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The art of tanning is that by which animal skins are converted into leather, a product possessing certain properties differing from those of the raw material, and eminently adapted to the purposes for which it is employed. Chemically considered leather is a compound of tannin and gelatine, possessing the requisites of durability, pliability, insolubility in water, and great power of resisting the action of chemical reagents.
The name of tan is applied to coarsely powdered bark which is obtained mostly from oak and hemlock trees, although all barks contain more or less tannin, and in some countries the extract of others is used.
To tan a skin is to saturate it with tannin in such a manner as to promote the slow combination of this principle with the gelatine, albumen, and fibrine contained in the former.
The principal steps in the manufacture of leather are,
1. The washing and soaking in pure water, for the purpose of cleansing and softening the skins, and preparing them for
2. The unhairing. - This is effected by the use of lime, or by sweating the hides, which dissolves or softens the bulbous roots of the hairs, and thus facilitates their removal by mere mechanical scraping with a blunt-edged knife.
When lime is employed, about 4 bus. are slacked and put into a large vat of clean water, capable of holding 40 hides or 2 hundred calf-skins; the lime is well mixed by a plunger, and the hides or skins are then put into it and allowed to remain from 7 to 10 days for the former, and 10 to 14 days for the latter, drawing them out daily to facilitate the process.
When the hair will slip they are taken out of the lime and plunged into clean water, from which they go to the beam where the hair is scraped off with a long curved blunt-edged knife; they are immersed in water again and taken back to the beam, and all the flesh removed from the inside of the hide or skin with a sharp knife similar in shape to the one used in unhairing, after which they are ready for
3. Bateing. - As it is all important to have the skins soft and in a condition to absorb the tanning liquor readily; this is accomplished by putting them in water impregnated with pigeon's dung, 1 bu. being enough for the number of hides or skins above named. This is called a bate, and acts by means of the muriate of ammonia which it contains, the lime taking the acid becomes muriate of lime, which is soluble and easily worked and washed out of the skins, while the ammonia passes off in a gaseous state.
Hides intended for sole-leather should remain 24 hours in this bath, when they may be worked out and are ready for the bark extract; calf-skins, or other upper leather, should remain in the bath from 3 to 5 days, according to the weather (a longer time being required in cold than in hot weather), and during this time they are taken out 2 or 3 times and placed on the beam where they are scraped, first on the grain side, and lastly on both flesh and grain with a worker similar to the one used in unhairing, after which they are ready for
4. Tanning. - When the hides or skins are taken from the beam-house they are put into vats containing a weak solution of ground bark, and should be handled two or three times a day until they are evenly colored, when they should go into a stronger liquor, or ooze, where they may remain a week, being taken out daily and allowed to drain off, at the same time strengthening the ooze. They may now be considered ready for laying away. For this purpose a vat is half filled with a very strong extract of bark, and the hides or skins are carefully laid in, one at a time, each being covered with finely-ground bark to the depth of half an inch, until all are thus laid away. About a foot in thickness of spent tan is put on for a heading, and the vat covered with boards.
The hides or skins may be allowed to remain in this their first layer for two weeks, at the expiration of which time they must be taken up, washed clean in the liquor, and the same process repeated, using a new liquor and fresh bark, as the strength has been absorbed from the other. As the tanning proceeds the extract is exhausted more slowly, and from 3 weeks to 1 month may be allowed for each successive layer, after the first - 3 layers being enough for calfskins, and 4 to 6 for sole-leather, according to the thickness of the hides.
When the sole-leather is tanned it is taken out of the vats, washed clean, and hung up to dry in the rolling loft. When nearly dry it is rolled on the grain with a brass roller until it is quite smooth, hung up again, and thoroughly dried, and is ready for the market.
When calfskins are sufficiently tanned they should be rinsed in the liquor in the vats, and hung over poles and slightly hardened, being careful not to expose them to the direct rays of the sun in the summer months. Put into piles, so that they will not dry out, dampening any part that may have become too dry. They are now shaved over a currier's beam, during which process the rough flesh is taken off, and the butts and heads are leveled and the rough edges trimmed off. The skins are then rinsed off, slicked on a marble table with a steel slicker and stiff brush on the flesh side, the dirt and coloring matter of the bark stoned, brushed, and slicked out on the grain side. They should then be hung up, by a loop cut in the head, for a few hours, that the water may be partially dried out of them: they must be then taken down and placed in a pile, and are ready for stuffing.
