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.