Fig. 53.
Fig. 53.
Fig. 54shows the improved tool-carriage introduced by C. Holmes of Boston, in which the brush and sleekers or stones are controlled by handles which are stationary instead of moving rapidly with the slide, as in the older form. Spiral springs are also substituted for the older elliptical ones.
Fig. 54.
Fig. 54.
Fig. 55.
Fig. 55.
The Fitzhenry machine has also been constructed so as to work in any direction over a fixed table, being driven by a small direct-acting steam-cylinder supplied by jointed pipes. But probably the most perfect scouring and setting machine which has yet been introduced is the Lockwood Automatic Scourer, which may also be regarded as a development ofthe Fitzhenry machine. This has been some years in use in America with great success, and has received considerable improvements, but has only very recently been introduced into England by Messrs. Schrader and Mitchell of Glasgow, who have kindly furnished the annexed illustration (Fig. 56). In this machine the table is fixed, and the tool-carriage can be moved over it in every direction. The large projecting carriage, or cross-head, which supports it, travels on a horizontal rail, which may be observed below and behind the table. Motion is given to it by a screw which is driven in either direction by the pulleys at each side of the cross-head. In a similar way the tool-carriage is traversed forwards or backwards by a second screw at right angles to the first, and by a most ingenious interlocking arrangement both screws are controlled by a single handle. The tool-carriage or "trundle frame" can also be turned like a turntable, so as to deliver its stroke in any direction, the tool-holder beingdriven by a horizontal crank in the centre of the frame, and immediately above the tools. Though the machine is complicated, and necessarily expensive, it has not been found either in America or Scotland difficult to work or liable to get out of order, while both the quantity and quality of its work are all that can be desired.Fig. 55is Gläser's scouring machine.Fig. 57illustrates the latest English scouring machine, Messrs. Haley and Co.'s Climax Scourer, which is also ingenious and effective. In it the table instead of the tool-holder is movable by screws driven by belts thrown into gear by a handle, and it is provided with two tables of which one is in work while the hides are being changed and spread on the other. The oscillating tool-holder, instead of being actuated by the rise and fall of the connecting-rod, is moved by an adjustable eccentric.
Fig. 56.
Fig. 56.
Fig. 57.
Fig. 57.
In the case of strap-butts, the currying is, of course, far less elaborate. They are well scoured out, heavily stuffed, and stretched in screw-frames, to prevent their giving afterwards when in use.
In Germany, Switzerland, and Austria, a method of stuffing strap-butts is frequently employed, which, so far as I am aware, is not in use in England. It is calledEinbrennenor"burning in," and consists in applying very hot tallow to the dry leather. The butts are washed free from liquor in a tumbler, boarded to soften them thoroughly, scoured, set out with a sleeker, nailed on laths, and air-dried. They are then very completely dried in a room heated to 104°-113° F. (40°-45° C.), as if any moisture remains in the hide, the fibre will be destroyed by the heat of the melted tallow. The tallowing generally takes place in the same room, as a high temperature is required to allow it to soak in, and the leather would greedily reabsorb moisture if exposed to damp air. The tallow is heated, generally by steam in a jacketed pan, to 167°-212° F. (75°-100° C.). There are two ways of applying it. The melted tallow may be applied on a table to the flesh side of the butt with a ladle, and rubbed on with a brush or rag. In this case, as soon as the tallow has sufficiently soaked in, the butts are placed in water to prevent its striking through to the grain. The second way is to have the pan of sufficient size and suitable shape, and for two men to draw the butt through the melted tallow with tongs, and more or less rapidly according to the quantity it is desired that the leather should absorb; and in some cases the process is repeated once or more. In this case, it is useless to wet in water, and the butts are allowed to cool gradually in pile.
The leather is now impregnated with grease, but it is far from being properly stuffed. Instead of the grease being spread over the finest fibres in a minute state of division, it simply fills the spaces between the larger fibres. To remedy this, the butts are well softened in water (which, if they have been drawn through the tallow and allowed to cool, must be tepid), and are then worked in a damp condition in a drum tumbler, by which they brighten in colour and become uniformly stuffed. They are then allowed to lie in a pile a day or two, are stoned and worked out with the sleeker, and hung up to dry. When in right temper they receive a final setting out with the sleeker, and when dry are either rolled or glassed. For further details, Nos. 256 and 257 of 'DerGerber,' 1885, must be consulted, where the matter has been exhaustively treated by Eitner, in his papers on "Extract-Gerberei."
Fig. 58.
Fig. 58.
In England, curried leathers are generally sold by weight, which leads to the use of glucose and other materials to add to the weight. In America, all upper leathers are sold by measure, and this is now ascertained by a very ingenious machine (Fig. 58). The skin is laid on a latticed table, and a frame, from which rows of bullets are suspended, is let down upon it. The total weight of the frame is indicated by a spring balance, and as the bullets which are over the skin are supported by it, the diminution of weight indicates the measurement. Several modified forms of this machine are now made.
ENAMELLED, PATENT, OR JAPANNED LEATHER.
