TANNING, the art of converting hides and skins into leather. This art has been practised for many centuries in Britain; but some improvements have been made on it, especially in France, suggested by the discoveries of modern chemistry. These improvements we shall briefly notice after having described the method lately practised in the neighbourhood of London, where the best British leather is manufactured. The general principles on which the improvements are founded, will naturally come to be considered, after describing the processes themselves.
The leather tanned in England is generally divided by the manufacturers into three kinds, butts or backs, hides, and skins. Butts are made from the stoutest and heaviest ox hides, and are used chiefly for the soles of stout shoes and boots. Hides, or crop-hides, are made from cow hides, or the lighter ox hides, and are employed for ordinary soles. The term skins is applied to all the other kinds of leather, comprehending that made from the skins of calves, seals, dogs, kids, &c.
Butts are tanned as follows. After the horns are taken off, the hides are laid smooth in heaps for two days in summer, and five or six in winter; they are then hung on poles in a close room, called a smoke-house, in which is kept a smouldering fire of wet tan; this occasions a small degree of putrefaction, by which means the hair more easily comes off, by spreading the hide on a sort of wooden horse or beam, and scraping it with a crooked knife. The hair being taken off, the hide is thrown into a pool of water, to cleanse it from the dirt, &c. which being done, it is again spread on the wooden beam, and the grease, loose flesh, extraneous filth, &c. carefully taken off: the hides are then put into a pit of strong liquor, called ooze, prepared in pits kept for the purpose, by infusing ground oak bark in water, which is termed colouring. The hides are then removed into another pit, called a scouring, which consists of water strongly impregnated with vitriolic or sulphuric acid, or a vegetable acid prepared from rye or barley. This operation
Tanning. operation is called raising. The hides are then taken out of the scouring, and spread smooth in a pit usually filled with water, called a binder, with a quantity of ground bark strewed between each. After lying a month or six weeks, they are taken up, and the decayed bark and liquor being drawn out of the pit, it is again filled with strong ooze, when they are put in as before, with bark between each hide. They now lie two or three months, at the expiration of which the same operation is repeated; they then remain four or five months, when they again undergo the same process, and after being three months in the last pit, are completely tanned, unless the hides are so remarkably stout as to require an additional pit or layer. The whole process requires from 11 to 18 months, and sometimes two years, according to the substance of the hide, and discretion of the tanner. When taken out of the pit to be dried, they are hung on poles; and after being compressed by a steel pin, and beaten out smooth by wooden beetles, the operation is completed.
3 Of tanning hides. Hides are thus managed. After the horns are taken off, and the hide is washed, they are put into a pit of water, saturated with lime, and having mixed with it a quantity of the same substance, where they remain a few days, when they are taken out, and the hair scraped off on a wooden beam, as before described; they are then washed in a pit or pool of water, and the loose flesh, &c. being taken off, they are removed into a pit of weak ooze, where they are taken up and put down two or three times a day, for the first week; every second or third day they are shifted into a pit of fresh ooze, somewhat stronger than the former, till at the end of a month or six weeks they are put into a strong ooze, in which they are handled once or twice a week with fresh bark for two or three months. They are then removed into another pit, called a layer, in which they are laid smooth, with bark ground very fine, strewed above each hide. After remaining here two or three months, they are generally taken up, when the ooze is withdrawn, and the hides put in again with fresh ooze and fresh bark, where, after lying two or three months more, they are completely tanned; except a very few stout hides which may require an extra layer. They are then taken out, and hung on poles, and being smoothed by a steel pin, are, when dry, ready for sale.
4 Of tanning skins. Skins are to be washed in water, &c. and put into lime pits as before mentioned, where they are taken up and put down every third or fourth day for two or three weeks, in order to destroy the scarf-skin. The hair is then scraped off, and the excrescences being removed, they are put into a pit of water impregnated with pigeons dung, called a grainer, which in a week or 10 days soaking out the lime, grease, and saponaceous matter, softens the skins, and prepares them for the reception of the ooze. They are then put into a pit of weak ooze, in the same manner as the hides, and being frequently handled, are by degrees removed into a stronger, and still stronger liquor, for a month or six weeks, when they are put into a very strong ooze, with fresh bark ground very fine, and at the end of two or three months, according to their substances, are sufficiently tanned; when they are taken out, hung on poles, and dried.
The lighter sorts of hides, called dressing hides, as
well as horse hides, are managed nearly in the same manner as skins, and are used for coach work, harness work, &c.
