350 



The World's Commercial Products 



OAK TREE 



bark of the stem or root, the 

 rind or husk of the fruit, or 

 the heart-wood, though in a 

 few cases it occurs in large 

 quantities in the living parts 

 of plants, e.g., in the leaves 

 and roots. 



As a general rule a part of 

 a plant is unsuitable for use 

 as a tanning agent unless it 

 contains at least ten per cent, 

 of tannin, though this alone 

 is not sufficient to give a 

 material value for this pur- 

 pose ; it must in addition 

 contain non-tannin extrac- 

 tive matter, which is useful 

 in producing what tanners 

 call a " well-filled " leather. 

 It must further be free from dark or undesirable colouring matters, as otherwise the value of 

 the leather produced will be prejudiced by its colour, which should be at most a pale 

 russet-brown. Though it is not customary to use tanning agents containing less than ten 

 per cent, of tannin, a method has been devised for the utilisation of such materials as 

 oak-wood and chestnut-wood, which contain only three or four per cent. This consists in 

 extracting from these the whole of the soluble matter they contain and concentrating this 

 extract till it solidifies. In this way " tanning extracts " containing as much as thirty 

 per cent, of tannin may be obtained from oak-wood. 



This process, first devised in order to facilitate the exploitation of materials poor in tannin, 

 has, during the last few years, been greatly extended until at the present time practically all 

 important tanning materials can be bought in the form of extracts. At first the manufacture 

 of these was confined to the industrial countries in which leather manufacture was principally 

 carried on, but lately the tendency has grown to make these extracts where the tanning- 

 materials are produced. In this way the exporter pays transport charges only on the material 

 actually used by the tanner and can use the inert matter left after extraction as fuel and in 

 other ways. 



Tanning Materials derived from Oaks 



t - 



Oak Bark. This was at one time practically the only tanning material used in the United 

 Kingdom for heavy leathers, but of recent years its use, though still very large, has become 

 more restricted owing to the fact that the bark is expensive as a tanning agent and cannot 

 compete in price with many materials of exotic origin now available. Oak bark is a bye- 

 product of the oak timber industry, and is only collected from felled trees. The trees are usually 

 cut down from April to June, and the bark is at once detached, roughly cleaned from lichens, 

 moss, etc., and dried. If is sold either in pieces about a yard long (" long rind bark "), or is 

 chopped into. small pieces ("hatched bark"). English oak bark contains from twelve to 

 fifteen per cent, of tannin, and is richest. when collected from trees from thirteen to twenty 

 five years old. . A good deal of oak. bark is also obtained from Belgium, Holland, France, and 

 Sweden }! the. Belgian -being considered the. best' of "these, imported varieties. 



Oak-wood. The heart- wood of the common oak contains from two to five per cent, of 

 tannin, and is therefore unsuitable for use in the crude state. The sawdust, shavings, and waste 

 pieces are, however, utilised, as already described, for the manufacture of "oak-wood extract," 



