TANNING

TANNING is an art, of which a full account, according to the general practice in London and its vicinity, has been given under the proper title in the Encyclopædia. But since that article was written, the superior knowledge which has been obtained of the tanning principle, as well as of the composition of the skins of animals (See Vegetable and Animal SUBSTANCES, Suppl.), has suggested to scientific artists various methods of shortening the process by which leather is manufactured. M. Seguin is said to have thrown much light upon the art of the tanner as it is practised in France; and in 1795 Mr William Desmond obtained a patent for practising Seguin's method in England. He obtains the tanning principle by digesting oak bark, or other proper material, in cold water, in an apparatus nearly similar to that used in the salt-petre works. That is to say, the water which has remained upon the powdered bark for a certain time, in one vessel, is drawn off by a cock, and poured upon fresh tan. This is again to be drawn off, and poured upon other fresh tan; and in this way the process is to be continued to the fifth vessel. The liquor is then highly coloured, and marks, as Mr Desmond says, from six to eight degrees on the hydrometer for salts. He calls this the tanning lixivium. The criterion to distinguish its presence is, that it precipitates glue from its aqueous solution, and is also useful to examine how far other vegetable substances, as well as oak bark, may be suitable to the purpose of tanning. The strong tanning liquor is to be kept by itself. It is found by trials with the glue, that the tanning principle of the first digester which receives the clear water, is, of course, first exhausted. But the same tan will still give a certain portion of the astringent principle, or gallic lixivium, to water. The presence of this principle is ascertained by its striking a black colour when added to a small quantity of the solution of vitriol of iron or green copperas. As soon as the water from the digester ceases to exhibit this sign, the tan is exhausted, and must be replaced with new. The gallic lixivium is reserved for the purpose of taking the hair off from hides. Strong hides, after washing, cleaning, and fleshing, in the usual way, are to be immersed for two or three days in a mixture of gallic lixivium and one thousandth part by measure of dense vitriolic acid. By this means the hair is detached from the hides, so that it may be scraped off with a round knife. When swelling or raising is required, the hides are to be immersed for ten or twelve hours in another vat filled with water and one five-hundredth part of the same vitriolic acid. The hides being then repeatedly washed and dressed, are ready for tanning; for which purpose they are to be immersed for some hours in a weak tanning lixivium of only one or two degrees; to obtain which, the latter portions of the infusions are set apart; or else some of that which has been partly exhausted by use in tanning. The hides are then to be put into a stronger lixivium, where in a few days they will be brought to the same degree of saturation with the liquor in which they are immersed. The strength of the liquor will by this means be considerably diminished, and must therefore be renewed. When the hides are by this means completely saturated, that is to say, perfectly tanned, they are to be removed, and slowly dried in the shade. Calf skins, goat skins, and the like, are to be steeped in lime-water after the usual fleshing and washing. These are to remain in the lime water, which contains more lime than it can dissolve, and requires to be stirred several times a day. After two or three days, the skins are to be removed, and perfectly cleared of their lime by washing and pressing in water. The tanning process is then to be accomplished in the same manner as for the strong hides, but the lixivium must be considerably weaker. Mr Desmond remarks, that lime is used instead of the gallic lixivium for such hides as are required to have a close grain; because the acid mixed with that lixivium always swells the skins more or less; but that it cannot with the same convenience be used with thick skins, on account of the considerable labour required to clear them of the lime; any part of which, if left, would render them harsh and liable to crack. He recommends, likewise, as the best method to bring the whole surface of the hides in contact with the lixivium, that they should be suspended vertically in the fluid by means of transverse rods or bars, at such a distance as not to touch each other. By this practice much of the labour of turning and handling may be saved. Mr Desmond concludes his specification, by observing, that in some cases it will be expedient to mix fresh tan with the lixivium; and that various modifications of strength, and other circumstances, will present themselves to the operator. He affirms that, in addition to the great saving of time and labour in this method, the leather, being more completely tanned, will weigh heavier, wear better, and be less susceptible of moisture than leather tanned in the usual way; that cords, ropes, and cables, made of hemp or speartery, impregnated with the tanning principle, will support much greater weights without breaking, be less liable to be worn out by friction, and will run more smoothly on pulleys; inasmuch that, in his opinion, it will render the use of tar in many cases, particularly in the rigging of ships, unnecessary; and, lastly, that it may be substituted for the preservation of animal food instead of salt. Mr Nicholson, from whose Philosophical Journal we have taken this account of Mr Desmond's method of tanning, made some very proper enquiries at one of the first manufacturing houses in the borough of Southwark, concerning its value. He was told by one of the partners, that the principle upon which the new process is founded had been long known to them; but that they preferred the old and slower method, because the hides are found to feed and improve in their quality by remaining in the pit. He could gain no satisfactory information of what is meant by this feeding and improving; and, without taking upon us to decide between the advantages peculiar to Desmond's method and those of the common practice, we cannot help saying that this objection of the tanner at Southwark appears to us to be that of a man who either understands not the principles of his own art, or has some reason for opposing the progress of improvement, if it do not originate in his own house.