4 
Indian Forest Records . 
[Vol. III. 
(5) The percentage yield of the extract obtained from wooden 
vats is equal, if not superior, to that furnished by a battery 
of autoclaves. 
(6) In open vats even if the tan liquor is heated to 100°C, it 
develops less colour and contains less insolubles, and conse¬ 
quently is more quickly clarified and more readily decolour¬ 
ised than in the case of tan liquors extracted under the 
pressure entailed by the use of autoclaves. In autoclaves 
the non-tannins increase with the pressure owing to the 
transformation of tannin into non-tannin, and to the solu¬ 
tion in the tan liquor of the non-tannins which are rendered 
soluble through hydration. 
(7) The open system of vats yields extracts containing a higher 
percentage of tannin than those obtained by using auto¬ 
claves wherein the temperature rises to 127°C, this high 
temperature undoubtedly destroying an appreciable quantity 
of tannin. 
The “ leaching ” is done at different temperatures according to the 
material treated, but generally a temperature of 70° to 80°C is high 
enough. To extract all the available tannin, the material should be 
leached seven times. The number of leaches may be reduced in the case 
of materials not very rich in tannin and of those that yield their tannin 
without difficulty under the action of water, but experience has shown 
that rich materials like Mangrove barks and myrabolams have to be 
leached seven times. 
The tan liquors are concentrated by passing them through a series of 
wooden vats filled with the raw material till they show a Sp. Gr. of 9° to 
11°B. The concentrated liquors thus obtained are next transferred to 
a large tank made of wood or copper—the so-called “ liquor tank.” This 
tank is kept on a higher level so as to allow the liquor to flow to the 
refrigerators by gravity. 
Cooling .—A very important step in the manufacture of tannin 
extracts is the cooling of tan liquors below 18°C. In practice the 
temperature should be reduced to 15° to 18°C and should nevei be 
allowed to exceed the latter limit. For this purpose, based on the 
latest experience, tubular refrigerators are recommended as the most 
suitable (see Plate I). These refrigerators are vertical and consist of five 
cc elements 33 with tubular bundles. Their interior condensing surface is 
m 
