CHAPTER IX.

[R]1grm.perlitreis very approximately equal to 1 oz. per cub. ft.

The action of lime on the hide has already been spoken of to some extent. It is throughout a solvent one. The hardened cells of the epidermis swell up and soften, therete malpighiand the hair-sheaths are loosened and dissolved, so that, on scraping with a blunt knife, both come away more or less completely with the hair (constituting "scud," as some English tanners name it, Ger.gneistorgrund). The hairitself is very slightly altered, except at its soft and growing root-bulb, but the true skin is vigorously acted on. The fibres swell and absorb water, so that the hides become plump and swollen, and, at the same time, the "cement-substance" (coriin) is dissolved, the fibres become differentiated into finer fibrils, and the fibrils themselves become first swollen and transparent, and finally corroded, and even dissolved. This swelling of the fibres is produced both by alkalies and acids, and is probably due to weak combinations formed with the fibre-substance, which have greater affinities for water than the unaltered hide. It is useful to the tanner, since it renders the hide easier to "flesh" (i. e. to free from the adhering flesh), on account of the greater firmness which it gives to the true skin. It also assists the tanning, by opening up the fibre, and so exposing a greater surface. This is advantageous in dressing leather which is afterwards tanned in sweet liquors, and must have the cement-substance dissolved and removed for the sake of flexibility; and, in the case of sole-leather, it is necessary for the sake of weight and firmness that the hide be plumped; but it is probable that the effect is produced with less loss of substance and solidity by suitable acidity of the liquors. A more certain advantage of lime is that it acts on the fat of the hide, converting it more or less completely into an insoluble soap, and so hindering its injurious effects on the after tanning process, and on the finished leather. If strong acids are used later on, this lime soap is decomposed, and the grease is again set free. In sweated or very low-limed hides this grease is a formidable evil.

The customary method of liming is simply to lay the hides flat in milk of lime in large pits. Every day, or even twice a day, the hides are drawn out ("hauled"), and the pit is well plunged up, to distribute the undissolved lime through the liquor. The hides are then drawn in again ("set"), care being taken that they are fully spread out. How much lime is required is doubtful, but owing to its limited solubility, an excess, if well slaked, is rather wasteful thaninjurious. Great differences exist in the quantity of the lime used, the time given, and the method of working. Lime, as we have seen (p. 140), is only soluble to the extent of about 1·25grm.perlitre, or (as 1 cub. ft. of water weighs about 1000 oz.) say 11/4oz. per cub. ft., or, in an ordinary lime-pit, not more than1/4lb. per hide. Only the lime in solution acts on the hide, but it is necessary to provide a surplus of solid lime which dissolves as that in the liquor is consumed. Jackson Schultz prescribes 1 bush. (56 lb.) of fresh lime to 60-70 hides, and 3-4 days as sufficient time to unhair and plump them; while a well-known English tanner states that, after working for 6-10 days through a series of old limes, the hides (presumably wet-salted South Americans) should have 4 days in a fresh lime, made with 3-12 lb. of lime per hide. 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. Most likely a compromise between the two is the most desirable, but about 2-4 lb. of lime per hide, according to weight, should be amply sufficient; while a week for market hides, and 14 days for heavy salted, will loosen the hair and plump the pelt as much as is requisite. This is on the supposition that the limes are kept at a uniform average temperature of about 60° F. (15° C.) in winter and summer. If they are heated to 80°-90° F. (27°-32° C.), of course much less time is required; but there are no published experiments showing the relative weights made by the two processes, and, from the fact that warmed limes are principally used for descriptions of leather where weight and solidity are not of primary importance, it may be concluded that, in this direction, theresults are unsatisfactory. Hides do not plump in warm limes.

Another undecided point is whether the best results are obtained by making fresh limes for every pack, or by strengthening up the old ones. 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.

