TANNING, is the the art of preparing raw hides or skins for the hand of the currier, or for immediate use without any further operation.
The former part of this definition includes all leather used for the upper part of shoes, coaches, coach-harness, saddle-leather, &c. and the latter the manufacture of bin or backs, &c.
A considerable improvement has been made in the method of tanning leather by the late Dr Macbride of Dublin, from whose account of his discovery in the Philosophical Transactions, the following is extracted. "The use of tanning is two-fold; first, to preserve the leather from rotting; and, secondly, to render it impervious to water.
"An infusion of any strongly-astringent vegetable will serve to tan leather, so far as to prevent its rotting; but if this vegetable does not contain a good deal of gum-resin, it will not answer for enabling it to keep out water: and hence it is that oak-bark, which is more abundant in the gummy-resinous part than any of our common indigenous astringents, is preferred to all other substances for the purpose of tanning.
"The tanners prepare their bark by gently drying it on a kiln, and grinding it into a very coarse powder. They then either use it in the way of infusion, which is called ooze; or they strew the dry powder between the layers of hides and skins, when these are laid away in the tan-pits.
"The ooze is made by macerating the bark in common water, in a particular set of holes or pits, which, to distinguish them from the other holes in the tan-yard, are termed letches.
"The first operation of the tanner is to cleanse his hides from all extraneous filth, and remove any remains of flesh or fat which may have been left behind by the butcher.
"The hair is next to be taken off; and this is accomplished either by steeping the hides for a short time in a mixture of lime and water, which is termed liming; or by rolling them up close and piling them in heaps, where they quickly begin to heat and putrify. The hair being loosened, is scraped off; and the tanner proceeds to the operation called stressing, which consists in a further scraping with a particular kind of knife contrived for the purpose, and cutting away the jagged extremities and ossal parts, such as the ears and nostrils.
"The raw leather is then put into an alkaline ley, in order to discharge the oil or fat, and render its pores more capable of imbibing the ooze. The tanners of Ireland generally make their ley of pigeon's dung: but a more active one may be prepared from kelp or pot-ash; taking care, however, not to make it too strong of the ashes, nor to allow the leather to remain too long in the ley.
"The oil being sufficiently discharged, the leather is ready for the ooze, and at first is thrown into smaller holes, which are termed handlers; because the
hides or skins, during this part of the process, are taken up from time to time, and allowed to drain: they continue to work the leather in these handlers, every now and then stirring it up with the utensil called a plunger, which is nothing more than a pole with a knob at the end of it, until they think proper to lay it away in the vatts. In these holes, which are the largest in the tan-yard, the leather is spread out smooth, whereas they toss it into the handlers at random; and between each layer of leather they sprinkle on some powdered bark, until the pit is filled by the leather and bark thus laid in stratum super stratum: ooze is then poured on, to fill up interstices; and the whole crowned with a sprinkling of bark, which the tanners call a heading.
"In this manner the leather is allowed to macerate, until the tanner feels that it is completely penetrated by the ooze; when this is accomplished (which he knows by cutting out a bit of the thickest part of the hide) the manufacture is finished, so far as relates to tanning, since nothing now remains but to dry the goods thoroughly, by hanging them up in airy lofts built for the purpose. Such in general is the process for tanning calf-skins, and those lighter sorts of hides which are called butts; but the large, thick, heavy hides, of which the strongest and most durable kind of sole-leather is made, require to have their pores more thoroughly opened before the ooze can sufficiently penetrate them. For this purpose, while the hides are in the putrescent state, from being allowed to heat in the manner already mentioned, and well soaked in an alkaline ley, they are thrown into a sour liquor, generally brewed from rye, in order that the effervescence which necessarily ensues may open the pores.
"The tanners term this operation raising, as the leather is considerably swelled, in consequence of the conflict between the acid and alkali. This is an English invention; for it appears from M. De la Lande, who was employed by the Royal Academy of Sciences to write on the art of tanning, that the foreign tanners know nothing of this branch of the business: indeed, their whole process, according to his account, is slovenly, and even more tedious than our common method, and must make but very indifferent leather.
