was also the earliest major center for its production and processing. The Indigofera tinctoria variety of Indigo was domesticated in India. Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product.

Indigo is among the oldest dyes to be used for textile dyeing and printing. Many Asian countries, such as India, China, Japan and South East Asian nations have used indigo as a dye (particularly silk dye) for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Greece, Rome, Britain, Mesoamerica, Peru, Iran, and Africa.

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, indikón (ινδικόν, Indian). The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

In Mesopotamia, a Neo-Babylonian cuneiform tablet of the 7th century BC gives a recipe for the dyeing of wool, where lapis-colored wool (uqnatu) is produced by repeated immersion and airing of the cloth. Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.

Indigo remained a rare commodity in Europe throughout the Middle Ages. Woad, a chemically identical dye derived from the plant Isatis tinctoria (Brassicaceae), was used instead. In the late 15th century, the Portuguese explorer Vasco da Gama discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands, China, and Japan. Importers could now avoid the heavy duties imposed by Persian, Levantine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Spain imported the dye from its colonies in South America. Many indigo plantations were established by European powers in tropical climates; it was a major crop in Jamaica and South Carolina, with much or all of the labor performed by enslaved Africans and African-Americans. Indigo plantations also thrived in the Virgin Islands. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry.

Indigo was the foundation of centuries-old textile traditions throughout West Africa. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba of Nigeria and the Mandinka of Mali particularly well known for their expertise. Among the Hausa male dyers, working at communal dye pits was the basis of the wealth of the ancient city of Kano, and they can still be seen plying their trade today at the same pits.

In Japan, indigo became especially important in the Edo period when it was forbidden to use silk, so the Japanese began to import and plant cotton. It was difficult to dye the cotton fiber except with indigo. Even today indigo is very much appreciated as a color for the summer Kimono Yukata, as this traditional clothing recalls Nature and the blue sea. In colonial North America there were three commercially important species: the native Indigofera caroliniana, and the introduced Indigofera tinctoria and Indigofera suffruticosa.

Newton used "indigo" to describe one of the two new primary colors he added to the five he had originally named, in his revised account of the rainbow in Lectiones Opticae of 1675.

Because of its high value as a trading commodity, indigo was often referred to as Blue Gold.

Era of synthetic indigo
In 1897, 19,000 tons of indigo were produced from plant sources. Largely due to advances in organic chemistry, production by natural sources dropped to 1000 tons by 1914 and continued to contract. These advances can be traced to 1865 when the German chemist Adolf von Baeyer began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from isatin) and a second synthesis in 1880 (from 2-nitrobenzaldehyde). The synthesis of indigo remained impractical, so the search for alternative starting materials at BASF and Hoechst continued. The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain aniline provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897. In 2002, 17,000 tons of synthetic indigo were produced worldwide.

Indigo white
Indigo is a challenging dye because it is not soluble in water. To be dissolved, it must undergo a chemical change (reduction). Reduction converts indigo into "white indigo" (leuco-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with oxygen in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and had many opportunities to injure workers. In the 19th century, English poet William Wordsworth referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem "The Prelude". Speaking of their dire working conditions and the empathy that he feels for them, he wrote,

Doubtless, I should have then made common cause With some who perished; haply perished too A poor mistaken and bewildered offering Unknown to those bare souls of miller blue

A preindustrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine. A more convenient reductive agent is zinc. Another preindustrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic bacteria was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki. Examples of clothing and banners dyed with these techniques can be seen in the works of Hokusai and other artists.

Direct printing
Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as pencil blue because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.

The second method was known as china blue due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of iron(II) sulfate, with air-oxidation between each immersion. The china blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.

Around 1880 the glucose process was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the china blue method.

Since 2004 freeze-dried indigo, or instant indigo, has become available. In this method the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.

Chemical properties
Indigo is a dark blue crystalline powder that sublimes at 390–392 °C. It is insoluble in water, alcohol, or ether but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. The chemical formula of indigo is C16H10N2O2.

The molecule absorbs light in the orange part of the spectrum (λmax = 613 nm). The compound owes its deep colour to the conjugation of the double bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is nonplanar.

Chemical synthesis
Given its economic importance, indigo has been prepared by many methods. The Baeyer-Drewson indigo synthesis dates back to 1882 but was impractical. The first practical route is credited to Pfleger in 1901. In this process, N-phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. This highly sensitive melt produces indoxyl, which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating N-(2-carboxyphenyl)glycine to 200 °C in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo. The preparation of indigo dye is practiced in college laboratory classes.

Indigo derivatives
The benzene rings in indigo can be modified to give a variety of related dyestuffs. Thioindigo, where the two NH groups are replaced by S atoms, is deep red. Tyrian purple is a dull purple dye that is secreted by a common Mediterranean Sea snail. It was highly prized in antiquity. In 1909 its structure was shown to be 6,6'-dibromoindigo. It has never been produced on a commercial basis. The related Ciba blue (5,7,5′,7′-tetrabromoindigo) is, however, of commercial value.

Treatment with sulfuric acid converts indigo into a blue-green derivative called indigo carmine (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.

Safety and the environment
Indigo has a low oral toxicity, with an of 5000 mg/kg in mammals. In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in Lesotho.