The grease called dubbing is composed of equal parts of cod-liver oil and melted tallow, and when ready the skin is laid on a wooden table and slicked on the flesh side. The stretch is in this manner taken out, and the skin should be perfectly smooth on the table before the dubbing is coated on, for which purpose a brush or pad is used, the quantity put on varying, according to the thickness of the skin. They are next hung up by the hind shanks, and allowed to dry. When entirely dry they must be taken down, and piled flesh to flesh and grain to grain, and should remain for a week or two, so as to become an even color, and also to absorb the strength of the grease. When ready to finish the grease must to slicked off on a finishing table (made of cherry or mahogany wood), and the skins are softened by rolling them with a board having fine grooves cut in it. The surface of the flesh side is smoothed by shaving over it with a currier's knife which has a very fine and delicate edge turned on it, so that the smallest quantity only is taken off. This process is termed whitening. The skins are then stoned on the grain side, and all wrinkles and breaks taken out, and a fine grain is turned on them with a smooth board, or with very fine grooves cut in it. They are matched for size, laid down in a pile, the larger ones in the bottom, and blacked on the flesh side with a compound of lampblack, tanner's oil, and dubbing, and a small quantity of water, to prevent it striking through. As they are blacked they are laid over a strip. They must now be pasted, to prevent the black rubbing off. The paste is composed of wheat flour and boiling water, stirring in a small quantity of soap and tallow, and is applied with a brush, coating them with as small a quantity as possible. They are hung up by the loop in the head and dried, then glassed with a polished glass slicker on the flesh side, and are ready for the last process, gumming. The gum used is gum tragacanth, dissolved in water, and is applied with a sponge, on the flesh side. hung up, and when thoroughly dry, they are ready for sale, or cutting into boots and shoes.
Sheep-skins, which are used for a variety of purposes, such as gloves, book-covers, etc., and which, when dyed, are converted into mock Morocco leather, are dressed as follows: They are first to be soaked in water and handled, to separate all impurities, which may be scraped off by a blunt knife on a beam. They are then to be hung up in a close warm room to putrefy. This putrefaction loosens the wool, and causes the exudation of an oily and slimy matter, all which are to be removed by the knife. The skins are now to be steeped in milk of lime, to harden and thicken; here they remain for 1 month or 6 weeks, according to circumstances, and when taken out, they are to be smoothed on the fleshy side with a sharp knife. They are now to be steeped in a bath of bran and water, where they undergo a partial fermentation, and become thinner in their substance.
The skins, which are now called pelts, are to be immersed in a solution of alum and common salt in water, in the proportion of 120 skins to 3 lbs. of alum and 5 lbs. of salt. They are to be much agitated in this compound saline bath, in order to become firm and tough. From this bath they are to be removed to another, composed of bran and water, where they remain until quite pliant by a slight fermentation. To give their upper surfaces a gloss, they are to be trodden in a wooden tub, with a solution of yolks of eggs in water, previously well beaten up. When this solution has become transparent, it is a proof that the skins have absorbed the glazing matter. The pelt may now be said to be converted into leather, which is to be drained from moisture, hung upon hooks in a warm apartment to dry, and smoothed over with warm hand-irons.
The goat-skins being first dried in the air, are steeped in water 3 days and nights, then stretched on a tanner's horse, beaten with a large knife, and steeped afresh in water every day; they are then thrown into a large vat on the ground, full of water, where quicklime has been slaked, and there lie 15 days, whence they are taken, and again returned every night and morning. They are next thrown into a fresh vat of lime and water, and shifted night and morning for 15 days longer, then rinsed in clean water, and the hair taken off on the leg with the knife, returned into a third vat, and shifted as before for 18 days; steeped 12 hours in a river, taken out, rinsed, put in pails, where they are pounded with wooden pestles, changing the water twice; then laid on the horse, and the flesh taken off; returned into pails of new water, taken out, and the hair-side scraped; returned into fresh pails, taken out and thrown into a pail of a particular form, having holes at bottom; here they are beaten for the space of an hour, and fresh water poured on from time to time; then being stretched on the leg, and scraped on either side, they are returned into pails of fresh water, taken out, stretched, and sowed up all round, in the manner of bags, leaving out the hinder legs, as an aperture for the conveyance of a mixture described below.
