Professional leather dyers, for the sake of economy, often use a little sulphuric acid in the dye-bath, a practice which is one of the chief causes of the short life of modern leathers.
With other animal fibres, such as wool and silk, the Acid colors take quite as readily as the Basic; but with leather, there is some little difference, according to the way in which the leather has been prepared.
For our purposes it is hardly worth while to discuss the dyeing of chrome leather or of chamois leather. The leather almost universally employed for hand work has been bark tanned, excepting where very white goods are used, of rather light quality. These are generally tawed with alum, and for this reason have a greater affinity for the Acid colors than when the fibres have already been fully charged with tannic acid, which at the same time, it will be remembered, acts as an excellent mordant for the Basic colors.
Dyestuffs.—Accordingly, while Acid colors may be used, they do not act nearly so readily as the Basic colors. For this reason, except for special shades such as a clear sky-blue or a pure scarlet, which can hardly be obtained excepting by the use of Acid dyes, or where special fastness to sunlight is required, the best Basic colors, such as Methylene Blue, Methylene Heliotrope, Thioflavine T (for yellow) and Safranine (for red) are usually employed. For black, it is well to use one of the many Leather blacks, made by mixing together powerful Basic dyes. For brown, the standard leather color, used in enormous quantities for gloves and the like, is the well-known Bismarck Brown, or for more orange shades, the closely related dyestuff, Chrysoidine. And, although neither of these colors is as fast to light as the Basic dyes mentioned above, they give very satisfactory results. These colors should be dissolved in water acidified with a little acetic acid.
The greatest pains must be taken in each case to see that the color is all in solution, and that no specks of undissolved color are allowed to come in contact with the leather. The leather must be very carefully and thoroughly moistened by soaking, if necessary over night, in lukewarm water softened, if the surface of the leather seems to demand it, with a few drops of ammonia water.
Dyeing Leather and Staining Leather.—As regards the application of the color; dyers generally make a distinction between leather that isdyed and leather that isstained.
Indyeing leather the moistened goods are placed in a tray or pan (agateware is most convenient for small pieces) and floated backwards and forwards in the dye-liquor, which should be deep enough to fully cover them. The liquor is usually about lukewarm on starting, and may be heated very gradually and gently to about 120° or 130°, if desired. For light shades, however, this is not at all necessary, and indeed the color, as a rule, penetrates deeper and is laid on more evenly when the bath is kept cold. The leather is kept in the dye-bath until the desired shade is reached, which should be at the end of half an hour or so.
When dyed in this manner, the dyestuff has a chance to soak into the leather, and so, when finished, the color is not so liable to be affected by rubbing or by wear. The leather should come out evenly coated on both sides, shaded effects if desired being produced later, by the staining process.
Stained Leather.—In staining leather, on the other hand, the color solution is applied directly to the surface of the damp—not wet—goods by means of a brush or soft sponge, or a little pad of cloth. Accordingly, no matter how carefully the leather has been softened and moistened beforehand, the color does not penetrate far, and is found only on the particular surface where it has been applied.
For flat, even shades, the dyeing process is usually preferable, but by staining, it is possible for the craftsman to work on the surface of the leather, as an artist does on paper with water colors, and beautiful effects can be produced. Oil paint is often used for decorating leather, and when applied skilfully in thin layers, the effects are good. But staining with dyestuffs is usually preferable, as showing more of the grain of the leather, and being more transparent.