3.Basic Colors.—Basic dyes may be used on silk as on wool, leather, or any other animal fibres for direct application, the dyestuff dissolved with a drop of acetic acid, being added to the paste, and then brushed in and, preferably, lightly steamed to sink the paste down into the fibres. These dyes, however, with but few exceptions, are not fast to light, and applied in this way are not fast, either, to washing. By adding some reagents to the paste, however, a Basic stencil paste can be formed which gives colors on silk which will stand active scrubbing excellently.
The Basic Stencil Paste is prepared by mixing with the paste a solution containing the Basic color, dissolved in acetic acid, and also containing a considerable quantity of tannic acid. As long as there is free acetic acid present in this mixture the color remains in solution, but directly the acid is driven off, an insoluble compound remains, formed by the combination of the tannic acid with the color base. This happens on steaming, and the insolubility of the product is still further increased by passing it through a weak bath or wetting it with a weak solution (half a teaspoonful to the quart) of tartar emetic.
Accordingly, to use this stencil paste on silk or, indeed, on cotton, the slightly dampened goods are stencilled with the paste, thinned if desired with water and a little acetic acid. Then directly they are dry enough so as not to run they are well steamed, then the gum rinsed off with a little warm water, and the goods moistened with the tartar emetic. After this they can be washed with soap with little or no danger of running.
(c) Wool.—Wool is rarely stencilled, although stencil patterns can be produced very well on it by using acid colors with a little oxalate of ammonia (about the same amount as the dyestuff), dissolved in a drop or two of water, and thickened with a little gum tragacanth. When this paste is applied with a brush, and then dried, the result is not fast at all, merely a distinct stain; but if steamed at once the oxalate of ammonia decomposes, leaving oxalic acid, which, combining with the color and melting down with it in the fibres, makes the dyestuff adhere quite firmly.
(d) Cotton and Linen.—It is much more difficult to stencil satisfactorily on vegetable goods, such as cotton and linen, than on the animal fibres above mentioned, because they are expected to stand very much more severe treatment. The fastness to washing needed for a handsome silk scarf is far less than for a cotton shirtwaist, or linen table-cover, and unless the results on the latter are at least as fast as the average calico print, the result is considered a failure.
There are three classes of dyes which can be used in this connection, the Basic dyes, the Sulphur dyes, and the Indigo or Vat dyes. The Basic stencil pastes have just been described, in connection with silk stencilling, and when carefully used they will give very fair results on cotton, and even on linen, provided it is free from dressing, and is not too coarse and thick. It is hardly worth while trying to fasten Basic dyes, by hand stencilling, upon such materials as heavy, coarse Russian crash, for instance, such as friends and students have frequently brought in to experiment with. But for light, thin materials, and especially for mercerized goods, poplins and the like, it is possible, with a little practice, to get effects that are fast to ordinary washing.
On the other hand, this method of stencilling has certain disadvantages. It is rather complicated, needing the use of a fixing bath of tartar emetic, a very active poison, by the way, although more uncomfortable than actually dangerous when taken by mistake in one dose, because of the severe vomiting it produces almost immediately. And then, too, the results at best are not really fast to light, and in the case of light pinks and yellows are distinctly fugitive.
Vat Color Stencil Pastes.—Many experiments have been made in our laboratory to work out a satisfactory stencil paste, so that Indigo and other Vat dyes could be applied, simply and easily, with no more difficulty than the usual one of brushing the paste in carefully, and then steaming as soon as possible. In these stencil pastes the Indigo and the other Vat dyes are reduced with the aid of caustic alkali and hydrosulphite before being mixed with the paste, and some special precautions are taken to prevent, as far as possible, the oxidation of the dyestuff before it gets well into the fibre. But, as the ordinary hydrosulphite is apt to decompose on standing, especially when it is wet, it is always best, just before using, to mix well with the paste a little fresh reducing agent, dissolved in a drop of hot water. The reducing agent that should be used for this purpose is not the ordinary hydrosulphite of soda, used for vat dyeing, but a compound of sodium hydrosulphite, “Stencil Salt,” which has the property of keeping better than the other, and also of not acting as a reducing agent until it is heated. This, then, is stirred into the Vat color stencil paste, just before using, and then, when the goods are steamed, the heat and moisture combined will enable it to reduce the color, which will be carried into the fibres in a reduced and dissolved condition. After steaming well for five minutes the color should be developed by a bath in hot soapsuds, after which the goods should be rinsed and dried. With care this process will give very satisfactory results, perfectly fast to both light and washing, after the first loose color has been washed off.
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.