FOREST UTILIZATION 103 



IV. Prepare the liquors in the leech house The liquors 

 contain often from 5% to 6j4% of tannin only. Cold 

 water extracts only part of the tannin from either bark 

 or wood. Very hot water may extract all, extracting 

 with it, however, undesirable coloring matters and kill- 

 ing the fermenting microbes. 



V. The tannage itself is either "Acid hemlock tannage" or 

 "Non-acid hemlock, oak and union tannage." 



(a) Acid hemlock tannage consists of: 



1. Coloring in a dilute solution of tannin. 



2. Placing skin for 2 to 4 days in a sulphuric 



bath (of 10% to 30%) by which the 

 hide is swelled to a great thickness. 



3. Placing the hide in a strong, concentrated 



solution of tannin. 



(b) 'Non-acid hemlock, oak and union tannage 

 (2-3 hemlock, 1-3 oak bark) : 



1. Treat the hide, to begin with, with very 



weak solutions of tannin. 



2. Gradually increase thereafter the concen- 



tration of the liquors. If a hide is at 

 once hung in a strong liquor, its outer 

 layers only are tanned. The hide will 

 not swell, and the inner layers will fail 

 to be impregnated. 



VI. The operations finishing the process of manufacture are: 



Washing; scouring off the so called bloom; stuffing 



(which means bathing in grease) ; drying; dampening 



and rolling under pressure; redrying; glossing on a 



brass bed by brass rollers. 



XXXIII. CHARCOAL BURNING IN CHARCOAL KILNS. 



Distillation of wood. 



Destructive distillation of wood, under reduced admission of air, 

 yields chemically the following proportion of substances : 



I. 25 % of non-condensable gases, viz. : 



carbon monoxide acetylene 



carbon dioxide propene 



marshgas ethylene 



II. 40% of condensable vapors, vir. : 



acetone formic acid 



furfurol butyric acid 



methyl alcohol crotonic acid 



methylamine capronic acid 



acetic acid propionic acid 



