If the trouble is unevenness, while the shade is satisfactory, the color can be dissolved off in the boiling soap bath and then, on breaking the bath with a little acid, the same dye can be laid right on again, it is to be hoped this time in a satisfactory manner. The question of dyeing silk fast to washing, and also of dyeing silk black, will be dealt with in the next chapter.
Chapter XII
SILK—II
BLACK DYEING OF SILK. WEIGHTING AND ADULTERATION
OF SILK. DYEING SILK WITH COLORS
FAST TO WASHING
The dyeing process described in the last chapter, while well suited for dyeing silk bright and lustrous colors, is not so well adapted to dyeing it black. To be sure, there are several good fast acid blacks, such as Silk Patent Black, 2R, Kalle, or Neutral Wool Black, B, Cassella, or Cashmere Black, 3BN, Elberfeld, or Amido Black, 4024, Metz, which, dyed in full shades in a broken bath of soap or boiled-off liquor, will give fairly good results. But the best of these are not always quite satisfactory, the resulting color generally showing a tendency to be a deep full grey rather than a perfectly true lustrous black.
Salt Colors.—Silk may also be dyed black with some of the good Salt colors—but unless the dyer takes the trouble to after-treat the goods by the troublesome process of diazotizing and developing, the results are no better, if indeed as good as those resulting from the Acid blacks mentioned above.
Sulphur Colors.—These have very often been tried on silk without much success, because for dark colors like blacks, it is necessary to boil the goods in the dye-liquor for some time and to have the latter very concentrated. Unfortunately the sodium sulphide, necessary for dissolving the sulphur dyes, is a powerful alkali, and hence readily attacks an animal fibre, like silk. It is possible, however, by the abundant use of glucose (Karo syrup, etc.) to greatly protect the silk from this tendering action. It is also possible for a dyer fairly well trained in chemistry, to very carefully neutralize the dye-bath by the cautious addition of acid sodium sulphite, until the dye-liquor is no longer alkaline and yet the dyestuff is not precipitated. This process, however, is hardly fitted for an amateur, and has not proved very successful even among the professionals.
Logwood Blacks.—Nearly all professional dyers continue to use the old vegetable dyestuff, logwood, about which some information was given in the first chapter.
To dye with this it is customary to use one of the many good logwood extracts on the market. Great care must be taken in the proper mordanting of the silk before it goes into the bath. For this purpose the silk is impregnated first with iron salts, and later with tannin, and in some processes, with salts of chromium or of tin, before entering the logwood bath. In all cases, therefore, silk dyed black with logwood contains a certain amount, say 15% to 20% of its weight, or 2-3 ounces to the pound, of foreign ingredients. When carefully done this does not injure the material at all, and the “pure dyed” logwood blacks are perfectly satisfactory both for shade, lustre, and durability.
WEIGHTING OF SILK
This moderate increase of weight, however, which is hardly enough to replace the weight of the gum lost in the stripping process, was far from satisfying the demands of the manufacturer for a cheaper raw material. And accordingly both dyer and dyeing chemist have exhausted all their energies and skill in trying to increase this percentage of cheap foreign matter in the finished silk, to the utmost limit of what the market will stand.