to dye with indigo the pigment must be returned to its original form, colorless indoxyl. dyevats are set with indigo, alkaline water and a reducing agent,either a mineral or bacteriological ferment. the stable pigment is stripped of its oxygen and splits into indoxyl radicals. the presence of a purplish scum, called the flower, on the surface of the liquid signals that the dye is ready for colouring.

fibre is put to the dye, an amber-colored fluid, and absorbs the reduced indigo. when the fibre is aired, the indoxyl 'breathes', taking on Oxygen and converting to stable indigo blue. this conversion takes place partly within the molecular structure of cellulose fibres, which adds to the relative permanence of indigo.