The indigo stencil paste, as prepared, will keep well reduced for quite a long time, and it is frequently quite unnecessary to add any fresh reducing agent to it. If, when taken from the tube or bottle, it looks yellow or yellowish green, it can be applied at once to the cloth, and, if steamed just as soon as possible, it will generally penetrate quite satisfactorily. With the other colors of the series, however, it is hard to tell by the color whether they are reduced or not, and hence the fresh reducing agent, Stencil Salt, should always be added to them. The cloth for stencilling with these pastes, as with the Basic pastes, should not be too thick or heavy, and must be washed quite free from dressing, or the result will not be satisfactory. It should also be slightly dampened, if only by holding over boiling water for a moment or two, so as to help the color to penetrate. _Sulphur Stencil Paste._—We have also found very satisfactory results from pastes made with one of the Sulphur colors, dissolved in a little sodium sulphide and sodium carbonate, and stiffened with a little gum. The presence of a reducing agent helps to keep the color reduced; and, when quickly applied and rapidly steamed, the colors will sink into the fibre and adhere firmly. The chief drawback with these pastes is the lack of a good red. =Black Stencil Paste.=—So far as can be learned, the Japanese use for their stencilling an Indigo paste made on the same general principles as the one just described. Besides this, which is a very favorite color of theirs, they use a red and also a very full black dye, both of which are fast to washing and to light. What the composition of these last pastes may be it is hard to tell. In our laboratory we have made careful experiments on the subject of stencilling black, and have worked out a method that we consider satisfactory by the use of a modification of the well-known Aniline Black process. [Illustration: FIG. 19—LARGE AND HANDSOME JAPANESE STENCIL, SHOWING USE OF NET] _Aniline Black._—It was noticed, early in the history of dyestuffs, that if aniline was mixed with strong oxidizing agents, and carefully heated, it would undergo a series of color changes resulting, finally, in black. This color, so-called “Aniline Black,” was at one time manufactured and used for a black pigment; but it was soon recognized that its real value would only be developed when it could be formed, in the fibre itself, by the oxidation of aniline or some compound of aniline upon the fibres. After many years of experimenting this problem was solved, and for fifteen or twenty years the blacks most used on cotton and linen by the calico printers, as well as by the dyers, have been one or another of the forms of Aniline Black.