vat dyeing

Mainly used to colour cellulose, such as cotton, yarns or cloths made from cellulose yarns. Vat dyes are insoluble in water so they require to be made soluble before dyeing the fibre. Using an alkaline solution of caustic soda and sodium hydrosulphite the dye is converted, by chemical reduction, to a leuco alkali-soluble. At this point the colour will differ from the final dyed colour. The dye, having entered the fibre, is exposed to air which oxidizes the dye in the fibre back to its insoluble state. This is a dyeing process when the dye is accepted into the fibre in a reduced or vatted form, when oxidized the colour is fixed firmly to the fibre. The basic principles of vat-dyeing are:

• The conversion of the insoluble vat dye into the soluble sodium-leuco form by reduction or vatting.
• The conversion of the absorbed dye, in the cloth or yarn, back to the insoluble state by oxidation.
• Dyed or printed yarn or cloth treated in a hot detergent bath to produce a stable shade with maximum fastness.

Indigo is a natural vat dye and has been used extensively in India and west Africa for many centuries.