Notes Upon Indigo

by John L. Hayes

After leaving the sulphuric acid vat the pieces are rinsed in the water vat, then in river water, and afterwards should be soaked in a sulphuric acid bath at 40 Beaumé, for the purpose of dissolving the last traces of the peroxide of iron adhering to the fibre. The fabric is then washed in water and finally passed through a soapy water at 40° R.

Solid or precipitated blue, Schutzenberger’s receipt.—The process consists in printing indigo white precipitated in a vat, in a thick paste to dissolve it on the tissue by a passage through an alkaline bath (lime or soda), and of reprecipitating it by oxidizing it as soon as it has entered the fibre.

It is then the china blue process, minus the reduction which is made before printing, and consequently minus the sulphate of iron vat.

Indigo white is too alterable to be printed with success, so it is generally precipitated in combination with a stannic hydrate (hydrate of a salt of tin), which gives it body and preserves it from a too rapid oxidation.

The stannic indigotate in paste, or as it is generally called precipitate of indigo, is prepared by turning into the clear portion of a strong copperas vat an acid solution of protochlorate of tin, and filtering it upon woollen filters,—as much as possible away from the air. It would be better to prepare a strong tin vat by heating a mixture of indigo, caustic soda, and protochlorate of tin, and to precipitate by chlorohydric acid. [9]

The deposit is made into a paste with gum water; a salt of tin is often added to prevent oxidation. It is important to prevent the transformation of the indigo white into indigotine before printing. This indigotine would not fix itself on the fabric. Moreover, after printing, it is necessary to hasten the dissolution of the indigo white to enable it to penetrate the fibre. It is sufficient for this end to pass it through milk of lime. The stannic combination is immediately destroyed; the colorable matter unites itself with the lime, and the color passes into a pale gray with apple green. The indigo white becomes momentarily soluble; but the presence of the excess of lime and the thickening, as well as the attractive affinity of the thickening, prevent any running.

The piece on issuing from the lime water is placed in running water, when reoxidation commences, which this time fixes the color. The piece is finally passed through a sulphuric acid bath to absorb the lime, and washed.

By adding to the color a salt whose base precipitates in the milk of lime and oxidizes in the running water, and replacing the simple acid bath by an acid bath with yellow prussite, the intensity of the blue is increased through the formation of Prussian blue.

Although we have seen beautiful effects from the application of the solid blue of indigo on prints at our Pacific Mills, the colors produced by Prussian blue and aniline are so much more brilliant and easy of application that the use of indigo in printing goods for ordinary consumption is likely to decline rather than increase. It will be otherwise if we should ever manufacture for the East India markets. Here is a field still open for our manufacturers. Mr. Watson, in his beautiful work on “The Costumes of the People of India,” remarks that “British manufacturers have hitherto failed to appreciate Oriental tastes and habits, and hence supply but an insignificant part of the clothing of the two hundred million persons that form the population of what is commonly spoken of as India.” The great defect, he observes, is the want of stability of color in the cotton fabrics introduced,—this stability being an imperative demand in the Oriental markets.

The applications of indigo to cotton fabric are altogether secondary, in our mind, to its relations to the woollen manufacture. If we have felt called upon to say a word in behalf of the most ancient and best ally which the fibre of wool has ever had, it is because the vividness of color of the new products of coal, and the fascination which the application of the recent discoveries of science always possesses, is threatening the eclipse of the more ancient sober and solid dyes. Let the new colors have their place as auxiliaries, not as substitutes for the ancient dyes. Let them serve to give a bloom [10] to goods, but let the foundation be the good old dyes which the experience of ages has proved to be the most unalterable by light and air. The recent wonderful discovery of alizarine, or artificial madder, in coal tar products, has led practical men to expect too much from science. The opinion is quite prevalent among manufacturers that artificial indigotine has already been obtained from the same source. And some manufacturers are sanguine that the difficulties of indigo dyeing will thus be resolved. It is not improbable—for what is impossible to modern chemistry?—that this result will yet be partially obtained. But we have looked over all the recent foreign chemical reviews, and personally consulted some of our best chemists, and we can find no authority for the prevailing opinion that artificial indigotine has been produced. If the production of artificial indigotine should be realized, the only benefit would be the possible cheapening of the material. The difficulties of the indigo vat would still remain; for we cannot too often repeat, that in the very difficulties of the process, or in the insolubility of blue indigotine by ordinary agents, consists the excellence of the dye.