Fig. 40.
Fig. 40.
It is one of the great attractions of extracts that they avoid almost all the expense and labour inseparable from the exhaustion of other tanning materials. It is usually necessary to dissolve the fluid extracts in water or liquor of as high a temperature as has been employed in their preparation, as otherwise, from some unexplained chemical change, a large portion of the tannin is precipitated, probably as an anhydride of the tannin. Gambier is usually dissolved by boiling or steaming, but is said to give a better colour when dissolved cold. This may be accomplished in a rotating latticed drum, sunk in a pit of liquor.
Where circumstances permit, it is a great advantage to place the taps either on a higher or a lower level than the layers and handlers, so that liquors may be run one way without pumping.
SOLE-LEATHER.—Treatment in the Tan-house.
Onfirst coming into the yard, the butts are usually suspended by the shoulder or butt ends from sticks placed across the pits. They should be kept in almost constant movement, either by raising and shaking them by hand, or by supporting them on frames, which are rocked, or otherwise worked. Perhaps the best device for this purpose is the "travelling handler" of W. N. Evans, which consists of a frame supported on wheels, and worked slowly backwards and forwards by power. This frame should extend the length of a range of pits sufficient to take in at least a 3 days' stock of butts, which should be tied to sticks resting crossways upon it. It should have a stroke of 1-2 ft., repeated, say 6 times a minute. The power required is very small.
The American rocker consists of a wooden frame balanced on its centre, and made to oscillate by power. It is a cheap and efficient machine, its defects being that the butts at the ends are much more moved than those in the centre, and that their upper parts, being lifted out of the liquor, are liable to become blackened.
The suspender pits should be supplied with old handler liquors, which, if the tannage is a mixed one, may range from 12° to 20° barkometer, as a large proportion of the weight consists only of lime-salts, gallic acid, and other worthless products. It must here be explained that the barkometer (also called "barkrometer" or "barktrometer") is a hydrometer, graduated to show the sp. gr. thus—20° Bark. = 1·020 sp. gr. In using it the temperature of the liquor must be at or near 60° F. (15° C.). It is, of course, affected by any other matters in solution, precisely the same as bytannins. In the Lowlights Tannery the waste liquors are constantly about 12° Bark., and contain tannin equal to less than 0·2 per cent. (expressed as crystal oxalic acid), and gallic acid and similar matters equal to O·6 to 0·7 per cent. If the tannage is pure bark, it may perhaps be advisable to let the strength be somewhat less, but something depends on whether the exhausted liquors are returned with all their impurities to the "taps" or liquor-brewing pits, or whether the liquors are made with water, and hence purer. In any case, the free acid in the suspenders should always be sufficient in quantity to neutralise the lime brought in by the butts, or bad colour will certainly result, making itself visible in the shed, or as the tanning proceeds. If the butts, when first brought into liquor, take a lemon-yellow colour, especially in places that have been imperfectly exposed to it, this is an indication of danger which must not be disregarded. It may be met either by cleansing the butts more thoroughly before bringing into the yard, or by adding acid (acetic, hydrochloric, or sulphuric) to the liquor. If this be done, great care must be taken not to over-do it, and an acid free from iron must be used. The use of sulphurous acid for the purpose has been patented, and presents some advantages. Sulphites have been observed by the writer to give a pink or purple reaction even with very dilute infusions of valonia (seep. 112); but any coloration from this cause would probably disappear as the tannage proceeds. The difficulty can, however, often be remedied, either by altering the way of working the liquors, so as to bring more sour liquor down to the suspenders, or by using a larger proportion of materials capable of yielding acetic acid by fermentation, such as myrobalans. It is a common error to call all the free acid of sour liquors "gallic," as this is scarcely present in pure bark-yards, and at the best is a very feeble acid. The most abundant acid is usually acetic, though butyric, lactic, and other acids are frequently present in varying proportions, according to the tanning materials employed. In the English process, with its comparatively short layers,in which the butts almost float in strong liquors, but little souring takes place, and we have nothing comparable to the German "sour bark" and "sour liquor" from long layers with weak liquor, and much dusty material. These contain large quantities of acetic and lactic acids, and plump almost like vitriol. Though the American tanners generally use the latter, their hemlock liquors sour much more intensely than those of English yards. It must always be borne in mind, in comparing English with American and Continental tanning, that, in the first, the opening up of the fibre is effected by lime, and the swelling is maintained in the liquors, not so much by acids, which are only present in very small proportion, as by the careful and gradual working forward into infusions stronger and stronger in tannin; while in the two latter, lime, if used at all, is simply employed to loosen the hair, and the swelling and differentiation of the fibre is first accomplished in the liquors either by vegetable or mineral acids. Hence good results cannot be expected in English yards from such processes as sweating or painting with sodium sulphide, which does not plump, without a radical modification of the whole tanning process. This point has been ably treated by Eitner in a series of papers onExtract-gerberei, published during the last few years in 'Der Gerber,' which will well repay attentive perusal by English as well as German tanners.
