_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.