The number of materials that may be used for the process of fat-liquoring is almost legion, but the principal are soaps, oils, egg-yolk, tallow, flour, Tragasol, Irish moss, china clay, and starch. Unless the special preparations made by leather trade chemical firms be used, much care has to be exercised in selecting the most suitable ingredients for each class of leather. For example, the mixture that would give good results on vegetable-tanned leather might be quite unsuitable for chrome leather. Then, again, the selection of material is based on the style of finish required, a dull finish necessitating heavy fats and greases which would be quite unsuitable for bright leathers, and particularly for those of the chrome-tannage. Among the hundreds of recipes that have been published from time to time, tallow has only found a place in one or two mixtures, yet it is one of the finest materials for strengthening and lubricating the fibres of leather. If its splendid properties were fully known, it would be very largely used for fat-liquoring purposes. Of course, it is well known to the currying trade, having for several centuries formed the basis, together with cod oil, of the dubbin used in stuffing waxed leather and belting butts. The following is a useful recipe for a fat-liquor for box calf: 2 lb. Marseilles soap and 3 lb. neatsfoot oil for every 100 lb. of leather. The soap is cut into small pieces and dissolved in hot water, and the oil is added slowly and thoroughly mixed by stirring vigorously. Where possible, emulsification should be done in a machine, in order to atomise the ingredients. The finer they can be brought into a state of division, the better they will be absorbed by the leather. Other suitable fat-liquors for box leathers are the following: (1) Turkey red oil, 2 per cent. of weight of leather and neutral soap 1 per cent.: (2) neatsfoot oil, 2 lb.; tallow, 1 lb.; and Tragasol, 1 lb.: (3) cod oil, 2 lbs.; Marseilles soap, 1 lb.: (4) neatsfoot oil, 2 lbs.; egg-yolk, 1/2 lb., or six fresh yolks. The process is completed in 20-30 minutes. In the case of fancy colours or browns, the leather should generally be well-drained and warmed up again in the drum in hot water (170° F.) before fat-liquoring, but the precaution is unnecessary for black leathers. After the fat-liquoring process, the leather is removed from the drum, and piled up on the horse to drain, and left for several hours or overnight, to allow it to assimilate the fatty ingredients. It is then submitted to an operation known as sleeking or striking out, with the object of removing surplus water and so facilitating drying, and making the grain side smooth. Machinery is now almost universally employed, especially for chrome leather. A good type of striking out machine is shown in Figure 34.