Indigo dye

Indigo dye is an organic compound with a distinctive blue color (see indigo). Historically, indigo was a natural dye extracted from the leaves of certain plants, and this process was important economically because blue dyes were once rare. A large percentage of indigo dye produced today, several thousand tonnes each year, is synthetic. It is the blue often associated with denim cloth and blue jeans.

Piece of indigo plant dye from India, c. 2.5 inches (6.35 cm) square.
Skeletal formula of indigo dye
Ball-and-stick model of the indigo dye molecule
Other names
2,2'-Bis(2,3-dihydro-3- oxoindolyliden), Indigotin
3D model (JSmol)
ECHA InfoCard 100.006.898
RTECS number DU2988400
Molar mass 262.27 g/mol
Appearance dark blue crystalline powder
Density 1.199 g/cm3
Melting point 390 to 392 °C (734 to 738 °F; 663 to 665 K)
Boiling point decomposes
990 µg/L (at 25 °C)
R-phrases (outdated) R36/37/38
S-phrases (outdated) S26-S36
Related compounds
Related compounds
Tyrian purple
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).


Indigo plant extract sample
Indigo dye

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 pants requires 3–12 g of indigo. Small amounts are used for dyeing wool and silk.

Indigo carmine, or indigo, is an indigo derivative which is also used as a colorant. About 20 thousand tons are produced annually, again mainly for blue jeans.[1] It is also used as a food colorant, and is listed in the United States as FD&C Blue No. 2.

Natural indigoes

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, notably the Indian subcontinent. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as I. sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia. Dyer's knotweed (Polygonum tinctorum) was the most important blue dye in East Asia until the arrival of the Indigofera species from the south, which yield more dye. In Central and South America, the species grown is anil. In Europe woad containing the same dye was used for blue-dying. Several plants contain indigo, but low concentrations make them difficult to work with and the color is then more easily tainted by other dye substances, typically leading to a greenish tinge.

Natural sources also include mollusks: the Murex sea snails produce a mixture of indigo and dibromoindigo (red) which together produce a range of purple hues known as Tyrian purple. Light exposure during part of the dying process can convert the dibromoindigo into indigo resulting in blue hues known as royal blue or hyacinth purple.


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 indigotin, the insoluble blue chemical that is the endpoint of indigo dye. 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 to convert the glycoside indican present in the plant to the blue dye indigotin.[2] They 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.

History of natural indigo

Indigo-Historische Farbstoffsammlung
Indigo, historical dye collection of the Technical University of Dresden, Germany

Indigo is among the oldest dyes to be used for textile dyeing and printing. The oldest known fabric dyed indigo dating to 6,000 years ago was discovered in 2009 at Huaca Prieta, Peru.[3] Many Asian countries, such as India, Japan, and Southeast Asian nations have used indigo as a dye (particularly silk dye) for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Britain, Mesoamerica, Peru, Iran, and Africa. Indigo was also cultivated in India, which was also the earliest major center for its production and processing.[4] The I. tinctoria species was domesticated in India.[4] Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product.[4]

India 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 (Ἰνδικόν, Indian). The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

Indigo cake
Cake of indigo, about 2 cm

In Mesopotamia, a neo-Babylonian cuneiform tablet of the seventh 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. A chemically identical dye derived from the woad plant (Isatis tinctoria) 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. Many indigo plantations were established by European powers in tropical climates. Spain imported the dye from its colonies in South America, and it was a major crop in Haiti and Jamaica, with much or all of the labor performed by enslaved Africans and African Americans. In the Spanish colonial era, intensive production of indigo for the world market in the region of modern El Salvador entailed such unhealthy conditions that the local indigenous population, forced to labor in pestilential conditions, was decimated.[5] 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.

