Recovered fats may be separated into a tolerably firm grease suitable for use instead of tallow in currying, and an oil not unlike neatsfoot oil, by melting, allowing to cool slowly to a soupy consistency to promote the crystallisation of the harder fats, and forcing the mixture through flannel cloths in a filter press. The temperature at which the filtration should take place is generally 20-25° C. The oil is, of course, “tender,” or liable to solidify in cold weather; and the more so the higher the temperature at which filtration takes place. The tallow is obtained in cakes. If from fresh fleshings, it will be white and with little odour, but that from dried glue-stuff is usually brown and of unpleasant smell, while recovered grease from curriers’ shavings or “moisings” is always dark in colour.
If the fleshings are to be sold wet, they should be preserved in a sweet lime liquor; if to be dried, they are washed carefully in a fresh lime, spread on frames, and frequently turned over so that they may dry evenly and rapidly. Heat, if employed at all, is in most cases only used at the end of the drying operation, but some tanners dry from the first in a room the temperature of which is a few degrees higher than the normal, and which is provided with good ventilation. For the purposes of the glue manufacturer, the roundings and larger pieces are more valuable than the fleshings, and should be treated with correspondingly greater care by the beamsman and his assistants.
Bate-Shavings are very valuable as sizing materials. They should be well washed in water, or with a very dilute solution of sulphurous acid, and are then laid out in thin layers to dry. They may also be partially dried by pressing between latticed boards in a screw or hydraulic press, and are then best finished as cakes. On the manufacture of sulphurous acid compare [p. 25].
Horns are usually kept until the “slough,” “pith,” or internal bone can be knocked out, having become loosened through drying and putrefaction. If kept dry, practically no longer time is required, and the smell and other annoyances incidental to storing in a damp place are avoided. The sloughs may be removed by steaming, but the horns are somewhat damaged by this treatment. The sloughs are principally ground for “bone-meal,” but some are boiled for glue, either without preparation, or after decalcifying with dilute hydrochloric acid.
The actual horn itself, which is quite incapable of making glue, is used chiefly in the manufacture of combs, buttons, and similar articles. The value of horns is to a considerable extent dependent on their size, small horns being unprofitable to work up for the articles above mentioned.
Spent Tan.—The tan as it is obtained from the leaches after extraction has, naturally, no value for the tanner except as a fuel. Spent tan cannot be profitably sold as manure, as its worth in this respect is extremely small. In those places where white lead is still made by the Dutch process, oak-bark is used to cover up the earthen pots, and commands a good price. It is, however, essential that oak-bark only should be used, as many other tanning materials give off products which injure the colour of the white lead. The quantities of tan used for hot-beds, and for deadening the noise of traffic in the streets, are so small that they are of no practical account in the disposal of this product. Spent tan is not nearly so good as wood for the manufacture of paper, and an attempt to distil it and thereby obtain pyroligneous acid and wood-spirit did not result in any commercial success. On the Continent, fine-ground tan is usually pressed into briquettes for use as domestic fuel, but it would be hard to obtain a market for these in England.
On the whole, in spite of its low heating value, spent tan is best utilised as a fuel. For this purpose specially constructed furnaces are necessary on account of the dampness of the tan, and its low calorific value, which varies, however, with the particular materials: thus while oak-bark and valonia are only poor fuels, hemlock and myrobalans are much better on account of the resin and lignine they contain.
The first successful furnaces for raising steam with wet tan were introduced in the United States, and consisted of large arched combustion chamber with abundant grate-area, and with four or six feed-holes in the fire-brick top which formed a floor on which the spent tan was laid, and where to some extent it was dried by the waste heat. The flames and furnace gases were conducted under the boilers, the flue being very large and deep so as to collect the light ash which was drawn in great quantities from the furnace, and the gases then returned through the tubes of the boiler, afterwards passing down the sides and going to the chimney. The wet fuel was fed in through the firing holes alternately, so that only a part of the grate-space was covered at once with wet fuel; which was speedily ignited by the heat from other parts of the furnace, and especially from the vaulted arch.[187] The large grate-area was a necessity not only on this account, but because of the light weight of the fuel and its low calorific power, which involved the need of burning a large volume. [Fig. 99] represents a furnace of similar principle constructed by Messrs. Huxham and Browns. Furnaces of this type are, the author believes, still largely in use in the United States, but in Germany “step-grates” sloping from the furnace-doors towards the back, are now preferred. In these the combustible material rests upon the flat surfaces of the grate, while the air enters by the spaces between the steps without the fuel being able to fall through. [Fig. 100] represents the furnace on this principle constructed by the Moenus Co. of Frankfort.
[187] Detailed drawings and particulars are given in Jackson Schultz’s ‘Leather Manufacture in the United States,’ New York, 1876.
