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 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.
Fig. 40.
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.