The Principles of Leather Manufacture

by H. R. Procter

The products which are of no direct value to the tanner and currier in the manufacture of leather, and which are nevertheless obtained in fairly large quantities, are of very varying characters. In the present chapter, the most important of them will be described, and some of their uses mentioned.

Hair is removed from the skin of the animal in the process of depilation ([p. 143]) in the form of a wet sodden mass, containing a considerable amount of lime when the skin has been through the lime-pits.

As white hair is the more valuable, care should be taken in the unhairing to keep it separate from the coloured. It is washed first in plain water to get rid of as much of the lime as possible, and then in water containing a little acid. Hydrochloric acid is often used for this purpose, but sulphurous acid ([p. 25]) is preferable as it has a slight bleaching action on the hair. The acid neutralises and renders soluble the lime which still remains in the hair, so that it can be easily removed by washing with water. In many tanneries, hair-washing machines are used. The washed hair is dried by laying it out on frames; or preferably, the greater part of the water is first removed by a centrifugal drier, or by pressing, and the drying is completed in a drying room, the temperature of which is a few degrees higher than that of the outside air, and which is provided with a fan or some other appliance for mechanical ventilation. Tables of wire gauze on which the hair is spread, and through which the warm air of the room is drawn by a centrifugal fan, are the most effective.

Coloured hair is sometimes washed and treated like the white hair, but is usually sold direct to plasterers, in which case there is no necessity to remove all the lime and other impurities which the hair contains. A considerable amount of hair is also sold to iron founders, who use it in preparing cores and in loam-casting. The loose lime may be effectively beaten from dried hair by passing it through a disintegrator with one of the grates removed.

Fleshings and Glue-stuff.—The various scraps of fat and flesh, more or less free from actual hide substance, are usually worked up for glue, though if they cannot be sold for a fair price it will pay to boil them in order to recover the fat they contain. If this is to be done, the fatty portions may be thrown out at the beam and not mixed with the fleshings as in the ordinary way. Before boiling, the fat is treated with sulphurous, sulphuric or hydrochloric acid, sufficient to neutralise the lime present. The boiling should be carried on very gently, so as to allow the fat to rise without emulsifying with the gelatinous matter. For boiling, open steam may be used, but in this case the size formed will have little value; on the other hand, if sulphurous acid has been used and a wooden vat with a copper steam-coil be employed, really good glue may be obtained, and the slight trace of bisulphite which it may contain will prevent its putrefaction. Except under special conditions it will not pay to make glue on a small scale in England, as its value depends much on its appearance, and the necessary plant is somewhat expensive. In some places, however, size can be sold to advantage. [Fig. 98] shows a glue-boiling plant.

After separation of the fat by skimming, the clear size is run off from the residual matter into wooden cooling troughs about 5 feet long by 9 inches deep and 15 inches wide, in which it is allowed to set ([Fig. 92], [p. 425]). Great care is required that both size and coolers are quite sweet and free from putrefaction, the coolers being frequently washed with sulphurous acid solution or fresh milk of lime. The jelly is cut out in blocks, and sliced into cakes of appropriate thickness by means of a series of frames like slate-frames which fit over the block of glue, and between which a wire or thin blade stretched on a saw-frame is inserted to cut the glue into sheets. In some factories a machine is used, with a series of parallel blades against which the glue-block is pushed. The sheets are afterwards separated by girls and laid to dry on nets, on which they are frequently turned. When dry, the cakes may be washed with warm water to remove any adhering dirt, but this causes some loss of weight, and in many cases it pays better to dry in a stove until quite hard, then grind in a disintegrator and sell as “size-powder,” in which appearance counts for little if the colour and strength of the size are good.

Fig. 98.—Glue Boiling.

Fat.—The fat, whether obtained in the manufacture of glue, or by boiling the fleshings and shavings for its recovery alone, is skimmed from the surface of the heated liquor, and should afterwards be freed from gelatinous matter by washing it with hot water in a tub and running off the upper layer after allowing the water to settle out. The fat thus obtained is a light-coloured grease of buttery consistence.

There are various other sources of waste fats which may be considered here. If glue is made from dried glue-stuff without previous treatment with acid, the fat skimmed off the pans, though dark in colour, will be neutral or alkaline, and a considerable additional quantity of fat and free fatty acids may be obtained by reboiling the “scutch” or refuse with open steam in lead pans with the addition of water and enough sulphuric acid to render the contents of the pan distinctly acid. This grease will be dark and of unpleasant smell from volatile fatty acids, but its odour may be to a considerable extent improved by blowing air and steam through it, and washing with water, or by heating to a temperature somewhat above the boiling-point of water for a considerable time. The same sort of treatment may be applied to the fat pressed out of sheepskins, and to that obtained by boiling currier’s shavings with water and a little acid.