Into this bath the material is placed, and stirred around until thoroughly saturated—the temperature being kept about 120° F. for heavy goods, to assist penetration. Light goods can be dyed equally well in a lukewarm, or even a cold bath. The goods are then taken out, wrung lightly by hand, and are carefully passed two or three times through the wringer, to get the color evenly distributed. They are then shaken out and hung up in the air to oxidize. In fifteen or twenty minutes, after the color has changed, they should be rinsed well in two or three waters, to get rid of all traces of the caustic alkali, and then boiled for several minutes in a soap bath, to wash off the loose dyestuff and prevent rubbing. This after-treatment with boiling soapsuds is of even more importance in the case of the other Vat dyes than it is with indigo, for with most of them the oxidation is not completed in the air—and so the color is developed as well as brightened by the soap bath.
It is very important, when working with these Vat colors, to remember that hot solutions of caustic alkali are very hard on the hands and that, therefore, rubber gloves are extremely useful, if not essential. Stains left on hands, clothes, and utensils, although difficult to remove by washing, are almost instantly dissolved by warm solutions of hydrosulphite with a little soda or other alkali in them.
Results.—Colors produced by synthetic indigo are clear and clean, but not brilliant. If the slightly purplish shades of natural indigo are desired, they can be obtained with special brands—Indigo R, or Indigo RR, Metz—or by mixing small quantities of Algol Red B, Elberfeld, or Thio Indigo Red B, Kalle, with the indigo before reducing it. It is generally supposed that the characteristic shade, the so-called “bloom” of natural indigo, was due to the presence of small quantities of a reddish dyestuff, known as indigo red. As a matter of fact, however, the method of dyeing has more to do with this than the composition of the dyestuff.
For instance, if the indigo is very thoroughly reduced in the vat before the goods are immersed, as is generally the case in the modern hydrosulphite method, and the bath is made up with fresh reducing agent for each dyeing, the resultant color will be a very clear, rather greyish, shade of blue without any purple lustre. If, however, the dyestuff is not very perfectly reduced, as was generally the case with the old fermentation vats, and the bath, from standing in the air, has a heavy scum on the top, and is greenish rather than clear yellow in color below the surface, then the dyed fabrics will be apt to show the marked purplish tone which is so characteristic of the older indigo dyeings.
Uses.—While of less importance than it used to be before the discoveries of the last few years, the use of indigo for dyeing cotton, especially for the craftsman, is not to be neglected. It furnishes, easily and rapidly, in one bath, without either boiling, mordanting, or after-treatment, exceedingly pleasant, soft shades which are fast to both light and washing. For resist dyeing, such as Tied and Dyed work, Resist Stencil work, and Batik, it will be found particularly useful, because the fabric can be dyed in the cold.
Indigo possesses, however, certain disadvantages, especially for the professional dyer, which it shares with the other Vat dyes described below, and which prevent it, and the other Vat dyes, from being used as widely as the Salt colors or even the Sulphur colors. In the first place these dyes are all of them expensive. They cost more than most others, pound for pound of the dry color, and full shades need much larger proportions of them in the bath.
Then it is difficult to dye to shade with them, because the color, as a rule, alters so much when exposed to the air. In practice, when dyeing large quantities of goods to the same shade, it is customary to divide the materials into several lots of the same weight; and to make a strong “stock solution” of the dyestuff, properly reduced with alkali and hydrosulphite. By making up a fresh vat for each lot of goods, using exactly the same volume of water and of “stock solution,” and working each lot for the same length of time and at the same temperature, even results can be produced with much less trouble than by dyeing to shade by the eye.
Another drawback is that indigo-dyed goods, especially of the heavy full shades, are apt to “rub.” This can best be avoided by always using a well-reduced bath; by washing with boiling soap after each dip; and by building up the deep shades by successive dippings in moderately weak vats, rather than by obtaining the shade, once for all, by using a very strong, concentrated dye-liquor.
For many hundreds, and even thousands, of years, indigo has been universally recognized as the most permanent and most valuable blue dyestuff for cotton and indeed for woolen goods. For the latter purpose it is now but little used, thanks to the introduction of the exceedingly fast dyestuffs of the Acid and Mordant classes. But for cotton it is still considerably used, for fast shades.