The skins thus sewed are put to luke-warm water, where dog's excrements have been dissolved. Here they are stirred with long poles for an hour, left at rest for 12 hours, taken out, rinsed in fresh water, and filled by a tunnel with a preparation of water and sumach, mixed and heated over the fire till ready to boil; and, as they are filled, the hind legs are sewed up to stop the passage. In this state they are let down into the vessel of water and sumach, and kept stirring for 4 hours successively; taken out and heaped on one another; after a little time their sides are changed and thus they continue 1 1/2 hours till drained. This done, they are loosened, and filled a second time with the same preperation, sewed up again, and kept stirring 2 hours, piled up and drained as before. This process is again repeated, with this difference, that they are then only stirred 1/4 of an hour; after which they are left till next morning, when they are taken out, drained on a rack, unsewed, the sumach taken out, folded in two from head to tail, the hair-side outwards, laid over each other on the leg, to perfect their draining, stretched out and dried; then trampled under foot by two and two, stretched on a wooden table, what flesh and sumach remains scraped off, the hair-side rubbed over with oil, and that again with water.
They are then wrung with the hands, stretched, and pressed tight on the table with an iron instrument like that of a currier, the flesh-side uppermost; then turned, and the hair-side rubbed strongly over with a handful of rushes, to squeeze out as much of the oil remaining as possible. The first coat of black is now laid on the hair-side, by means of a lock of hair twisted and steeped in a kind of black dye, prepared of sour beer, wherein pieces of old rusty iron have been thrown. When half-dried in the air they are stretched on a table rubbed over every way with a paumelle, or woodentoothed instrument, to raise the grain, over which is passed a light couche of water, then sleeked by rubbing them with rushes prepared for the purpose. Thus sleeked, they have a second couche of black, then dried, laid on the table, rubbed over with a paumelle of cork, to raise the grain again; and after a light couche of water, sleeked over anew; and to raise the grain a third time, a paumelle of wood is used.
After the hair-side has received all its preparations, the flesh-side is pared with a sharp knife for the purpose; the hair-side is strongly rubbed over with a woollen cap, having before given it a gloss with barberries, citron or orange. The whole is finished by raising the grain lightly, for the last time, with the paumelle of cork; so that they are now fit for the market
After steeping, stretching, scraping, beating and rinsing the skins as before they are at length wrung, stretched on the leg, and passed after each other into water where alum has been dissolved. Thus alumed, they are left to drain till morning, then wrung out, pulled on the leg, and folded from head to tail, the flesh inwards.
In this state they receive their first dye, by passing them after one another into a red liquor, described hereafter. This is repeated again and again, till the skins have got their first color; then they are rinsed in clean water, stretched on the leg, and left to drain 12 hours; thrown into water through a sieve, and stirred incessantly for a day with long poles; taken out, hung on a bar across the water all night, white against red, and red against white, and in the morning the water stirred up, and the skins returned into it for 24 hours.
The following is the quantity and proportions of the ingredients required for the red color, for a parcel of 36 skins:
Cochineal, 130 drs.; ground suchet (crocus indicus), 45 drs.; gutta gamba, 15 drs.; gum Arabic, 10 drs.; white alum, pulverized, 10 drs.; bark of the pomegranate tree, 10 drs.; citron juice, 2 drs.; common water, 120 lbs.
The alum is gradually added to the other articles, and boiled in a copper for about 2 hours, till one-tenth part of the water is consumed.
Soften the hide in water; then spread it on a tanner's beam, and let it be wrought with a knife on the flesh-side, and subjected to the action of lime-water. In the succeeding process it is treated as goat-skins for making morocco, i. e. put it into hot water, with dog's dung, to purify the animal juices; then let it be again wrought with a knife on both sides, on a tanner's beam; afterwards put it into blood-warm water with bran; and, finally, tan it with sumach.
Calf-skins steeped in a weak bath of carbonate of potass and water, are well cleaned and scraped, to have the hair, etc., removed. They are now immersed in another bath, containing dog and pigeon's dung in water. Being thus freed from the alkali, they are thrown into a mixture of oat-meal and water, to underdo a slight fermentation. To tan these hides it is necessary to use birch bark instead of oak bark; and during the operation they are to be frequently handled or agitated. When tanned, and perfectly dry, they are made pliable by oil and much friction; they are then to be rubbed over gently with birch tar, which gives them that agreeable odor peculiar this kind of leather, and which secures them against the attacks of moths and worms. This odor will preserve the leather for many years; and, on account of it, Russia leather is much used in binding handsome and costly books. The marks, or intersecting lines on this leather, are given to it by passing over its grained surface a heavy iron cylinder, bound round by wires.