Beside considerable quantities of ammonia, old limes contain tyrosin, leucin or amidocaproic acid, and some caproic acid, the disagreeable goaty odour of which is very obvious on acidifying an old lime-liquor with sulphuric acid, by which considerable quantities of a partially altered gelatin are at the same time precipitated. Very old limes, especially in hot weather, often contain active bacteria, which may be seen in the microscope under a good1/4-in. objective. Their presence is always an indication that putrefaction is going forward, and leather out of such limes will generally prove loose and hollow-grained. Spherical concretions of calcium carbonate may also be seen under the microscope, resembling on a smaller scale those found in Permian limestone, and caused perhaps in both cases by crystallisation from a liquid containing much organic matter. It is probable that in many tanneries the ammonia would pay for recovery from the lime-liquors, which would be easily done by steaming the old limes in suitable vessels, and condensing the ammoniacal vapours in dilute sulphuric acid. (Some appliances suitable for this purpose are described in the Journal of the Soc. of Chem. Industry, iii. 630.) For methods of estimation of ammonia, seep. 103.

Several variations in the above-described method of liminghave been proposed. The hides may be suspended on laths, or by strings attached to pegs or notches, and the liquor agitated by plunging in place of hauling. Probably this is an actual improvement, especially if some mechanical agitating contrivance be substituted for hand plunging. It has, however, the drawback that much room is required, though this may be, to some extent, compensated by the hides liming more quickly. The method has been long in use in America, and had been tried in several places in England before the patent of Messrs. Conyers and Pullein was obtained. Two other American labour-saving methods in connection with liming may be mentioned here. One is to have the liming-vat double the ordinary size, and, instead of hauling the hides, to simply draw them from one side to the other by two strings, which are attached to the fore and hind shank of each hide, either by sharp iron hooks or by loops. The strings are looped over iron rods at the four corners of the pit, or have simple knots, which are placed in notches sawn in wood. Of course, while the hides are at one side of the pit, the other side may be plunged or warmed. The other method (Fig. 24) is to have a spindle sunk below the surface of the liquor, and with discs A, at each end, to which the hides or sides are attached by hooks set round the edges. The hides are turned over by revolving the spindle with a handspike inserted in the holes C, at the ends of the cross-arms B, and the whole spindle is also capable of being raised and lowered in the liquor, in the slot D. In Germany, hides are frequently suspended on laths radiating from a central upright revolving spindle in a round vat (Drehkalk).

An American plan, sometimes known as the "Buffalo method," is described by Jackson Schultz. The hide is prepared in the usual way, and is then thrown into a strong lime for 8-10 hours, when it is taken out and immersed in water heated up to 110° F. (43° C.), in which it remains 24-48 hours. The warm water soaks, softens, and swells the roots of the hair, and much the same result is obtained as in "scalding" pigs. So little lime really permeates the inner fibre that, aftera slight wheeling, the hides may be thrown into cold water, and allowed to cool and plump, preparatory to taking their places in the handlers. The process is strongly recommended for sole-leather, particularly where great firmness of fibre is desired. The tanner who tries it must be satisfied if he gets 20-30 sides a man unhaired and fully ready for the liquor per diem. Of course this process may be varied to any extent by giving more liming, and less hot water, and this is frequently done in America. About 3-4 days' cold liming in good limes, and with hauling if possible twice daily, followed by 12-24 hours in water at 86°-95° F. (30°-35° C.), which should be changed at least once, will give good results. The hides are of course less plump than usual, but if properly managed in the handlers will swell well in the tan-house. Grease is obviously less thoroughly "killed" than in the ordinary method, and especial care must be used that the hides are well worked on the beam, both on grain and flesh. In this method, and indeed in all liming processes, much is gained if the fat can be fleshed off green.

Fig. 24.

On the Continent and in America, the prevalent mode of loosening the hair, at least for sole-leather purposes, is called"sweating," and consists in inducing an incipient putrefaction, which attacks the soft parts of the epidermis and root-sheaths, before materially injuring the hide-substance proper. The old European method of "warm-sweating" consisted simply in laying the hides in pile, and, if necessary, in supplying heat by covering them with fermenting tan; but as this crude and dangerous process is everywhere being supplanted by the American plan, where sweating at all is adhered to, it is not necessary to do more than describe the latter. This is called "cold sweating," but really consists in hanging the hides in a moist chamber, kept at a uniform temperature of 60°-70° F. (15°-21° C.); or in some cases slightly warmer.