Theseare terms used to designate those leathers, whether of the ox, the horse, the calf, or the seal, which are finished with a waterproof and bright varnished surface, similar to the lacquered wood-work of the Japanese. The name "enamelled" is generally applied when the leathers are finished with a roughened or grained surface, and "patent" or "japanned" are the terms used when the finish is smooth. Though generally black, yet a small quantity of this leather is made in a variety of colours.
In America, large thin hides are principally used for the purpose. They are limed and bated in the usual way, stoned after bating, and tanned with hemlock and oak barks in a paddle tumbler, which is run for 10-15 minutes in each hour. When one-third tanned, they are levelled on the flesh, and split with the belt-knife splitter,Fig. 52. After splitting, the portions are drummed with strong gambier liquor for1/4hour, and then tanned out with bark. The grains are scoured with the Fitzhenry or Lockwood machine (Figs.53and56). They are then lightly oiled and stretched on frames which can be enlarged by screws or a sort of knuckle-joint at the corners. When quite dry, they are grounded with a mixture of linseed-oil with white lead and litharge, boiled together and thickened with chalk and ochre. This is dried in closets heated by steam, into which the frames are slid face downwards, the heat being gradually increased from 80° to 160° F. (27° to 71° C.). If it be desired to employ a higher temperature, the leather is first saturated with a solution of 2 oz. each of borax and alum in 1 gal. water, when temperatures of 230°-250° F. (110°-120° C.) maybe used. The remaining treatment is much as above described, but a little turpentine is used to make the paint work freely. The final varnish is composed of 20 parts spirit of turpentine, 20 linseed oil, 10 thick copal varnish, and 1 of asphaltum or other colouring material. This must be mixed 2-3 weeks before use, and applied with a brush.
The splits are also often enamelled, and as a preparation receive a dressing of linseed-oil boiled to a jelly and thinned with turpentine or naphtha. This is applied with a stiff brush after the splits are stretched on the frames and are still damp, so that it does not penetrate the leather, but forms a sort of artificial grain.
Leather destined to be finished in this way requires to be curried without the use of much dubbing, and to be well softened. The English practice is to nail the skins thus prepared, and quite dry, on large smooth boards, fitted to slide in and out of stoves maintained at a temperature of 160°-170° F. (71°-77° C.), coating them repeatedly with a sort of paint composed (for black) of linseed-oil, lamp-black, and Prussian blue, well ground together. Each coating is allowed to dry in the stoves, before the next is applied. The number of coatings varies with the kind of skin under treatment, and the purpose for which it is intended. The surface of every coat must be rubbed smooth with pumice; finally, a finishing coat of oil-varnish is applied, and, like the preceding coats, is dried in the stove. The exact degrees of dryness and flexibility, the composition of the paint, and the thickness and number of the coats, are nice points, difficult to describe in writing.
This branch of the leather industry, so far as it relates to calf-skins, is carried on to a larger extent, and has been brought to greater perfection in Germany and France than in England. In the former countries, the heat of the sun is employed to dry some of the coatings. The United States have also brought this style to a high degree of excellence, especially in ox-hides. There, use is said to be made of theoils and spirits obtained from petroleum, and without doubt, French and German emigrant workmen have materially assisted in attaining this high standard.
Leather finished in these styles is used for slippers, parts of shoes, harness, ladies' waist-belts, hand-bags, &c., and has now maintained a place among the varieties of leather for a long period of years.
MOROCCO LEATHER.
Moroccoleather is produced from goat-skins. Rough-haired or "blue-back" seal-skins are also used, and produce an excellent article; while an inferior description, called "French morocco," is produced from sheep-skins. The skins are unhaired by liming in the usual way, and are then baited with a mixture of dogs' dung and water. The tanning is done chiefly with sumach, at first in paddle-tumblers, and then in handlers, lasting about a month in all. Sheep-skins are usually tanned through in about 24 hours, by being sewn up into bags, grain-side outwards, and nearly filled with strong sumach infusion. A little air is then blown in, to completely distend the skin, and they are floated in a sumach bath, and kept moving by means of a paddle. After the first day's immersion, they are thrown up on a shelf, and allowed to drain; they are then again filled with sumach liquor; when this has a second time exuded through the skin, they are sufficiently tanned, and the sewing being ripped open, they are washed and scraped clean, and hung up to dry, making what are called "crust-roans." The dyeing is sometimes done by brushing on a table, grain-side upwards, but more usually the skins are folded closely down the back, flesh-side inwards, so as to protect it as much as possible from the influence of the colour, and then passed through the dye-bath, which is now generally of aniline colours. The original oriental method of manufacture for red morocco was to dye with cochineal before tanning, and this is still customary in the East, but is quite obsolete in this country. A grain orpolish is given to the leather, either by boarding, or by working under small pendulum rollers, called "jiggers," which are engraved either with grooves or with an imitation of grain. A well-cleaned sumach-tanned skin is capable of being dyed in the finest shades of colour; and this branch of the manufacture of leather has been brought to great perfection.
RUSSIA LEATHER (Ger.,Juchtenleder).