The principal objections to this old method of tanning are, that it is extremely tedious, and very expensive. Various means have been suggested for introducing a cheaper and more expeditious method of tanning. Among the earliest of these we may notice that of Dr Macbride. This method consists chiefly in the use of sulphuric instead of acetic acid, for raising or distending the pores of the leather, and in substituting lime water, or a solution of lime, for what has been called the milk of lime, or a considerable quantity of lime diffused in water. According to a report made to the committee of commerce of the Dublin society, it appeared that Dr Macbride's method produced a saving of more than 20 per cent. to the manufacturer, while the hides were completely tanned in a much shorter time. It does not appear, however, that this method ever came into general use.
The experiments of M. Seguin, made in the end of the 18th century, on the nature of the tanning principle, led him to suggest a method of tanning which is certainly much more expeditious than the old method. It has been adopted in England by Mr William Desmond, and by his directions has been practiced with considerable success, by some of the principal tanners in Warwickshire, Staffordshire, and some of the neighbouring counties. The following directions, communicated by Mr Desmond to the editor of the Philosophical Magazine, will sufficiently explain this new process.
Provide five vessels, called digesters, of any convenient materials and dimensions, with an aperture at the bottom of each. Let them be placed near each other, and elevated on stilles or otherwise; so that a small vessel may be placed under them. Fill the digesters with tan, viz. the bark of certain trees, such as of oak, cut small, or ground to a coarse powder. Pour water on the tan in the first digester, where it may stand some time, or be immediately drawn off. This liquor is to be poured on the tan in the second digester; from that to the third, and so on, until it comes through the tan in the last digester. The liquor is then highly coloured, and marks from 6° to 8° on the hydrometer for salts. This liquor is to be used for tanning the thickest hides, and may be called the tanning lixivium. If you take a small quantity of it in a glass, and pour on it a few drops of a solution of animal glue, the clear liquor becomes turbid, and a whitish substance falls to the bottom. The precipitate thus obtained, is a sure indication that the liquor contains the tanning principle; for this reason, that glue being of the same nature with the skins or hides of which it is made, whatever substance unites itself indissolubly with the former, will do so likewise with the latter.
This solution is made by dissolving a little common glue in water over a moderate fire; by means of it, not only oak bark, but also the bark of several other trees, as well as different shrubs, and plants, all which may be called tan, are found to contain the tanning principle; and by employing the solution as before, it will be always easy to ascertain whether any given substance contains this principle.
In the course of these lixiviations it may be observed, 1. That the liquor running from the first digester, at length
length loses its colour. If in this state a little of it be put into a glass, and the former experiment be repeated, the liquor no longer becomes turbid, but remains clear, which shews that it contains no more of the tanning principle; but if a few drops of a solution of sulphate of iron be poured into the same glass, the liquor becomes thick and black, which is not to be poured on the tan in the second digester, but afterwards used for taking off the hair or wool. It is known by the name of gallie lixivium, as it appears to contain the same principles with galls.
The liquid sulphate of iron is obtained by dissolving a small quantity of iron in diluted sulphuric acid, or by dissolving green copperas in water. This solution serves to ascertain such substances as contain the gallic principle. Lime water will also produce this effect.
When the liquor ceases to grow black by the mixture of the sulphate of iron, it will be in vain to pour any more water on the tan in the first digester. This tan being thus exhausted, must be removed, and new tan put in its place.
The liquor, after running through all the digesters, at last grows weak. All the liquor that marks from 6° to 8° on the hydrometer, must be added to the stock of tanning lixivium. What proceeds afterwards from the last digester is to be poured on the new tan in the first. Then the fresh water is to be conveyed on the tan in the second digester, and the liquor of the first set aside, while it marks 6° or 8° on the hydrometer, and added to the tanning lixivium, which must always be carefully separated from the gallic. In this manner, the tan in all the digesters may be renewed, and the lixiviations continued.
The number of these lixiviations, as well as the mode of making them, may be varied at pleasure; the essential point is to repeat them so as to give the liquor a sufficient degree of concentration, which may be determined by the hydrometer, and proportioned to the quickness required in the operation, and the thickness of the hides and skins to be tanned; all which experience will soon teach. As all kinds of tan are not equally good, it will sometimes happen that six or more filtrations will be necessary to obtain a lixivium of 6° or 8°, in which case the number of digesters must be increased, and the same method pursued as above; and when a weaker lixivium is wanted, three or four filtrations will be sufficient.
The person directing these lixiviations should be provided with the solution of glue and sulphate of iron, already described, in order to ascertain the qualities of the different lixivia, as well as with a hydrometer properly graduated, to determine their degree of concentration or specific gravity.