The "sweating-pit" now in use is sometimes of wood, but usually consists of a building of brick or stone, protected from changes of temperature, both above, and at the sides, by thick banks of soil or spent tan. If soil be used, it will form an excellent bed for vines, &c., which are fertilised by the ammonia penetrating from below, which is evolved in large quantities and which assists the unhairing process by its action on the epidermis.[S]Though called a "pit," it is undesirable that it should be actually below the level of the ground, but should be arranged so that the hides can be wheeled in and out in barrows. It is lighted and ventilated by a lantern roof above a central passage, and should be divided into chambers, each capable of suspending a pack of hides. By means of sprinklers above and steam-pipes below, the chambers may be cooled or warmed, as required, and the air kept so moist that globules of condensed water collect on all parts of the hides, which are suspended from tenterhooks.

[S]Hides have been unhaired by the action of gaseous ammonia alone, but the method does not seem suited for technical use.

The process is principally used in America for dried hides, but may be employed either for wet or dry salted, after complete removal of the salt. It is imperatively necessary that dried hides should be completely softened before sweating. As the sweating process advances more rapidly in the upperthan in the lower part of the pit, and as the thick portions are more resistant than the thin ones, the hides, after about 3 days' sweating, require constant attention in changing their positions, and in checking the forward ones by taking down and laying in piles on the bottom of the pit.

The usual treatment for sweated hides, when the hair is sufficiently loosened, is to throw them into the stocks, and work out in this way the slime and most of the hair. This has the disadvantage of working out too much of the dissolved gelatin, and of fulling the hair so firmly into the flesh, that it is difficult again to remove it. To overcome these evils, some American tanners now pass the hides, after sweating, through a weak lime. This, to a great extent, prevents the hair fixing itself in the flesh, and tends to counteract the injurious effect of the vitriol (which is almost invariably used in plumping sweat stock) on the colour of the leather. By this process, 10,000 Texas and New Orleans wet-salted hides gave an average yield of leather of 73 per cent. on their green weight, and the leather was excellent in quality (Schultz). If sweated or very lightly limed hides are imperfectly worked on the grain, greasy spots are apt to remain, which will not colour in the liquors ("white spots"). These may be made to colour by scraping and working the grain with a knife, or by the application of a solution of soda or soda ash, and would probably be avoided by the use of soda ash in the soaks on greasy parcels of hides.

It must be clearly understood that all sweating depends on partial putrefaction. This is proved both by the plentiful production of ammonia in the pits, and by the fact that antiseptics, such as salt or carbolic acid, entirely prevent sweating till they are removed. Although the process undoubtedly has advantages, and especially so in the treatment of dried hides, it is an open question whether it gives the extreme gains over liming in weight and firmness, which are claimed by some of its advocates.

An unhairing process, largely coming into use on the Continent, depends on the action of alkaline sulphides, andparticularly sodium sulphide, upon the hair. While all the methods already spoken of involve the softening and destruction of the hair-sheaths, either by lime or by putrefaction, the sulphides are peculiar in attacking the hair itself; when strong, they disintegrate it rapidly and completely into a sort of paste. From very early times to the present day, arsenic sulphide ("rusma") mixed with lime has been used in unhairing skins for glove-leather and similar purposes. About 1840, Böttger concluded that the efficacy of arsenic sulphide was due simply to the sulphydrate of lime formed by combination of the sulphur with the lime, and proposed lime sulphydrate, formed by passing sulphuretted hydrogen into milk of lime, as a substitute for the poisonous and expensive arsenic compound. It proved a most effective depilatory, but has never obtained much hold in practice. This is probably due to the fact that it will not keep, oxidising rapidly on exposure to the air; hence it must be prepared as it is required, which is both troublesome and expensive. A minor objection is the unpleasant smell of sulphuretted hydrogen, which is inseparable from its use.

