The process of dyeing is least troublesome and most effective when done in the drum tumbler. The leather is run in warm water until the temperature reaches 140°-150° F (60°-66° C.) and the dyeing materials are added gradually in a box at the side of the drum whence it passes through the axle or journal into the tumbler. Although chrome leather is not materially affected by boiling water in contradistinction to vegetable-tanned leather, which cannot be treated with water above 60° C. without injury, it is inadvisable to dye it in boiling solutions, the above-mentioned temperature being the most satisfactory. There are two methods of dyeing blacks in the leather trade: (1) the logwood-ammonia, and (2) the aniline black. The former is the cheaper of the two and quite satisfactory, although many dyers seem to prefer the aniline process. The latter certainly gives a deeper black, but it is not at all necessary to make the flesh side of box calf black, and many buyers prefer the back blue, although, of course, the grain side of the leather must be finished a jet black. The crude logwood is generally subjected to a process of fermentation or "ageing." The new wood has to be placed on stone or cemented flooring in a warm room and frequently turned over until fermentation has ceased, which usually takes about a week. It is then cut into small chips by machinery and packed in bags. This is the form in which it always used to enter the tannery, but it is now generally prepared in paste or crystal extract by makers of tanning and dye-wood extracts, a business which grows in importance every year. The colouring matter of logwood is haematoxylin, which is converted into haematin by oxidation; hence, the extract is often referred to as haematin crystals. The paste may be used where it is desirable to increase the substance of the leather, but most dyers prefer the crystals, which are easily dissolved in hot water. One lb. of crystals, to which is added just enough ammonia to change the brown colour to violet blue, suffices for each 100 lb. of leather dyed. The colouring matter is absorbed by the time the drum has been running half an hour, leaving clear water behind. The next process is known technically as "fat-liquoring," which, as its name implies, consists in lubricating the fibres of the leather with fatty or oily matters. This is a very important process, for, when suitable ingredients are used, the pliability, strength, and waterproof quality of the leather are greatly increased. The making of special fat-liquors for different kinds of leather has become quite an important business, and most makes can be depended upon for the specified purposes. It is a debatable point as to whether fat-liquoring should precede or follow dyeing. Generally speaking, it is better to adopt the latter course, as the fatty ingredients help to fix the colouring matter on the fibres of the leather. The dye is also less liable to fade when fat liquoring follows the colouring.