In tanning cow and ox hides with this lixivium, they should first be washed in running water, well cleaned, and fleshed in the usual way. For removing the hair, the hides are to be steeped for two or three days in a vat filled with the gallic lixivium, and a mixture of sulphuric acid, marking 66° on the hydrometer for acids, and in the proportion of one to a thousand, or one pint to 125 gallons. During this steeping, the hair is separated from the hides in such a manner, that it may be easily known when they are to be taken out of the vat, that is, when the hair is quite loose. It is to be scraped off with a round knife on the horse or beam.
When raising is necessary, the hides are immersed for 10 or 12 hours in a vat filled with water, and of its volume of mineral acid, of the same quality with the former, and the operation of raising is finished. The hides are repeatedly washed, and the round knife is used, after which they are prepared for tanning.
The rest of the process consists in tanning, properly so called; for which purpose, the hides are to be steeped some hours in a weak lixivium of only 1° or 2°; to obtain which, that is to be taken which runs from the second digester, or some already used for tanning. They are next 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 being then much diminished, it must be renewed; and when the hides are completely saturated, or fully tanned, which is known by cutting off a bit of the edge, remove the leather, and let it dry slowly in a shady place.
For calf skins, goat skins, &c. these are first fleshed with the knife, and worked in running water like the others. They are then steeped in lime water, in which there should be more lime than the water can dissolve at once. What is not dissolved will subside, but must be mixed with the water, by stirring it several times a day. In two or three days the skins are to be removed; when the hair is found quite loose, it is scraped off on the horse. They are then washed and pressed well, till the water running from them is perfectly clear, and the lime totally extracted. They are first steeped in a weak lixivium, then tanned as above; but the tanning lixivium must not be nearly so strong as that for hides.
Lime is used for these soft skins instead of a mixture of gallic lixivium and sulphuric acid, because the acid always swells the leather more or less, and because the lime may be more easily extracted from them, by washing and compressing them, than from the thick hides, which, when limed, are harsh and apt to crack, if the lime be not wholly extracted before they are tanned.
Among the different methods of immersion which may be practised in the course of these operations, the best way seems to be that of suspending the hides and skins vertically in the lixivia, by means of transverse rods or bars, and at such a distance asunder as not to touch each other in any one point. If they are laid out the one over the other, they will require frequent handling, in order that all the parts may be equally saturated, and to prevent the folds or plaits that would otherwise be formed in them. In some cases it will be found expedient to mix fresh tan from time to time with the lixivium, which will depend on the state and quality of the hides and skins to be tanned, as well as on the purposes for which they are intended. All these considerations must be left to the judgment of the manufacturer; but they do not change the principle on which this mode of tanning is founded.
Mr Desmond asserts, that besides the very great savings in point of time and labour, the leather tanned according to the above method being more completely saturated, will be found to weigh heavier, to wear better, and to be less susceptible of moisture, than the leather tanned in the usual way*. * Philos. Mag. xi. 20.
In explaining the principles on which the several parts of the tanning process depend, we must first remark, that the principal object of tanning is, to combine
Tanning. bine the gelatinous part of the hides with the tanning principle of astringent vegetables as intimately as possible, and thus produce that compound which we call leather, and which is insoluble in water. The chief part of the process therefore consists in steeping the hides in a solution of tannin till they are sufficiently impregnated with the tanning principle; and to this operation the others are subservient, only as they prepare the hides to be more easily acted on by the tanning principle.
The infusions of oak bark, when chemically examined, are found to contain two principal substances, one precipitable by solution of gelatine made from glue or isinglass, and this gives a dense black, with solution of common sulphate of iron; the other not precipitable by solution of gelatine, but precipitating the salts of iron of a brownish black, and the salts of tin of a fawn colour.
The former of these is the tanning principle, or the tannin of Seguin; it is essential to the conversion of skin into leather. The latter is the colouring or extractive matter; it is capable of entering into union with skin, and it gives to it a brown colour; but it does not render it insoluble in boiling water.
It has been generally supposed that the infusion of oak bark contains a peculiar acid, called gallic acid; but some late experiments render this opinion doubtful; and this principle, if it exists in oak bark, is in intimate combination with the extractive or colouring matter.
In the common process of tanning, the skin, which is chiefly composed of gelatine, slowly combines in its organized form with the tannin and extractive matter of the infusions of bark; the greater proportion of its increase of weight, however, is owing to tannin, and from this substance the leather derives its characteristic properties; but its colour, and the degree of its flexibility, appear to be influenced by the quantity of colouring matter that it contains. When skin, in large quantity, is suffered to exert its full action on a small portion of infusion of bark, containing tannin and extractive matter, the fluid is found colourless. It gives no precipitate to solution of gelatine, and produces very little effect on the salts of iron or of tin. The tanning principle of oak bark is more soluble in water than the extractive matter; and the relative proportion of tannin to extractive matter is much greater in strong infusions of oak bark than in weak ones; and when strong infusions are used for tanning, a larger proportion of tannin is combined with the matter of skin.