"When the raising is accomplished, the leather is put into the handlers, and worked in them for the requisite time; then laid away in the vatts, and there left to macerate until the tanning is found to be completely finished, which, for the heaviest kind of leather, such as this of which I am now speaking, requires from first to last full two years.
"It is this tediousness of the process which enhances the value of leather; and the returns being so slow, the trade of tanning never can be carried on to advantage but by persons possessed of a large capital: therefore, one sure way of increasing the number of tanners, and of course of bringing down the price of their manufacture, is to shorten the process; and if at the same time we can improve the quality of the leather, and save somewhat in the expence of tanning materials, the public will be essentially benefited in respect to one of the necessary articles of life.
"All this can be done by pursuing the following method, which may be introduced into any common tan-yard.
“With respect to time, it is possible, in the way that I have found out, to finish leather in a fourth part of what is required in the ordinary process; for I have repeatedly had calf-skins tanned in a fortnight or four weeks, which in the common way could not be done in less than from two to four months.
“I shall not pretend, however, to affirm, that that business can be carried on in the large way with such expedition; because a great deal of this abridgment of time was probably owing to frequent handling and working of the leather: but I am confident, and know it from four years experience, that in the ordinary course of business, and in a common tan-yard, the tanner may save at least four months out of twelve, produce better leather, and find his bark go much farther than in the old way of tanning.
“As the new method of tanning depends on this principle, ‘That lime-water extracts the virtues of oak-bark more completely than plain water;’ the first thing in which the tanner is to be instructed, is the making of lime-water.
“1. Provide a large vessel, in the nature of a cistern, whose depth shall be at least twice its diameter, and of a capacity adapted to the extent of the tan-yard.
“2. This cistern must be fixed in a convenient corner of the yard, under a shed; and should stand so as that the liquor which is to be drawn off from it may run freely into the letches.
“3. There must be a cock fixed in the side of the cistern, about a foot from the bottom, to let off the contents; and there must be a hole in the bottom of it, of five or six inches diameter, which is to be stopped with a plug. Let this hole open over a gutter.
“4. The cistern must be covered with a flooring of boards, strong enough to bear a man’s weight; and from side to side of this lid there must be an opening of two or three feet wide.
“5. If it can be so contrived that a water-pipe may be led into the cistern, it will save the servants a good deal of trouble; but if this cannot be done, a pump must be fixed in the most convenient way, for the purpose of filling it from time to time.
“6. The cistern being once fixed (which is all the additional apparatus that the new method of tanning requires) the making of lime-water will be found extremely simple and easy.
“7. You are first to fill the cistern with water; and then, for every hog’shead that it may contain, throw in ten or a dozen pounds weight of unslaked lime.
“8. Mix the lime thoroughly with the whole body of the water, by stirring it exceedingly well from the bottom, with a bucket and plunger, until you perceive that the lime is completely diffused, and the whole mixture grows as white as milk; leave it then to settle for a couple of days, that the undissolved part of the lime may entirely subside, and the water become perfectly limpid, and clear as rock-water. Your lime-water will then be fit for immediate use.
“9. The cock, as already mentioned, is to be fixed at least twelve inches from the bottom of the cistern, in order that only the limpid part of the lime-water may run off; and the use of the hole in the bottom, which is ordered to be stopped with a plug, is to
let off the gross and insoluble remains of the lime, as often as it may be found necessary to clean out the cistern. Tanning.
“10. When the first brewing (as it may be termed) of lime-water is all expended, you are to fill up the cistern with water a second time; stir up the lime from the bottom with the bucket and plunger, so as to mix it thoroughly with the whole body of the water, as before directed, and then leave it to subside for the requisite time. Thus you will have a second brewing of lime-water; and you may go on in the same manner to make a third, fourth, fifth, or perhaps a sixth or more brewings, from the original quantity of lime; provided you shall find the lime-water continue sufficiently strong.
“11. There are two ways of knowing when lime-water is sufficiently strong. The one is by the taste, and this a little practice will teach you to distinguish; the other is, by observing a certain solid scum, like the flakes of very thin ice, which collects and forms itself on the surface of the lime-water.—As long as you find this solid scum floating on the top of the water in the cistern, so long you may conclude that there is no necessity for throwing in fresh lime.