Man wearing an indigo-dyed tagelmust

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.[6]

In Japan, indigo became especially important during the Edo period. This was due to a growing textiles industry,[7] and because commoners had been banned from wearing silk,[8] leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.[9]

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.[10]

In North America indigo was introduced into colonial South Carolina by Eliza Lucas Pinckney, where it became the colony's second-most important cash crop (after rice).[11] As a major export crop, indigo supported plantation slavery there.[12] In the May and June 1755 issues of The Gentleman's Magazine there appeared a detailed account of the cultivation of indigo accompanied by drawings of necessary equipment and a prospective budget for starting such an operation authored by South Carolina planter Charles Woodmason. It later appeared as a book. [13] [14] By 1775, inidigo production in South Carolina exceeded 1,222,000 pounds. [15] When Benjamin Franklin sailed to France in November 1776 to enlist France's support for the American Revolutionary War, 35 barrels of indigo were on board the Reprisal, the sale of which would help fund the war effort.[16] In colonial North America, three commercially important species are found: the native I. caroliniana, and the introduced I. tinctoria and I. suffruticosa.[17]

Because of its high value as a trading commodity, indigo was often referred to as blue gold.[18]

Peasants in Bengal revolted against unfair treatment by the East India Company traders/planters in what became known as the Indigo revolt in 1859, during the British Raj of India. The play Nil Darpan by Dinabandhu Mitra is based on the slavery and forced cultivation of indigo.

The demand for indigo in the 19th century is indicated by the fact that in 1897, 7,000 km2 (2,700 sq mi) were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg is 2,586 km2 (998 sq mi).[1]

Synthetic indigo

Indigoproduktion BASF 1890
Production of Indigo dye in a BASF plant (1890)

In 1865 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). (It was not until 1883 that Baeyer finally determined the structure of indigo.[19]) The synthesis of indigo remained impractical, so the search for alternative starting materials at Badische Anilin- und Soda-Fabrik (BASF) and Hoechst continued. Johannes Pfleger[20] and Karl Heumann eventually came up with industrial mass production synthesis.[21]

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, at which time 19,000 tons of indigo were being produced from plant sources. This had dropped to 1,000 tons by 1914 and continued to contract. By 2011 50,000 tons of synthetic indigo were being produced worldwide.[22]

Developments in dyeing technology

Leucoindigo structure
Indigo white (leuco-indigo)
Yarn dyed with indigo dye

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 felt for them, he wrote,

A preindustrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia. 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.

Pot of freeze-dried indigo dye

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.[23]

Chemical properties


Indigo is a dark blue crystalline powder that sublimes at 390–392 °C (734–738 °F). 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).[24] The compound owes its deep color 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. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route is highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but was impractical for industrial-scale synthesis. Johannes Pfleger[20] and Karl Heumann (de) eventually came up with industrial mass production synthesis.[21] The first commercially 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 (392 °F) 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.[1] The preparation of indigo dye is practiced in college laboratory classes according to the original Baeyer-Drewsen route.[25]

Heumann's original synthesis of indigo
Indigo Synthesis V.1
Pfleger's synthesis of indigo

Indigo derivatives

Structure of Tyrian purple
Indigo carmine
Structure of indigo carmine.

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 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. Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble using transient protecting groups such as the tBOC group, which suppresses intermolecular bonding.[26] Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.

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.

Indigo as an organic semiconductor

Indigo and some of its derivatives are known to be ambipolar organic semiconductors when deposited as thin films by vacuum evaporation.[27]

Safety and the environment

Indigo has a low oral toxicity, with an LD50 of 5000 mg/kg in mammals.[1] In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in Lesotho.[28]

The compound has been found to act as an agonist of the aryl hydrocarbon receptor.[29]

