The process of tanning is essentially the same in all skins. It consists merely in immersing the skin for a sufficient length of time in an inflision of oak bark, or other vegetable astringent, until it is completely saturated with it. Hence the art of preserving the hides of animals by this method is one of the most ancient and universal of all manufactures, no apparatus whatever being required to perform it, except a pit or hole for water, in which the tanning vegetable may be put, and the skin thrown in along with it. Almost equal simplicity is observed in the most improved methods of tanning, the art mainly consisting in judiciously regulating the strength of the tanning infusion, and in the manipulation of stirring the hides in such a manner, that all that are in a pit may be equally impregnated.
The substance used in this country is chiefly oak bark, which is ground into a coarse powder, and is thrown into pits with water, by which an infusion of the tan, and other soluble parts, is made, which is technically called wee. The bides, (previously prepared in one or other of the ways before mentioned,) are first put into small pits, with a very weak ooze, where they are allowed to macerate for some weeks, with frequently stirring, or handling, as it is termed. As the process of tanning proceeds, the strength of the different oozes is gradually increased, after which, the half-tanned hides, (if of the thick kind, intended for sole leather, and which require very complete tanning,) are put into larger pits, with alternate layers of ground bark, in substance, till the pit is filled, over which a heading of bark is also laid, and the interstices filled up with a weak ooze to the brim. The hides are by this arrangement supplied with a quantity of fresh tan in proportion as they absorb the tan, previously dissolved in the water. By this mode of tanning, the thickest leather takes fifteen months before it is thoroughly tanned throughout; which is ascertained by cutting a piece off the edge of the hide, when it should appear uniformly throughout its thickness of a nutmeg-brown colour, and any portion that is not tanned will exhibit a whitish or pale-coloured streak in the middle.
M. Seguin, a French chemist, investigated the process of tanning with great assiduity, and came to the conclusion that by condensing the tanning principle so as to accelerate its action, leather might be tanned in a less number of days than it usually takes of months. To effect this, his process is simple. He pours water upon the powdered tan, contained in an apparatus nearly similar to that made use of in saltpetre works. This water, by going through the tan, takes from it a portion of its tanning principle, and by successive filtrations dissolves every time an additional quantity of it, till at last the bark rather tends to deprive it of some than to give up more. Seguin succeeded in bringing these solutions to such a degree of strength that he could, according to his own state ment, completely tan a calf-skin in twenty-four hours, and the strongest oxhide in seven or eight days. These solutions containing a greater quantity of the tanning principle, impart (it is said) to the skin as much of it as it can absorb, so that it can then easily attain a complete saturation of the principle, and pro duce leather of a quality much superior to that of most countries famous for their leather.
When a patent for Seguin's method was taken out in this country, Mr. Nicholson stated, that from information acquired from the manufacturers, he fonnd that they bad previously been sufficiently acquainted with the powers of strong tanning infusions, and that it had even been proposed to employ them as to abridge the process, but the leather thus produced so was by no means equal to that produced in the old way. The advantage of the slow and gradual pro changed; appears to be, that the whole substance of the skin is penetrated and equally chan ; while in the more rapid method the external must be more acted on, and the texture probably more unequal. It appears also from Sir IL Davy's experiments, to combine with a larger quantity of the extractive matter con tained in the astringent infusion ; and hence, too, the advantage of the immer sions in the weak liquors, as these contain more of this than the strong infusions. It must be confessed, however, that for any thing theory can discover, the com mon process appears to be unnecessarily protracted, and some advantage might probably be derived from adopting some of the manipulations of Seguin.
To accelerate the process of tanning, warm infusions of the tanning liquor, instead of cold, have been employed, and we are informed with some degree of success. With the same object in view, it has likewise been attempted to make leather by forcing the tanning liquor into the pores of the skin by mechanical pressure. The first of these attempts was made by Mr. Francis Gibbon Spils Walsall, in Staffordshire, who took out a patent for his process in 1824, which he thus describes in his specification :—" My invention consists in the introduction of the tan liquor, by means of mechanical force, into the pores or substance of the skin or hide, which I effect in the following manner :—The skin or hide being cleansed, and otherwise prepared in the usual ways for the action of the tan liquor, is to be carefully examined, and any holes that may be found are to be sewed, or otherwise secured, by means which are well known, so as to prevent the liquor from running through ; after which it is in a state to be exposed to the action of the tan liquor, in conjunction with nical pressure, which I effect in the following manner. I provide three frames,
of similar shapes, made of wood, copper, or any other suitable material (I may mention that the use of iron for this purpose, unless covered with a coating of paint, should be avoided, as its effects would be to blacken the skin or hide), and furnished at the sides with ears or loops, for the reception of screw-bolts, the object being, by means of the outer frames, to press two skins or hides, one on each side, against the middle frame, and through an aperture in this middle frame to the tan liquor under pressure into the space thus formed between the two bides, the effect of which will be to produce a continued filtra tion orpercolation of the liquor ; and in consequence of which, the tanning process rapidly takes place. The middle frame differs from the others in having two pipes let into it at the top, and a cock let into it at the bottom. One of the exterior frames being laid fiat down, with its inner surface uppermost, a skin or hide, previouslyprepared as aforesaid, is laid or stretched over it; the middle frame is then laid on, taking care that the edges of the skin or hide shall be every where griped or nipped between the two frames ; a second skin or hide, prepared as aforesaid, is then to be laid on the middle frame ; and lastly, the other exterior frame is to be laid on, care being taken that thee of the second skin or hide shall be every where griped or nipped between middle frame and the last exterior frame. The frames and skins are then to be secured by means of screw bolts, entering into screwed holes, in the ears or loops. The frames are then to be raised upright; one of the pipes is to be secured to a pipe communicating with a cistern containing tan liquor; the other pipe is to be left open for the escape of air, and the cock at the bottom is to be closed. The cock of the pipe communicating with the cistern being opened, the liquor contained in the cistern will flow down, and will occupy the space between the two skins or hides, driving out the air. When the liquor has risen into the pipe for the escape of the air, showing that the space is filled, its cock is to be closed; upon which the tan liquor between the skins or hides being subjected to hydrostatic pressure, by means of the communication with the cistern (and which may be produced, increased and varied by methods well known), will be forced through the pores or substance of the skins or hides, and will appear in the form of dew, or small drops, on their outward surface. The time required for completing the tanning will vary according to the density of the skins or hides, the strength of the liquor, the amount of the hydrostatic pressure, and other circumstances. 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 appa ratus 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 lig. 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 is a 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. Spllsoury's. Neither of these gentlemen, however, according to our information, have as yet succeeded in bringing their plane 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.