Oak bark is obtained from the smaller branches and young stems of the British oak, Quercus robur, Linn. (N.O. Cupuliferae), and is collected in spring from trees growing in Britain. The oak, and particularly the two forms formerly distinguished as Q. pedunculata, Ehr., and Q. sessiliflora, Salisb., is largely cultivated for its bark, which is almost universally employed as a tanning material. For this purpose the young plants or shoots arising from the stools of felled trees are allowed to grow until they are about fifteen to twenty years old, when they are about 10 to 15 centimetres thick, and still retain their smooth bark. The latter is stripped in May and dried, first on hurdles and afterwards in a warm room. Quercus, U.S.P., consists of the dried bark of Quercus alba, Linn., collected from trunks or branches from ten to twenty-five years old, and deprived of the periderm. Oak bark occurs in pieces about 25 millimetres wide, 2 millimetres thick, and of varying length. The cork is glossy and silvery grey, often marked with darker, transverse lenticels. The inner surface is brown, and coarsely striated. The bark breaks with a fibrous fracture, the section exhibiting under the lens a thin cork, and a narrow cortex separated by a line of sclerenchymatous cells from the bast, in which radially and tangentially arranged groups of bast fibres can be distinguished. It is odourless, but has a sweetish taste, with an astringent after-taste. It yields about 6 per cent. of ash. Bark of about the age and appearance indicated is richer in quercitannic acid than either older or younger bark. Older bark may be distinguished by the formation of outer bark, the smooth silvery cork becoming fissured, dark, and dull. Younger bark may be recognised by its smaller size.
Constituents.—The chief constituent of oak bark is quercitannic acid (C17H16O9), of which it should contain from 15 to 20 per cent. Bark of lower grade contains about 12 per cent., whilst trunk bark contains from 5 to 8 per cent., or if the outer bark has been removed 8 to 10 per cent. Oak bark also contains a phlobaphene, oak red, which is produced from quercitannic acid by hydrolysis, and may be regarded as its anhydride; gallic acid, ellagic acid, quercite, laevulin, phloroglucin, and starch are also present.
Action and Uses.—Oak bark is an astringent employed as Decoctum Quercus for external use in gonorrhoea, leucorrhoea, and vaginitis, as a rectal injection in haemorrhoids, and as a gargle in sore throat.
Dose.—½ to 2 grammes (8 to 30 grains).
- Decoctum Quercus, B.P. 1885.—DECOCTION OF OAK BARK.
- Oak bark, bruised, 6.25; distilled water, sufficient to produce 100. Add the bruised bark to 120 of the water, boil for ten minutes, strain, and makeup to the required volume, if necessary, by passing distilled water through the strainer.