After this is done, pour on the hides as much of the solution as the hollow surface which they will then present will hold, and continue to fill them up as it runs off through the pores of the hide for the space of from three to fifteen days (the time in proportion to the thickness of the hide or skin), in which time they will be tanned, except the extreme parts or edges, which cannot be brought so fully under the process as the other parts of the hides; and in order perfectly to tan them it is necessary to lay them in vats after the common mode, for three or four weeks."
In 1832 Mr. William Drake, of Bedminster, near Bristol, specified a patent "for an improvement in tanning hides and skins," the novelty of which consists in applying the tanning liquor on one side only of the skin, and causing it to ooze through the skin to the other side, whence the aqueous portion of the liquor is abstracted by evaporation; the results of which process are stated to be, that the skins are more thoroughly and uniformly tanned, and that the operation is completed with cold liquor in ten days instead of ten months. The specification states that the skins are to undergo the usual primary process of liming; they are then to be immersed and well handled in a vessel containing backwoard (a weak solution of tan) until thoroughly saturated, which removes the lime and prepares them for a stronger impregnation. Thus prepared, the skins (excepting such as are intended for butts and middlings) are to be rounded; then two of them are to be laid face to face, and be carefully sewn together with waxed thread at their edges, so as to form a kind of bag impervious at the junction, leaving a small opening at the shoulder for the insertion of the neck of a funnel shaped vessel; but the patentee observes, it would be better to sew between the skins a collar adapted to receive the end of the funnel.
As bags so formed would bulge out when filled, they are to be confined between two gridironlike frames of parallel bars, adapted to compress the bag in such a manner as to produce internally a vertical stratum of liquid of about an inch in thickness between the two skins; and as the skins are thickest towards their middles, this variation is compensated for by cutting away a portion of the vertical wooden bars from a straight into a hollow curved line. The skins are suspended by loops to the bags, which traverse the upper horizontal bars of the frames, and the two frames are duly drawn together by four screw bolts passing through the extremities of the top and bottom bars. The funnel being inserted into the aperture between the skins, it is charged with strong tan liquor sufficient to distend the bag, and leave a surplus quantity to supply the loss by evaporation after the moisture has penetrated to the outside of the bags; a small gutter at the bottom of and between the frames receives whatever liquors may drop from the skins, and conducts it into a vessel, by which it is returned whenever necessary into the funnel reservoir above.
To prevent the compression of the vertical bars from forming permanent indentations and ridges in the skins, the patentee directs that the bags be occasionally shifted a little laterally.
To facilitate the evaporation, and consequently the absorption of fresh solutions of tan, the operations are recommended to be conducted in chambers artificially warmed, and the liquor which oozes through the skin, and is received into the gutters, is directed to be conducted into vessels acting the part of refrigeratories, in order that cold liquor may always be supplied to the skins; (but how this liquor is to be preserved cold in a warm chamber, the specification does not explain). When the skins are sufficiently tanned, a stitch or two of the sewing at the bottom of the bag is opened, and the liquor is received into, and carried oft" by, the gutter underneath.
The claim to invention in this patent consists in the mode of accelerating the penetration of the tanning liquor by exposing the outer sides of the skins to evaporation. The process seems to be well calculated to economize time, but there is one defect in the arrangement for which we would suggest a remedy. The skins being laid vertically, the pressure of the column of liquid will cause a much more rapid absorption of the tan in the lower than in the upper part of the skins; and if no injury be sustained by the lower, by continuing the process until the upper is fully saturated with tan, there is, at the least, a loss of time. It is also probable that the liquor is stronger at the bottom than at the top of the bag. From both these causes, therefore, we should not expect that the leather produced would be uniform in its quality. To obviate these defects, we recommend the patentee to suspend his frames midway upon revolving axes, and to fix at each end of his bag a charging vessel with a stop-cock, or Borne other simple contrivance to answer the same purpose: the bags may then be reversed at pleasure, swinging them round upon their axes into any desired position, and the lateral shifting between the bars will take place of itself.
If there were only one charging vessel with a stop-cock to it, it would suffice; as by turning the frame half-way round it would serve for a discharging aperture.
Mr. Jacquemart, of Leicester Square, London, recently introduced, under a patent right, a process of tanning, which is stated to be especially applicable to the skins of small animals, such as hares, rabbits, cats, and sea-rats. Upon reading the specification, however, we did not find any thing essentially different from that which is well known, and in use, except that he adds a small quantity of orpiment to the other ingredients employed in tanning; 5 or 6 ounces are mentioned as a proper quantity for a hundred of the small skins. He commences the process by removing the hair from the skins; first taking off the long hair and afterwards the short; and to facilitate this operation, he steeps the skins in water slightly acidulated, (using sulphuric acid, in very small quantities,) or in the milk of lime; and in either of these the skins are suffered to remain till the matter which fixes the hair to the hide is decomposed. After the hair has been removed, the skins are to be again steeped in water containing a very small proportion of sulphuric acid, in order to raise or thicken them.