Of English barks, Sussex and Hampshire are considered the best, and contain up to 12-14 per cent. of tanning matter; a coppice bark from Wastdale, Cumberland, is however, recorded to have yielded 19 per cent. tanning matter (Hellon).
Probably each of the two varieties of oak gives best bark where it thrives best (v. Höhnel).
Belgian bark is sometimes equal to English, and contains 10-12 per cent. tanning matter. Dutch bark as exported is generally inferior and not cleaned; Swedish is bright, but very poor.
Oak-bark contains a tanning matter, quercitannic acid, giving green-blacks with iron salts, and possibly containing both catechol and pyrogallol groups, but its constitution is not fully understood. It yields both red anhydrides and ellagic acid; and gallic acid has been obtained by the action of hydrochloric acid, though not by fermentation in the tannery. The tannin is not a glucoside, but the fact that a sugar, lævulose, is also present in the bark has led some observers to erroneous conclusions regarding the constitution of the tannin. The unpurified infusion of the bark of Q. robur gives a blue-black with iron-salts, from the presence of a colouring matter; but those of most other oaks give green-blacks.
Most tannin is contained in the living part of the bark. The yield diminishes in trees over twenty-five years, and coppice barks, from absence of ross, are often strong, and also contain less colouring matter and more fermentable sugar.
Warm and rich soils seem to yield the best barks.
The brighter the colour of the fresh cut “flesh,” the better the bark. Dark brown inner side shows that bark has been exposed to rain, which deteriorates strength and colour; but a very light colour is thought by some to indicate poorness in tannin. White lichen is said to be a mark of poor bark, and probably indicates a damp and unfavourable situation.
Oaks are generally cut when the sap is rising (15th April to 15th June), and the buds open and new soft cells begin to grow, for the bark is then more easily detached.
Experiments in France have shown that the bark of timber felled at other seasons may be loosened by steaming, and it is said there is no practical loss of tannin. Superheated steam, produced in a small boiler in the woods, is used.
The bark is peeled with tools of various forms, the branch and knotted places being loosened by beating with a mallet. The bark must be peeled immediately the tree has been felled.