“12. But when the scum ceases to appear, or you find from the taste that the lime-water is not so strong as it ought to be, you must then take out the plug from the bottom of the cistern, and clear it by sweeping away the gross remains of lime: and after you have cleaned the cistern, begin your brewings of lime-water anew, and proceed in the manner already directed, as to stirring up the lime, and leaving it to settle for the necessary time, so as to have your lime-water perfectly limpid. In this manner you may go on from year to year, and constantly keep yourself in stock with respect to lime-water.
“13. It is this lime-water which is now to be used in making your ooze instead of the plain common water; and this is all the difference between the old and the new method of tanning; for when your ooze is prepared, by steeping your bark in lime-water (in the letches, as you do at present, only running it through two letches) you are to make use of it in the very same way that you have hitherto used the common ooze, there not being the least variation required with respect to any of the previous management before the skins or hides are fitted for the ooze. Every thing that relates to cleaning, liming, fleshing, &c. is to be conducted precisely as in the old or common method of tanning; and the goods are to be worked in the handlers for the requisite time, and then laid away in the vats, with layers and headings of bark, just as you now practise; and when you observe that the leather is sufficiently penetrated with the ooze, that is to say, completely tanned, you will take it up, dry it, and afterwards dress it according to the different uses for which it is intended. You are always to observe, however, that the ooze is to be turned from one letch on another before it is used, otherwise it will be apt to blacken the leather.
“14. What has been hitherto said relates only to butts and calf-skins: as to foal-leather, which is prepared for the ooze by steeping it in some four liquor, in order to open its pores, and raise it (according to the
Tanning. the tanner's phrase), the new method requires a different practice from the old one.
"15. In the old method, the tanners made use of fourings brewed generally from rye or some other grain. But these liquors are not only troublesome to brew and to ferment, but they are always uncertain as to the degree of fourness or strength, which depends on the state of the weather and other variable circumstances: these liquors are moreover exceedingly apt to rot the leather; and, without great care, may injure it very materially in its texture.
"16. To obviate these inconveniences, you are to imitate the bleachers of linen, who make use of a four prepared by diluting the strong spirit of vitriol (vulgarly, but improperly, termed oil of vitriol) with a sufficient quantity of plain water.
"17. It was not without much difficulty that the bleachers could be prevailed on to quit their old fourings, made either, like yours, of rye or barley, or of four butter-milk, from a groundless fear that the vitriolic fouring would corrode their cloth; but the experience of many years has convinced them of their error, and now no other fouring is used. In like manner the tanners at first may some of them be afraid to use the vitriol; but a little practice will show how far superior this fouring is to what they have hitherto used. They will never find it subject to any change, in respect to strength, from variations of weather or different degrees of heat; and so far from tending to rot the leather, it gives unusual firmness; and the soals which are raised by the vitriolic fourings are remarkably sound, and always free from the slightest degree of rottenness. Besides, the same four may do for many parcels of leather, by adding a little vitriol to it; and it need only be thrown away, when it becomes too dirty for use by the frequent succession of hides.
"18. A wine-pint of the strong spirit of vitriol, which will not cost more than nine or ten pence, is sufficient for fifty gallons of water to prepare the fouring at first: therefore all you have to do, in raising the soals, is only to prepare them before-hand in the usual way; and, when they are fitted for the fouring, mix up a quantity of vitriol and water, according to the number of hides that you require to have raised, still observing the proportion of a pint to fifty gallons, which will be enough if the vitriol be of the due degree of strength. The hides may lie in the fouring till you find them sufficiently raised: for they will be in no danger of rotting, as they would be in the common corn-fourings, which in time might turn putrid, and rot the leather; whereas the vitriolic fouring keeps off putrefaction.
"19. When you find your hides sufficiently raised, put them directly into the ooze, and go on with the tanning as in the old way; and you will see that the lime-water ooze penetrates raised leather even faster than it does butts or calf skins, allowance being made for their different degrees of thickness.
"20. Let it be now supposed that you have your cistern fixed, your lime-water prepared, and some letches full of lime-water ooze, which has been run through two letches in order that the lime-water may completely spend its force on the bark; you are not to throw away what common ooze you have in stock
in the yard, but only as it shall be spent; then, indeed, you are to throw it away, and supply its place with the lime-water ooze.