See also


  1. ^ a b c d Elmar Steingruber "Indigo and Indigo Colorants" Ullmann's Encyclopedia of Industrial Chemistry 2004, Wiley-VCH, Weinheim. doi: 10.1002/14356007.a14_149.pub2
  2. ^ Schorlemmer, Carl (1874). A Manual of the Chemistry of the Carbon compounds; or, Organic Chemistry. London. Quoted in the Oxford English Dictionary, second edition, 1989
  3. ^ Splitstoser JC, Dillehay TD, Wouters J, Claro A (2016-09-14). "Early pre-Hispanic use of indigo blue in Peru". Science Advances. 2 (9): e1501623. Bibcode:2016SciA....2E1623S. doi:10.1126/sciadv.1501623. PMC 5023320. PMID 27652337. Retrieved 2016-09-15.
  4. ^ a b c Kriger & Connah, page 120
  5. ^ Fowler, Walter (6 August 1991). The Formation of Complex Society in Southeastern Mesoamerica. CRC Press.
  6. ^ Kriger, Colleen E. & Connah, Graham (2006). Cloth in West African History. Rowman Altamira. ISBN 0-7591-0422-0.
  7. ^ Eiko Ikegami (28 February 2005). Bonds of Civility: Aesthetic Networks and the Political Origins of Japanese Culture. Cambridge University Press. p. 284. ISBN 978-0-521-60115-3.
  8. ^ John H. Sagers (20 July 2018). Confucian Capitalism: Shibusawa Eiichi, Business Ethics, and Economic Development in Meiji Japan. Springer. p. 27. ISBN 978-3-319-76372-9.
  9. ^ Trudy M. Wassenaar (3 November 2011). Bacteria: The Benign, the Bad, and the Beautiful. John Wiley & Sons. p. 105. ISBN 978-1-118-14338-4.
  10. ^ Quoted in Hentschel, Klaus (2002). Mapping the spectrum: techniques of visual representation in research and teaching. Oxford, England: Oxford University Press. p. 28. ISBN 978-0-19-850953-0.
  11. ^ Eliza Layne Martin. "Eliza Lucas Pinckney:Indigo in the Atlantic World" (PDF). Retrieved 2013-08-24.
  12. ^ Andrea Feeser, Red, White, and Black Make Blue: Indigo in the Fabric of Colonial South Carolina Life (University of Georgia Press; 2013)
  13. ^ Claude E. Jones,"Charles Woodmason as a Poet". The South Carolina Historical Magazine, Vol. 59, No. 4 (October 1958), pp. 189-194.
  14. ^ David S. Shields. Oracles of Empire: Poetry, Politics, and Commerce in British America, 1690-1750. (Chicago: University of Chicago Press, 2010), pp. 69, 249
  15. ^ Walter B. Edgar, ed. The South Carolina Encyclopedia. Columbia, SC: University of South Carolina Press, 2006), p. 9.
  16. ^ Schoenbrun, David (1976). Triumph in Paris: The Exploits of Benjamin Franklin. New York: Harper & Row. p. 51. ISBN 978-0-06-013854-7.
  17. ^ David H. Rembert, Jr. (1979). "The indigo of commerce in colonial North America". Economic Botany. 33 (2): 128–134. doi:10.1007/BF02858281.
  18. ^ "History of Indigo & Indigo Dyeing". Wild Colours and natural Dyes. Retrieved 30 December 2015. Indigo was often referred to as Blue Gold as it was an ideal trading commodity; high value, compact and long lasting
  19. ^ Adolf Baeyer (1883) "Ueber die Verbindungen der Indigogruppe" [On the compounds of the indigo group], Berichte der Deutschen chemischen Gesellschaft zu Berlin, 16 : 2188-2204 ; see especially p. 2204.
  20. ^ a b
  21. ^ a b
  22. ^ "Chemists go green to make better blue jeans". Nature. Nature. Retrieved 19 February 2018.
  23. ^ Judith McKenzie McCuin. "Directions for Instant Indigo". Archived from the original on 2004-11-16. Retrieved 2008-05-06.
  24. ^ Wouten, J.; Verhecken, A. (1991). "High-performance liquid chromatography of blue and purple indigoid natural dyes". Journal of the Society of Dyers and Colourists. 107: 266–269.
  25. ^ McKee, James R.; Zanger, Murray (1991). "A microscale synthesis of indigo: Vat dyeing". Journal of Chemical Education. 68 (10): A242. Bibcode:1991JChEd..68..242M. doi:10.1021/ed068pA242.
  26. ^ Głowacki, Eric Daniel; Voss, Gundula; Demirak, Kadir; Havlicek, Marek; Sünger, Nevsal; et al. (2013). "A facile protection–deprotection route for obtaining indigo pigments as thin films and their applications in organic bulk heterojunctions". Chemical Communications. 49 (54): 6063–6065. doi:10.1039/C3CC42889C. PMID 23723050.
  27. ^ Irimia-Vladu, Mihai; Głowacki, Eric D.; Troshin, Pavel A.; Schwabegger, Günther; Leonat, Lucia; Susarova, Diana K.; Krystal, Olga; Ullah, Mujeeb; Kanbur, Yasin; Bodea, Marius A.; Razumov, Vladimir F.; Sitter, Helmut; Bauer, Siegfried; Sarıçiftçi, Niyazi Serdar (2012). "Indigo - A Natural Pigment for High Performance Ambipolar Organic Field Effect Transistors and Circuits". Advanced Materials. 24 (3): 375–80. doi:10.1002/adma.201102619. PMID 22109816.
  28. ^ "Gap alarm". The Sunday Times. 2009-08-09. Retrieved 2011-08-16.
  29. ^ Denison MS, Nagy SR (2003). "Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals". Annu. Rev. Pharmacol. Toxicol. 43: 309–34. doi:10.1146/annurev.pharmtox.43.100901.135828. PMID 12540743.

