Natural Indigo (Neel) Dye — Bihar and Bengal

India was the world's dominant indigo producer until synthetic aniline indigo displaced it after 1897. A small revival is underway in Bihar, Rajasthan, and Bengal. Indigofera tinctoria is grown as a crop, cut before flowering, bundled, and steeped in fermentation vats of water for 12–14 hours. The yellow-green liquid is then aerated by beating or paddling to oxidise indigo precursors into insoluble indigo particles that precipitate. Sediment is boiled, filtered, and dried into cakes of natural indigo. The dye requires a vat — sodium hydrosulphite or traditional fermented bran vat — to reduce it to soluble form for fiber dyeing. Natural indigo gives a characteristic blue with subtle green undertone that fades gracefully unlike synthetic indigo. Bihar Sharif and Patna districts have active small-scale revival cooperatives.