Myrabolanes or myrobalams, the fruit of an Indian shrub, contains about as large a percentage of tannin as valonia, and gives a similar bloom, and ex cellent colour ; but it can only be used very sparingly on butts, since it produces a soft and porous leather.
Divi-divi is a S. American bean, which contains much of a brown tannin in the pod, being con siderably stronger than valonia. It makes a heavy and solid but somewhat horny leather. Its great danger arises from a tendency to sudden fer mentation, which produces brown or red stains on the leather.
Mimosa-bark is the product of several Australian acacias, and is probably nearly as strong as valonia. It gives a bard 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.
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 iu 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. 908 is illustrated such a mill, as made by Newell and Barker, of Warrington, combining both utensils. Fig. 909 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.
Now that a large variety of other materials besides bark are required by tanners, the mill just described is not always sufficient for the purpose. Myrobalams 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-hark to bones or brick-dust, and their disadvan tages, 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 rev. a minute. Wilson's is shown in Fig. 910, as an example. It is one of the oldest tanners' dis integrators, and probably still one of the best. In the figure, it is opened, showing the disc with its steel beaters attached. When myro balams are only required roughly crushed, a machine with fluted rollers (Fig. 911) acts better than a disintegrator, making less 'dust, and requiring less power.
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 with wooden cross-pieces carries the bark forward, on the same principle as the elevators of corn-mills. 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 the 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. 912, 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 luathc r tanneries, where the hark is resinous and almost unlimited in quantity, sufficient steam may be raised with tan wet from the locks ; but in England, where material is more sparingly used, it is advisable partially to dry ft before burning. This is accomplished by powerful roller-presses, as shown in Fig. 913.