Further reading

External links

Al-Dana, Syria

Al-Dana (Arabic: الدانا‎) is a town in northern Syria, administratively part of the Idlib Governorate, located north of Idlib, 38 kilometers west of Aleppo, and just east of the border with Turkey. Nearby localities include Sarmada to the southwest, Tell al-Karamah to the south, Atarib to the southeast, Turmanin to the northeast, Salwah to the north and Qah to the northwest. According to the Syria Central Bureau of Statistics, al-Dana had a population of 14,208 in the 2004 census. The town is also the administrative center of the al-Dana nahiyah consisting of thirteen villages with a combined population of 60,058. Al-Dana was notable for its indigo dye industry which has since disappeared.


β-Nitrostyrene is an aromatic compound and a nitroalkene used in the synthesis of indigo dye and the slimicide bromo-nitrostyrene.


Cross-conjugation is a special type of conjugation in a molecule, when in a set of three Pi bonds only two pi-bonds interact with each other by conjugation, the third one is excluded from interaction. Whereas a normal conjugated system such as a polyene typically has alternating single and double bonds along consecutive atoms, a cross-conjugated system has an alkene unit bonded to one of the middle atoms of another conjugated chain through a single bond. In classical terms, one of the double-bonds branches off rather than continuing consecutively: the main chain is conjugated, and part of that same main chain is conjugated with the side group, but all parts are not conjugated together as strongly. Examples of cross-conjugation can be found in molecules such as benzophenone, divinylketones, p-quinones, dendralenes, radialenes, fullerene, and Indigo dye. The type of conjugation affects reactivity and molecular electronic transitions.

Hexaplex trunculus

Hexaplex trunculus (also known as Murex trunculus, Phyllonotus trunculus, or the banded dye-murex) is a medium-sized sea snail, a marine gastropod mollusk in the family Muricidae, the murex shells or rock snails.

The snail appears in fossil records dating between the Pliocene and Quaternary periods (between 3.6 and 0.012 million years ago). Fossilized shells have been found in Morocco, Italy, and Spain.This sea snail is historically important because its hypobranchial gland secretes a mucus used to create a distinctive purple-blue indigo dye. Ancient Mediterranean cultures, including the Minoans, Canaanites/Phoenicians, Hebrews, and classical Greeks created dyes from the snails. One of the dye's main chemical ingredients is dibromo-indigotin. The dye will turn indigo blue, similar to the color of blue jeans, if exposed to sunlight before the dye sets.

Huaca Prieta

Huaca Prieta is the site of a prehistoric settlement beside the Pacific Ocean in the Chicama Valley

, just north of Trujillo, La Libertad Province, Peru. It is a part of the El Brujo Archaeological Complex, which also includes Moche (culture) sites.

Huaca Prieta was occupied as early as 4700 BC, before the ceramics were introduced. It consisted of a huge mound of ash, stones, textiles, plants and shells, with some burials and constructions.


Indican is a colourless organic compound, soluble in water, naturally occurring in Indigofera plants. It is a precursor of indigo dye.


Indigo is a deep and rich color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine. It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum.

The color indigo is named after the indigo dye derived from the plant Indigofera tinctoria and related species.

The first known recorded use of indigo as a color name in English was in 1289.

Indigofera suffruticosa

Indigofera suffruticosa, commonly known as Guatemalan indigo, small-leaved indigo (Sierra Leone), West Indian indigo, wild indigo, and anil, is a flowering plant in the pea family, Fabaceae.

Anil is native to the subtropical and tropical Americas, including the southern United States, the Caribbean, Mexico, Central America, and South America as far south as northern Argentina. This species has been widely introduced to other parts of the world and today has a pantropical distribution. It is an erect branching shrub growing to 1 m (3.3 ft) tall with pinnate leaves, and is commonly found growing in dry, highly disturbed areas such as roadsides and fallow fields.

