INDIGO, probably the oldest dye known to man, is one of the most important members of this group. Natural indigo extracted from the plant 'Indigofera tinctorie' was used by the Egyptians in 200 BC. The first synthetic indigo was introduced to the textile trade in 1897 and had the effect of completely replacing the natural product.

Although the vat dyes may be divided into three chemical groups, they are similar in that they are insoluble in water and become water soluble when reduced in the presence of an alkali. After dyeing, the fabric is oxidized and the dye again becomes water insoluble. Because of the time consuming and costly procedure in reducing vat dye into a water-soluble complex, dye manufacturers have produced a stabilized water-soluble vat dye.

This dye can be applied to cotton and viscose rayon by the methods used by applying direct cotton dyes. After the dyeing, a simple treatment restores the vat dye to its normal insoluble state. Solubilized vat dyes have an affinity for cellulose and animal fibres.

USE: Vat dyes are used in cotton dyeing where high wash and boil fastness required. Because of the high alkali concentration in the dye bath, pure vat dyes cannot be used on animal fibres, (wool, natural silk, and various hairs). Bright red is absent in vat dye range. Solubilized vat dyes, not requiring the presence of alkali, can be used for dyeing on animal fibres. Because they are dyed at low temperatures, they are used in Indonesian batik dyeing for green shades.