The butts should at first be brought into the weakest liquor; a circulation system, by which the liquors are all pumped in at one end of a set of suspenders, and run out at the other, the butts being moved forward in the opposite direction, seems to have much to recommend it. In this case, the top of one pit should be connected by a wooden box with the bottom of the next.
It is usually advisable to run away the first liquor into which butts are brought from the lime-yard, as it is very completely spent, and highly charged with lime salts and impurities. Whether other exhausted liquors are to be retained or rejected is largely a question of climate, and modeof working. In hot weather, such liquors, charged with organised ferments (moulds,bacilli, andbacteria), are apt to cause ropiness, and other fermentive diseases of the liquors. This danger may be lessened by boiling all spent liquors, so as to kill the ferments, before running on the taps, or prevented by the free use of antiseptics, such as carbolic acid. Small doses of carbolic acid, however, are useless; at least1/10per cent. must be employed; and it must be borne in mind that antiseptics prevent souring as well as other fermentations, and hence, where they are employed, other means must be adopted to maintain the necessary acidity. Such liquors are very liable to darken if boiled.
The suspender liquors should be acid enough freely to redden litmus-paper. The present author has published a simple volumetric method for the determination of the free acid; 10cc.of the carefully filtered liquor is placed in a beaker, and clear lime-water is run in from a burette till permanent cloudiness is produced. The quantity of lime-water employed is that which the acid is capable of neutralising, without producing discoloration of the leather, and care must be taken that the lime introduced with the butts does not exceed this proportion. The explanation of the reaction is that dark-coloured tannates of lime are formed, which are dissolved by the free acid so long as it remains in excess. It must be remembered that this process estimates all acids capable of retaining tannates of lime in solution, including some so feeble as to have practically no plumping effect. A liquor may have acidity equal to severalcc.of lime-water, and yet react absolutely alkaline to methyl-orange (seep. 9), a colour which is distinctly reddened by small excess of acids, even so weak as gallic, which is barely acid to the taste. Hence, the acidity of a liquor available for plumping may be taken as represented by the lime-water required to change the red of methyl-orange to yellow, and if the liquor does not redden methyl-orange it is incapable of plumping. If 5 or 10 drops of orange solution be added to the pale filtered liquor from suspenders, there is no difficulty in approximately hitting the point of change, but greataccuracy is not to be expected. If the liquor will not filter clear, kaolin (seep. 119) may be used to clear it. It is well to test the lime-water occasionally on 10cc.of decinormal sulphuric or oxalic acid (p. 96), to make certain of its constancy. Lime-water should be kept in a bottle with excess of lime, shaken occasionally, and a small quantity filtered off as required. Liquors are frequently miscalled "sour" which are not acid, but putrid. Such liquors will not plump, but reduce and soften hides placed in them. (Compare alsop. 185). Suspender liquors usually consist mainly of liquors from the handler shift. If liquors be used direct from the leaches, they generally produce harsh grain and bad colour.