Anil is commonly used as a source for indigo dye, and if mixed with Palygorskite clays, can produce Maya blue, a pigment used by the Mesoamerican civilizations.

Indigofera tinctoria

Indigofera tinctoria, also called true indigo, is a species of plant from the bean family that was one of the original sources of indigo dye. It has been naturalized to tropical and temperate Asia, as well as parts of Africa, but its native habitat is unknown since it has been in cultivation worldwide for many centuries. Today most dye is synthetic, but natural dye from I. tinctoria is still available, marketed as natural coloring where it is known as tarum in Indonesia and nila in Malaysia. In Iran and areas of the former Soviet Union it is known as basma. The plant is also widely grown as a soil-improving groundcover.

True indigo is a shrub one to two meters high. It may be an annual, biennial, or perennial, depending on the climate in which it is grown. It has light green pinnate leaves and sheafs of pink or violet flowers. The plant is a legume, so it is rotated into fields to improve the soil in the same way that other legume crops such as alfalfa and beans are.

Dye is obtained from the processing of the plant's leaves. They are soaked in water and fermented in order to convert the glycoside indican naturally present in the plant to the blue dye indigotin. The precipitate from the fermented leaf solution is mixed with a strong base such as lye.

The rotenoids deguelin, dehydrodeguelin, rotenol, rotenone, tephrosin and sumatrol can be found in I. tinctoria.Marco Polo (13th century) was the first European to report on the preparation of indigo in India. Indigo was quite often used in European easel painting, beginning in the Middle Ages.


In chemistry, indoxyl is a nitrogenous substance with the chemical formula: C8H7NO. Indoxyl is isomeric with oxindol and is obtained as an oily liquid.

Indoxyl is obtained from indican, which is a glycoside. The hydrolysis of indican yields β-D-glucose and indoxyl.

Indigo dye is a product of the reaction of indoxyl by a mild oxidizing agent such as atmospheric oxygen.

Indoxyl can be found in urine and is titrated with Obermayer's reagent. Obermayer's reagent is a dilute solution FeCl3 in hydrochloric acid.


Isatin or tribulin is an indole derivative. The compound was first obtained by Otto Linné Erdman and Auguste Laurent in 1841 as a product from the oxidation of indigo dye by nitric acid and chromic acids. The compound is found in many plants, such as Isatis tinctoria, Calanthe discolor and in Couroupita guianensis.Schiff bases of isatin are investigated for their pharmaceutical properties.Isatin forms a blue dye (indophenin) if it is mixed with sulfuric acid and crude benzene. The formation of the indophenin was long believed to be a reaction with benzene. Victor Meyer was able to isolate the substance responsible for this reaction from crude benzene. This new heterocyclic compound was thiophene.

Kru people

The Kru or Kroo are a West African ethnic group who originated in eastern Liberia and migrated and settled along various points of the West African coast, notably Freetown, Sierra Leone, but also the Ivorian and Nigerian coasts. The Kru were famous for their skills in navigating and sailing the Atlantic. Their maritime expertise evolved along the west coast of Africa as they made livings as fishermen and traders. Knowing the in-shore waters of the western coast of Africa, and having nautical experience, they were employed as sailors, navigators and interpreters aboard slave ships, as well as American and British warships used against the slave trade.Kru people were more valuable as traders and sailors on slave ships than as slave labor. To ensure their status as “freemen,” they initiated the practice of tattooing their foreheads and the bridge of their nose with indigo dye to distinguish them from slave labor.Their history is one marked by a strong sense of ethnicity and resistance to occupation. In 1856 when part of Liberia was still known as the independent Republic of Maryland, the Kru along with the Grebo resisted Maryland settlers' efforts to control their trade. They were also infamous amongst early European slave raiders as being especially averse to capture.

The Kru are one of the many ethnic groups in Liberia, comprising 7% of the population. It is also one of the main languages spoken. The Kru are one of the three main indigenous group players in Liberia's socio-political activities along with the Krahn and Mano people.

