The best way of doing this is by the use of the Direct Cotton or Salt dyes, described in Chapter III, which, it will be remembered, only dye wool or silk at a high temperature, at or near the boiling point and, preferably, in an acid bath, but, when once on, are very hard to dislodge. The selected ones are very fast to light and present a great range of bright, attractive colors, which are nearly, if not quite, as brilliant as those produced by the Acid dyes.
They are applied in a boiling bath containing a little acetic acid, and a good deal of salt, especially for full shades. For lighter shades, the presence of salt is hardly necessary. The goods are to be finished just as with the Acid dyes, with a soap bath followed, if the scroop is desired, by a weak bath of acetic acid.
The results, when carefully done, are very good. They possess, however, one disadvantage for the amateur dyer. These colors are quite hard to strip, and so, the desired effect must be produced the first time, or not at all. It is not possible to strip an unsatisfactory shade in a hot soap bath, and dye it over and over again without injury, as in the case with Acid dyes. They are best stripped by soaking in a bath of sodium hydrosulphite, and then washing.
Embroidery Fast Colors.—While the above process gives shades fast enough against all ordinary washing, it sometimes happens that silk must be dyed fast enough to withstand exactly the same treatment that coarse cotton or linen goods are subjected to, without bleeding or staining. The salt dyes are not quite fast enough for this, particularly because, not having been converted in the dyeing process into a special insoluble condition, if they should be detached from the fibre by strong or hot soaping, they would be liable to stain the neighboring tissues and not wash off quite clear.
One of the hardest tests that colored silk is called upon to stand is when, in small quantities, it is used with a large amount of white linen or cotton goods. Thus, for instance, when monograms are embroidered in red or blue silk upon white towels or napkins, and the latter are scrubbed, week after week, in the regular wash, the color must be fast, indeed, not to show some evidences of running. Hence the term “embroidery fastness” as applied to this class of dyes. Thanks, also, to the amiable practice of the modern laundress of lightening her labors by the addition of bleaching powder and other strong chemicals to the washtub, it is very important that a silk dyed “embroidery fast” should be able to withstand the action of these agents as well as of soap. Up to the last few years these colors were only obtained by the use of the Alizarine dyestuffs, the full rich scarlet so often used for this purpose being the modern form of the old, madder-dyed, Turkey red of our forefathers.
But, during the last few years, the troublesome and tedious mordanting processes necessary for the proper development of color by the Alizarine dyes, have been replaced, for craftsmen, and, indeed, by most professional dyers, by the much simpler and shorter processes of vat dyeing. As long as Indigo was the sole representative of the class, it was of very little use for silk dyeing. But since the introduction of the splendid series of new vat dyes, the Algol, Ciba, Helindone, Indanthrene, and Thio Indigo colors, which, dyed in a single bath, give a whole range of brilliant shades, wonderfully fast to light and to washing, the necessity for mordant colors has very largely disappeared.
DYEING SILK WITH VAT DYES
It must always be remembered when working with silk, wool, leather, or any other animal material, that such materials are extremely sensitive to the action of alkalies, especially when hot or caustic, while they are but slightly injured, if at all, by the action of dilute acids. For this reason it is always better, whenever possible, to dye silk with the Acid dyes or the Salt dyes, in an acid or neutral bath, rather than to use dyestuffs like the Vat dyes or the Sulphur colors, which need an alkaline dye-liquor. Furthermore, the silk is likely to have a more brilliant lustre when dyed with a color which fastens to it by chemical affinity, from a solution, rather than one where the color is fixed because the oxygen of the atmosphere changes it into an insoluble powder, while in the pores of the silk. It is, however, perfectly possible to dye silk full shades with the Vat dyes and even—though this is not often advisable—with the Sulphur dyes, by using some simple precautions.
The best Vat dyes for silk are Indigo itself, and its substitution products, like Brom-Indigo, Elberfeld, or the Thio Indigo dyes, Kalle, or else the rather closely related colors like the Helindones, Metz, and the Ciba colors, Klipstein. It is of importance to use only those which are shown in the table on page 102, as dyeing in a cold, or at most, a lukewarm bath.
The dye-bath should be made with a considerable amount of dyestuff, so as to avoid the necessity of keeping the goods in it long. And the amount of caustic alkali should be kept as low as possible, consistent of course with dissolving the reduced dyestuff. It has been found in practice that the presence of glue or gelatine in the bath, or even of glucose (molasses, corn syrup, Karo syrup, etc.), protects the silk, wool, and other animal fibres greatly from the action of alkalies. It should, therefore, be added in quantities of two or three large tablespoonfuls to the gallon of dye-liquor.