In England, the tanning ma terial is usually exhausted in pits called " leeks," " 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 ma terial 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. 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 hark, which is open and porous, but is scarcely adapted to such materials as valonia or myrabolams, 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 leek, 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 fermentation, with its attendant discoloration and loss of tannin.
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. 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.
Construction of Tanneries.—The old-fashioned method of sinking pits is to make them of wood, and carefully puddle them round with clay, which should be well worked up before use. Loam mixed with water to the consistence of thin mortar may also be employed, the pits being filled up with water, to keep them steady, at the same rate as the loam is run in. Probably the best materials for pit-sides are the large Yorkshire flagstones. Where theae are not attainable, very durable pits may be made of brick, either built with Liaa lime, and pointed with Portland cement, or built entirely with the latter. Common lime cannot be used, as it spoils both liquors and leather ; and even cements with too large a percentage of lime are unsatisfactory. Brick and common mortar are, however, suitable for lime-pita.
If possible, both latches and handler-pita abould be provided with plugs and under ground pipes, communicating with a liquor well some feet below their levels. Glazed fireclay is very suitable both for pipes and plug-holes, which should be in the pit corners. Iron should, as far as possible, be avoided wherever it can come into contact with liquor, as it discolours the leather.
Management of Sole-leather in the Tan house.—After suspension in water, the hides
are usually taken at once into weak tan liquors ; but occasionally they are treated with dilute acids before the actual tanning, both to remove the lime still remaining in the pelt, and to plump them. If the sole object be the former, hydrochloric acid is probably the most suitable, since the calcio chloride formed is very soluble, and, in small quantities, hydrochloric acid has not the bad effect, often produced by sulphuric, of causing a dark layer to form immediately beneath the grain during tanning. If acid is to be used for plumping, it is, no doubt, best to use it mixed with the early liquors, where its influence is modified and restrained by the tannin ; hut it seems probable that, where lime is used for unhairing, the fibre is sufficiently opened up by this agent to receive the tanning, and that, in a well-managed yard, the natural acids of the liquors are sufficient, in conjunction with suitable tanning materials, to give the hides all the weight and substance of which they are capable.
On first coming into the yard, the butte are usually suspended by the necks from sticks placed across the pits. They should be kept in almost constant movement, either by raising and shaking them by hand, or by supporting them on frames, which are rocked, or otherwise worked. Perhaps the best device for this purpose is the "travelling handler" of W. .N. Evans, which consista of a frame supported on wheels, and worked slowly backwards and forwards by power. This frame should extend the length of a range of pits sufficient to take in at least a 3 days' stock of butts, which should be tied by the neck end to sticks resting crossways upon it. It should have a stroke of 1-2 ft., repeated, say 6 times a minute.
The suspender pits should be supplied with old handler liquors, which, if the tannage is a mixed one, may range from 12°-l6° barkometer, as a large proportion of the weight consista only of lime-salts, gallic acid, and other worthless products. If the tannage is pure bark, it may perhaps be advisable to let the strength he 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 neutralize the lime brought in by the butts, or bad colour will certainly result, making itself visible in the shed, or as the tanning proceeds. If the butts, when first brought into liquor, take a lemon-yellow colour, especially in places that have been imperfectly exposed to it, this is an indication of danger which must not be disregarded. It may be met either by cleansing the butts more thoroughly before bringing into the yard, or by adding acid (acetic, hydrochloric, or sulphuric) to the liquor. It 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 myrobalams. It is a common error to call all the free acid of sour liquors "gallic," as this is not even present in pure bark-yards, and at the best is a feeble acid, scarcely reddening litmus, or capable of swelling hides. The most abundant acid is usually acetic, though butyric, lactic, and other acids are fre quently present. It must here be explained that the barkometer (also called " barkrometer " or " barktrometer ") is a hydrometer, graduated to show the sp. gr. Bark. = 1.020 ap. gr.