the art of manufacturing leather from raw hides and skins.
Before we detail the process, it may be proper to observe, that raw hides and skins being composed of minute fibres interlacing each other in every direction, the general operation of tanning consists chiefly in expanding the pores, and dissolving a sort of greasy substance contained in them; and then, by means of the astringent and gummy-resinous properties of oak bark, to fill and reunite them, so as to give firmness and durability to the whole texture. But this theory has been controverted by some chemists, who suppose that the animal jelly contained in the skin is not dissolved, but unites during the process with the astringent principle of the bark, and forms a combination insoluble in water.
The process of tanning varies considerably, not only in Method of different countries, but even in different parts of the same country. The following is the method most approved and practiced in London and its vicinity, where the best leather is generally allowed to be manufactured.
The leather tanned in England consists chiefly of three sorts, known by the name of butts or backs, hides, and skins.
Butts are generally made from the stoutest and heaviest ox hides, and are managed as follows: After the horns are taken off, the hides are laid smooth in heaps for one or two days in the summer, and for five or six in the 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 is easily got off, by spreading the hide on a fort of wooden horse or beam, and scraping it with a crooked knife. The hair being taken off, the hide is thrown into a pit or pool of water to cleanse it from the dirt, &c. which being done, the hide is again spread on the wooden beam, and the grease, loose flesh, extraneous filth, &c. carefully scrubbed.
scrubbed out or taken off; the hides are then put into a pit of strong liquor called ooze or tannin, prepared in pits called ketches or taps kept for the purpose, by infusing ground bark in water; this is termed colouring; after which they are removed into another pit called a scowering, which consists of water strongly impregnated with vitriolic acid, or with a vegetable acid prepared from rye or barley. This operation (which is called raising), by diffusing the pores of the hides, occasions them more readily to imbibe the ooze, the effect of which is to attract and condense the fibres, and give firmness to the leather. The hides are then taken out of the scowering, and spread smooth in a pit commonly 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 filled again 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 to be remarkably stout as to want 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 beat out smooth by wooden hammers called beatles, the operation is complete; and when thoroughly dry, they are fit for sale. Butts are chiefly used for the soles of stout shoes.
The leather which goes under the denomination of hides is generally made from cow hides, or the lighter ox hides, which are thus managed. After the horns are taken off, and the hides washed, they are put into a pit of water saturated with lime, 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 (which is technically termed handling) 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 between each hide. After remaining here two or three months, they are generally taken up, when the ooze is drawn out, 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 few very stout hides, which may require an extra layer; they are then taken out, hung on poles, and being hammered and smoothed by a steel pin, are, when dry, fit for sale.
These hides are called crop hides; they are from 10 to 18 months in tanning, and are used for the soles of shoes.
SKINS is the general term for the skins of calves, teals, hogs, dogs, &c. These, after being washed in water, are put into lime-pits, as before mentioned, where they are taken up and put down every third or fourth day, for a fortnight or three weeks, in order to dilate the pores and dissolve the gelatinous parts of the skin. The hair is then scraped off, and the flesh and excrescences being removed, they are put into a pit of water impregnated with pigeon-dung (called a grainer or malfiring), forming a strong alkaline ley, which in a week or ten days foaming out the lime, grease, and sapaceous matter (during which period they are several times scraped over with a crooked knife to work out the dirt and filth), 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 substance, are sufficiently tanned; when they are taken out, hung on poles, dried, and fit for sale.
These skins are afterwards dressed and blacked by the currier; and are used for the upper-leathers of shoes, boots, &c.
The lighter sort 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. &c.
As the method of tanning above described, and all others schemes to in general use, are extremely tedious and expensive in their operation, various schemes have at different times been suggested to shorten the process and lessen the expense.—Though most of these schemes have ultimately proved unsuccessful, yet it in a work of this kind it may be expected that we should not pass them over wholly unnoticed.
Some have imagined, and perhaps justly, that cold water alone is not an adequate menstruum for extracting the resinous qualities of bark, however assisted by the maceration of the bark and of the skin; a decoction, instead of simple infusion, has therefore been recommended as a more effectual mode of obtaining those properties.
The late Dr Macbride of Dublin having been concerned in a leather manufactory, published in 1779 a new method of tanning. His projected improvements may be briefly clasped under two heads: the one recommending the use of vitriolic instead of vegetable acid, brewed from rye or barley; the other substituting lime-water, for the purpose of extracting the virtues of the bark, instead of the water commonly used by tanners. With respect to the first, it is generally acknowledged that the vitriolic acid is very proper for raising or diffusing the pores of the hides intended for butts, as its operation is not only more simple and certain than the acid formerly used, but as it tends more effectually to render the texture of the leather firm and durable; it is therefore still preferred by the most skilful taners. As to lime-water instead of water, it has been found inefficacious; and if the utmost care and attention be not observed, the leather is liable to suffer much injury. Even the shortening of the time and lessening of the expense (which were its chief recommendations) being very problematical, it is now almost generally exploded.
A very ingenious chemist has observed, that it is necessary, on account of a chemical combination between the astringent principle and the animal substance in the process of tanning, that tree acetates should be given to the pure air; and therefore supposes that the process could not be conducted properly in close vessels.
The methods of tanning in different provinces of France are so various, so complicated, and so contrary to the acknowledged principles of the manufacture, that it would be an endless and useless task to endeavour to detail them: we shall therefore content ourselves with a general reference to M. de la Lande's elaborate Treatise on this subject.
