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 recently made on it, suggested by the discoveries of modern chemistry. These we shall briefly notice, offer having described the method generally practised.

The leather tanned in England is generally divided by the manufacturer into three kinds; butts or backs, hides, and skins. Butts are made from the stoutest and heaviest ox-hides. The butt is formed by cutting off the skin of the head for glue-pieces, the neck, which is tanned as a shoulder, and the shanks and a strip of the belly on each side, which, with the shoulder, are tanned in the poor tanning fluids left by the remaining central part or butt of the hide. Hides, or crop-hides, are made from cow-hides, or the lighter ox-hides, whole, and employed for ordinary soles, whereas butts are used for boots and the stoutest shoes. The term skins is applied to all the other kinds of leather, comprehending that made from the skins of calves, seals, dogs, kids, &C.^[1. Hides are imported in Urge quantities, and sold as wet salted, dry salted, or dry hides. The two latter require some preparation in order to soften them; they are thrown into water, and frequently taken out and brushed and stretched, by being placed upon a cylindrical table and smoothed with a blunt knife by degrees: their horny character is lost, and they become pliant and soft. In some places dry hides, after depiling, are softened by working them with water in a pair of stocks like those of the fuller. A dry hide of 30 lbs. should produce as much leather as a wet salted one of 60. ]

The first operation to which they are subjected is depilation, which removes not only the hair, but also the scarf-skin. This is effected variously in different countries. In England the most common plan is to throw the hide or skin into a strong solution of slaked lime, with lime in excess. Here, in a few days, more or less according to the proportion of lime present, the hair is easily detached, the [7:21:75]hair-sheath having been dissolved, and at the same time the scarf-skin with the hair is scraped off upon a sloping rest, with a species of knife with two handles. The lime has also combined with the fat of the hide to form a calcareous soap, to dissolve which, and the excess of lime, it is well washed, sometimes in a running stream. The hide is fleshed, or deprived of the loose or extraneous flesh or cellular tissue; and if butts are to be made, the head, shoulder, and bellies are cut off. The central part or butt weighs about two thirds of the whole leather. The hair was formerly taken off by making a sour liquor from fermented vegetable matter, in which the hide lay for several days; they were also smoked in a damp state for the same purpose; but both those methods are now abandoned. They are still sometimes sweated, that is, they are laid in heaps, kept wet and warm: but in America the sweating is performed cold; the hides are hung up wet in a damp underground cellar, and are kept wet for ten days or a fortnight. In either of the three last processes, incipient putrefaction takes place sooner or later, when the hair and scarf-skin are easily removed; but the fatty matter remains, and in some cases prevents the hide from taking the tan. The tanning either follows at once, or is preceded by what is called abating or grainering. For this process, a quantity of pigeon’s dung is steeped in water. In this mixture the hides are steeped for a week or ten days, with occasional removals and strikings. The excess of lime is removed by the lithic acid of the dung; and the ammonia generated by the putrefaction of the mixture tends to form an ammoniacal soap with any remaining fat of the hide: but as the gelatine of the hide exists in two states; one, the principal, hard, or fibrous portion,—and the other (which is more soluble) contained between the fibres, and more affected by agents and putrefaction; this softer portion is removed by grainering, and the leather, when tanned, is light and porous, and more readily permeable to water.

Some tanners are anxious that their leather should look thick when completed. To “raise” the hide, they use a solution of sulphuric acid, containing 1 / 500 th part of acid: in this the hides remain ten or twelve hours, when they are found to be thickened, or to have substance; but as nothing is added to the gelatine by the process, it is only the appearance which is altered, and no difference of thickness is found in the leather after it has been under the shoemaker’s hammer.

The hide or skin thus prepared is ready for tanning. Formerly in England, and at present on the continent, it is done by stratification. A bed of bark is made upon the bottom of the pit; upon this is laid the hide, then bark, then a hide, until the pit is full; water is sometimes pumped in, and the pit left for some months; it is then emptied, and the same hides returned with fresh bark and water for a few months longer: this is repeated again and again, until the tanning is completed; the time varying from one to four years for heavy leather.

About the end of the eighteenth century, Seguin professed to give a theory of tanning. He showed that astringent solutions contained gallic acid, which precipitated sulphate of iron black, but did not precipitate gelatine; whereas the tannin present threw down gelatine as well as sulphate of iron; and from this he deduced that leather was a compound of gelatine and tannin. Upon this he founded a new process. He recommended solutions of tannin, made by pumping water successively upon the vegetable used, contained in latches or spenders, until it arrived at as great a degree of strength as the series of spenders permitted. In this way he was continually throwing away some bark as expended, and replacing it with fresh, which was next to be the strongest tap.

