Indigo dye

Indigo dye is traditional vat dyeing with the dye indigo, chemically 2,2-bis(indolin-3-ylidene). The dye is water insoluble; it is reduced to its water-soluble yellow-green leuco form by alkaline reduction with sodium dithionite or biologically with bacteria. In this form the cotton yarn is immersed; on air contact the leuco indigo oxidises back and deposits as tiny insoluble blue crystals in the fibre.

Characteristic for denim dyeing is ring dyeing: on rope or slasher dye ranges the warp yarn is dipped for only a few seconds into several indigo baths so the dye colours only the outer fibre zone and the core stays white. During wear and washing the outer colour layer abrades, the white core shows through and creates the typical vintage look with light edges, fades and whiskering.

Historically indigo came from woad (Isatis tinctoria) in Europe and the Indian indigo plant (Indigofera tinctoria). Today the dye is almost entirely produced synthetically (BASF process since 1897). More sustainable methods like Advanced Denim (Archroma) or E-Flow reduce water, chemistry and dye loss to wastewater significantly.