From the suspenders, the butts are transferred to the "handlers," where they are laid flat in the liquor. They are usually pulled over by hooks, which are very apt to scratch the grain. Sometimes strings are used, attached to the corners and held in notches or on pegs at the edge of the pit. Other tanners place a frame below the pack, with ropes at the four corners, by which it is raised sufficiently for the men to grasp the top butts with their hands. This is only practicable in pits of ample size. In American yards, the handling is almost universally performed by tying the sides with strings or fastening them in a long band by drawing the slit tail of one side through a hole in the nose of the next, and inserting a wooden "key." The string of the sides is then wound from one pit to another over a skeleton reel (Fig. 41). This method is also used in the lime-yard, and is frequently employed in England to handle offal, but it is not well adapted for butts.Fig. 42shows the application of mechanical power in a Chicago yard for the same purpose, by means of Ewart's drive-chain, which is manufactured in this country by Ley's Malleable Castings Co., at Derby, to whom I am indebted for the block.
Fig. 41.
Fig. 41.
The handlers are generally worked in sets, to each of which a fresh liquor is daily run, and the most forward pack is pulled over into it, and is often also dusted down with a little fine bark or myrabolans. The second pack follows into the liquor out of which the first has been taken; the third intothat of the second, and so on. Frequently the greenest packs are handled up a second time in the course of the day, and put down again in the same liquor. The strength of liquors, and the length of time for which butts are retained in the handlers, are varied; but a time of 1-2 months, andliquors of 20°-35° Bark. are usual. It is well to divide the handlers into at least two sets. Gambier is very useful, especially to the greener goods, and if hemlock and other extracts are employed, their appropriate place is in the forward handlers or earlier layers. New valonia liquors must be avoided, but old layer liquors of considerable strength (up to 40° Bark. where the handling is long continued) may be employed.
Fig. 42.
Fig. 42.
At the end of this period, the butts are taken to the "layers" or "bloomers," in which they are laid down with stronger liquors and much larger quantities of "dust"; the latter is usually bark or valonia, though mimosa is occasionally used. The liquors vary from 40° to 60° or 70° Bark. in strength in mixed tannage, and the duration of each layer from 10 days in the earlier stages to a month in the later ones. For the best heavy tannages, 6-8 layers are required. Each time the butts are raised, they should be mopped on the grain, to remove dirt and loose bloom. Strong valonia liquors, or heavy valonia dusting, causes a brown sandy crust to form on the freely exposed parts of the butts. This is removed in striking, but is sometimes very troublesome on rough dried dressing leather. In pure bark tannage, which, however, is gradually becoming extinct, the liquors used are of necessity much weaker, as it is extremely difficult to obtain liquors of more than 25°-30° Bark. from this material. The last layer, however, should always have liquors of the greatest strength which can possibly be obtained, or the leather will be deficient in firmness.
After receiving their last layer, the butts are well mopped or brushed and washed up in a clear liquor, and thrown over a horse to drain before going into the shed. In America, the Howard scrubber (Fig. 43) is generally employed instead of hand labour at this stage. It consists of 2 rotating wooden frames at the top of a pit, provided with brushes or birch-brooms, and, when in use, enclosed by a cover A, through a slit G in which the sides are inserted and drawn back, while water is supplied by the pump B. Sometimes the brush-drums are placed one above another, and the leather is passed in at the side.
Pl. VI.E. & F. N. Spon, London & New York."INK-PHOTO." SPRAGUE & CO. LONDON.OPENING, STRETCHING AND STORING WHITE SKINS.
Pl. VI.
E. & F. N. Spon, London & New York.
"INK-PHOTO." SPRAGUE & CO. LONDON.
OPENING, STRETCHING AND STORING WHITE SKINS.
Fig. 43.
Fig. 43.
In mixed tannages, where the colour is dark, the leather is frequently handled or suspended in a warm sumach or myrobalanes liquor, and occasionally in dilute sulphuric or oxalic acids. If these acids are not effectually removed before drying, the toughness of the leather will be destroyed, and in extreme cases the leather will become brittle and refuse to take black. In any case, strong acids are prejudicial to the durability of the leather. In America, alternate baths of vitriol and sugar of lead are frequently used for bleaching and weighting the leather, but the colour given is not durable.
The great point to aim at, in arranging the mode of work of a tannery, is to contrive that butts should always receive the strongest liquors they can bear with safety, and that the strength should constantly increase in a regular and systematic way. To attain this end, very frequent handling and change of liquor are requisite in the early stages, when the buttsrapidly absorb the tannin presented to them. As the process advances, the exterior part of the butt becomes thoroughly tanned, and the liquor only slowly reaches the interior, which is yet susceptible of its action, and hence longer layers in stronger liquors are permissible.