It was proposed to replace it by sodium sulphide, which, though at first said to be only effective when mixed with lime, so as to produce calcic sulphide, has since proved a powerful depilatory alone. Its use has been greatly extended on the one hand by its production on a large scale, and in the crystallised form (at first by reduction of sulphate by heating with small coal), and on the other, by the great interest which Wilhelm Eitner, the able director of the Austrian Imperial Research Station for the Leather Trades, has taken in its introduction. The substance, as manufactured by De Haen, of List, Hanover, is in small crystals, coloured deep greenish-black, by iron sulphide, which must have been held in suspension at the time of crystallisation. If the salt be dissolved in water, and the solution be allowed to stand, this is gradually deposited as a black sediment, leaving the supernatant liquor perfectly clear and colourless. Sodium sulphide is now manufacturedfrom tank waste in a much purer form by Schaffner and Helbig's process, of which Messrs. Gamble of St. Helens are sole licencees. The crystallised salt is SNa210Aq, and therefore contains 69·8 per cent. of water.

For sole-leather, the method recommended by Eitner is to dissolve 4-5 lb. of sulphide per gal. of water, making the solution into a thin paste (of soupy consistence) with lime or pipe-clay. This is spread liberally on the hair side of the hides, one man pouring it down the middle of the hide from a pail, while another, with a mop or cane broom, rubs it into every part. The hide is then folded into a cushion, and in 15-20 hours will be ready for unhairing, the hair being reduced to a paste. In the writer's experience, the concentrated solution here prescribed will completely destroy all hair wetted with it in 2-3 hours, and if left on longer, will produce bluish patches, and render the grain very tender. The hides should be thrown into water before unhairing, to enable them to plump, and to wash off the sulphide, which is very caustic, attacking the skin and nails of the workmen. There is no doubt that this process gives good weight, and tough and solid leather; but there are several difficulties attending its use. Unless the mopping is done with great care, it will fail to completely destroy the hair, and the patches of short hair left are very difficult to remove. The expense of the material and the loss of hair are also important considerations. 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, containing3/4lb. 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. After or before unhairing, the hides may receive a light liming, to plump them, or lime may be added to the solution of sulphide, whichby forming calcium sulphide, and liberating caustic soda, considerably increases the unhairing and plumping effect. The pit may be several times strengthened for successive packs, but the loosened hair must be fished out, or it will quickly spoil the solution. When hides have been suspended in sodium sulphide solution, the hair is very quickly loosened by a short liming. Squire, Claus, and J. Palmer have all taken out patents for the use of tank-waste as a depilatory. It consists of impure calcium sulphides, and when brought into the form of soluble sulphydrate, either by boiling in water, or by the oxidising action of the air, it will unhair hides. The conversion is, however, very imperfect in either case, and its action is uncertain and slow; while the iron present is apt to cause unsightly stains. It is probable that the weights obtained may somewhat exceed those by liming. Palmer employs sulphuric acid to plump the hide and remove stains, and then reduces it by a bate of whiting and water. He claims that this prepares the hide for rapid and heavy tanning, but the swelling and subsequent reduction almost certainly entail loss of weight and quality, and to get good results the bate should at most only be allowed to have a superficial effect. Professor Lufkin proposed the use of a mixture of various sulphides of lime and soda, formed by mixing 10 lb. each of soda ash and sulphur, kneading to a paste with a little moist slaked and then mixingwarmin a cask with 80 lb. stone lime slaked to a paste. This quantity will unhair 50 hides in the same way and in about the same time as an ordinary lime. The pelt is not much plumped and is easily reduced by a few minutes' wheeling in warm water. (J. S. Schultz.)

