COLORS NOT FAST.
In regard to all the combinations of indigo with sulphuric acid, including the carmines, it must be observed that their application does not constitute true indigo dyeing: the colors are not fast. It is not pure indigotine which is fastened on the tissues as in the vat dyeing, but another compound of indigo with the sulphur. Berzelius observes that “the color of soluble indigo is fully as alterable and fugacious as that of the colors extracted by the decoction of vegetable materials. By a long exposure to the sun the indigo blue is destroyed: it becomes green during evaporation, and changes its nature.” The carmines as well as the sulphur acids are easily decolorized by reducing agents, such as hydrogen and sulphuretted hydrogen, although they gradually assume their original color when exposed to the atmosphere. We are informed by some of the older dealers that imported cloths and merino stuffs known as “Saxony” were formerly largely sold in our shops, but that, notwithstanding their attractiveness to purchasers, they were objectionable on account of the instability of their color.