Clean the skins from wool or hair, by laying them in a vat of slaked lime-water for 5 or 6 weeks. During this operation the lime and water are to be twice changed, and the skins are to be shifted every day, and when taken out for good they are to be laid all night in a running water, to clear them from the forcing qualities of the lime; next lay them on a wooden leg by sixes, to get the flesh off; then they are to be laid in a vat with a little water, and to be fulled with wooden pestles for a quarter of an hour, after which rinse them wall in a full vat of water; place them next on a clean pavement to drain, and afterwards cast them into a fresh pit of water; rinse them again, and re-lay them on the wooden leg, with their hair outside, over which a whetstone is to be briskly rubbed, to fit them for further preparations. They are next to be put into a pit of water, mixed with wheaten bran, and stirred until the bran sticks to the wooden poles. They now arrive to a kind of fermentation, and as often as they rise on the top of the water, are to be plunged down at the same time the liquor, now highly fermented, is to be fined. When the skins have done rising, take them out, and scrape away the bran with a knife on the leg: when sufficiently drained give them their feeding. For 100 large sheep-skins, take 8 lbs. of alum, and 3 lbs. of seasalt, and melt the whole with water in a vessel. Pour the solution out, while lukewarm, into a trough in which is 20 lbs. of the finest wheat flour, with the yolks of 8 dozen of eggs, of which mixed materials is formed a kind of paste, somewhat thicker than children's pap; next pour hot water into the trough where the paste was, mixing 2 spoonfuls of the paste with it, with a wooden spoon, which will contain a sufficiency for 12 skins, and when the whole is well incorporated, put 2 dozen of the skins into it, taking care that the water is not too hot. After they have been in some time, take them severally out of the trough, and stretch them twice well out. After they have absorbed the paste, put them into tubs, and full as before. Let them lie in a vat 6 days, and hang them out to dry; in fair weather, on cords or racks. When dry, put them into bundles, just dipped in clean water, and drained; throw them into an empty tub, and having lain some time they are to be taken out and trampled under foot; hang them up a second time on the cords to dry, and finally smooth them upon a table ready for sale.
The skins, smeared with quicklime on the fleshy side, are folded lengthways, the wool outwards, and laid on heaps, to ferment 8 days; or if they had been left to dry after flaying, for 16 days.
Then they are washed out, drained, and half dried; laid on a wooden horse, the wool stripped off with a round staff for the purpose, and laid in a weak pit of slacked lime.
After 24 hours they are taken out, and left to drain 24 more; then put into another strong pit. Then they are taken out, drained, and put in again by turns; which begins to dispose them to take oil; and this practice is continued for 6 weeks in summer, or 3 months in winter; at the end whereof they are washed out, laid on the wooden horse, and the surface of the skin on the wool side peeled off, to render them the softer; then made into parcels, steeped a night in the river, in winter more, stretched 6 or 7, one over another on the wooden horse, and the knife passed strongly on the fleshy side, to take off anything superfluous, and render the skin smooth.
Then they are stretched, as before, in the river, and the same operation repeated on the wool side; then thrown into a tub of water and bran, which is brewed among the skins till the greater part sticks to them; and then separated into distinct tubs, till they swell, and rise of themselves above the water.
By these means, the remains of the lime are cleared out; they are then wrung out, hung up to dry on ropes, and sent to the mill, with the quantity of oil necessary to fill them; the best oil is that of cod-fish.
Here they are first thrown in bundles into the river for 12 hours, then laid in the mill-trough, and fulled without oil, till they are well softened; then oiled with the hand, one by one, and thus formed into parcels of 4 skins each, which are milled, and dried on cords a second time, then a third, then oiled again and dried.
This is reseated as often as necessary; when done, if any moisture remains they are dried in a stove, and made up in parcels wrapped up in wool; after some time they are opened to the air, but wrapped up again as before, till the oil seems to have lost all its force, which it ordinarily does in 24 hours.
The skins are now returned to the chamoiser, to be scoured, by putting them into a lixivium of wood-ashes, working and beating them in it with poles, and leaving them to steep till the lye has had its effect, then wrung out, steeped in another lixivium, wrung again, and this repeated till the grease and oil are purged out. They are then halfdried, and passed over a sharp-edged iron instrument, placed perpendicularly in a block, which opens and softens them; lastly, they are thoroughly dried, and passed over the same instrument again, which finishes the operation
Kid, and goat skins, are chamoised in the same manner as those of sheep, excepting that the hair is taken off by heat: and that when brought from the mill they undergo a preparation galled ramaling, the most difficult of all.