Various other depilatories have been proposed, but as they have not come into general use, brief mention of the most important will suffice. Anderson, in 1871, patented the use of wood-charcoal, applied in a similar manner to lime in the ordinary process. The hair is probably loosened simply by putrefaction, as in sweating, while the charcoal acts as a deodoriser, very little smell being produced, and the actionproceeding with considerable uniformity. John Palmer has patented a process for unhairing, in which the hides are alternately steeped in water and exposed to the air till the hair loosens. In this, very similar principles to those of the charcoal method are involved. Caustic potash and soda will loosen hair, but seem to have no decided advantage over lime, though it is quite possible that in skilful hands good results might be obtained. They are more costly, and their corroding action on the hide-substance is more powerful, but they form soluble soaps with the grease of the hide. Unless used in very dilute solution, the pelt is so swollen as to fix the hair, and the leather is dark-coloured and spongy. Soda-ash or crystals (sodic carbonate) may be used to strengthen ordinary limes, in which caustic soda is formed. The time of liming is shortened, the hides are more swollen, and the grease is better "killed" than when lime alone is used. The patent for Moret's "Inoffensive" claimed the use of the carbonate or caustic potash formed from calcined wool-washings, for unhairing. This is more costly than, and has no advantage over soda. I am not aware whether "Inoffensive," as now sold, has other constituents.

Whatever method of loosening the hair may be adopted, the next step is to remove it by mechanical means. This is usually accomplished by throwing the hide over a sloping beam, and scraping it with a blunt two-handled knife (Fig. 25), the workman pushing the hair downwards and away from him. The beam is now usually made of metal. The knife employed is also shown at C,Fig. 26.

When a hide is lightly limed, it is often easy to remove the long hair, but excessively difficult to get rid of the short under-coat of young hairs, which are found in spring, and which can sometimes only be removed by the dangerous expedient of shaving with a sharp knife. The reason of this difficulty is obvious: not only do the short hairs offer very little hold to the unhairing knife, but, as has been explained in describing the anatomical structure of the skin, their roots are actually deeper seated than those of the old hairs theyreplace. Several attempts have been made to unhair by machinery, but so far without such success as to lead to their general adoption. The fleshing-machine invented by Garric and Terson, and manufactured in this country by T. Haley and Co., of Bramley (Fig. 27), is furnished with a special wheel for unhairing. An American machine for the purpose, invented by J. W. Macdonald, and said to be capable of unhairing 800 sides a day, is shown inFig. 28.

Fig. 25.

Fig. 26.

Fig. 27.

Fig. 28.

When the hair is very thoroughly loosened, as by sweating, or destroyed, as by sodium sulphide, it is not uncommon to work it off by friction in the stocks; but it is very doubtful whether the saving of labour is not more than compensated by the loss of weight, consequent upon submitting the hide while its gelatin is in a partially dissolved condition, to such rough usage.

Fig. 29.

After unhairing, the loose flesh and fat are removed from the inner side of the hide by a sharp-edged knife E (Fig. 26), partly by brushing or scraping, partly by paring. It is necessary not only to cut off the visible adhering fat, but to work the hide well, so as to force out that contained in the loose areolar tissue, which would not only impede tanning, but is liable to soak completely through the hide, producing most unsightly blotches. Several machines have been introduced to supersede hand-fleshing, but with only partial success. One of the best is Garric and Terson's machine (Fig. 27), which gives a very level flesh, free from galls, and without so much loss of weight, but scarcely so clean as desirable, while the saving in labour is not great. Molinier's machine (Fig. 29), and that of Jones and Rocke, are well adapted for skins, but hardly capable of fleshing an entirehide. All these machines are very similar in principle, the working parts consisting of drums with oblique or spiral knives.

When unhaired and fleshed, the hides intended for sole-leather are, in England, almost invariably "rounded," or separated into (1) "butts," which are the best and thickest parts, and receive the most solid tannage, and (2) "offal," which is thinner, and for which a cheaper and more rapid tannage is sufficient.Fig. 30shows the customary division. Frequently the butt is divided down the centre, and the halves are then called "bends." A piece called a "middle" is sometimes taken between the butt and the shoulder.