It has been said, that every part of the oak tree contains a great portion of astringent, gummy-resinous matter, and will therefore tan leather as effectually as the bark itself. This opinion, which was first published in 1674 by the Honourable Charles Howard (Phil. Transf. vol. ix.), has since been countenanced by the celebrated Buffon; who adds, that Tanning. The bark of birch will answer the purpose of tanning even sole leather, which, it is well known, requires the strongest and most penetrating materials.
A long memoir, written by M. Gleditsch, recommends the leaves, branches, fruit, and flowers, of a vast number of plants as substitutes for oak bark. Heath dried and pulverized, gall nuts, and the bark of birch, are said by M. Gleditsch to be used in different provinces of Germany. Abbé Nollet informs us, that the leaves of myrtle are used by the tanners in Naples. In Corsica they make use of the leaves of wild laurel dried in the sun and beaten into powder, and in the island of St Kilda they tan with the tormentil root. In some parts of Italy leather is tanned with myrtle leaves. In Russia, it is said, that leather is tanned with the bark of willow; and it may here be observed, that a late writer has recommended the extract of bark to be made in America, in order to lessen the expense of freight, &c., in conveying the bark itself to Europe.
In the year 1765, the Society of Arts, &c., granted a premium of £1,100 for the discovery of a method of tanning with oak sawdust; which method has been adopted in Germany; and the Reverend Mr Swaine has lately revived the exploded substitute (mentioned by Gleditsch and others) of oak leaves.
The following proposal was communicated to the Bath Society for extracting the essence of oak bark:
Suppose (says the author) the operator has at hand a common family brew-house, with its necessary utensils; let him procure a ton of good oak bark ground as usual for the pit; and having placed a strainer to the mash-tub, fill it two-thirds with the bark; heat as much water, nearly boiling, as will sufficiently moisten it, and mix it well together. After it has stood about two hours, draw it off clear, and put it into a cask by itself. Make a second extract with a smaller quantity of boiling water than before, so as to draw off a quantity nearly equal to the first, and put that also into the same cask with the former.
These two extracts will probably contain in them as much of the virtues of the bark as the quantity of liquid will absorb.
A third extract, rather more in quantity than the other two, may be made from the same bark, and as soon as drawn off, should be returned into the copper again when empty, and employed for the first and second bath of a quantity of fresh bark, as the three extracts may be supposed to have carried off the virtues of the first. Then proceed as before till all the bark is steeped, and a strong liquid extract is drawn from it. The bark, when taken out of the copper, may be spread in the sun to dry, and serve as fuel in the succeeding operations.
The next process is, to evaporate the watery particles from the extract by a gentle heat, till it comes to the consistence of treacle. This may be done either by the air and heat of the sun, or by the still or iron pan over the fire.
Anthony Day, Esq., of London, obtained a patent, dated 17th July 1790, for a new method of tanning, "with half the bark in half the usual time." This plan chiefly consists in concentrating the bark into a strong extract, and in some mechanical improvements in the construction of the tan-yard. But neither the one nor the other have yet been adopted.
The 12th May 1795, a patent was granted to Mr Tucker of Wickham, Hants. He proposes that the vat, made of wood, be inclosed in a metallic coating or copper pit, completely foldered, to prevent the escape of any of the fluid. This is to be surrounded with a cage of brick-work, leaving an interstice of a few inches; and a fire is to be made in a grate near the bottom of the pit, to keep the ooze moderately warm, and thus to shorten the process. But the great expense of these triple pits and of the fuel, it is feared, will counterbalance any advantages which might otherwise be derived from this invention.
Monsieur Seguin of Paris has lately submitted to the French Convention a new method of tanning, which is said to possess wonderful advantages. He has certainly exploded the ignorant and absurd systems of the French tanners, which we have above hinted at, and has shown much ingenuity and chemical knowledge in the prosecution of his discoveries; but his leading principles seem, in fact, to be nearly similar to those which have been long known and practiced in England.
An ingenious manufacturer in London has, by the application of warm air, conveyed by means of flues from stoves properly constructed, and by other contrivances not generally known, considerably abridged the usual process of tanning. Some experiments have likewise been lately made with the bark of ash and of horse-chestnut.
A substitute for oak bark, the price of which has lately been enormous, is the grand desideratum in the manufacture of leather. Most of those above enumerated have hitherto been found ineffectual; but a patent, bearing date 16th January 1794, has been granted to Mr Ashton of Sheffield, Yorkshire, for his discovery of a cheap and expeditious method of tanning leather. This method chiefly consists in applying a preparation of mineral substances instead of oak bark. Those which, on account of their cheapness, are most to be preferred, are the dros of coal-pits, called fulphurstone or pyrites, and the yellow ferruginous earth or red ochre; and, in general, all affrangent, sulphureous, or vitriolated substances.
If this discovery, which is yet in its infancy, should prove successful, it may cause a material alteration in the process of this manufacture; and by reducing the expense, may ultimately be of great advantage to the public. Many other experiments are now making in England for the improvement of tanning; and as there are many persons of ingenuity and knowledge engaged in the leather manufacture, much may be expected from their industry and skill.
As the acts of Parliament respecting leather, &c., are very Acts of numerous, and many of them almost obsolete, we shall refer Parliament our readers to Burn's Justice, or to the Statutes at Large, &c., for We cannot, however, help remarking, that the act of James I., cap. 22, which prescribes the mode and manner in which leather shall be tanned, the materials to be used, and the time to be employed, is so palpably absurd and oppressive, that it ought to be immediately repealed.
The revenue arising from the duty on leather tanned in Great Britain (exclusive of oiled leather) is upwards of L. 100,000 per annum.