A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia.

Until the introduction of Indigofera species from the south, Persicaria tinctoria (dyer's knotweed) was the most important blue dyestuff in East Asia; however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical Indigofera tinctoria plant.

The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan, and Indigofera leaves contain as much as 0.2–0.8% of this compound.

This route was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis.

Johannes Pfleger[6] and Karl Heumann [de] eventually came up with industrial mass production synthesis from aniline by using mercury as a catalyst.

This highly sensitive melt produces indoxyl, which is subsequently oxidized in air to form indigo.

The oldest known fabric dyed indigo, dated to 6,000 years ago, was discovered in Huaca Prieta, Peru.

The dye was also known to ancient civilizations in Mesopotamia, Egypt, Britain, Mesoamerica, Peru, Iran, and West Africa.

[11] The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

The Bengali play Nil Darpan by Indian playwright Dinabandhu Mitra was a fictionalized retelling of the revolt.

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.

In the late 15th century, the Portuguese explorer Vasco da Gama discovered a sea route to India.

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.

However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry.

In central Europe, indigo resist dyeing is a centuries-old skill that has received UNESCO Intangible Cultural Heritage of Humanity recognition.

[13] 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.

This was due to a growing textiles industry,[18] and because commoners had been banned from wearing silk,[19] leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.

[22] 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.

[26] In colonial North America, three commercially important species are found: the native I. caroliniana, and the introduced I. tinctoria and I.

[28]) The synthesis of indigo remained impractical, so the search for alternative starting materials at Badische Anilin- und Soda-Fabrik (BASF) and Hoechst continued.

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.

When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property.

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: 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 pre-industrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia.

Another pre-industrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic bacteria was maintained.

The indigo was then reduced in a sequence of baths of iron(II) sulfate, with air oxidation between each immersion.

It is insoluble in water, alcohol, or ether, but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid.

Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents.

[33] Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.

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