Thisis tanned in Russia with, the bark of various species of willow, poplar and larch, either by laying away in pits, or handling in liquors, much like other light leathers, the lime being first removed by bating, either in a drench of rye- and oat-meal and salt, by dogs' dung, or by sour liquors. After tanning, the hides are again softened and cleansed by a weak drench of rye- and oat-meal. They are then shaved down, carefully sleeked and scoured out, and dried. The peculiar odour is given by saturating them with birch-bark oil, which is rubbed into the flesh-side with cloths. This oil is produced by dry distillation of the bark and twigs of the birch. The red colour is given by dyeing with Brazilwood; and the diamond-shaped marking by rolling with grooved rollers.
Much of the leather now sold as "Russia" is produced in Germany, France, and England. It is tanned in the customary way, occasionally with willow, but more generally with oak-bark, and probably other materials. Economy would suggest the use of such materials as, from their red colour, are objectionable for other purposes, and therefore cheap. The currying is in the usual manner, care being taken that the oil used does not strike through to the grain, which would prevent it taking the dye. The colour is given by grounding with a solution of chloride of tin (100 parts tin perchloride, 30 parts nitric acid, 25 parts hydrochloric acid, allowed to stand some days, and the clear solution poured off, and mixed with 12 volumes of water). The dye-liquor may be composed of 70 parts rasped Brazilwood, 3 parts tartar, and 420 parts water, boiled together, strained, and allowed to settle clear. The grounding and dyeing are done on a table with a brush or sponge (see Glove-kid dyeing,p. 229). The odour is communicated by rubbing the flesh-side with a mixture of fish-oil and birch-bark oil, which sometimes contains no more than 5 per cent. of the latter.
Pl. VII.E. & F. N. Spon, London & New York."INK-PHOTO." SPRAGUE & CO. LONDON.TREADING AND DYEING THE SKINS.
Pl. VII.
E. & F. N. Spon, London & New York.
"INK-PHOTO." SPRAGUE & CO. LONDON.
TREADING AND DYEING THE SKINS.
CHAMOIS OR WASH-LEATHER.
Thisleather, which is remarkable for its soft felty texture, which it retains even after wetting, although perfectly porous and free from greasiness in its finished state, is prepared by the action of oil on the raw skin. Wash-leather was formerly manufactured from sheep- and calf-skins, and from those of the chamois, and various deer (hence the name), from which, after liming, the grain was removed (frized) with a sharp knife, either with the hair, or after unhairing. The flesh-splits of sheep-skins are now generally employed for ordinary wash-leather, and of course no such process is needed, though buff-leather for belts and military purposes is still so manufactured. The skins receive a thorough liming, which, where softness is desired, is so conducted as very thoroughly to remove the cement-substance (coriin) from between the fibres; and this removal is frequently carried still further by a short bran-drench, which also secures the complete absence of lime. After the usual beam-work, the skins are pressed or wrung out to remove surplus water, and while still moist are oiled on a table and folded in cushions. Fish-, seal-, or whale-oil is generally used, and vegetable oils do not seem to answer even in mixture, with the exception perhaps of olive-oil. The skins are next stocked for 2-3 hours, shaken out, and hung up for1/2-1 hour to cool and partially dry. They are then again folded in bundles, and stocked for a short time, taken out, oiled again, and returned to the stocks; and this process is repeated, until the skins lose their original smell of limed hide, and acquire a peculiar mustard-like odour, and the water at first present has been entirely replaced by oil. The later dryingsare frequently conducted in a heated room, and when the oiling is complete, the skins are piled on the floor, and the oxidation of the oil, which has already commenced during the fullings and dryings above described, is completed by a sort of fermentation, in which the skins heat very considerably. During this process, they are carefully watched, and if the heat rises so high as to endanger the quality of the leather, the pile must be turned over, so as to cool the skins, and bring those which were originally outside to the centre. When the fermentation comes to an end, the skins are no longer susceptible of heating, and are of the well-known yellow or chamois colour. Where this colour is objectionable, the oxidation is sometimes completed by hanging the leather in a heated room instead of by piling. It is now necessary to remove the surplus oil, and this in France is done by oiling with any sort of oil, throwing into hot water, and wringing or squeezing. The oil obtained in this way forms themoëllonordégrasso much prized for currying purposes. The unoxidised oil still retained by the skins is removed by washing with soda or potash lye. In England and Germany, the whole of the uncombined oil is removed in this way, and is recovered from the lye, in which it exists in a partially saponified state, by neutralisation with sulphuric acid. It forms the "sod" oil of commerce. About half the oil employed is obstinately retained by the skin, and cannot be removed even by boiling with alkalies, while no gelatin is obtained by boiling water, to which the chamoised skin is much more resistant than ordinary leather. The nature of the tanning process does not seem to be well understood. It is generally stated that the fibres of the skin are unaltered, but are merely coated with the oxidised products of the oil. It is hard, however, on this hypothesis to understand their extraordinary indifference to water, even at a boiling temperature, which speedily converts kid and other tawed leathers into a solution which gelatinises on cooling; and it seems more probable to the present writer that some actual chemical combination isformed. Lietzman ('Herstellung der Leder,' p. 164) supposes that the whole of the gelatigenous tissue has been removed by liming and bating, and that only the very indifferent yellow elastic fibres (seep. 21) remain. This view, however, is quite untenable, in consideration of the very small proportion of these fibres originally present in the skin. Müntz, in his researches (seep. 17), showed that the fibres insoluble in boiling water scarcely exceeded 3 per cent, of the dried pelt. Dry gelatigenous fibre has a considerable resistance to heat, and it is possible that the action of the oil may consist in preventing the absorption of water. This, however, will not explain its resistance to alkalies. Cotton or other vegetable fibres moistened with oil, readily undergo oxidation, with so much evolution of heat as sometimes to cause spontaneous combustion; but the oxidation products are easily and completely removed by alkaline solutions, leaving the fibre in its original state, as indeed is noted by Lietzman (loc. cit.).
