material. Various means for shortening this process have been devised, such as forcing the tanning-liquor through the skin by pressure, sewing the skins together into a bag in which the liquor is suspended, and simply substituting for the dry bark which was formerly used liquid infusions of tanning materials, which are gradually increased in concentration as the process advances. The last-named method, though the slowest, is found to produce the best leather, and the process of tanning is still a tedious one, consuming weeks or even months. The general method employed in American tanneries is described by Sadtler as follows:
“The tan-house into which the cleansed and prepared hides or ‘butts’ now come is provided with rows of pits running in parallel lines, which are to contain the butts during their treatment with tan-liquor. The butts in most cases are first suspended in weak tanning infusions before they go into the first, or ‘handler’ pits. The object of this is to insure the uniform absorption of tannin by the skins, before subjecting them to the rough usage of ‘handling,’ which in the early stages of the process is liable to cause injury to the delicate structure of the skin. During this suspension the skins should be in continuous agitation to cause the tannin to be taken up evenly. Both the suspension and the agitation are accomplished generally by mechanical means. From the suspenders the butts are transferred to the ‘handlers,’ where they are laid flat in the liquor. They are here treated with weak infusion of bark, commencing at about 15° to 20° by the barkometer and are handled twice a day during the first two or three days. This may be done by taking them out, turning them over, and returning them to the same pit, or more generally by running them, fastened together, from one handler pit to another. The treatment of the butts in the handlers generally occupies about six to eight weeks, by which time the coloring matter of the bark and the tannin should have ‘struck’ through about one-third of the substance of the skin. Many of the butts will have become covered, moreover, with a peculiar ‘bloom’ (ellagic acid) insoluble in water. They are now removed to the ‘layers,’ in which they receive the treatment of bark and ‘ooze’ or tan-liquor in progressive stages until the tanning is complete. Here the butts are stratified with ground oak-bark or valonia, which is spread between each butt to the depth of about one inch, and a thicker layer finally on top. The pit is then filled up with ooze, which varies in strength from about 35° barkometer at the beginning to 70° at the end of the treatment. For heavy tannages six to eight layers are required, the duration of each ranging from ten days in the beginning to a month in the later stages. Each time the butts are raised they should be mopped on the grain to remove dirt and loose bloom.”
Many materials besides oak-bark are now used to make tanning infusion, and some of these, being stronger, have hastened the tanning process. Among the most important of these are valonia, the acorn of an evergreen oak found in Asia Minor and Greece, which contains three times as much tannin as the strongest oak-bark; the sumach; the divi-divi and algarovilla, pods of South American trees closely allied to logwood; and the larch, spruce, pine, and hemlock barks. Besides this group there are the tanning materials derived from abnormal growths, caused by the sting of insects or other injuries, as galls (q.v.) and knoppern. The so-called ‘union’ tannage is produced by a combination of oak and hemlock barks.
Undressed leather, after it is tanned, needs simply to be rendered smooth and compact, which is accomplished by scouring and compressing the surface with stones, brushes, the ‘striking-pin’ and rollers, all of which processes are effected by machinery. Dressed leathers must, in addition, be ‘stuffed’ with oils to increase their resistance to water and their flexibility; they must frequently be dyed or stained in black or colors and ‘grained.’ These processes are also performed by machinery. In 1860 a machine was invented for splitting leather to any desired degree of thinness. The practice previously was to shave the leather down, the shavings being wasted. The process of dressing tanned leather known as currying was formerly a separate industry, but is now carried on as a part of the general business of leather manufacture. A favorite oil used by curriers for stuffing leather is the degras, or superfluous oil pressed from shamoyed leather. The demand for this oil is so great that its manufacture has recently become a separate industry.
Chrome-Tanning. The possibility of tanning by the use of chromium compounds, instead of the older tanning materials, was discovered as early as 1856 by a German chemist, but the first process which attained commercial success was invented in 1884 by Augustus Schultz. The introduction of this process in Philadelphia caused it to become at once a great leather-manufacturing centre. Chrome-tanning consumes only a few hours, as compared with weeks or months required by the older method, and it produces a leather which is extremely soft and pliable, of close texture, and thoroughly resistant to water. At the close of the nineteenth century two-thirds of the glazed kids made in the United States were chrome-tanned, but the process had not been applied successfully to sole-leather. The process consists in treating the skins at first with a weak solution of bichromate of potash, to which sufficient hydrochloric acid is added to liberate the chromic acid. Of course pickled skins may be used without the necessity of adding free acid. After the skins have taken up a bright yellow color, through their entire texture, they are drained and transferred to a bath of hyposulphite of soda, to which some acid is added to liberate sulphurous acid, which reduces the chromic acid to green chromic oxide. The sulphurous acid is at the same time oxidized to sulphuric acid, until the whole of the chromic acid is reduced. The leather so produced is of a pale bluish-green color. The combination of the hide fibre, or corium, with the chromium oxide is apparently more stable than its combination with tannin, and yields less to boiling water. The leather can also be dyed successfully if the dye is applied while the skin is still wet, but so great is its water-repellent character that, once dried, it cannot be wetted sufficiently to dye properly.
Tawing consists in dressing the skins in antiseptic materials, so as to preserve them from decay; but by this operation no chemical change is effected in the gelatin of the skin; hence, scraps and other wastes of tawed leathers can