Notable ethnic Krus include the 25th President of Liberia George Weah, who is of mixed Kru, Gbee, Mano, and Bassa heritage, as well as his predecessor, former President Ellen Johnson Sirleaf , who is of mixed Kru, Gola, and German ancestry. Soccer star William Jebor is exclusively of Kru background, as is Christian Evangelist Samuel Morris who was originally known as Kaboo. Mary Broh, the current mayor of Monrovia, is of mixed Kru and Bassa ancestry. Didwho Twe, a judge and politician, who ran for President of Liberia in 1951 was of Kru heritage.

Midnight blue

Midnight blue is a dark shade of blue named for its resemblance to the apparently blue color of a moonlit night sky around full moon. Midnight blue is the color of a vat full of indigo dye; therefore, midnight blue may also be considered a dark shade of indigo. Midnight blue is identifiably blue to the eye in sunlight or full-spectrum light, but can appear black under certain more limited spectra sometimes found in artificial lighting (especially early 20th-century incandescent). It is similar to navy, which is also a dark blue.

Mima, Tokushima

Mima (美馬市, Mima-shi) is a city located in Tokushima Prefecture, Japan.

The modern city of Mima was established on March 1, 2005, from the merger of the former towns of Mima, Anabuki and Waki, and the village of Koyadaira (all from Mima District).


Nankeen, also called Nankeen cloth, is a kind of pale yellowish cloth, originally made at Nanjing, China from a yellow variety of cotton, but subsequently manufactured from ordinary cotton that is then dyed. Also in the plural a piece or variety of this cloth.The term blue nankeen describes hand-printed fabric of artistic refinement and primitive simplicity, which originated on the Silk Road over three thousand years ago.

Hand-carved stencils, originally made from wood but now from heavy paper, are prepared and a mix of soya bean flour and slaked lime is applied through the openings of the stencil onto the 100% cotton fabric. When dry, the fabric is then dipped numerous times into the large tubs containing the indigo dye. After the desired colour is achieved and the fabric has dried, the paste is scraped off, revealing the white patterns on the blue cloth. The fabric is then washed, dried, and ironed before fabrication.

Navy blue

Navy blue is a very dark shade of the color blue.

Navy blue got its name from the dark blue (contrasted with white) worn by officers in the British Royal Navy since 1748 and subsequently adopted by other navies around the world. When this color name, taken from the usual color of the uniforms of sailors, originally came into use in the early 19th century, it was initially called marine blue, but the name of the color soon changed to navy blue.An early use of navy blue as a color name in English was in 1840 though the Oxford English Dictionary has a citation from 1813.

In practice, actual blue uniforms of the United States Navy and other navies have become outright black in color, in order to combat fading.

Persicaria tinctoria

Persicaria tinctoria is a species of flowering plant in the buckwheat family. Common names include Chinese indigo and "Japanese indigo." It is native to Eastern Europe and Asia.

The leaves were a source of indigo dye. It was already in use in the Western Zhou period (c. 1045–771 B.C.), and was the most important blue dye in East Asia until the arrival of Indigofera from the south.

Plantation economy

A plantation economy is an economy based on agricultural mass production, usually of a few commodity crops grown on large farms called plantations. Plantation economies rely on the export of cash crops as a source of income. Prominent crops included cotton, rubber, sugar cane, tobacco, figs, rice, kapok, sisal, and species in the genus Indigofera, used to produce indigo dye.

The longer a crop's harvest period, the more efficient plantations become. Economies of scale are also achieved when the distance to market is long. Plantation crops usually need processing immediately after harvesting. Sugarcane, tea, sisal, and palm oil are most suited to plantations, while coconuts, rubber, and cotton are suitable to a lesser extent.

Tokushima (city)

Tokushima (徳島市, Tokushima-shi, Japanese: [tokɯꜜɕima]) is the capital city of Tokushima Prefecture on Shikoku island in Japan.

As of October 1, 2016, the city had an estimated population of 258,237 and a population density of 1,400 persons per km². Its total area is 191.23 km². The city is situated in the north-eastern part of Tokushima Prefecture at the mouth of the Yoshino River. In terms of layout and organization, Tokushima displays the typical characteristics of a Japanese castle town. Tokushima was developed under the Hachisuka clan. Its prosperity was built on a strong indigo dye industry.

The modern city of Tokushima was founded on October 1, 1889. At the time, it was the 10th largest city in Japan. The city is served by Tokushima Airport (recently renamed to Awa Odori Airport), in nearby Matsushige.

Types of dyes
Traditional textile dyes
Craft dyes

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