The finishing processes consist in staking during drying to retain softness, and in whitening and smoothing the flesh (or sometimes both sides) on the fluffing wheel. Skins for gloves, &c., are bleached like linen, by sprinkling and exposure to the sun; or more rapidly by treatment with a weak solution of potash permanganate, and subsequently with sulphurous, or very dilute sulphuric acid, to remove the brown manganous oxide formed (Barreswil, Dingl. Polyt. Jour., 161, 312). Gaseous sulphurous acid from burning sulphur may also be used for bleaching. The "dyeing" of chamois leather is generally done with ochres and similar colouring matters, and may be removed by washing. Treatment with egg-yolk in water, or with an emulsion of olive-oil with a little soap, and rubbing, or stretching, will restore softness to chamois leather which has become stiff by washing.
CROWN LEATHER, OR PRELLER'S LEATHER.
Theprocess of manufacture of this leather, which has obtained a firm position as the most suitable material for certain classes of belting, picker-straps, &c., was discovered about 35 years since by Theodor Klemm, a cabinet-maker in Wurtemburg and founder of the present well-known firm of leather manufacturers, Gebrüder Klemm of Pfullingen. Klemm, at that time in poor circumstances, sold his patent in Paris to an Englishman, Preller, who started a manufacture of it in Southwark and adopted a crown as his trade-mark. Since this time the manufacture has spread, first to Switzerland and then through Germany; but in England, to the writer's knowledge, it is confined to one or two firms.
The process of manufacture of crown leather is in principle intermediate between that of calf-kid (seep. 223), and the pure oil-tanning, if we may call it so, of which the chamois leather (seep. 210) is typical. It depends on impregnating the raw hide with a mixture of fats and albumens, to which salt or saltpetre is added to prevent putrefaction. The process as described in the original patent was as follows,—The hides were unhaired by liming or painting (with sulphides), and cleansed as usual, no plumping lime being given. After unhairing they were allowed to dry some little time in the air till no longer plump, and were then worked in a tumbler drum, without water, till uniformly soft. They were then spread on a table and brushed over on the flesh-sides with a mixture of 23 parts of ox-brain, 61/2of butter, 28 of soft fat, and 4 of salt or saltpetre, with 26 of barley-flour and 121/2of milk, of which the leading 4 ingredientswere first to be mixed and the flour stirred in, the milk being last added. The hides were then returned to the tumbler, which was provided with tubular axes, through which a portion of exhaust steam was admitted to warm the drum. After tumbling some hours, the drum was opened, and the hides were examined. If the tanning was not complete, the hides were hung in the air for a time to dry, and the process was repeated till a cut showed that the mixture had completely penetrated the hide.
From Eitner's researches it appears that the essential tanning ingredients of the mixture above described are the fat (and butter which acts simply as fat) and the albuminous matter of the milk (casein), brains (albumen, &c.), and flour (gluten); the starch serving at most to assist in the emulsification of the fats. Eitner treated crown leather with dilute potash solution to remove the albumen and fats, and after washing and drying obtained a material like an insufficiently stocked chamois leather. On again stuffing with a quantity of fat equal to that removed, but without the albumen, the leather became dark and quite greasy, so that by sharp bending oil could be pressed out. Good results may be obtained in crown leather manufactured with fats and flour only, without the use of milk or brains, so that it is obvious that the same purpose is served by either vegetable or animal albumenoids. The most important point in the purposes for which crown leather is employed is toughness, and this is given by the unaltered hide-fibres, which are merely preserved by the coating of oily matter with which, like those of chamois leather, they are surrounded. The albumen serves the purpose of filling the spaces between the fibres, and giving solidity and firmness, so that the belts may keep their shape, and not stretch inordinately. It also serves to make the leather waterproof, and fit it for water-bags for military purposes (as it gives no taste to the water) and for hose-pipes. The albumen, which much resembles the hide-fibres in composition, is like them preserved by the fats.
For the modern process of manufacture, good, even andwell-flayed hides are selected, and unhaired either by sweating, or by a very short liming, which must be assisted by rockers or some mechanical mode of moving the hides, so as to get them unhaired in the shortest possible time and with the least injury to the fibre. Sodium sulphide (seep. 147) may be employed with great advantage. The fleshing and scudding are performed as usual, according to the mode of unhairing adopted. The hides are then very commonly rounded, and the bellies tanned in the usual manner; but sometimes the whole hide is made into crown leather.