Fig. 30.

After rounding, it is necessary to get rid of the lime, as completely as possible, before taking into the tan-house. For this purpose, the butts are usually suspended in fresh water for 12-24 hours, and frequently shaken up in it to remove adhering lime and dirt. If the water is hard, it is best to add to it, before putting in the butts, a few pailfulsof clear lime-water, to precipitate the lime bicarbonate,[T]which would otherwise cause a deposit of chalk on the surface of the butts; this would not only make the grain harsh, but afterwards, by combining with the tannin of the liquors, would cause bad colour. For the same reasons, it is important that limey hides should be as little exposed to the air as possible, as the latter always contains a small amount of carbonic acid, which renders the lime insoluble.

[T]Lime softens water containing lime bicarbonate in solution by combining with half the carbonic acid, when the whole is precipitated as normal carbonate or chalk. CaO + CaCO3. H2CO3= 2CaCO3+ OH2. This is Clark's process. See alsop. 84.

This suspension in water is frequently considered sufficient for sole-leather, but it removes the lime very imperfectly. 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. It is essential to use acids nearly free from iron, as it may be precipitated on the butts and give a bluish colour in the liquors, and the acid must be of such a strength as neither to allow the iron to be precipitated, nor, on the other hand, perceptibly to plump the butts, which in this stage would endanger buff and colour. 100cc.may neutralise 15-20cc.of lime-water for this purpose. Hydrochloric acid and chlorides have a tendency to prevent plumping, and are therefore better adapted for dressing than for sole leather. Great care must also be taken to prevent putrefaction, or the use of putrid solutions, if firmness and plumpness are desired.

SOLE-LEATHER.—Tanning Materials.

Beforedescribing the management of the hides in the tan-house, it is necessary to say a few words about one or two of the principal materials used, and the methods of preparing them for use. Further details of their nature and origin have been given in the section on Tannins,p. 23.

Oak-bark is one of the oldest of tanning materials, and the leather produced by its aid is still considered for many purposes the best. For sole-leather, its weakness in tannin (8-12 per cent.), the slowness of its action, and the light weight of the leather produced, render it unavailable alone except for the very finest class of work. It is, however, generally used in admixture with stronger and cheaper materials, such as valonia.

Valonia, the acorn-cup of an evergreen oak growing in Greece and the Levant, is perhaps the most important of materials to the English sole-leather tanner. It contains 25-35 per cent. of a tannin somewhat similar to oak-bark, and, like it, communicating a light-coloured bloom to the leather, but giving much greater firmness and weight, and a browner colour.

Myrabolanes or myrobalans, the fruit of an Indian shrub, contains about as large a percentage of tannin as valonia, and gives a similar bloom, and excellent colour; but it can only be used very sparingly on butts, since it produces a soft and porous leather.

Divi-divi is a South American bean, which contains much of a brown tannin in the pod, being considerably stronger than valonia. It makes a heavy and solid, but somewhat horny leather. Its great danger arises from a tendency to suddenfermentation in thundery weather, which, produces brown or red stains on the leather. At all times it is liable to give a bluish or violet colour, which is most obvious in the interior of the leather, and which resists both acids and alkalies.

Mimosa-bark is the product of several Australian acacias, and is probably nearly as strong as valonia. It gives a hard and heavy leather, but of a dark-red colour.

Hemlock-extract is a deep-red syrupy extract of the bark of the hemlock pine of America.

Chestnut-extract is a similar product from the rasped wood of the Spanish chestnut. Its colour is paler and yellower than that of the hemlock, and hence it is often employed to correct the red tone produced by the latter.

Oakwood extract is an analogous preparation from oak saw-dust.

Grinding and Exhaustion of Tanning Materials.