"21. In a very few days you will perceive the difference between the activity of the two oozes, the new and old, with respect to penetrating the leather: and thus, without any kind of loss or waste, you will get rid of all your old liquors, and come speedily into a full stock of the ooze made with lime-water; and after you have got the new method established, your business will go in a regular course, and one parcel of goods will succeed another, as fast as you can manufacture and dispose of them.
"22. Though it is possible to tan small parcels of leather, by way of experiment, by the use of lime-water ooze, in a fourth part of the time which is required if only common ooze be made use of; yet the business of a large tan-yard cannot be carried on with so much expedition: but even in large works, and in the common course of business, soal-leather can be completely tanned and finished in from eleven to fifteen months, according to the different weight and thickness of the hides; butts, in from eight to twelve months; and calf-skins, in from six to twelve weeks: in general, the tanner may save at least a third of the time that has hitherto been required.
"23. The leather which is manufactured in the new way is of a superior quality to that of the old tanning, especially the soal-leather, which wears remarkably well, and never shows the least sign of rottenness.
"24. Let it always be remembered, that the lime-water is never to be used but when it is sufficiently strong, and as clear as rock-water.
"25. Whenever you make fresh ooze, you must always use fresh lime-water, and run the ooze through two letches; and the lime-water ooze, when spent from laying on the leather, is never to be returned back upon the bark which is in the letches, (as you now return your spent ooze), but must always be thrown away, as being entirely useless; for which purpose you must contrive a gutter in the tan-yard to carry off the spent ooze.
"26. The letches ought to be under cover, lest the rain get into them and weaken the ooze; and if the handlers are sheltered, it will be so much the better; but it is of no importance to cover the vats, provided, when the leather is laid away in them, they are kept constantly full to the brim.
"27. You must always take care to have a sufficient stock of unslaked lime by you, (for if it be slaked it will not answer to make lime-water): therefore get your lime fresh, if possible, from the kiln, and immediately pack it in any kind of old dry casks. Weigh one of these casks, and it will enable you to ascertain the quantity of lime necessary to be thrown into the cistern each time you begin a fresh brewing of your lime-water, and thus save you the trouble of repeated weighings; not that there need be much nicety about the quantity of lime, a score of pounds over or under making no sensible difference in the strength of the lime-water.
"28. Any expence you may be at in procuring lime, which even in the largest tan-yards can amount but to a trifle, will be amply compensated by the saving of
Tanning, of bark; because that lime-water so completely exhausts the bark, and makes it go so much farther than when the ooze is made only of plain water. As a proof of this, you may make a pretty strong ooze from the tan or spent bark, which you now consider as completely exhausted, by infusing in it lime-water."
The leather, when only tanned, is not sufficiently soft and pliable to answer a number of purposes. The currier's province is to reduce the leather to the proper thickness, pliability, and colour, requisite for the different uses to which it is applied: and though there is a material difference in the method of manufacturing the several kinds, and a good deal of dexterity required; yet what could be here said of them would be of little use to those who are unacquainted with the business. We shall therefore reduce all we are to say on this subject to a general detail of the process.
The leather, after it has dripped some time from the tan-pit, is shaven on an upright beam with a knife whose edge is turned on the one side, and with which the currier can take down the leather in the same manner as a wright can take a shaving from a piece of wood with his plane. After the currier has thus levelled the hide or skin to his purpose, (which is always done on the flesh-side), he spreads it out on a stone or table made for the purpose, and there scours it on the grain-side from all the loose tan, and other stuff that may hurt the beauty of the grain. He then puts on a certain quantity of oil on both grain and flesh-side, for the purpose of softening the fibres, and at the same time making them tough, and hangs it up to dry. When it is sufficiently dry, there is a thin shaving again taken off the flesh-side, in order to clean or brighten it up; and then it is rubbed backward and forward upon a table by the currier with a nicked or furrowed board, keeping the leather always doubled at the place where he rubs, till it be made soft and pliable to his intention. This last part of the operation is properly called currying, and it is from this that the business gets its name. As to the colour, the leather is fair or dark in proportion to the quantity of oil laid on, if the tanning has been properly performed, and the currier do not neglect some material part of his duty. Curriers seldom dye any colour but black, which is done with copperas and a very little logwood.