DYEING went ]Norco. As indigo blue i, insolu ble in water, it must first be converted into indigo white, wide]] is soluble in alkaline liquids. This ehange is produced by the ehemieal addition of hydrogen, so that the blur, goes over into the white, This is effeeted in (-old vats. which are emplc.yed for the dyeing of cotton and linen, by metallic reducing agents, such as of iron, zinc dust.. and alkaline sulphites: while wool and silk are dyed in warm vats, where the reduction is caused by a fer mentation of vegetable substances. preferably woad, in :u manner analogous to that by whirl' indieane is converted into indigo white. These vats are always alkaline. When the reduction of the indigo is emitph.ted, the liquid is colorless, with a light bluish scum on top. The secured materials are then drawn through the vats and exposed to the air while drying. The atmospheric oxygen immediately reeonverts the white into the blue, by removing the extra hydrogen atoms.
Indigo onto is also soluble in finning sulphuric acid, forming 'indigo-sulphottic acid' or Saxon blue. which was formerly known as a dyestnff, but has been replaced in modern practice by its sodium sail, indigo carmine. The solution of this carmine produces a hlue precipitate upon the fibre with an Minn mordant, but the color is neither a. deep nor as fast a. that of the un altered intligolin from the reducing-vats. With artificial indigo, the final synthesis van be pro duced on the fibre itself, as when ortho-nitro phenylpropiolie acid and potassium xanthogenate are applied separately and the doubly impreg nated cloth is then steamed, whereupon indigo blue is formed within the fibre. and the by produets can lie washed out.