Source, Etc

The British oak, Quercus robur, Linne (N.O. Cupu-liferce), is widely diffused over Europe and largely cultivated for its wood and especially for its bark, which, from the large proportion of tannin it contains, is valued highly for tanning. Although the astringent properties of the bark have long been known, and although it has been used from time immemorial in preparing leather, it never appears to have been much used in medicine.

In the collection of oak bark the trees are usually felled when they have reached an age of twelve to thirty-five years, and in the early spring when the buds are opening. Longitudinal incisions are made through the bark, which can then be removed in strips, and after drying is ready for the market.

Young bark is preferable to old, because as the trees increase in age the outer portions are cut off by the production of layers of cork in the bast (formation of outer bark), and the tannin in the portions thus cut off undergoes certain changes. Bark from older stems is also collected and freed from its dead outer portions, but such bark is not so valuable for tanning or fit for medicinal use.

From the stools that are formed when the trees are felled adventitious shoots arise, and these, when they have attained a sufficient age, are cut and peeled.

While the bark is young (up to about twenty years old) it possesses a smooth, glossy, silvery cork; such bark is to be preferred and should alone be used medicinally.

Description

Oak bark usually occurs in channelled pieces 10 to 20 cm. in length and 2 to 3 cm. in breadth. The outer layer is a thin, smooth, shining, silvery-grey, firmly adherent cork, which in young barks is marked with darker transverse lenticels and" in older barks is frequently longitudinally fissured and bears darker spots and patches; beneath the cork is a reddish brown cortex.

Description 216Fig. 141.   Oak bark. A, outer surface, showing the smooth, glossy cork and transverse lenticels. B, inner surface. Natural size.

Fig. 141. - Oak bark. A, outer surface, showing the smooth, glossy cork and transverse lenticels. B, inner surface. Natural size.

The inner surface is strongly striated longitudinally and fibrous, and varies in colour from yellowish to reddish brown.

The bark breaks with a short fracture in the outer part (cork and cortex), but is coarsely fibrous in the inner part (bast). Under a lens the section exhibits a thin cork, a narrow yellowish cortex, occupying about one fourth of the total width, separated by a pale line (sclerenchymatous cells) from the reddish brown bast, which is chequered by tangentially arranged groups of bast fibres. Touched with dilute solution of ferric chloride the section assumes a black colour.

The bark has a scarcely perceptible odour, but a strongly astringent taste.

The student should observe

(a) The glossy silvery cork,

(b) The line of sclerenchymatous cells,

(c) The striated fibrous inner surface; and should compare the bark with

(i) Willow bark, which usually possesses a dull greenish brown cork, paler inner surface, and no line of sclerenchymatous cells;

(ii) Witch-hazel bark, which has a dull grey cork and reddish pink colour.

Constituents

The principal constituent of oak bark is the tannin, quercitannic acid, C17H1609 (Etti), of which it contains from 5 to 20 per cent, according to the age of the bark. Oak bark, as described above, should contain 15 to 20 per cent.; trunk bark contains only 5 to 8 per cent., or if the dead outer bark has been removed, 8 to 10 per cent.

The bark also contains gallic acid, ellagic acid, quercite, phloro-glucinol, etc.

Quercitannic acid has been obtained as an amorphous, yellowish brown or reddish white powder readily soluble in water and alcohol. When boiled with dilute sulphuric acid it is converted into oak-red, C38H16017, a reddish brown substance insoluble in water, alcohol and ether. Whether dextrose is simultaneously produced is doubtful, probably it is not, and oak-red may be regarded as the anhydride of quercitannic acid.

Gallic acid, C7H605, (trihydroxybenzoic acid), is colourless and crystalline; it is produced when gallotannic acid is boiled with dilute sulphuric acid and occurs in a number of plants.

Ellagic acid, C14H608, is pale yellow and crystalline; it forms a deep yellow solution with caustic alkalies and dark blue with ferric salts.

Quercite C6H1205, is a sweet, crystalline pentahydric alcohol also found in acorns.

Phloroglucinol (phloroglucin), C6H3(OH)3, occurs in several plants and may be obtained from many resins, tannins, etc, by fusing with caustic potash.

Use

Oak bark has been used medicinally as an astringent, but is not much prescribed at present.

Note

Quercus Suber, Linne, the cork oak (Mediterranean, Algeria) yields commercial cork. The cork first formed is unsuitable for technical purposes and is sold as ' virgin ' cork. That which is subsequently produced from the phellogen is stripped from the tree every 8 or 10 years, steamed, scraped and pressed into flat strips.

Other Substances used for Tanning

The following are the chief substances used in the tanning industry together with the approximate percentages of tannin contained in them:

Per cent.

Oak bark, bark of Quercus robur, Linne ...

8 to 13

Valonia, acorn cups of Ęgilops, Linne

25 to 35

Hemlock, bark of Tsuga canadensis, Carr, ..

10 to 14

Mangrove, bark of Rhizophora mangle, Linne ..

9.to 33

Wattle, bark of Acacia dealbata, Link; and of A. decurrens,

Willdenow ..............................

24 to 30

Catechu, extract from Acacia Catechu ...

45 to 55

Gambier, extract from Uncaria Gambier ..

36 to 40

Sumac, leaves of Rhus Cotinus, Linne; and of R. coriaria, Linne

15 to 33

Quebracho, bark of Aspidospermum Quebrachocolorado, Schlechtendal .. ...

14 to 33

,, Extract ....

55 to 60

Myrobalans, fruit of Terminalia Chebula, Retz.

20 to 40

Divi-divi, fruit of Coesalpinia coriaria, Willdenow .

30 to 50

Galls ..

60 to 70

Tamarisk Galls, Tamarix gallica, Linne ...

50 to 54