Indigo is the blue matter extracted from a plant Indigofera

tinctoria and other species, growing in Asia, South America and

Egypt. It reaches the market in a fine powder, which is insoluble in

water. There are two ways of dyeing with Indigo. It may be dissolved

in sulphuric acid or oil of vitriol, thereby making an indigo extract.

This process was discovered in 1740. It gives good blue colours but is

not very permanent, darker colours are more so than the paler. It does

not dye cotton or linen.

The other method is by the Indigo vat process which produces fast

colours but is complicated and difficult. In order to colour with

indigo it has to be deprived of its oxygen. The deoxidized indigo is

yellow and in this state penetrates the woollen fibre; the more

perfectly the indigo in a vat is deoxidized, the brighter and faster

will be the colour. For wool dyeing the vats are heated to a

temperature of 50 deg.C. Cotton and linen are generally dyed cold.