This is the plan now generally followed in England; but in practice it is varied, some using ground bark only, others terra japonica, valonia, or divi-divi, or mixtures of two or more; some using steam to facilitate the solution, others steaming only the backward latches; and some using only cold water. In some yards clean water only is used to extract the soluble matter, while in others the ooze, exhausted as much as it can be by hides, is made to perform the office instead of water. Every tanner has also his own particular strength of liquor to work at, the strongest being about sixty degrees, while others do not go beyond ten degrees. It is these variations which cause so great a variety among the samples of leather in the market.^[2. Some tanners, instead of laying their hides flat in the ooze, suspend them vertically. They are penetrated more quickly, but require frequent moving to prevent the fold from being an objection to their future sale; and a pit will not hold so many suspended as it will of laid bides. Skins, particularly light ones, are sometimes tanned by sewing them up as bags, filling them with fluid, and then throwing them into pits; but although they thus tan quickly, they require a great deal of room. ]

This improvement of Seguin proved a most important one; and although not equal in practice to his original ideas, yet it has shortened the period to about half of that previously occupied. The principal difficulty experienced in its use occurs in the estimation of the real quantity of tanning material actually in solution. He proposed the use of a solution of gelatine, isinglass, or glue, which was to be dropped into the fluid as a test of the presence of the tannin. In the hands of an experienced chemist this is a tolerable means of arriving at the quantity; but it proved too difficult for the tanner of the day. He therefore judges by the astringent taste of the solution, and its darkness of colour; or depends upon its strength from a certain weight of bark, &c. which may have been used.

Upon the principle that substances dissolved in water increase its density, an instrument is used which is a variety of gravimeter—a floating bulb with a stem, graduated into certain fractional parts of the bulk of fluid displaced by it. This is generally called a barkometer; and although it only indicates and measures differences of gravity, and consequently does not necessarily indicate whether it is tan, or gum, or sugar, in solution, yet in tanning solutions it affords a tolerable approximation to the relative strengths of solutions from the same substances. Each degree upon the stem is equal to 1 / 1000 th part of the weight of water. Thus, 1° shows the specific gravity of the fluid to be 1∙001, and 60° are equal to specific gravity 1∙060, water being 1∙000. But this instrument is but of little use in oozes which have been long upon hides, as tannin is converted by long exposure into gallic acid, acetic acid, and carbonic acid gas: these act upon the soluble or interstitial gelatine, and by dissolving it in the poor ooze, give the fluid an apparent and not a real value by the instrument. Thus, if a spent liquor of 5° or 6° by the instrument be tasted, or tested, it will be found to have no astringent taste; it will give no precipitate to gelatine while at the bottom, and in suspension it will contain gelatine, gallate of lime, and acetate of lime. Again, temperature alters the apparent strength; and if an ooze indicating 30° be heated gradually, it will be found to appear weaker as the temperature rises, until at 190° Fahrenheit it will settle at 0, or no strength at all.^[3. When tannin in solution is exposed to the air for a short time, the following changes take place: every two atoms of tannin absorb six atoms of oxygen from the air, and are thus converted into three atoms of gallic acid, three atoms of acetic acid, and three atoms of carbonic acid gas. The gas flies off in part; the other two acids remain, and give the acid taste and properties to the tanners’ poor liquors; [table] Becoming [table] So that it may be said that the air takes three atoms of carbon from two atoms of tannin, and leaves the remainder in the state of gallic and acetic acids. The letters C, H, and O, in the table, mean carbon, hydrogen, and oxygen. ] ®
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The hide or butt, to tan it, is brought from the beam-yard and introduced into a poor and milky liquor, in which it is "handled” for four or five days; that is, it is lifted by hooks from the pit, laid with others in a heap on the side, where it is allowed to drain, and is then returned into the pit. The lime is intended to be taken out here by the gallic and acetic acids of the poor ooze, as it is found in practice that the presence of lime tends to darken the colour of the future leather: this effect is produced by all alkalies and alkaline earths. The hide is next successively handled into, and out of, all the handlers, increasing in strength; and in this way at length, having after some months got to the strongest handler, it is found to be stained through, though not tanned, when it is removed to the troughs which contain still stronger liquors: these it successively passes through, being handled at longer intervals. Upon arriving at the end of this series, it is laid away in a pit of very strong liquor to colour, having had powdered bark of valonia sprinkled over its surface. When the process is deemed complete it is taken out, and it will be found converted into leather; and a portion of its gelatine which has been dissolved from its interior is, by Combination with a portion of tannin from the strong solution, deposited upon its surfaces, where it is found in the form of a yellow deposit, technically known as “bloom,” or “pitching,” which disguises the under colour of the leather, just as if it were covered with yellow paint. This, prejudice says, must be on its surface or it is not saleable; but it is so much quality and weight lost to the consumer, as he pays for it on the outside of his leather, to be scraped off by the shoemaker in the operation of buffing; and the leather is so much the worse, as, if it had remained incorporated with the leather, this would not have been so porous, or so permeable to water. The health of the wearer of the shoe is perhaps sacrificed by wet feet occasioned by the desire of his shoemaker to see a yellow paste on his leather, which in work he scrapes off with a piece of glass.