Before tanning materials can be exhausted, it is almost invariably necessary to crush or grind them, so as to enable the water to get freely at the tannin, which, in most cases, is enclosed in the cellular tissue of the plant. It may be thought that for this purpose it would scarcely be possible to crush too finely, but in practice, a very fine powder is extremely difficult to spend, as it cakes into compact and clay-like masses, through which liquor will not percolate. The object, therefore, is to grind finely enough to allow the liquor ready access to the interior, but not so finely as to prevent liquids running through the mass. The mill most usually employed for this purpose consists of a toothed cone, working inside another cone, also toothed on its interior, precisely like those of a coffee-mill. As bark is frequently delivered "unhatched," or in long pieces, it is necessary to crush it preparatory to grinding, and this is usually accomplished by rollers composed of toothed discs, called breakers. InFig. 31is illustrated such a mill, as made by Newall and Barker, of Warrington, combining both utensils.Fig. 32shows a section of the well-known American "keystone" mill, in which the preliminary breaking is accomplished by the arms A; the bark is then finely ground by the toothed cones N, and discharged at the spout R by the revolving shover M.Fig. 33shows a somewhat similar mill, made by Gläser of Vienna, in which the axis is horizontal, and driven directly by a belt. It is better to drive bark-mills by a belt than by toothed gearing, as in event of iron getting into them there is less danger of breakage. In America, a cheap cast-iron coupling is frequently used, weak enough to give way before serious damage is done. Safety "friction" clutches are generally ineffective. American bark-mills are run faster than English, up to about 80 rev. per minute, and where the bark is to be used immediately it is frequently damped by a smalljet of steam below the mill, which lays dust, and prevents danger of fire. Bark which is damp before grinding can scarcely be ground in these toothed mills, but must be dried, or a disintegrator used.

Fig. 31.

Fig. 32.

Fig. 33.

Now that a large variety of other materials besides bark are required by tanners, the mills just described are not always sufficient for the purpose. Myrobalans and mimosa-bark have proved specially troublesome, the former from its very hard stones and clogging character, and the latter from its combined hardness and toughness. "Disintegrators" of various makes have proved admirably adapted for grinding both of these materials, their advantage being the universality of their reducing powers, ranging from oak-bark to bones or brick-dust, and their disadvantages, the somewhat considerable power they consume, and the rather large portion of fine dust they make. Their principle is that of knocking thematerial to powder by rapidly revolving beaters, which, in the smaller mills, are driven at so high a speed as 2500-3000 rev. a minute. Wilson's is shown inFig. 34, as an example. It is one of the oldest tanners' disintegrators, and probably still one of the best. In the figure, it is opened, showing the disc with its steel beaters attached. When myrobalans are only required roughly crushed, a machine with fluted or toothed rollers (Fig. 35) acts better than a disintegrator, making less dust, and requiring less power. Such a machine also crushes valonia very satisfactorily.

Fig. 34.

Fig. 35.

In England, the tanning material is generally carried from the mill, to the pits where it is exhausted, in baskets or barrows; in America, this is frequently accomplished by a"conductor," or horizontal spout, in which a double belt, or malleable iron "drive chain,"[U]with wooden cross-pieces, carries the bark forward, on the same principle as the elevators of corn-mills.Fig. 36shows the conveyors used in a Chicago tannery. Another American plan is to use circular tubs for extraction. These are mounted on wheels, and are worked on a railway, coming up to the mill to be filled, and thence under a series of sprinklers like those used by brewers, and finally "dumping" their contents before the boilers, which are heated solely by wet bark, burnt in a peculiar furnace with brick chambers. This furnace for burning wet bark seems worthy of extended adoption in Europe, as spent tan is frequently not only valueless, but costly to get rid of. Full details and scale drawings may be found in Jackson S. Schultz's book on 'Leather Manufacture' and inFig. 37is shown a modification of it, patented by Huxham and Brown, which has been very successfully used in burning wet tan, either alone or with a portion of coal. In American sole-leather tanneries, where the bark is resinous and almost unlimited in quantity, sufficient steam may beraised with tan wet from the leaches; but in England, where material is more sparingly used, it is advisable partially to dry it before burning. This is accomplished by powerful roller-presses, as shown inFig. 38. Gläser, of Vienna, constructs tan-burning furnaces on a different principle from the American, the essential point being the use of a "ladder-grate" (Treppenrost), on which the burning tan is exposed to a draught of air playing over its surface.Fig. 39shows a portable stove of this construction. Gläser also makes furnaces of larger size for heating air for drying-rooms, and for boiler purposes. The essentials of successful tan-burning are good draught, a large grate-surface, and a high temperature of the combustion-chamber, and hence the ordinary Cornish or Lancashire boiler, with its limited grate-area, surrounded by the comparatively cool boiler-tube, is peculiarly ill-adapted for the purpose. The writer has profitably burnt a mixture of wet tan and very small coal in such a boiler by the aid of a steam-jet under-grate blower, but such a method can only be regarded as a makeshift in default of better appliances.

