SOLE-LEATHER.—Tanning Materials.

Before describing 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 sudden fermentation 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. In Fig. 31 is illustrated such a mill, as made by Newall and Barker, of Warrington, combining both utensils. Fig. 32 shows 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. 33 shows 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 small jet 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.

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 the material 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 in Fig. 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.

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. 36 shows 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 in Fig. 37 is 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 be « 163 »
« 164 » raised 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 in Fig. 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. 39 shows 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.

In England, the tanning material is usually exhausted in pits called "leaches," "latches," or "taps." These, in large yards, 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 on till 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.

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 the diffuseur used 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 next by 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 in Fig. 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 valonia per 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.

It is one of the great attractions of extracts that they avoid almost all the expense and labour inseparable from the exhaustion of other tanning materials. It is usually necessary to dissolve the fluid extracts in water or liquor of as high a temperature as has been employed in their preparation, as otherwise, from some unexplained chemical change, a large portion of the tannin is precipitated, probably as an anhydride of the tannin. Gambier is usually dissolved by boiling or steaming, but is said to give a better colour when dissolved cold. This may be accomplished in a rotating latticed drum, sunk in a pit of liquor.

Where circumstances permit, it is a great advantage to place the taps either on a higher or a lower level than the layers and handlers, so that liquors may be run one way without pumping.

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