The theory of the formation of the bloom is this. As soon as ooze has penetrated into a hide, it loses its tanning material, but by capillary attraction is detained; this exhausted ooze acts by maceration on the finer and more soluble interstitial gelatine, and dissolves it. In handling, about one twelfth of this flows out; the remaining eleven twelfths accompany the hide into the next stronger solution, of which only one twelfth is absorbed directly, and a small portion is slowly exchanged by endosmosis and exosmosis. The small portion of strong solution which passes into the pores of the hide contributes to tan the hard fibrous portions not dissolved; and the small portion of weak solution passing out of the hide by exosmosis, gives up its dissolved gelatine to the tan of the stronger solution outside, to form tannate of gelatine, which partly adheres to the surface as bloom, and partly falls to the bottom of the pit as pitching. It is to be hoped that such an exposition will tend to explode this absurdity, and lead to the adoption of a system of tanning in which the exhausted ooze shall be so quickly removed by pressure as not to allow it to dissolve the finer and better portions of the skin. Some years since Mr Spilsbury endeavoured to introduce a new principle in tanning, by which some part of this action should be prevented, by bringing strong ooze at once into contact with the inside of the hide. He obtained a patent for a plan of tanning by infiltration. He fastened two hides in clamps, so that the frame represented four sides of a box, and the two hides two sides. By filling this box with strong tanning ooze, and forcing it through the pores by the pressure of a column of the fluid, he expected to tan them promptly and well; but he was not aware that a large excess of tannin dissolves gelatine, and thus tannate of gelatine was found on the outside of his box in long masses of slime, while the leather had lost as much in weight, was porous, and was tanned very much more in the thin than the thicker parts, as the fluid always passed through the easiest channel. Many other inconveniences attending the process caused it soon to be abandoned; but the erroneous principle upon which it was founded not having been made public, three other patentees have followed in the same track. Mr Drake of Bedminster sewed hides together so as to form a bag, which he placed inside a hollow frame-work of wood to support the bag, and then filled it with ooze. Mr William Cox of Bedminster sewed up a hide as it was on the animal’s back, laced a canvass support round, and then filled it with ooze; and Mr Chaplin sewed his hide into a bag, filled it with ooze, but laid it in a reclining position, turning it periodically. Three if not the whole of these plans are now abandoned by the patentees themselves.

Seguin’s process of tanning by solutions, although a great improvement upon the old method, is still tedious, and most expensive. Where warm oozes are used it lasts six or seven months for sole-leather; where cold oozes are in use, it extends to twelve months, consequently the tanner can turn his money only once a year: he must have capital enough to pay for twelve months’ hides, bark, &c., labour and contingent expenses, besides keeping a stock of leather; and when his capital has been turned at the end of twelve or more months, it must pay him in one single profit, the interest, &c. of twelve months. This has confined the trade to a few wealthy individuals, who look upon tanning as an investment for capita], rather than as a business which might be improved by science; and being in comfortable circumstances, they are not driven to personal exertion and close application, which would be required of less wealthy tradesmen. It is from these circumstances that tanning has been more stationary than any other manufacture, and the few improvements which have been made in it have not been made by tanners. A more recent patent appears at present to occupy considerable attention. It is founded upon the principles put into action in washing a sponge. The old tanner takes his sponge (the hide) full, or nearly so, out of one ooze, and inserts it nearly full into the next. The patentees of the roller and belt system squeeze, by a simple and self-feeding press, their hide, before they drop it into the fresh ooze; and this very simple modification bids fair to revolutionize the trade, and it is already very extensively introduced into this and foreign countries.