As crown leather is naturally almost white, it is usual at this stage to colour the hide with bark or other liquors. As in this case simply colouring and not tanning of the grain is required, high-coloured liquors, made by steaming materials with much colour and little tannin, are preferable. For this purpose wood extracts, such as chestnut, quebracho, or oak-wood are said to be very suitable, and beech, pine or alder bark may also be used. In practice, chestnut and hemlock extracts, and occasionally cutch are employed; but the last named is not to be recommended. A chestnut liquor of 71/2° Tw. or 5° B. (34° Bark.), with constant handling or in a paddle-tumbler, will give a satisfactory colour and grain in 1-2 hours. This rapid colouring is preferable to the slower process, which occupies 24 hours in weaker liquors. If sweated, the hides are now plumped with sulphuric acid, but only to a very moderate extent. This process is best performed in a paddle-tumbler; about 31/2oz. of sulphuric acid are required per hide, and a time of 6-12 hours according to the water employed. The liquor may be several times used, strengthened with the necessary quantity of acid. Limed hides do not require further swelling. The hides are washed through clean water, and hung up to dry somewhat.
The hides are next spread on a table, flesh-side uppermost, and covered with a layer of the tanning paste nearly1/4in. thick. The composition of this paste may be varied according to the relative prices of different materials, and the amount of hard fats must be regulated according towhether or not appliances are provided for heating the tumbler. A good mixture is 7 parts common wheat-flour, 7 of horse-grease, 1 of salt, and 1-2 of tallow. If too soft, more tallow may be employed. The salt is first added to the horse-grease, then the melted tallow, These fats are added little by little to the flour till a uniform paste is obtained. Another good mixture is 27 parts wheat-flour, 25 of bone-grease, 4 of tallow, and 4 of salt. Another recipe gives 28 lb. fine white flour made to a paste with 13-14 pints water and then worked up to a uniform mass with a tepid mixture of 28 lb. beef tallow and 28 lb. hard horse-fat (Pferdekammfett). These mixtures are all for use in warmed drums; a specimen of one used in a factory where the mixture was simply trodden in cold into the leather in open tubs is as follows:—7 parts flour, 9·4 of horse-fat, 2·8 of fish-oil, 7 of ox-brains and 0·7 of salt. The hides are next folded in bundles and placed in the drum; or in stocks, which are occasionally used for the purpose. If a drum be used, it must be of large diameter, 8-9 ft., provided with pegs inside, and should make about 25 revolutions per minute, so as to work the hides with considerable force. Much more care is needed in warming the drum, than is required in ordinary stuffing, and this is best accomplished by warmed damp air. This may be arranged by the use of an air-pump, which draws air through water warmed by exhaust steam, and forces it through the hollow axles of the drum (or drums); or a simple aspirator consisting of a cask filled with water may be connected to one axle, so that as the water runs out it will draw air through the drum from the opposite axle, which is connected with a cask half filled with hot water through which air is allowed to bubble. Probably the same effect could be reached in a still simpler and cheaper manner by the use of a steam-jet blower, such as Körting's. In any case the drum must be warmed to a temperature of 82°-104° F. (28°-40° C.). Warm dry air may also be used, but is not so suitable, as it dries the hides too much. The hides are tumbled 8-12 hours, hung up till half-dry, and theprocess is repeated. For very heavy hides, 4 tumblings may be required. In the later tumblings, a lower temperature, 95° F. (35° C.), may be employed, and the time extended to 15 hours.
The currying of crown leather is very simple. It is set out on flesh and grain, and boarded to raise the grain. Mossner, before currying, washes 2 hours in water and brushes with tepid soda solution (1 in 60). The yield of weight is small, only amounting to about 30-40 per cent. of the raw hide employed, and hence the price per lb. must be considerably higher than that of tanned leather to yield a profit. The above information is mostly drawn from articles by W. Eitner ('Der Gerber,' iv. 1et seq.) and Franz Kathreiner ('Gerber Zeitung,' 21st December, 1875).
MINERAL-TANNED LEATHER.
Theinvention of the earliest form of mineral tanning, that with alum and salt, dates from remote antiquity; but as it is in large measure the type of all that has been since done, it deserves examination in some detail, at least as regards principles. In practice it is used alone in curing skins with the hair on, and for making white leather for laces and other purposes; and, in combination with oil and albumen, which, as we have seen, are the tanning agents in the case of "crown leather," it forms the process for producing calf and glove kids, as will be described under those headings (pp.223,225).