The varied requirements of the trade render it difficult to give any practical information as to the selection of tanning materials. As a general rule, it is important at the outset to give the required colour; and if materials undesirable in this respect are to be used for the sake of cheapness, they should be introduced in the form of liquors in the middle stages of the process, i. e. in the later handlers or earlier layers. Materials used as dust generally have more effect in producing bloom and colouring the leather, than those used in liquors at this stage. Some information as to the respective qualities of the different tanning materials will be found in the chapter on Tannins; but even practical men are very deficient in accurate information on these points, since many materials are never used alone, but invariably in connection with others which mask their effects.
The use of extracts, and the demand for low-priced leathers, to compete with the American tannages, has introduced still more rapid methods than those described, and very fair-looking heavy leather has been tanned in 5-10 weeks. These tannages are very various, but their main feature is the free use of hot liquors, composed principally of extracts and gambier. This treatment imparts great firmness, or more properly speaking, hardness; but the leather is deficient in toughness, and the grain usually cracks on bending sharply. Extract properly used is, however, capable of making excellent leather; it is employed in at least one of the highest priced tannages in the country.
It may be noted here, that when Continental writers speak of extracts and extract tannage, what we should call liquor tannage only is meant, and not specially the use of the concentrated extracts, to which alone in England the term is applied.
SOLE-LEATHER.—Treatment in the Shed.
Fig. 44.
Fig. 44.
Thebutts, after being treated as above described, are frequently oiled lightly on the grain, and are taken into the drying-lofts, where they are hung on poles till about half dry. They are then laid on the floor in piles, and covered up till they heat or "sweat" a little, which facilitates the succeeding operation of "striking." This is performed by laying the butt over a horizontal "beam" or "horse," and scraping its surface with a triangular pin, shown at D inFig. 25. This pin has an even, though tolerably sharp, edge, and is so used that it stretches and smooths out the grain, without breaking it; and at the same timeit removes a portion of the white deposit called "bloom," which has been mentioned. Common goods are frequently struck by the machine introduced by Priestman, of Preston Brook, shown inFig. 44; but the work is not very uniform, and the leather is much compressed and stretched. For offal, the machine is a very useful one, and perfectly satisfactory.
Fig. 45.
Fig. 45.
Fig. 46.
Fig. 46.
Butts are now generally struck by the very ingenious machine of Wilson, whose name has also been mentioned in connection with the disintegrator, and which is shown inFig. 45. The arms carry blunt brass or steel knives or sleekers, and work outwards from the centre, while the butt is carried backwards and forwards over the drum. Stones may be substituted for the sleekers, when it is required to remove the bloom. The machine requires a firm foundation, as its reciprocating motion causes considerable vibration.
Fig. 47.
Fig. 47.
After a light oiling and a little further drying, the butt is laid on a flat "bed" of wood or zinc, and is rolled with a brass roller loaded with heavy weights. Various machines are also in use for this purpose. InFig. 46, is shown Wilson's spring butt-roller, in which the pressure is produced bysprings immediately above the roller, which works backward and forward over a flat table, beneath a fixed girder. In the later patterns of this machine the roller is automatically reversed by a mechanical finger before coming to the edge of the butt.Fig. 47shows an adaptation of the American pendulum roller, which is specially suited for refinishing Singapore kip sides and the commoner class of goods, giving great firmness and a high gloss.Fig. 48represents a machine in which the roller is fixed, and works over a brass drum; it is specially adapted for offal, and, when used for butts, is apt to make them "baggy." In this machine, the reversing motion is obtained by using two belts, one being crossed.
Fig. 48.
Fig. 48.
