BAEK FOR TANNING. 153 



CHAPTER III.— UTILIZATION OF THE BARK OF 

 TREES AND SHRUBS. 



The principal uses of barJi are for tanning and dyeing, and for fur- 

 nishing fibres and drugs. On a small scale, it is also employed as 

 covering material, as fuel, and for some minor industrial purposes. 



Section I. — Baek foe tanning. 



Tannin is the generic name given to a large class of organic 

 bodies, mostly uncrystallizable, which often differ widely in chemi- 

 cal composition and reaction, but have the common property of 

 precipitating gelatine from its solution and forming insoluble com- 

 pounds with gelatine-yielding tissues. By virtue of this property 

 they convert animal hide, which is easily putrescible, into insolu- 

 ble imputrescible leather. They all form blackish-blue or blackish- 

 green compounds with ferric salts, and when treated with alkalies, 

 they give solutions which oxidize rapidly, usually becoming suc- 

 cessively orange, brown, and black. Associated with catechu-tan- 

 nio acid a white crystalline body is found, called catechin, which 

 does not precipitate gelatine. The tannins are met with chiefly 

 in parenchymatous tissues, especially in bark and young wood, in 

 the pericarp or other coverings of green fruits, and in the excres- 

 cences called galls. Leaves that darken in drying, like those of 

 the tea plant, of Anogeissus latifolia, &c., are rich in tannin. 



From the tanner's point of view, tannins may be divided into 

 two principal classes, viz., those which produce a light fawn-colour- 

 ed deposit (the bloom) on leather and those which do not. To the 

 first of these belong the tannin of myrabolams, to the second the 

 tannin of cutch. 



Tannin is a substance so easily soluble in water that bark intend- 

 ed for it has to be taken off with the greatest care. Bark may 

 be removed (1) from a standing tree, or (2) from fresh-felled 

 wood, or (3) from wood that has become more or less dry. In the 

 first two cases it should be possible to strip the bark off the wood 

 without difficulty ; hence the barking must be effected when the 

 union between the bark and wood is weakest, that is to say, at the 

 beginning of the season of vegetation, from the moment the buds 

 begin to swell up to the time the leaves attain their full size. Dur- 

 ing this period, the cambium is exceptionally active, and the tissues 