[U]Such chains with attachments for elevators and conveyers, are manufactured in this country by Ley's Malleable Casting Company, in Derby.

Fig. 36.

Fig. 37

In England, the tanning material is usually exhausted in pits called "leaches," "latches," or "taps." These, in largeyards, are made capable of holding about 50 cwt. of material. The new material is first flooded with a pretty strong liquor. When this has gained as much strength as possible, it is pumped off, and is followed by a weaker one, and so ontill the material is exhausted. Much of the economy of a tan-yard depends on the way, systematic or otherwise, in which this is done. It is customary to complete the exhaustion with hot liquors, or water, but opinions differ on the expediency of the practice. By the use of heat, however, stronger liquors and more rapid spending are attained; and with some materials, such as mimosa, complete exhaustion is impossible in the cold.

Fig. 38.

Fig. 39.

The worst tap is frequently boiled by inserting a steam-pipe; but if heat is used at all, it would probably be better to heat a strong liquor by a steam-coil, and run it on the new material, which would be softened and swollen, and yield a much larger proportion of its strength to the first liquor; while it is stated by Eitner that the colouring matters of tanning materials are much less soluble in strong than in weak infusions. Boiling weak old liquors containing lime is specially prejudicial, causing great darkening and discoloration.

Careful tanners also cast their material over from one pit into another, before throwing away, so as to lighten it up, and allow the liquor to penetrate to every part. In bark-yards, latches are frequently worked in series, which are connected by pipes, so that the liquor flows from the bottom of one upon the top of the next stronger. This is an excellent plan for bark, which is open and porous, but is scarcely adapted to such materials as valonia or myrabolans, which have a tendency to form compact masses, through which the liquor does not circulate. The same objection, in an almost higher degree, must be urged against the Allen and Warren, or sprinkler leach, in which the liquor, distributed on the surface by a rotary sprinkler, is allowed to percolate downwards, and run freely away at the bottom. In this case, it is almost sure to form channels, instead of flowing uniformly, and, in addition, the material is constantly exposed to the action of the air, which causes oxidation, with its attendant discoloration and loss of tannin. Various attempts have been made to exhaust tanning materials in closed vessels.Dr. Kohlrausch applied thediffuseurused in extracting beet-root sugar, and which consists of a series of closed copper vessels in which the coarsely ground material is placed, of which the bottom of one is connected with the top of the nextby a pipe, through which the liquor is forced by steam pressure. This apparatus is in use at the large tannery of Gerhardus, Flesch, and Co., of Vienna, and is said to give satisfaction, though it is very costly, and the liquors produced are not of great strength. Gläser, of Vienna, has patented an apparatus of which a model is illustrated inFig. 40, in which the materials are used finely powdered, and very rapidly exhausted by the combined action of heat and mechanical agitation. Of its mechanism I have not been able to obtain any detailed description, but it is said to be capable of exhausting 9 tons of valoniaper diem, to 2 per cent., giving only 70° liquors, clear and of good colour, while good bark is exhausted to 0·5 per cent. giving 30° liquors. The cost of the apparatus is very heavy, but if the results claimed are realised in practice it would pay well for an extensive tannery. I have not been able to ascertain where it is to be seen in use.

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