The leather is now frequently coloured on the grain with a mixture, for which each tanner has a recipe of his own, but usually consisting mainly of yellow ochre with size orliquor and oil in order to give a gloss, and to hide uneven or dull colour, and, when sufficiently dry, is well brushed by hand or power, rolled a second time, and dried-off in a room gently heated by steam. This is the Bristol method of finishing. In the Lancashire district, butts are generally struck out much wetter, and "stoned," so as to remove the whole of the bloom, and show the natural brown "bottom" of the grain. When sufficiently dry, they are struck a second time, to set the grain, and rolled as described, the painting being omitted. This method has the disadvantage of requiring more labour, and causing a loss of weight; but leather so got up brings a higher price, as the finish is only applicable to such tannages as make a fair colour. The usual London plan is a compromise between the Bristol and Lancashire methods; the leather is sammed, or tempered by partial drying and piling before striking; stoning is not resorted to, but the bloom is thoroughly removed from the surface with the pin and scrubbing-brush. Colour is not generally used.
It is very important, and especially so with heavy mixed tannages, that the drying should be conducted in the dark, and not too rapidly. No artificial heat should be used, except in frosty weather, to wet leather; and it should be carefully protected from harsh drying winds. After the leather is finished, it should be dried off in a well-ventilated drying-shed, heated to about 70° F. (21° C.). The same observations apply to the drying of rough dressing-leather, except that artificial heat should be avoided. Frost makes dressing-leather porous, and prevents it carrying a proper quantity of grease in currying. On the construction of drying-sheds, seepp. 243-54.
DRESSING LEATHER.
Hideswhich are intended for purposes where softness and flexibility are required, as for instance, for the upper-leathers of boots, and for saddlery purposes, are called "dressing" or "common" hides, or, if they are shaved down to reduce their thickness before tanning, they are denominated "shaved" hides. Hides for this purpose are limed much in the same way as has been described for butts; but if they are required very soft and flexible, a somewhat longer liming is permissible. After unhairing, fleshing, and washing in water, they are usually transferred to a "bate," composed of pigeon- or hen-dung, in the proportion of about 1 peck to 25-30 hides.
In this they are retained for some days, being handled frequently. They completely lose their plumpness, and become soft and slippery; the caustic lime is entirely removed; and the remaining portions of hair-sheaths and fat-glands are so loosened that they are easily worked out by a blunt knife on the beam. This final cleansing process is called "scudding." The theory of the action of the "bate," or "pure," as it is sometimes called, is somewhat imperfect. It is frequently attributed to the action of ammonia salts, and phosphates, contained in the fermenting dung. Ammonia salts certainly will remove caustic lime, free ammonia being liberated in its place, and weak solutions of ammonia sulphate or chloride will rapidly reduce hides, and remove or neutralise the lime. The phosphates in dung are mostly, if not entirely, in the form of lime phosphate, which is quite inert. In point of fact, the process seems to be a fermentive one, the active bate swarmingwithbacteria; to this, rather than to its chemical constituents, its action must be attributed. Thebacteriaact not only on the organic constituents of the dung, but on those of the hide, producing sulphuretted hydrogen, together with tyrosin and leucin, and other weak organic acids, which neutralise and remove the lime, and, at the same time, soften the hide by dissolving out the coriin, and probably also portions of the gelatinous fibre. The truth of this theory is supported by the fact that, in warm weather, the activity of the bate is greatly increased, and that, if one pack of hides is over-bated, the next following is much more severely affected, the hides having in fact themselves furnished food for the multiplication of the bacterian ferment from the destruction of their own tissues. It also explains the effective use (as a substitute) of warm water with a very small portion of glucose, which, in itself, would be insufficient to dissolve the lime, but with a small quantity of nitrogenous matter, forms an excellentnidusfor the multiplication of these organisms. An American invention for bating is the use of old lime-liquor neutralised with sulphuric acid, an idea which is much more scientific than would at first sight appear. Old lime-liquors, as we have seen (p. 143) contain much ammonia and weak organic acids, such as caproic, amidocaproic (leucin), and tyrosin. On adding sulphuric acid, the lime forms an inert sulphate, and the sulphate of ammonia and the weak organic acids which remain dissolved are just what are required in a chemical bate. The lime-liquor should of course be filtered or settled clear before using, and enough acid added barely to neutralise the lime, and the liquor again settled or filtered. By this means both the dissolved gelatin and the iron of the acid will be got rid of. The liquor might then be slightly acidified before use. The writer has no experience of the method, but imagines that used as described it might be worth trying, although it would have a very unpleasant smell. In this connection may be mentioned the fact that, when brandrenches are used, in which lactic acid is developed, the butyric fermentation is liable, in hot weather, to take its place, and as butyric acid is a powerful solvent of gelatinous tissue, and the dissolved tissue itself feeds the fermentation, rapid destruction of the skins is the result. Cleanliness, scalding out of the drench vats, and washing the bran before using with cold water to remove adhering flour, are useful precautions.
