Indigo dyeing

Indigo dyeing is the prototype of vat dyeing and has been practised for over 6000 years — remains of indigo-dyed cotton were identified at Huaca Prieta (Peru). The dye was originally extracted from Indigofera tinctoria (tropics) and Isatis tinctoria (woad, Europe); BASF has produced it synthetically since 1897. In 2011 the denim industry alone consumed over 95 percent of the annual world output of about 50,000 tonnes.

In industrial indigo dyeing the warp beam (rope dyeing) or flat sheet (slasher dyeing) is pulled several times through reduced indigo vats (sodium dithionite, caustic soda). Between each dip the dye oxidises in air and develops its deep blue colour. Typical are 6-8 dip-and-sky cycles for classic denim colour depth.

Characteristic is ring dyeing: indigo does not fully penetrate the cotton fibre but only deposits in the outer layers. This is what creates the typical whisker, honeycomb and fade effects during later wear. In denim only the warp is indigo-dyed, the weft remains white — hence the white inside of a pair of jeans.