The state of the skin with regard to its impregnation with tannin may be easily ascertained by cutting it transversely with a sharp knife, as the tanned part will appear of a nutmeg colour, while the unimpregnated part retains its whiteness. Though the impregnation of the skins with tannin be an essential part of the process, something more is required to give the leather its proper degree of strength and pliability. The infusions of oak bark, especially the weaker infusions, contain, besides tannin, more or less of extractive matter, which is absorbed by the skins during the tanning process. Hence it appears, that a solution of tannin alone would not convert the skins into leather; and that as concentrated infusions of oak bark contain a less proportional quantity of extractive matter, they are not so well calculated for the purposes of tanning as the weaker infusions.
This is an important conclusion, as it shews that the vulgar opinion of tanners respecting the propriety of the old methods, and what they call feeding the leather, is founded on rational principles. In fact it appears, that, though, in the quick method, recommended by Seguin and Desmond, the leather may be more expeditiously, and perhaps more completely impregnated with tannin, it is deficient in strength and pliability, from the want of its due proportion of extractive matter.
Having thus explained the principles on which the material part of the tanning process depends, we must briefly notice the rationale of the preliminary operations.
Chaptal has shewn, that when skin is immersed in a tanning liquor, without having been previously freed from its cuticle or scarf-skin, the impregnation of tannin takes place only on the flesh side. This shews the necessity, especially in the thicker hides or butts, of removing the cuticle, before steeping the hides in the tanning liquor. The small degree of putrefaction to which the butts are subjected, has this effect, and the steeping of the hides and skins in lime water contributes to the same end; for though lime does not seem to be capable of dissolving the cuticle, it renders it friable, so that it is easily removed by the instruments employed for scraping off the hair. Not only the cuticle, but likewise the soft matter of the extremity of the hair is acted on by lime; and this effect must considerably tend to facilitate the process of depilation. The same substance mixing with the fat on the fleshy side of the skins, forms a soapy compound, which, with other extraneous matter, is removed by the subsequent washings.
It has been supposed that the acids in which the skins are steeped, previous to their immersion in tanning liquors, have the effect of opening their pores, and thus rendering them more easily penetrable by the tanning principle and extractive matter. We believe that this opinion is erroneous, as we cannot see how acids, the obvious effect of which seems to be that of contracting animal matter, can enlarge the pores of the skins. It is probable that they produce some other advantageous effect not yet sufficiently understood, in preparing the skins for being more perfectly acted on by the tanning liquors.
The principal effect of the grainer, or the pigeons dung employed in the thinner skins, seems to be that of promoting putrefaction, and rendering the skins less elastic, though the alkali evolved during the fermentation of the dung, may assist in removing the fat on the flesh side of the skins.
As from the present great demand, and consequent scarcity of oak timber, oak bark has become a very expensive article, it may be proper to enumerate a few of the principal vegetable substances, especially those indigenous to Great Britain, that may be substituted for it. Of these the bark of the Scotch fir appears to be most deserving of attention, and was some years ago employed by a gentleman in Ireland with great success. Several species of willow afford a good substitute for oak bark, particularly the Leicester willow, of which the entire bark produces a greater quantity of solid extract than the entire bark of oak. Next to these may be mentioned the bark of the common elm, the root of tormentil (tormentilla vulgaris, Lin.) which has been long employed in the north of Scotland as an article of domestic
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Taur. 1000
meotic tanning. To these may be added the herb avens (geum urbanum, Lin.), several species of cinquefoil, and of bistort, common ladies mantle (alechemilla vulgaris), and the root of the common water-flag (iris pseudacorus, Lin.). Of plants not indigenous to Britain, but generally cultivated here, we may particularly notice the horse-chestnut, the bark of which is a strong astringent, and might be employed, we think, with great advantage in tanning. The most powerful tanning substance, however, with which we are acquainted, is the juice or extract of the mirasol catechu, commonly called Japan earth, one pound of which will tan as much leather as seven or eight pounds of oak bark.
Our limits will not permit us to extend this article, by describing the processes for tanning employed in other countries. On the method pursued in Russia, our readers may consult Tooke's View of the Russian Empire; and of the French method of tanning, an ample account has been given in a publication by De Lalande. Another on the same subject may soon be expected from Chaptal. The most complete work on British tanning, and on other processes to which leather is subjected, with which we are acquainted, is a small volume entitled The Art of Tanning and Currying Leather, published by the Dublin Society in 1780. Several useful papers on this subject may also be found in Nicholson's Philosophical Journal, and Tillich's Philosophical Magazine.
For an account of other processes connected with the leather manufacture, see LEATHER and CURRYING.