The time required for completing the tanning will vary according to the density of the skins or hides, the strength of the tan liquor, the amount of the hydrostatic pressure, and other circumstance When the skins or hides are found to be tanned, they are to be removed from the frames, and their outer edges, as far as they were squeezed or nipped between the frames, must be pared off; the skins or hides are then to be dried, and prepared for market in the usual manner. It is not my intention to claim, under this patent, the exclusive use of the frames, screw bolts, pipes, or any article of apparatus herein mentioned; or the use of any particular kind of tan liquor, or any mode or process of preparing and of finishing the skins or hides, save and except the application of the machines or engines herein described or set forth, or any imitation of them, for the purpose of causing the tan liquor or liquors to pass, by filtration, or percolation, through skins or hides. The apparatus herein described for effecting this purpose is such as I have employed with success, and consider, upon the whole, to be best; but particular local situations, or other circumstances, may render it expedient to change the shape of the frames, or their vertical positions, for some other; or to enclose between the middle frames and either of the exterior ones, two or more skins or hides, instead of the single one, as above mentioned.
Fig. 1 is a front view, and Fig. 2 is a side view. The same letters of reference indicate the same parts in each figure; a a is one of the exterior frames; b b is the other exterior frame; e e are two hides, secured between the exterior frames and the middle frame, by means of the screw bolts; d is the cistern containing the tan liquor; e is the pipe through which the tan liquor descends from the cistern into the space or cavity between the two hides, and which will vary in length according to the amount of hydrostatic pressure intended to be given; f is the exit pipe, through which the air escapes when the liquor is running down through the pipe e; g isa cock for the purpose of discharging from between the skins."
Shortly after Mr. Spilsbury enrolled his specification, another person took out a patent for a slight deviation in the apparatus, but on the same principle as Mr. Spilsbury's. Neither of these gentlemen, however, according to our information, have as yet succeeded in bringing their plans into practical operation, owing, we understand, to the curious circumstance, that the pressure has a tendency to drive the gelatin out of the skin, and to convert it into a very hard and inflexible material, not at all applicable to the ordinary uses of leather.
In 1827, Messrs. Knowlys and Duesbury obtained a patent for improvements in tanning, having a similar object in view, and, as it appears to us, with an arrangement better calculated to succeed. The skins were to be suspended vertically in a large air-tight vat, which, as well as the skins, were to be completely exhausted of air, previous to saturating them with the tan liquor, which the skins will, in consequence, more readily imbibe. A large aperture, or manhole, is made in the top of the vat, for a workman to descend and hang up the skins, which are stretched from side to side upon hooks, at a regular distance apart, and kept in vertical and parallel positions by leaden weights, at their lower edges. This being done, a weak infusion of tan is admitted, until it covers the hides; the workman then closes the man-hole by the cover, which is rendered air-tight by a proper packing upon its rebated edges; the air is next exhausted by the air-pump as far as may be deemed necessary; in this state the vessel is to remain for a day or two, when the air may be re-admitted by a stop-cock, and the liquor pumped out through a pipe at the bottom of the vessel.
The hides are then to remain to drain, and in contact with the air for a few hours, after which a second infusion of tan, stronger than that first used, is let in to cover the hides, and the process repeated as often as may be found necessary to completely tan the hides, increasing the strength of the liquors at every successive operation.
Our transatlantic brethren are not behind us in attempts to improve the old system of tanning. In the Journal of the Franklin Institute, we find the following specification of an American patent, granted to Osmond Cagswell, in 1831, which seems to be well deserving of the attention of the British tanner:- "The improvement consists in applying a solution of oak or other bark to hides or skins, in such manner, as that when the glutinous particles of the hide have absorbed and become mixed with the tanning or astringent principle, the other part of the solution (viz. the water) may pass off, and leave the hide free to receive more of the solution, and so on till it is tanned. The object is to expedite the process of tanning, and, consequently, to diminish the amount of capital necessary to be employed in the business. The apparatus, and mode of application, is as follows: Make a frame of timber, of a square form; the width to be made as great as the width of the hides, parts of hides, or skins, that are to be tanned; the height and length to suit convenience.
Near the bottom, or ground of said frame, a light floor is to be formed of the length and breadth of the frame; said floor to incline to one side, so as to carry off the liquor after it has passed through the hide; the sides and ends to be raised from two to four inches above the floor, by fastening strips of plank on the inside of the frame; this will appear like a box, - say four feet wide, two inches high on one side, and four on the other, and twenty feet long; (these boxes may be fixed one above another, about twelve inches apart, to the top of the frame;) said boxes to be filled with sawdust, or any other soft porous substance that will not prevent the solution from running through the hide, and, at the same time, absorb and carry it off after it has passed through. On this surface (of sawdust) the hides, sides, or skins, (after having been prepared in the usual mode for tanning, except that the flesh is to be taken off clean,) are to be smoothly spread out, and, in order to keep on them a sufficient quantity of the solution, make sacks of coarse cotton or other cloth, an inch or more in diameter; fill them with the same material that the boxes are filled with, and place them around under the edges of the hides, which will raise said edges equal to the diameter of the sacks.