156 UTILIZATION OF BARK. 



bined great heat and humidity, such as obtain in the hold of a 

 ship, would cause it to decompose in a much shorter time. This 

 plan would save cost of packing and carriage in a very remarkable 

 degree, so that our forests, in which thousands of tons of bark now 

 go annually to waste, could supply not only local tanners, but 

 tanners in the most distant countries.* To Captain Wood is due 

 the credit of attempting to introduce this practice into India and 

 thereby utilizing the vast resources of our forests in tanning bark. 

 He has experimented with sdl and Terminalia tomentosa. The bark 

 is chopped up in pieces about 4 inches square and boiled in earthen 

 pots, the strained liquor being completely desiccated or reduced to 

 the consistency of treacle. The extract prepared by the Oudh 

 Forest Department has the defect of containing too much dark 

 colouring matter ; but this is, no doubt, due in a great measure to 

 portions of the dead bark having been boiled with the lighter- 

 coloured living bark, a practice to which must also be attributed 

 the very poor outturn in tannic acid (only 2^ per cent, of the dry 

 weight of the bark). With a proper system of manufacture, the 

 yield would be at least doubled, and the percentage of tannic acid 

 in the desiccated extract would certainly reach 50, instead of only 

 33, as at present. The price of chestnut wood extract, containing 

 30 per cent, of tannic acid, varies in London from £12 to £15 a 

 ton. There is no reason to suppose that a great many of our 



* The practice of preparing a tanning extract by boiling chips of wood of the 

 sweet chestnut is a very old one, and for many years now the system has been 

 very successfully applied to oak wood in Europe. "Within the last 30 years an 

 extract of the hemlock {Abies canadensis) bark has taken the place of the crude 

 bark in the exports from Canada and the United States. The mode of preparation 

 is an instructive one for us in India, who have to devise some way of turning to 

 profitable account the thousands of tons of bark which go to waste every year. 

 The bark, in pieces | to 1 inch thick and several inches long, is soaked for about 

 15 minutes in water at 200° Fah, It is then fed into a hopper, which conducts it 

 to a 3-roller machine, something like a sugarcane mill, through which it passes, 

 coming out lacerated and compressed. It next falls into a vat of hot water, where 

 it is agitated by a wheel in order that the tannin from the crushed cells may be 

 dissolved in the water. Hence it is raised by a series of buckets, on an endless 

 chain, to another hopper, whence it is fed to another 3-roller mill. Here it 

 receives its final compression, and comes out in flakes or sheets, like coarse paper, 

 and almost free from tannin. The buckets are made of coarse wire, that the 

 water may drip through. To avoid the blackening action of iron, wherever this 

 metal is brought into contact with the solutions, it is thickly coated with zinc. 

 The solution is evaporated to a solid consistency, generally in vacuum pans. 

 About 2 tons of bark yield 500 Bis. of extract, containing 18 to 25 per cent, of a 

 deep-red tannin, giving considerable weight and firmness to leather. 



