Indian Forest Records. 
[VOL. III. 
of this evaporator. The liquid rises through a tube 7 metres long, by a 
phenomenon called grimpage or “ climbing.” The ratio between the volume 
of the liquid that passes up the tube from bottom to top and the volume 
of steam which escapes from the top has been calculated to be 1 : 1,000. 
Globules are formed in the bottom of the tube, and the liquid begins 
to rise in thin layer up the sides of the tube under the action of the 
steam. The steam keeps on circulating at a speed of 25 metres per 
second. Under these conditions, the evaporation is very intensive and 
the tan liquors are placed under conditions best suited to prevent the ex¬ 
tract being damaged. Practical experience has also shown that the 
employment of Kestner's evaporators has always yielded a superior pro¬ 
duct. The superior efficiency of this plant has been proved beyond 
doubt. 
Different Processes of Decolourisation. 
Since 1879, a long series of patents has been taken out* and it is not 
possible to describe all the processes of decolourisation here. It is only 
proposed to glance at the available literature on the subject. The chief 
decolourising materials are (1) sulphurous acid, or bisulphites, (2.) blood 
or albumen, (3) oxalic acid and alumina, (4) animal charcoal, (5) haloid 
salts of organic acids, (6) sulphurous acid gas under pressure, (7) 
aluminium hyposulphite, (8) barium chloride, (9) lead nitrate, (10) lactic 
acid, etc. All the chemical processes of decolourisation, however, 
entail some loss of tannin, and some of them also affect injuriously 
the quality of the extracts. For example, the clarification of tan liquors 
with blood which is one of the most rational processes causes a loss of 10 
per cent, of tannin in the extract. The nitrate process is still less to be 
recommended because the eventual formation of free nitric acid destroys 
a considerable amount of tannin owing to secondary reactions. 
A satisfactory process of decolourising the tan liquor has been 
patented by George Klenk (English patent 25,063, December 1901). 
This process 1 consists in the addition of aluminium sulphate and sodium 
bisulphite as follows :— 
Add to the hot tan liquors in a vat fitted with an agitator a solution 
of sulphate of alumina, and after the mixture of these two liquors, add 
thereto bisulphite of soda (38° to 40°B), stirring constantly. The aver¬ 
age proportions used are 8*8 lbs. of aluminium sulphate and 33 to 40 lbs, 
1 See Jour. Soc. Chem. Iud., Yol, XXI, 1902, p. 1462. 
225 
