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 statement, completely tan a calf-skin in twenty-four hours, and the strongest ox-hide 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 produce 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 had previously been sufficiently acquainted with the powers of strong tanning infusions, and that it had even been proposed to employ them so as to abridge the process, but the leather thus produced was by no means equal to that produced in the old way. The advantage of the slow and gradual process appears to be, that the whole substance of the skin is penetrated and equally changed; while in the more rapid method the external must be more acted on, and the texture probably more unequal. It appears also from Sir H. Davy's experiments, to combine with a larger quantity of the extractive matter contained in the astringent infusion; and hence, too, the advantage of the immersions 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 common 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-bury, of 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 1 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 proper state to be exposed to the action of the tan liquor, in conjunction with mechanical 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 introduce the tan liquor under pressure into the space thus formed between the two hides, the effect of which will be to produce a continued filtration or percolation 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, previously prepared 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 the edges of the second skin or hide shall be every where griped or nipped between the 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.