Messrs Herapath and Cox of Bristol, the patentees, say, that to tan 100 butts per week, they erect in a tanyard, at present capable of tanning fifty per week, six or eight large pairs of rollers, one pair over each pit. The lower roller is thirty inches in diameter, covered with horse-hair cloth; the top one eighteen inches, with levers for weights, covered with woollen cloth, each covering nailed on. For each pair of rollers there are from fifty to 100 hides connected together, head to head and tail to tail, each by about four strings. Now the first butt or hide is inserted between the rollers loaded to the proper pressure; and upon turning the bottom [7:21:77]roller (which requires very little force, as it is a slow motion), all parts of the butt are pressed as pulled through by the rollers; the next comes in order by the assistance of the strings; and so on, until the whole of the hides in the belt have been pressed, and deposited in the fluid on the other side of the pit. By a detent the motion is now reversed, when the belt is again pressed, and returned to its original position for a fresh supply of ooze. This simple plan brings strong ooze so frequently into the interstices of the hide, that the centre of a stout butt is not only stained through, but tanned, in between one and two months; calfskins and kips are tanned in from twenty to thirty days; and the liquors are so rapidly exhausted, that they are reduced from 45° to nothing in thirty-six days, losing at first more than 2° per day; and the shortness of this exposure to the air prevents much of the tannin from being decomposed. At Nailsea, near Bristol, 100 butts per week are tanned, with six latches, six rolling pits, twenty-four handlers for the offal, and six layers; one horse power drives the rollers, and two boys manage them. The following are given to us as the results produced in the first yard which was put entirely on the patent plan, as compared with those of the same yard on the old plan. Double the work is done, and half the capital drawn out as useless. The saving on bark, labour, and general cost of manufacture, is 1 1 / 8 d. per lb. The increase in the weight of butt leather is as 34 lh. to 28 lb.; 46½lb. of leather being produced from a 60 lb. wet salted hide. The butts are sent to market within four months from the time the hides are delivered to the yard. The profits from quick return, great weight, and small expense, are eight times as great as on the old plan, at similar prices of hides, bark, and leather. The leather is more elastic and more impervious to wet than any other that is made.

Oak bark was formerly the only substance used in tanning; but large quantities of valonia and terra japonica are now imported for the purpose; and recently a bean pod called divi-divi has excited attention. These substances have lessened the demand for oak bark, of course lessening the price, and rendering it not so needful to cut down oak for the sake of the bark. Valonia gives to leather great solidity and weight: the colour inclines to gray, while there is a considerable quantity of bloom on its surface. The colour of terra japonica leather is dark fawn, inclining to red; it is light, spongy, and very pervious to water: while that from oak bark is well known to be a light fawn, and does not resist water so well as that from valonia. These differences arise from the varying proportions of tannin, extractive colouring matter, and vegetable acids present; and as the tanner has the prejudices of his buyers to consult, he selects that material, or mixture of materials, which best suits his locality. There are other vegetable productions which contain tannin. Sir H. Davy estimates 1 lb. of catechu (terra japonica) as equal to 2¼ of galls, 3 of sumach (which also contains oxalic acid), 7½ of the bark of the Leicester willow, 8½ of oak bark, 11 of the bark of the Spanish chestnut, 18 of elm bark, and 21 of common willow bark. Few barks are entirely free from it; and recently patents have been taken out for using the bramble and the hop-bine in tanning; but they possess the serious inconvenience of requiring very much latch-room.^[4. Terra japonica is the dried extract of acacia catechu, and obtained at Kauhauna in Hindustan. The name catechu means in the native language the juice of a tree. Dr Thomson says it is composed of Tannin 54·5 Peculiar extractive matter. 34· Mucilage 6∙5 Insoluble matter, chiefly sand and lime S∙ 100· ]

In trying the quantity of tannin by Seguin’s process, that is, by precipitating it with solution of gelatine, 480 grains of the bark in coarse powder should be acted on by half a pint of boiling water. The mixture should be frequently stirred, and suffered to stand twenty-four hours; the fluid should then be strained through a linen cloth, and mixed with an equal quantity of solution of gelatine, made by dissolving glue, jelly, or isinglass, in hot water, in the proportion of a dram of glue or isinglass, or six table-spoonfuls of jelly, to a pint of water. The precipitate should be collected by passing the mixture of the solution and infusion through folds of blotting paper, and the paper exposed to the air till its contents are quite dry. Every 100 grains of precipitate contains 40 grains of tannin nearly.

When butts or crops are tanned, they are taken out and hung up to dry; but once or twice during drying they are placed upon a bench which is cylindrical and inclined to the horizon. Upon this they are struck or smoothed with a square bar, the surface being occasionally wetted. When the drying is sufficiently advanced, they are rolled with a brass roller about nine inches wide, loaded with from 15 cwt. to 30 cwt. This operation is called finishing.