If the removal of the lime be the only object aimed at in bating, the ordinary process is most wasteful, as well as disgusting, from the loss of pelt it entails. It is easy to find chemical reagents which will remove the lime; but the resultant leather has been found wanting in softness, and it is probable that the solution of the inter-fibrillar matter is in many cases advantageous. Probably one reason for the non-use of such chemicals is their expense. Maynard has patented the use of sulphurous acid for the purpose. If sugar, glucose, or ammonia salts be used, and the alkalinity of the solution nearly neutralised after each lot of hides by common vitriol, the same liquor may be used again and again. In this case, if iron is contained in the acid it will be precipitated by the ammonia and must be settled out. The writer is convinced, from his own experience, that with suitable tannage such bating would yield better weights and quite as satisfactory leather for many purposes as the ordinary mode. French tanners, by the free use of water, and careful working at the beam, and the employment of very weak liquors at the commencement of tanning, make excellent dressing leather without bating and this is also true of the celebrated French calf.
The bating required may be shortened, and probably with advantage, by washing the hides with warm water in a "tumbler," or rotating drum,Fig. 49, prior to putting them into the bate, or the whole bating may be done in the tumbler. After a short bating, also, the hides may be softened and cleansed by stocking for 15-20 minutes. Warm bates act much more rapidly than cold ones.
Fig. 49.
Fig. 49.
Various machines have been proposed to take the place of hand-labour in the beam work, and, at least as regards the smaller skins, with considerable success. As a type of these, may be mentioned Molinier's hide-working machine,Fig. 29, which consists of a drum covered with helical knives, rotating at a speed of about 500 rev. a minute, over a cylinder coated with india-rubber. The skin is allowed to be drawn in between these drums, and the two being pressed together by a treadle, it is drawn out by a mechanical arrangement in a direction contrary to the rotation of the knives, which scrape off the flesh, or work off the hair.
After bating, "shaved" hides are reduced in thickness in the stronger parts by a shaving-knife, on an almost perpendicular beam. The workman stands behind the beam, and works downwards. The knife is represented at A,Fig. 26, and is a somewhat peculiar instrument. The blade is of softish steel, and after sharpening, the edge is turned completely over by pressure with a blunt tool, so as to cut atright angles to the blade. There is an obvious economy in shaving before tanning, since the raw shavings are valuable for glue-making, while, if taken off by the currier, they are useless for this purpose. The hide also tans faster.
Instead of shaving, the untanned hide is frequently split, by drawing it against a rapidly vibrating knife. The piece removed is tanned for some inferior purpose, if sufficiently perfect. In sheep-skins, which are split by a special machine, the grain-side is tanned for French morocco or basil, while the flesh-side is dressed with oil, and forms the ordinary chamois or wash-leather (seep. 210). Such a machine is shown inFig. 50.
Fig. 50.
Fig. 50.
Tanned leather is frequently split by forcing it against a fixed knife, as in the American "Union" machine,Fig. 51. This is however being gradually superseded by the band-knife splitting machine,Fig. 52, in which an endless steel blade travels over two pulleys like a belt, and is kept constantly sharpened by a pair of emery-wheels seen below the machine. I am indebted for the block to Messrs. Haley and Co., who have made great numbers of these machines.
Fig. 51.
Fig. 51.
