The Oil of Birch Bark, which is so much used in Russia for currying eather, to which it gives a peculiar odour, and a power of resisting moisture beyond any other dressing, is prepared in the following manner: - A large earthen pot is filled with the thin white paper-like external bark of the birch-tree, carefully separated from the coarse bark; the mouth of this pot is closed with a wooden bung perforated with several holes. The pot thus prepared is then turned with the mouth downwards, and luted with clay to the mouth of another pot of the same size, which is buried in the ground. The upper pot is now surrounded with fuel, and a fire is made and continued for several hours, according to the size of the pot. When the operation is completed, and the apparatus cooled and unluted, the lower pot is found to contain a quantity of liquid equal to about 60 per cent. by weight of the bark employed; the liquid consisting of a brown oil mixed with pyroligneous tar, swimming in an acid liquor. In some places iron pots have been substituted for the earthen pots, the mouths being separated by an iron plate pierced with holes. The peculiar odour of the oil is supposed to be owing to a resinous matter which is melted out of the bark, and drops into the lower pot during the process of distillation.
In conducting this operation on the large scale, a number cf these double pots may be placed in the horizontal bed of a reverberatory furnace, with the lower pots imbedded up to their necks in sand; by which arrangement a great economy of fuel and labour will be attained.