488

DYEING AND CALICO PRINTING.

of indigo, this application takes only place for goods which are not to bewashed, the blue tinge imparted being not fixed at all.

Woollen and silk fibres do not behave in the same way with sulphindigoticand sulphopurpuric acid : the diredt affinity of sulphindigotic acid for animalfibres is limited by certain conditions. When an excess of alkali is presentwith this acid, or even when it is in the state of a neutral salt, it does not dyewool, and this is readily explained by the fadts above stated, as alkalies extradlthe colour from wool, this process being made use of in the preparation ofso-called distilled blue. Although sulphindigotic acid unites diredtly withwool, this property of the fibre is increased by the use of such mordants asalumina and oxide of tin. The dyeing with the substance just named shouldalways be executed in presence of an excess of acid, since the greater thatexcess the greater the brilliancy of the hue, and the more quickly the processis finished. In former days the so-called Saxon blue, a solution of indigo insulphuric acid, diluted with water, was used; now-a-days indigo-carmine isemployed, while for printing on wool the so-called acetate of indigo abovementioned is preferred.

The dyeing of the wool with indigo-carmine is very readily effedted in awarm bath, containing the carmine, alum, and sometimes also cream of tartar.The alum enhances the solidity and fastness of this blue.

When the colour produced upon wool has been obtained by means of a sulph-indigotic acid prepared with a large excess of fuming sulphuric acid (viz.,more than twelve times the weight of acid to that of indigo), such blue im-perfedtly resists treatment with alkalies, and even repeated washing with soap,especially if during the dyeing operation heat be applied to the liquid in thedye-beck,

Silk is dyed in a precisely similar manner, but it is first alumed. Sulph-indigotic acid forms the ground colour for many mixed shades, as, for instance,green, grey, violet, and black; it is largely used in its modern form of indigo-carmine, because it is so readily fixed. The printing operations are con-duced in a manner similar to that of dyeing: the solution of indigo-carmineis slightly acidulated with oxalic acid, thickened with gum, alum being some-times added in small quantity. Indigo-carmine is also largely employed forthe production of such colours as green, violet, grey, and it is added to anumber of others to alter their tone.

Indigo-blue, that is to say the indigo-carmine and sulphindigotic acid, isreadily detected upon wool or silk by the following tests:—A moderately strongand tepid solution of crystalline carbonate of soda dissolves the blue colour,the liquid becoming blue-coloured. Chlorine, nitric, and chromic acids destroythe colour, as do also concentrated caustic soda, and a freshly-made solutionof zinc in sulphurous acid; on the other hand, this dye is not acted upon bysulphuric, hydrochloric, and the stronger vegetable acids. According toM. C. Kcechlin,* indigo-purple is violet when printed, but becomes blue afterhaving been steamed, and remains so, owing to the adtion of the sulphurousacid which the woollen tissue retains from the brimstone vapours to which itis exposed in bleaching.

We end this account of indigo with a brief statement of the mode employed

* “ Bulletin de la Socictc Industriclle de MulhoUBe,” vol. xxiv., p. 335.