It consists in this, that as soon as brought from the mill they are steeped in a fit lixivium; taken out, stretched on a round wooden leg, and the hair scraped off with the knife; this makes them smooth, and in working cast a fine nap. The difficulty is scraping them evenly.
Take a teaspoonful of alum, and 2 of saltpetre, both finely powdered; mix them well, sprinkle the powder on the flesh side of the skins, then lay the two salted sides together, leaving the fur outward; roll the skin exceedingly tight, and tie it round with pack-thread; hang it in a dry place for some days, then open it, and if sufficiently dry scrape it quite clean with a blunt knife, and keep it in a dry situation. This finishes the process.
It may not be generally known, that the bitter apple bruised and put into muslin bags, will effectually prevent furs from being destroyed by moths.
This article is manufactured from sheep skins, cleared from lime. The skin is stretched on a frame where the flesh is pared off with an iron circular knife; it is then moistened with a rag, and whiting spread over it; the workman then, with a large pumice-stone, flat at the bottom, rubs over the skin, and scours off the flesh. He next goes over it with the iron instrument as before, and rubs it carefully with the pumicestone without chalk; this serves to smooth the flesh side. He drains it again by passing over it the iron instrument as before; he passes it over the wool side, then stretches it tight on a frame. He now throws more whiting and sweeps it over with a piece of woolly lambskin. It is now dried, and taken off the frame by cutting it all round. Thus prepared it is taken out of the skinner's hands by the parchment maker, who, while it is dry, pares it on a summer (which is a calfskin stretched on a frame), with a sharper instrument than that used by the skinner, who, working it with the arm from the top to the bottom of the skin, takes away about half its substance, which leaves the parchment finished.
Soak and wash the skins well and often in soft water for 24 hours; then remove them for the same period into a bath composed of 1 1/2 lbs. of white vitriol, 1 lb. of cream of tartar, and 1 oz. of sal ammoniac, dissolved in 20 galls. of water. Next add 10 lbs. of vitriolic acid, 1 lb. of nitric acid and 1 pt. of spirit of salt, in which steep the skins for a short time to purge away the old lime; next wash them clear of the acid, and rinse them as dry as possible, without damaging the skins. They are then to be put into a tanning liquor, composed of 20 lbs. of oak bark, 7 lbs. sumach, 5 lbs. of elm-bark, 3 lbs. of sassafras, and the same quantity of lignum vitae shavings, portioned to 20 galls. of water, and previously warmed for 12 hours and cooled down to a newmilk warmth, before the skins are immersed.
This is a species of parchment made of the skins of abortives, or sucking calves; it has a much finer grain, and is white and smoother than parchment but is prepared in the same manner, except its not being passed through the limepit. The article is used for binding superior books, and covering of drum heads.
Pyroligneous acid may be used with success in preserving leather from the attacks of mould, and is serviceable in recovering it after it has received that species of damage, by passing it over the surface of the hide or skin, first taking due care to expunge the mouldy spots by the application of a dry cloth. This remedy will be of equal service if applied to boots, shoes, etc., when damaged in the same manner.
The following colors may be imparted to leather, according to the various uses for which it is intended.
Blue is given by steeping the subject a day in urine and indigo, then boiling it with alum; or, it may be given by tempering the indigo with red wine and washing the skins therewith.
Another. - Boil elderberries or dwarf elder, then smear and wash the skins therewith and wring them out, then boil the elderberries as before in a solution of alum-water, and wet the skins in the same manner once or twice; dry them, and they will be very blue.
Red is given by washing the skins and laying them 2 hours in galls, then wringing them out, dipping them in a liquor made with ligustrum, alum and verdigris; in water, and lastly in a dye made of Brazil-wood boiled with lye.
Purple is given by wetting the skins with a solution of roche alum in warm water, and when dry, again rubbing them with the hand, with a decoction of log-wood in cold water.
Green is given by smearing the skin with sap-green and alum-water boiled.
Dark green is given with steel-filings and sal ammoniac, steeped in urine till soft, then smeared over the skin, which is to be dried in the shade.
Yellow is given by smearing the skin over with aloes and linseed-oil, dissolved and strained, or by infusing it in weld.
Orange color is given by smearing it with fustic berries boiled in alum-water, or, for a deep orange, with turmeric.
Sky-color is given with indigo steeped in boiling water, and the next morning warmed and smeared over the skin.
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