Amidoazotoluol Garnet.—Amidoazotoluol produces with beta-naphthol a fine garnet red in the usual way.
The developing bath is made from 14 oz. amidoazotoluol, mixed with 1½ pints of sodium nitrite solution containing 1½ lb. per gallon, when well mixed add 1 pint of hydrochloric acid diluted with 2 pints water, when this is well mixed add sufficient water to make up a gallon, then add 1 lb. acetate of soda.
The cotton is passed through this dye-bath, then washed well, passed through a weak acid bath, then soaped well, washed and dried.
(8) DYEING COTTON BY IMPREGNATION WITH DYE-STUFF SOLUTION.
Indigo is a dye-stuff which requires special processes for its application to the cotton or wool fibre.
Its peculiarity is that in the form in which it comes to the dyer it is insoluble in water, and to enable it to be dissolved and therefore to be used as a dye, the indigo has to go under a special treatment. The colouring principle of indigo is a body named indigotin, to which the formula C16H16N2O2 has been given. When indigo is mixed with substances like lime and copperas, lime and zinc, zinc and bisulphite of soda, which cause the evolution of nascent hydrogen, it takes up this body and passes into another substance which is called indigo white that has the formula C16H12N2O2, leuco, or white indigo; this substance is soluble in water, and so when it is formed the indigo passes into solution and can then be used for dyeing. But indigo white is an unstable substance on exposure to air, the oxygen of the latter attacks the hydrogen which it has taken up, and indigotin is reformed, the indigo white changing again into indigo blue.
Indigo dyeing consists of three operations: