Short answers, pulled from the story.
Natural indigo dye is made from the leaves of plants in the Indigofera genus, particularly Indigofera tinctoria. The leaves contain a colorless precursor called indican, which breaks down during fermentation to eventually yield blue, water-insoluble indigo after exposure to air. Most indigo produced today is synthetic, made from aniline-based chemical processes.
The oldest known fabric dyed with indigo dates back approximately 6,000 years and was discovered at Huaca Prieta in Peru. Indigo was also used for centuries in India, China, Japan, Mesopotamia, Egypt, and West Africa.
Indigo was called blue gold because of its high value as a trading commodity. It was scarce in Europe during the Middle Ages, imported as a luxury item from India via Arab merchants, and later became a major colonial cash crop underpinning plantation economies in South Carolina, Haiti, Jamaica, and Central America.
Adolf von Baeyer published the first laboratory synthesis of indigo in 1878. Commercial-scale synthesis was developed by Johannes Pfleger, credited with the first practical industrial route in 1901, and by Karl Heumann, whose 1897 method used N-(2-carboxyphenyl)glycine. By 1897, BASF had an economically viable process in production.
Indigo is used for denim because it is insoluble and binds to the surface of cotton fibers rather than penetrating them fully. This surface-level attachment allows stone washing and acid washing to selectively remove dye and create the faded color gradients associated with denim. Around 80,000 tonnes of synthetic indigo are produced annually as of 2023, mostly for this use.
The Indigo revolt of 1859 was an uprising in Bengal in which indigo cultivators rebelled against exploitative conditions imposed by European merchants and planters. The Indian playwright Dinabandhu Mitra dramatized the events in the Bengali play Nil Darpan, a fictionalized retelling of the revolt.