There are several methods of applying Aniline Black on furs, the most important being
1. One-bath Aniline Black 2. Oxidation Aniline Black 3. Diphenyl Black 4. Aniline Black by Green's Process
A typical formula for this method is the following given by Beltzer:
Aniline salt 10 kg. Sodium chlorate 1.5 kg. Copper sulphate 0.7 kg. Vanadate of ammonia 10 gr.
All these substances are dissolved hot in 50 liters of water, and allowed to cool, forming solution A. Aniline salt is aniline oil which has been neutralized with the exact quantity of hydrochloric acid to form the hydrochloride. It forms white or greyish crystalline lumps very easily soluble in water. The sodium chlorate is the oxidizing agent, and the copper sulphate and the vanadate of ammonia are the oxygen carriers.
15 kg. of sodium bichromate are also dissolved in 50 liters of water, forming solution B. The bichromate is also an oxidizing agent and serves to complete the oxidation of the aniline to the black.
Immediately before using, solutions A and B are mixed together, both being cool. In general practise it is customary to mix only small quantities at a time, as a considerable precipitate forms when the whole batch is mixed at once, the precipitate being so much waste dye substance. Usually a liter of A and a liter of B are mixed at a time, and the furs brushed with the mixture. The brushing must be varied according as the hair is hard and stiff, or soft and tender. The hair must be thoroughly impregnated in all directions, and the penetration must not be too deep to affect the leather. With experience and dexterity satisfactory results can easily be achieved. After the skins have been properly treated, they are dried at a temperature of about 35 degrees centigrade. When dry, they are returned to the dye bench, where they receive another application of the dye mixture, and are again dried. This operation may be repeated as often as six or seven times before a sufficiently intense black is obtained. Another way of producing the desired depth of shade with fewer applications is by using more concentrated dye mixtures. Each method has its disadvantages, the greater number of brushings requiring the expenditure of more time and labor, and the greater concentration of the bath resulting in a considerable loss of dye substance due to the formation of a large precipitate when the two solutions are mixed, and moreover, not all furs can be treated with concentrated mixtures. The best results with this method usually require the application of six coats of a mixture of moderate concentration.