Drying the Hides.
The hides, when taken out of the solution of tan, must be dried with the usual precautions, that is to say, so slowly, that the skin does not shrink on the flesh side. With respect to thinner hides, for the upper leather of shoes, Mr. S—— begins by washing and taking off the flesh in the manner already described, or, as is done in the common way for strong soal leather; he then takes off the hair by means of clear lime-water; he does not make them undergo the operation of swelling, but puts them immediately into weak solutions of tan, the strength of which he gradually increases, but without ever bringing it to the degree of contraction, which he gives it when it is to be used in tanning thick leather; two, three, or four days, are enough for tanning the thinner kind of leather. Leather which is not sufficiently impregnated with the tanning principle, is generally known by a white speck or streak, which is observable in the middle of its substance. We can affirm that those hides which were tanned in our presence, in a few days, were completely tanned, as the above mentioned white streak was not perceivable; we may also add, that Mr. S——'s method has the advantage of affording the opportunity of observing and examining, from time to time, the progress of the operation; for this purpose nothing more is necessary but to take a slip off the hide out of the vat, and cut off a corner of it, the white streak already spoken of will appear more or less thick, until the tanning is completed; it has been generally supposed, that the tan in the tanpits had no other effect upon the leather than that of hardening and bracing the fibres of the skin, which has been relaxed by the preliminary of tanning. Mr. S——, however, examined the operation more closely, and discovered that there existed in the tan a principle which was soluble in water, by which the tanning was brought about. That this principle afterwards became fixed in the leather in consequence of a particular combination between the said principle and the skin; and this combination produced a substance that was not soluble in water; all this has been demonstrated by Mr. S——, in the most evident manner. It is well known that if leather, which has not been tanned, is boiled in water, it is in a short time almost entirely dissolved therein. This solution, by being concentrated, produces a jelly, or size, which, by farther evaporation, and being dried in the air, becomes what is called glue. Mr. S—— having, in the course of his experiments, examined the effects of a solution of tan upon a solution of glue, observed that they were hardly mixed together before a white felamentous precipitate took place, owing to a combination of the glue with the tanning principle contained in the solution of tan. This precipitate is insoluble in water, either hot or cold, and acquires colour by being exposed to the light. The foregoing experiment furnishes a true explanation of the process of tanning; for it will easily be conceived that the solution of tan acts upon the hides (from which glue is produced) in the same manner as it acts upon glue; this is what really happens in common tanpits, and Mr. S——'s new method, in which the solution of tan gradually penetrates the hides, and as it penetrates combines with it, producing a gradual change of colour that is very observable, till at last the colour of the hide is changed throughout, and it acquires a compact texture and marbled appearance, like that of a nutmeg: by this it plainly appears, that a precipitation also takes place in the action of tanning, although the hide is not dissolved, but merely swelled so as to enable the solution to penetrate it more easily. The property which animal jelly, or glue, possesses, of being precipitated by a solution of the tanning principle, furnishes a means of discovering what substances may be useful in tanning: nothing more is necessary than to make a solution or infusion of the vegetable substance supposed proper for that purpose, and that upon being mixed with a solution of glue, will show by the greater or less quantity of precipitate produced, what probability there is that such substance might be advantageously employed in tanning.
Another Remark.
Lime-water also offers an excellent means of discovering such substances. If lime-water be added to a solution of tan, the mixture instantly produces a copious precipitate; and if a sufficient quantity of lime-water be added to neutralize the whole of the tanning principle, then the supernatant liquor, although still possessing colour, will not form any precipitate with glue; I mean in solution. In like manner the liquor separated from a precipitation, caused by the mixture of a solution of tan with one of glue, will not produce any precipitate with lime-water, if, during the precipitation, the tanning principle has been completely neutralized. This shows evidently that Doctor M'Bride's method of exhausting the tan by means of lime-water is defective, and that by so doing a loss of the tanning principle takes place, in proportion to the quantity of it contained or combined with the lime dissolved in the lime-water.
Another Remark.
As in summer the solution of tan is disposed to run into the vinous fermentation, and, of course, from that into the acetous, and have its principal changed, no more of the solution of tan should be prepared in the summer season than is wanted for immediate use. In winter, this precaution in not necessary, as in that season it will keep, and may be then prepared for exportation to any part of Europe and thus converted into a profitable article of commerce.
A table showing the time different hides took to be completed, in the operations of preparing and tanning.