After bating, scudding, and shaving, the hides are taken into the tan-house, where they are grained, either by frequent handling, or by working in a paddle-tumbler (a vat agitated with a paddle-wheel, and known in America as an "England wheel"), with a liquor of suitable strength. What this strength should be depends on whether a well-marked grain is required or not. The stronger the liquor, the more it contracts the hide, wrinkling the surface into a network of numberless crossing furrows, which form the well-known marking of "grain-leather." In bark tannage, the after management is much like that described with sole-leather, except that weaker infusions are employed, and acid liquors, which would swell the hide and produce a harsh leather, are avoided. In old-fashioned country yards, which produce some of the best bark-tanned shaved hides, the liquors rarely range above 10°-15° of the barkometer, and the time employed is 3-6 months. The hides, after passing through a set of handlers, of gradually increasing strength, in which they are at first moved every day, are laid away with bark liquor and a good dusting of bark, receiving perhaps 4-5 layers of 2-4 weeks each. Unfortunately, these tannages are so unprofitable that they are rapidly being supplanted by quicker and cheaper methods.
Fig. 52.
Fig. 52.
These more rapid and cheap tannages mostly depend on the use of "terra" (block or cube gambier) in combination with bark, valonia, mimosa, and myrobalanes. Liquors warmed to 110° or even 140° F. (43°-60° C.) are frequently employed, and a bright colour is finally imparted by handling in a warm sumach or myrobalanes liquor, which dissolves out much of the colour imparted by terra or extracts. The tannage is helped forward by frequent handling, by working in tumblers, or sometimes by suspension on rocking or travelling frames, after the American fashion.
To this class of tannage belongs that of East India kips, which is largely carried on in the neighbourhood of Leeds. These kips are the hides of the small cattle of India, and are imported in a dried condition, and with their flesh-side protected (and loaded) with a coat of salt and whitewash or plaster. They are usually softened in putrid soaks, and unhaired with lime, and are used in England for many of the purposes for which calf-skins were formerly employed. A variety of East India kips, called "arsenic kips," are treated (instead of plastering) with a small quantity of arsenic before drying, to prevent the ravages of insects, which are often very destructive to these goods. Many kips tanned in India have also been imported of late years, and have greatly interfered with the profits of English tanners.
In yards where the leather is intended to be sold uncurried, it is taken up into the drying-sheds, well oiled on the grain with cod-liver oil, and either simply hung on the poles to dry, or stretched with a "righter," a tool shaped somewhat like a spade-handle, and finally set out with it to a smooth and rounded form. As in the case of sole-leather, too much light or wind must be avoided, and it is very difficult to use artificial heat successfully in the early stages of the process. It is, however, now very common for the tanner who produces such leather also to curry it, and, as this effects aconsiderable economy, both in labour and material, it is likely to become universal. When leather is to be sold rough, it is necessary to tan it in such a way as to give it a white appearance, from the deposit of "bloom" already mentioned; this being regarded by curriers as an essential mark of a good tannage, although the first step in the currying process is to completely scour it out. When the tanner curries his own leather, he of course aims at putting in as little bloom as possible, thus economising both tanning material and labour. In addition, the leather goes direct from the tan-house to the currying-shops, thus saving both drying and soaking again, and, it is said, giving better weight and quality. The tanner, too, is enabled to shave his hides or skins more completely, utilising the material for glue-stuff, which, had the leather been for sale in the rough, must have been left on to obtain a profitable weight.
CURRYING.
Ingeneral terms, the process of currying consists in softening, levelling, and stretching the hides and skins which are required for the upper-leathers of boots, and other purposes demanding flexibility and softness, and in saturating or "stuffing" them with fatty matters, not only in order to soften them, but to make them watertight, and to give them an attractive appearance.
It is obvious that great differences must be made in the currying process, according to the character of the skin and the purpose for which it is intended, since the preparation of French calf for a light boot, and of the heaviest leather for machine belting, equally lie within the domain of currying. In this case, however, as in that of tanning, the clearest idea of the general principles involved will be gained by taking a typical case, and afterwards pointing out the different modifications needed for other varieties. The French method of currying waxed calf is selected as an example, since the well-known excellence of this leather makes it interesting to compare the details with the methods ordinarily in use in this country.
After raising the skins from the pits, and beating off the loose tan, they are hung in the sheds till partially dry (essorage), great care being taken that the drying is uniform over the whole skin. In modern shops, this drying is usually accomplished at once, and in a very satisfactory manner, by means of a hydraulic press. If dried in the air, they must be laid in pile for a short time to equalise the moisture, and then brushed over on flesh and grain. The next process consists in paring off loose flesh and inequalities (dérayage).This is done on a beam, and with a knife similar to that used in bate-shaving, and shown in A,Fig. 26. This knife has the edge turned by rubbing with a strong steel, and is calledcouteau à revers.
