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.