Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/402

 384 LEATHER the tannin. Some tanners, especially Americans, who work the so-called acid process, plump their hides by the use of sulphuric acid, hanging them six or eight hours in a solution containing -g-^j-th of acid. The plumping is sometimes done as a preliminary operation, and again others add the acid to the colour pits, or the first pit into which the hides go for the tan ning process. Among non-acid tanners the plumping of sweat stock in which there is no lime is se cured iu the weak acid li quors of the colouring and handling pits. In the case of limed stock the hides, at the proper stage, are withdrawn from FIG. 2. Tanner s Beam. the pits and stretched over an unhairing beam (fig. 2), when with a working knife (fig. 3, a) a workman partly scrapes partly shaves off the hair and scarf-skin. Another workman in a similar way with a fleshing knife (fig. 3, V) removes the fatty compounds and flesh from the flesh side. For these operations seve ral machines have been adapted, working mostly with revolving knives or cutters, under which the hides or skins pass in a fully extended state. Such machines are, however, only applied to the smaller FlG - 3. Tanner s Knives and Tin. skins. The next step in the preparation of the hide is to remove from it as thoroughly as possible all traces of lime. This is partly accomplished by going over the hide on the beam with a scudding knife, pressing the combined lime and interfibrous mat ter out of the tissue. For more complete neutralization of lime in the larger hides the influence of the weak acid of the colouring pits is trusted to. Harness hides are washed by some means in pure water, the most con venient and generally adopted method being to place them in the dash wheel (fig. 4), FIG. 4. -Dash Wheel. in which they revolve and tumble about whilst fresh water is continually being poured on them within the revolving wheel. The hides now come to be trimmed and prepared for tanning in the shape in which they are intended ultimately to be sent into the market. An entire untrimmed hide (fig. 5) is termed a crop; a side is half a crop, the dividing line of the two sides being shown at EF; a butt is the back portion ABCD, and a bend is half a butt ABFE. G, G are belly pieces, and H, H the cheeks, both together FIG. 5. Divisions of a Hide. being the offal. When the shoulder (the upper part of the butt) is removed, what remains is a short butt. The actual tanning now commences, and the operations involved may be divided into a series of three (1) colouring, (2) handling, and (3) the laying away. The colouring consists in exposing the hides in a series of pits containing oozes which are almost entirely deprived of tannin, but in which some amount of gallic and acetic acids have been developed, and which, moreover, contain a large proportion of the colouring matter extracted from the tanning substances. In these pits (also called sus penders) the hides are sus pended over poles laid across the pit, and they are moved daily from one to another of a series of four or six, this stage usually occupying about a week. As the hides are moved forward in the series they are exposed to a liquor containing a small and steadily in creasing proportion of tannin, and this, it may be said, holds good till the hide reaches the last lay-away pit, in which the tanning is completed. The objects attained in the colouring pits are the superficial colouring or dyeing of the hide, some amount of plumping from the acids of the ooze, and a dissolving out of remaining traces of lime, principally by the acetic acid to which the hide is exposed. After colouring, the hides pass on to the handlers or handling pits, a round or series of which may consist of from four to twelve, according to the mode of working. In the handlers the hides are spread out horizontally; and in the first series they are &quot;handled&quot; once a day or more frequently if con venient. The handling consists of lifting the hides out of the pit by means of a tanner s hook (fig. G), piling them on the side till they drain, and returning them into the pit, the hide on the top in one handling going to the bottom in the next. This operation is continued throughout the series ; only as the hides advance the necessity and advantage of frequent handling decreases, while the strength of the tan liquor in which they are handled increases. The whole handling stage consumes on an average about six weeks. Finally, the hides are carried over into the layers or lay-aways. In these the stock is exposed to the strongest tanning liquors, and between the hides thin layers of the tanning bark or mixture are strewn. The object of this interstratification is to separate the mass of hides so as to secure the more ready permeation of the entire mass by the liquor, and also to feed and strengthen the ooze itself as its tannin is absorbed by the hides. In these layers the hides are allowed to rest for about six weeks, after which the pits are cleared out, charged with fresh ooze, and filled with the hides and tan as before. These processes may be re peated three or four times before the tanning is completed. When the process is deemed complete, each hide, on being taken out, will be found to be converted into leather, and a portion of its gelatin which has been dissolved from its interior is, by combination vith a portion of tannin from the strong solution, deposited upon its surfaces, where it is found in the form of a yellow deposit, technically known as bloom, or pitching, which disguises the under colour of the leather just as if it were covered with yellow paint. This, prejudice says, must be on its surface, or it is not saleable, but it is so much quality and weight lost to the FIG. 6. Tanner s Hook (without handle).