Next follows themise au vent. The skins are first placed in a tub with water or weak tan-liquor for 24 hours; they are then folded and placed in a tub with enough water to cover them, and beaten with wooden pestles for1/4hour. At the present day, stocks (foulon vertical), or a "drum-tumbler" (tonneau à fouler), a machine on the principle of the barrel-churn, usually take the place of this hand-labour. The skin is next placed on a marble table, flesh upwards, and with one flank hanging somewhat over the edge, and is worked with a "sleeker" or stretching-iron (étire), B,Fig. 26. The first 2 strokes are given down and up the back, to make the skin adhere to the table, and it is then worked out regularly all round the side on the table, so as to stretch and level it. The flesh is then washed over with a grass-brush (brosse à chien-dent), the skin is turned, and the other flank is treated in the same way. It is lastly folded in 4, and steeped again in water. The next process is the cleansing of the grain. The skin is spread again on the table, as before, but grain upwards, and is worked over with a stone (cœurse), set in handles, and ground to a very obtuse edge. This scours out the bloom; after washing the grain with the grass-brush, it is followed by the sleeking-iron, as on the flesh.
The next step is resetting (retenage). For this, except in summer, the skins must be dried again, either by press or in the shed. This is another setting out with the sleeker, and, the skin being dried, it now retains the smoothness and extension which is thus given to it. The skins are now ready for oiling in the grain, for which whale-oil or cod-liver oil is generally employed. Olive-oil, castor-oil, and even linseed-oil may, however, be used, and are sometimes made into an emulsion with neutral soap and water. After oiling the grain, the skins are folded and allowed to lie for 2-3 days before oiling the flesh.
The oiling on the flesh is done with a mixture ofdégrasand tallow, in such proportions as not to run off during the drying.Dégrasis the surplus oil from the chamois-leather manufacture, which in France is effected by daily stocking the skins with oil, and hanging in the air for oxidation. Thedégras(toise,moëllon) is obtained, not by washing the skins in an alkaline lye, as in the English and German method, but by simple pressing or wringing. This oil, altered by oxidation, is so valuable for currying purposes that skins are frequently worked simply for its production, being oiled and squeezed again and again till not a rag is left. It is generally mixed in commerce with more or less of ordinary fish-oil. Eitner recommends, where thedégrasis of indifferent quality, a mixture of 65 partsdégras, 20 of neutral soap (i. e. soap without the usual excess of alkali), and 15 of soft tallow. After oiling the flesh, which is accomplished by extending the skin on the marble table with the sleeker, and applying grease with a sheep-skin pad, it is hung to dry at a temperature of 65°-70° F. (18°-21° C.). After drying, the surplus oil is removed by a fine sleeker from both flesh and grain, and the skins are ready for "whitening" (blanchissage). This consists in taking a thin shaving off the flesh, and was originally accomplished by the shaving-knife on the currier's beam, and some curriers are still in favour of this method. It is now, however, usually done by a sleeker with a turned edge. The grain then undergoes a final stoning and sleeking, to remove the last traces of adhering oil, and the skin is grained by rubbing it in a peculiar way under a pommel covered with cork. It is then coated on the flesh with a mixture, of which the following is a specimen:—5 parts of lamp-black are rubbed with 4 of linseed-oil, and 35 parts of fish-oil are added; 15 parts of tallow and 3 of wax are melted together and added to the mixture; and, after cooling, 3 parts of treacle. This compound is put on with a brush, and allowed to dry for some days. Finally, the skins are sized over with a glue-size, which is sometimes darkened by the addition of aniline-black.
The preceding account will give some idea of the care and labour expended on these goods in France. In England, cheaper productions are more in vogue, and almost every process is accomplished by machinery. An illustration of the Fitzhenry or Jackson scouring-machine, which is largely employed both for scouring and setting out, is given inFig. 53. This is a simple and efficient machine, and has been largely used, both here and in America.