The peeled bark, in pieces up to three feet long, is laid on hurdles sloped in such a way that the rain runs off as much as possible, and in this way it is dried, but in wet seasons is much damaged. Bark so dried in the woods often retains 40-50 per cent. water, and must be stacked or stored so as to allow of further drying.
English bark is sometimes sold in “long rind,” and sometimes “hatched” or chopped in pieces about four inches long. Belgian and Dutch barks are generally hatched. Belgian tree bark is “cleaned” (and cleanings often mixed back with bulk), Dutch bark is not cleaned. Much sand and dirt is contained in most Continental bark: screenings of Belgian bark yielded a black liquor, and contained so much sand that they would not even burn!
Oak-bark extract is occasionally offered for sale, but is not usually genuine or of good quality, except that of the American chestnut-oak, Q. prinus, from which an excellent extract has been manufactured in the Alleghanies. Factitious extracts often contain myrobalans and quebracho.
Fig. 45.—Turkey Oak (Quercus cerris).
Oakwood contains only a very small percentage (from 2-4 per cent.) of a tannin, practically identical with that of chestnut, but different to that of oak-bark. It is stated by de Lof to reach 9-14 per cent. in old heart-wood; but this is doubtful. The wood retains the tannin in its interior for a long time. Wood of a Roman bridge built at Mainz 55 B.C. is stated by de Lof to have still contained 2·14 per cent. tannin in 1881 A.D. A good deal of imitated oakwood extract is undoubtedly made from chestnut wood, and unfortunately no very satisfactory way of distinguishing it is known, though oak-bark extract can be distinguished from oakwood by giving a precipitate at once, even in dilute solution, with bromine-water, while the wood gives a brown precipitate only after long standing. Precipitation by bromine-water is a general characteristic of catechol tannins, and hence a mixture of quebracho (a cheap catechol tan) with chestnut would simulate oak-bark in this respect. If a few drops of the non-tannin solution or an alcoholic extract from the “total soluble” of extracts containing quebracho or other catechol tannins be treated with concentrated sulphuric acid in a test-tube, a deep crimson will be produced, especially at the surface of the acid, which remains pink on dilution with water. With pyrogallol derivatives, such as genuine oakwood, a yellow or brown only is produced (J. Hughes). The test is very delicate. Another distinction is that bark extracts contain perceptible traces of manganese, but this cannot be relied on as many wood extracts also contain some, probably derived from the twig and branch bark which is used along with the wood. Oakwood extract is now manufactured on an enormous scale in Slavonia, and is used both by sole- and dressing-leather tanners, chiefly to increase the strength of the layer liquors. The extract is also used to increase the weight of leather after tannage by mopping on the flesh-side. All the best oakwood extract manufacturers contract to sell on analysis and colour estimation, and good Slavonian oakwood extract generally contains 26-28 per cent. of tanning matter, giving a tintometer measurement of 4-5° red, and 20-25° yellow, when a solution containing 1⁄2 per cent. of tanning matter is measured in a 1 cm. cell. For particulars of the manufacture of concentrated extracts see [p. 337].
Fig. 46.—Cork Oak (Quercus suber).
Q. cerris, Turkey Oak. Ger. Zerreiche. Common in southern Europe, a fine tree, but bark inferior to Q. robur. [Fig. 45].
Q. pubescens. Fr. Chêne velu; Ger. Weiss- or Schwarzeiche. In mountain districts and scattered in Southern Europe, about equal to Q. robur.