H. R. Procter

“ It is obvious that if the American authority is right, the English process is wasteful in the extreme, both in hide-substance and lime. Much depends on the amount of hauling which the hides receive, and the more frequently they are moved the better. It is probable, however, that it would be found impossible to unhair and flesh hides, to suit the English market, in cold limes with the quantity and time mentioned, and if the limes are steamed, it is quite likely that the destructive action on the pelt may be even greater than by the longer and slower process in the cold. ”

“ 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. ”

“ Air-drying is dependent on the condition that the air must be capable of taking up more moisture than it already contains. It is a matter of common experience that there are warm days when the air is so saturated with moisture in the form of invisible vapour, that scarcely any drying takes place; and similarly, cool dry days, when leather dries rapidly. The relative amount of moisture in the air is easily ascertained by the simple instrument known as the wet and dry bulb hygrometer; an instrument which ought to be in every drying-shed, especially where steam heat is used. ”

“ 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." ”

“ To do useful work, it is not necessary to possess a very elaborate or expensive instrument, but it is essential that the microscope be well made and good of its kind. As high powers are often required in the examination, both of hide sections and of ferments, which are the principal objects of investigation in a tannery, it is of the first importance that the fine adjustment should be perfectly steady, without vibration or backlash. ”

“ 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 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. ”

“ Another invention, of which we have as yet heard little definite in England, consists in drying at a low temperature by air artificially deprived of its moisture. This may be accomplished in several ways. Experiments have been made in drying in a closed chamber provided with trays of calcium chloride to absorb the moisture evaporated. Air when artificially cooled by compression and subsequent expansion, as in the case of ice-making machines, parts with a large portion of its moisture, which is condensed in the form of ice in the tubes of the machine. ”

“ 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. ”

“ Before speaking of actual processes of manufacture, it is necessary to devote some attention to the structure and chemical constitution of hide or skin, which forms the raw material. Although a great variety of skins are employed in tanning, they are all constituted on the same general type, and an anatomical description of the hide of the ox will apply almost equally to those of the calf, sheep, and goat ”

“ We may, therefore, divide tannins broadly into derivatives of catechol, which yield no bloom, and usually give greenish-blacks with iron acetate, and which include hemlock, mimosa, cutch, gambier, quebracho, &c; derivatives of pyrogallol, which give bluish-blacks with iron, deposit bloom in leather, and embrace galls, sumach, divi-divi, myrobalans, pomegranate rind, &c., and tannins which contain both pyrogallol and catechol, such as oak-bark and valonia, and which, as is well known, yield bloom, and give blue-blacks with iron. ”

“ 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 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. ”

“ A hard water probably softens dried hides more slowly, though it is possible that the observed difference may be due in many cases to the lower temperature of wells from which hard water is generally derived. In the actual limes, the hardness of the water can have no appreciable influence, though if sodium sulphide be used alone, a certain waste occurs from temporary hardness, which may render it advisable to add a little lime. It is in the washing of the hides from lime that the influence is first distinctly felt. ”

“ An old lime becomes charged with decomposing animal matter and with ammonia, and, within limits, loosens the hair more effectually than a new one. An experienced tanner states that, by using old limes, better weights are obtained, but that the leather is thinner than when a fresh portion of lime is used; and this is quite possible. If, however, the old lime-liquor be retained too long, it ceases to swell the hides as it should, and, in warm weather, the liming proper is complicated by a putrefactive process allied in principle to sweating. ”

“ To prevent freezing, and to enable leather to be dried in damp or cold weather, it became customary to provide sheds with ranges of steam-pipes on the floor; this, though decidedly a valuable addition, has not proved by any means an entirely satisfactory solution of the problem of leather drying. No sufficient means are provided for controlling the ventilation, and the upward currents of hot air dry the leather irregularly, and produce bad colour. A much more satisfactory shed is the American turret drier. ”

“ 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. ”

“ If hide-powder be employed, it must be moistened with a small quantity of water before adding to the infusion, and this water must be taken into account in the quantity of the filtrate employed for the titration of the "non-tannin." The digestion with the hide- or bone-powder must be continued till the filtered liquid does not give the faintest clouding with a drop of clear gelatin solution, and it is always very difficult to be sure that the tannin is so completely removed as with gelatin and salt. ”

“ In olden days, it was customary not only to wash the hides much more thoroughly in water, but to "scud" them (i.e. work them over with a blunt knife) , to remove lime, and the detritus of hair-roots and fat-glands, and this should never be omitted from sole-leather treatment where bright colour and clean buff are desired. Some tanners go so far as to bate best butts slightly with hen-dung, but with such treatment firmness and weight are lost. Washing in weak solution of sugar, or ammonic chloride or sulphate, or of sulphuric, or hydrochloric acid, may be adopted. ”

“ 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. ”

“ Oxalic acid is the easiest to make of any. A sufficient quantity of pure crystallised oxalic acid is powdered and pressed between filter paper, so as to absorb the moisture which occasionally is retained in cavities of the crystals. 6·3 grm. is then weighed out and dissolved in water, exactly as was done with sodic carbonate, forming a decinormal solution. It is used in Löwenthal's tannin estimation process and may also be employed to determine alkalies, but forms insoluble calcium oxalate with lime salts, and does not give a sharp reaction with methyl orange indicator. ”

“ The hides are rather difficult to flesh, unless previously plumped by a light liming, and it is necessary to swell them with acid or sour liquor in the tanhouse, as the sulphide has but little plumping effect.
Another method, which is much cheaper in labour and easier in execution, is to suspend in a solution of sodium sulphide, containing 3/4 lb. a hide or upwards; the hide should unhair in 24 hours. Very weak solutions loosen the hair, without destroying it; but it is always weakened, as the specific action of the sulphides is on the hair itself. ”

“ 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. ”

“ Of scarcely less importance than the water supply is the drainage of the yard. It not unfrequently happens that tanneries are prohibited from discharging their refuse liquors, limes, and soaks into rivers and watercourses, and it is sometimes a matter of extreme difficulty to find any other way of getting rid of them. In default of an outlet, recourse must be had to precipitation and filtration, but this is a costly expedient, and in fixing a site for a new yard it is far better to provide against such a possible contingency. ”