Machine for Extracting Tannin from Hemlock Bark.

Scientific American 22, 30.11.1867

Mr. Langley, at the November meeting of the Massachusetts Institute of Technology, described a machine for the above purpose, now in process of construction at the South Boston Iron Works, under his superintendence, and from his designs. By this machine much time and labor will be saved, and the old tedious process of long contact of the coarsely ground lark with the skins to be tanned considerably shortened.

The hemlock bark, in pieces of half an inch to an inch thick, and several inches long, is soaked for about fifteen minutes in water at 200° Fah.; it is then fed into a hopper, which conducts it to a threerolled machine, something like the rollers of a sugar or cane mill, through which it passes, coming out lacerated and compressed; it then falls into a vat of hot water, where it is agitated by a wheel, that the tannin from the crushed cells may be dissolved in the water; it is then raised by a series of buckets on an endless chain, somewhat in the manner of a grain elevator, to another hopper, whence it is fed to another series of three rollers; here it receives its final compression, and comes out in flakes or sheets, like coarse paper, and almost free from tannin. The buckets are made of coarse wire, that the water may drip through during the elevation. In order to avoid the blackening action of iron, wherever this metal is brought into contact with the solutions, it is thickly coated with zinc.

The extracts thus obtained are of a fine crimson color, highly concentrated—indeed almost saturated solutions of tannin; they require to be largely diluted, being from three to six times too strong for application to the skins; thus the tanning principle of a cord of bark, which the machine can treat in an hour, is concentrated into a barrel of the extract. Even supposing that the tanning process cannot be shortened, as far as the best quality of leather is concerned, any one will see the immense advantage of taking a machine to the hemlock woods, and bringing back tanning extract by the barrel instead of so many loads of bark. This process will open an immense and profitable commerce between this country and others whets tanning materials are not indigenous.