Careful researches by Reimer (Ding. Polyt. Jour., 205, p. 143et seq.) show (what has long been known in practice) that alum alone is not capable of making a pliable leather. The salt, nevertheless, does not enter into combination with the alum, or even with the hide. Its function is partially physical, increasing the diffusion of the solution, and partially chemical, as in the presence of acids (and salts of acid reaction) it precipitates the coriin, and prevents it from gluing the fibres into a horny mass as it dries. Prof. Knapp has shown that this is the first essential in producing leather, and that raw hide may be converted into a pliable material with all the properties of white leather by simply withdrawing the water with alcohol, in which coriin is not soluble, and by which it is therefore precipitated. This leather, containing when dried no added constituent, is of course at once reconverted into raw hide by soaking in water. Both the salt and a portion of the alumina is removed from tawed leather by soaking in water, and it then dries hard andhorny, and by boiling in water will yield a considerable percentage of gelatin. The alum is not absorbed as a whole. It is a double salt (alumina and potash sulphate or alumina and ammonia sulphate), and only the alumina sulphate is absorbed, potash (or ammonia) sulphate accumulating in the liquor. The alumina salt retained by the hide, especially in presence of much salt, contains slightly more than its normal proportion of alumina to acid, or in chemical language is to some extent basic. This is caused partly by the lime remaining in the skin from the unhairing process, which neutralises a portion of sulphuric acid, but in part is the result of the affinity of the hide-fibres for alumina, a certain small proportion of free sulphuric acid being left in the liquor. The accumulation of this and of potash sulphate is the reason why such liquors cannot be used perpetually by mere strengthening with alum, but must be frequently renewed. The attraction of hide-fibre for alumina sulphate is so strong, that in presence of a sufficient excess of hide it may be completely removed even from dilute solutions. Alumina acetate or sulphate may be substituted for alum with equally good results in practice, the only advantage of the latter being its easier preparation. Ferric and chromic salts and iron or chrome alum, may be substituted for common alum, and are absorbed in a similar manner, and in presence of common salt give equally pliable leathers, of a buff and pale greenish tint respectively. Without salt, the leathers are hard and brittle. In all these cases, the tanning agent may be to a large extent removed by simple washing with water. The tannage may be rendered more durable by passing the leather before drying through a weak bath of sodic carbonate or even lime-water, which precipitates the alumina, iron, or chrome in a basic form on the hide-fibres. Soap baths may also be used, by which aluminic, ferric, or chromic stearates and oleates are formed, possessing considerable toughness and resistance to water. So far as the writer is aware, no mineral tannage has yet been produced which will not yield gelatin when treated, first with dilute acid and then with boilingwater; but this is rather a gain than otherwise, as leather scraps might be utilised for glue. There seems no reason why good and durable leather, for boot-uppers and for many mechanical purposes, should not be fabricated with salts of iron and chromium in conjunction with salt. If eggs and flour were also used, products similar to calf-kid would be obtained. Iron-leathers may of course be blacked with infusions of galls or many tanning materials, or with logwood. Ferrous salts have no tanning properties.
If, instead of using neutral iron salts, basic ferric salts (which may be obtained by dissolving ferric oxide in solution of neutral ferric salts, or by oxidising ferrous sulphate with manganese black oxide, or nitric acid) be employed, much larger quantities are absorbed by the hide, and if this be fixed with soap baths and finished with a moderate quantity of oil, a gain of weight—approaching 50 per cent. of the finished leather, or about the same as that given by bark, may be obtained. The leather, however, has by no means the same resistance to wet and decay as bark-tanned leather, and invariably has a tendency to crack when sharply bent. The process has been most carefully worked out by Professor Knapp, and was patented and worked commercially for a short time in Brunswick, but apparently without financial success. Professor Knapp's method is as follows:—The iron solution is prepared by adding nitric acid to a boiling solution of ferrous sulphate (green vitriol) till the iron is completely oxidised to the ferric condition. To this, ferrous sulphate is again added so long as it continues to cause effervescence. The resulting solution is a clear orange, and of more or less syrupy consistence, and may be evaporated without decomposition or crystallisation to a transparent varnish. The hides are unhaired and prepared for tanning in the usual way, and are then handled in solutions of the iron salt, which are at first weak, and are gradually strengthened. Skins are tanned in 2-3 days, and the heaviest hides in a week. After tanning, the hides are stuffed in a drum ventilated through the axes, very similar to thatdescribed under "crown leather,"p. 213, with an insoluble iron-soap made by precipitating soap solution with the iron-liquor; or the iron-soaps may be formed in the hide by the alternate use of iron and soap solutions, as already described. The leather is finally saturated with a solution of stearin and paraffin, to render it waterproof.
A process which has been worked on a larger scale, is that of Dr. Heinzerling, introduced about 1878, with the usual promise of "complete revolution" in the leather trade; but which, in spite of the most determined and persevering efforts of the Eglinton Chemical Company, who own the English patent, has failed to take any very prominent position in commerce. The tanning materials employed are alum and salt, with a varying proportion of potash, soda, or magnesia bichromate. These salts have a very marked hardening effect on animal tissues, and, when mixed with gelatin and exposed to light or acted on by acids, become reduced, and at the same time render the gelatin insoluble even in hot water, a property which is made useful in many photographic processes. This is probably due to the formation of salts of chromium, which, as has been stated (p. 219) have a similar tanning effect, but perhaps more powerful, than those of alumina. However this may be, the effect of potash bichromate when exposed to light with gelatin, differs from that of the addition of chrome salts ready formed, the gelatin in the first case becoming incapable of even swelling in hot water, while in the second, though rendered insoluble, it becomes soft and swollen. The use of potash bichromate in tanning had been previously patented by Cavalin, who used it in conjunction with alum and salt, and with the addition of a portion of green vitriol, to give the leather a colour more similar to that of bark-tanned.
