It has long been a well-known fact that Russia leather owes its durability, as well as its peculiar odor, to the oil of birch-bark, with which it is dressed. The whole process seems to be pretty well understood, and has been for a long time, - the great difficulty in the way being the labor required in the preparation of the oil. It is only from the thin paper-like bark of the birch that the oil can be procured; the wood and the coarser bark of the birch yield only a stinking oil, totally unlike the oil of the external bark. Gray, in his "Operative Chemist," describes the process of preparing this oil, as follows: -
The Russians obtain this oil by filling a large earthen pot with the thin, whitish, paper-like external bark of the birch tree, carefully separated from the coarse bark, closing the mouth of this pot with a wooden bung pierced with several holes; and then turning it over and luting it with clay to the mouth of another of the same size. A hole being dug in the ground, the empty pot is buried in it and a fire is lighted round and over the pot containing the bark, which is continued for some hours, according to the size of the pot. When the apparatus is cooled and unluted, the lower pot contains the brown oil, mixed with pyroligneous tar, and swimming on an acid liquid.
In some places iron pots are used for this purpose, and the bark is hindered from falling into the lower pot by a plate of iron pierced with holes. Gray says that one hundred pounds of bark yield about sixty pounds of oil.
The waste of fuel in this process might be avoided by placing the pots in the side chamber of a reverberatory furnace, filling the chamber a little above the joining of the pots with sand, and then proceeding to distillation.
This oil is used in Russia for currying leather, to which it gives a peculiar odor and a power of resisting moisture far beyond any other dressing. Its use seems to have arisen from observing that the thin paper-like leaves of birch-bark remained after the coarser part of the bark and the timber of fallen trees had rotted. The oil appears to owe this quality to a resin, which, by this mode of distilling per descensum, is allowed to escape in a great measure from the action of the fire and drop into the lower pot.
Other barks - as those of the oak, willow, poplar, alder, as also poplar-buds, rue, and savine - have been tried, but the produce from them was only a stinking oil. Cork yielded an oil approaching, in some degree, that of birch-bark.
The genuine Russian birch-oil has been imported into this country, and has given very good results in the dressing of American leather.