The butte should at first he 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. Whether other exhausted liquors are to be retained or rejected is largely a question of climate, and mode of working. In hot weather, such liquors, charged with organized ferments (moulds, bacillus, and bacteria), are apt to cause ropiness, and other fermentive diseases of the liquors. This danger may be lessened by boiling all spent liquors, so as to kill the ferments, before running on the taps, or prevented by the free use of antiseptics, such as carbolic acid. Small doses of carbolic acid, how ever, are useless; at least per cent. must be employed.
The suspender liquors should be acid enough freely to redden litmus-paper. H. R. Procter has published a simple and instructive volumetric method for the determination of the free acid : 10 cc. of the carefully filtered liquor is placed in a beaker, and clear lime-water is run in from a burette till permanent cloudiness is produced. The quantity of lime-water employed is that which the acid is capable of neutralizing, without producing discoloration of the leather, and care must be taken that the lime introduced with the butts does not exceed this proportion. The explanation of the reaction is that dark-coloured tannates of lime are formed, which are dissolved by the free acid so long as it remains in excess.
From the suspenders, the butte are transferred to the " handlers," where they are laid flat in the liquor. The handlers are generally worked in sets, to each of which a fresh liquor is daily run, and the most forward pack is pulled over into it, and is often also dusted down with a little fine bark or myrabolams. The second pack follows into the liquor out of which the first has been taken ; the third into that of the second, and so on. Frequently the greenest packs are handled np a second time in the course of the day, and put down again in the same liquor. The strength of liquors, and the length of time for which butts are retained in the handlers, are varied ; but a time of 1-2 months, and liquors of 20°-35° are usual. A little gambler may be appropriately used in the
liquors.
At the end of this period, the butts are taken to the "layers" or bloomers, in which they are laid down with stronger liquors and much larger quantities of " dust " ; the latter is usually bark or valonia, though mimosa is occasionally used. The liquors vary from 40°-60° or 70° in strength in mixed tannage, and the duration of each layer from 10 days in the earlier stages to a month in the later ones. For the best heavy tannages, 6-8 layers are required. Each time the butts are raised, they should be mopped on the grain, to remove dirt and loose bloom. In pure bark tannage, which, however, is gradually becoming extinct, the liquors used are of necessity much weaker, as it is extremely difficult to obtain liquors of more than 25°-30° from this material. The last layer, how ever, should always have liquors of the greatest strength which can possibly be obtained, or the leather will be deficient in firmness.
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 con stantly increase in a regular and systematic way. To attain this end, very frequent handling and change of liquor are requisite in the early stages, when the butts rapidly absorb the tannin presented to them.
As the process advances, the ex terior part of the butt becomes thoroughly tanned, and the liquor only slowly reaches the interior, which is yet susceptible of its action, and hence longer layers in stronger liquors are permissible.
The varied requirements of the trade render it difficult to give any practical information as to selection of tanning materials. As a general rule, it is important at the outset to give the required colour ; and if materials undesirable in this respect are to be used for the sake of cheapness, they should be introduced in the fOrm of liquors in the middle stages of the process, i. e. in the later handlers or earlier layers. Materials used as dust generally have more effect in producing bloom and colouring the leather, than those used in liquors at this stage. Some information as to the respective qualities of the different tanning materials will be found in the article on Tannin ; but even practical men are very deficient in accurate information on these points, since many materials are never used alone, but invariably in connection with others which mask theil effects.