Dr. Heinzerling uses metallic zinc in the salt and alum solution to assist in the precipitation of amorphous alumina on the hide-fibres. The same material was used in a similar way by Jennings (No. 2295, 1861), but with the object of whitening the goods. Yellow or red prussiates of potash(potassic ferrocyanide or ferricyanide), are also sometimes mixed with the solution, in order to enable the leather to be blacked with iron-liquor, with which they produce prussian blue. To fix the tannage on the fibre, and prevent its washing out, the use of barium chloride, lead acetate, and of soap solution is claimed; the latter having been also patented for similar purposes by Knapp, and subsequently by Jennings and others.
In order to render the leather waterproof, it is finally saturated with solutions of paraffin, stearin, and other fats and hydrocarbons (resin is employed, though not named in the patent), in petroleum spirit and similar solvents.
Such is the original patent, which, it will be seen, is rather a combination of older processes than an original discovery. Whether it is still worked on the same lines the writer is unable to say, but he is aware that considerable improvements have been made in the finish and appearance of the goods. The leather in its present form possesses considerable resistance to water, is free from the brittleness so common in mineral tannages, and like other alumed leathers, considerably exceeds bark-tanned leather in toughness and elasticity. These make it valuable for many purposes, and among others, for machine-belting, although it has the disadvantage of elongating considerably while in use.
CALF-KID.
Calf-kidis used for light upper-leather, and belongs to a different class from any yet described, being "tawed" instead of tanned. In this respect, and in most details of its manufacture, it resembles glove-kid.
The process is as follows. Selected calf-skins, dried or salted, are the raw material, and after a suitable softening in fresh water, are limed for 2-3 weeks, or till the hair goes easily. They are then unhaired and fleshed in the usual manner, pured with a bate of dogs' dung, scudded, and again cleansed with a bran drench. In Germany, the bran drench is used alone, and is composed of 33 lb. bran to 100 medium skins. Before use, the bran should, especially in summer, be well washed, to free it from adhering meal. The temperature of the drench should not exceed 100° F. (38° C.), and the skins should remain in for 8-10 hours. Lactic acid is produced by fermentation; this removes lime, and is itself neutralised by the products of putrid fermentation which succeeds it.
The tanning is accomplished in a drum with a mixture of alum and salt; and after drying, the skins are again moistened, and worked in the drum with a mixture of oil, flour, and egg-yolk. In the German method, these two operations are combined. Eitner, who has written a series of articles on the process, gives 40 lb. flour, 20 lb. alum, 9 lb. salt, 250 eggs, or about 11/3gal. of egg-yolk,7/8pint (1/2litre) of olive-oil, and 12-16 gal. water, as a suitable mixture. The skins are worked in a drum-tumbler (preferably a square one, seePlate 5) for 20 minutes, then allowed to rest 10 minutes, and this process is twice repeated. The temperature must not exceed 100° F. (38° C.), and it is saidto be important that the drum should be ventilated by holes at the axis.
The skins are allowed to drain, are then rapidly dried at a temperature of 140°-160° F. (60°-71° C.), and, after "samming," or damping with cold water, are staked by drawing them to and fro over a blunt knife fixed on the top of a post (seePlate 6). They are then wetted down and shaved, either with the moon-knife or ordinary curriers' shaving-knife, and sometimes receive a second dressing of oil, flour, and egg, to soften them still further.
Dyeing black is accomplished either by brushing on a table, or by "ridging" or folding, grain-side outwards, and drawing quickly through baths of the mordant and colour. To prepare them for the colour, stale urine is generally employed. A deeper colour, and one less liable to strike through the skin, is obtained by adding1/4lb. potash bichromate to 4 gal. of urine, or the following mixture may be substituted with advantage, viz.1/2lb. Marseilles soap dissolved in boiling water, 5 or 6 egg-yolks added, and the whole made up to 4 gal. with water and1/4lb. potash bichromate. The colour used is infusion of logwood or its extract, or two-thirds logwood, which is best extracted by stale urine or old soak-liquor, with addition of a small quantity of soda (1 lb. to 25 lb. dye-wood). It is fixed and darkened by a wash of iron-liquor (1 of iron protosulphate in 75 of cold water). After being again dried, the skins are grounded with the moon-knife, and rubbed over on the grain with a composition containing oil, wax, &c., and are finally ironed with a flat-iron, to give them a fine and smooth surface. Eitner gives a recipe for the gloss:—1 lb. gum arabic,1/2lb. yellow wax,1/2lb. beef-tallow,3/4lb. Marseilles soap, 2 lb. strong logwood infusion, and 1 gal. water. The water is brought to a boil in an earthen pot, and then the soap, wax, gum, and tallow are added successively, each being stirred till dissolved before adding the next, and lastly the logwood. After boiling for an hour, it is allowed to completely cool, being incessantly stirred during the whole process.
GLOVE-KID.
Thisbranch of leather manufacture is mainly carried on in Germany, Austria, and France. In Germany and Austria, lamb-skins are principally employed; in France, kid-skins. For fine gloves, the skins of very young animals only can be used. The ordinary style of manufacture is as follows:—The soaking of the dried skins is effected in large wooden tubs (Kufen,Bottichen), and occupies on the average 3-4 days, according to the character of the soak-water, the size of the skins, and the time they have been stored. The skins, when thoroughly and uniformly softened, are unhaired, either by painting the flesh-side with a thin paste of lime, or in lime-pits. In unhairing by painting (Schwöden), the skins, after coating the flesh-side with lime, are folded together, so that the lime comes as little as possible into contact with the wool, and these bundles or "cushions" are placed in a tub, in which they are most frequently covered with water. After unhairing on the beam with a blunt knife, the skins must be limed for some days, in order that the leather may stretch well, a quality which the Germans denominateZug. By this method of unhairing, the wool is preserved uninjured, but it is not suitable for the finer sorts of leather. The unhairing in lime-pits is done either with gas-lime (Grünkalk), or, as is now almost exclusively the practice, with the so-called "poison-limes" (Giftäscher). These are prepared by mixing red arsenic (arsenic sulphide) with lime, while it is being slaked, and is at its hottest. The calcic sulphydrate (and perhaps sulpharsenite) thus formed hastens the unhairing, and gives the grain a higher gloss. Well-conducted establishments now avoid as much as possible the use of old limes, whichproduce a loose, porous leather, with a rough, dull grain. The liming lasts on the average 10 days, and is of the greatest importance. It is essential that the interfibrillary substance shall be dissolved, that the leather may have the quality known asStand, that is to say, may be strongly stretched in either length or breadth without springing back. It also depends upon the liming (and this is of special importance in the case of lamb-skins), whether the tissue of the fat-glands is well loosened, so that the fat, either as such, or as lime- or ammonia-soap, may be readily and completely worked out. Skins in which this is neglected can never be properly dyed.
When the hair (or wool) is well loosened, the skins are rinsed in water, and then unhaired on the beam with a blunt knife. The water employed in washing should not be much colder than the limes, or it will prevent the hair from coming away readily. The wool or hair is washed and dried for sale. The skins are thrown into water, to which a little lime-liquor has been added, to prevent precipitation of the lime in the skins by the free carbonic acid of the water, which would have the effect of making them rough-grained.
Next comes the first fleshing (Vergleichen) or "levelling." By this, the loose cellular tissue on the flesh-side is removed, together with the head, ears, and shanks, and the flanks are trimmed. The skins are then again thrown into water, softened with lime-liquor as above described, and then into a bate of dogs' dung. This is prepared by stirring up white and putrid dogs' dung with boiling water, and straining it through a sieve or wicker basket. The bate must be used tepid, and not too strong. The skins "fall" (lose their plumpness) in it rapidly, and become extremely soft and fine to the touch; and the fat-glands, remaining hairs, and other dirt, can now be very readily scudded out. So far no completely satisfactory substitute has been found for this somewhat disgusting mixture, but it has been noted that guano will produce similar effects. With regard to the mode of action of the dung bate, much has been speculated withoutproof, and exact analytical evidence is wanting; but, no doubt, a weak putrefactive action goes on, as may be deduced from the presence ofbacteria; further, the ammonia and weak organic acids present in the putrefying dung are capable of acting on fat and lime; and finally, a direct mechanical effect seems to be produced, difficult to describe, but favourable to the succeeding manipulation. Too strong bates, or too long continuance in them, produces evident putrefactive effects on the skins. (See alsop. 184.)
When the skins come out of the bate, they are stretched and worked (abgezogen) on the flesh with a sharp knife, and any remaining subcutaneous tissue is removed. This constitutes the second fleshing. They are then rinsed in warm water, and beaten with clubs (Stoss-keule), see Plates 3 and 4, in a tub, or worked in a tumbler-drum (Walkfass), in either case with a very little water only; and finally brought into a tank of water, not too cold, and kept in constant motion with a paddle-wheel.
The skins are next cleansed on the grain-side by working on the beam with plates of vulcanite with wooden handles, so as to remove fat, lime- and ammonia-soaps, and other lime compounds, together with all remaining hair or wool. The skins are now a second time washed in the "paddle-tumbler," first in cold, and then in tepid water; and after allowing the water to drain from them, they are transferred to the bran drench.
This is prepared by soaking wheaten bran in cold water, diluting with warm water, and straining the extract through a fine hair-sieve. Sufficient of the liquid must be employed to well cover the skins, and the temperature may range from 50° F. (10° C.) to 68° F. (20° C.). These conditions are favourable to bacterial activity, which comes into play, and, on the one hand, evolves formic, acetic, lactic, and butyric acids, which dissolve any remaining traces of lime, and on the other, loosens and differentiates the hide tissue, so as to fit it to absorb the tawing solution (Gare). Much care is required in the management of the bran drench, especiallyin summer, since the lactic readily passes into the butyric fermentation (see alsop. 186). The tawing mixture is composed (like that employed in the fabrication of calf-kid, q. v.) of alum, salt, flour, and egg-yolks, in a quite thin paste. The skins are either trodden in it with the feet, or put into a tumbler-drum with it (Fig. 48). Kathreiner pointed out, some years since (in vol. i. of 'Der Gerber'), that a mixture of olive-oil and glycerine might be partially substituted for the egg-yolks, in both the tanning and dyeing of glove-kid leather.
The tawed skins are now dried by hanging on poles, grain inwards. Rapid drying in well-ventilated, but only moderately-heated, rooms is essential to the manufacture of a satisfactory product.