32 ANIMAL PROTEINS 



exerts a powerful bleaching action on other tannages, and 

 which assists to brighten as well as lighten the rather dull 

 appearance of leathers largely tanned with valonia. It is 

 rather an expensive tannin, but most manufacturers find 

 that its results are worth its cost. 



Other pyrogallol tans are also used to a limited extent. 

 Algarobilla and divi-divi are the fruit pods of several species 

 of American ccesalpina. They are strong in tan (45 per 

 cent.) and yield a light-coloured and bright leather, but are 

 unstable tans, yielding much bloom. Babla is a small 

 pod yielding a mellow tannage and much gallic acid. Cela- 

 vinia is another pod containing no colouring matter and 

 giving an almost white leather. The tannin is closely 

 similar to that of oak galls. These last were once exten- 

 sively used for tanning in Austria. Willow bark is used for 

 tanning in Russia and Denmark. Valuable pyrogallol 

 tannins are obtained from oak wood and chestnut wood, 

 but the woods are not used in tanning as the percentage of 

 tan is so small. 



Catechol tans, often obtained from barks, contain 

 usually about 60 per cent, of carbon. They are seldom 

 used alone, for they usually have little or no sugar associated, 

 and hence their liquors do not either " sour " or " plump." 

 They can be used alone if artificially acidified, but without 

 acidifying or blending would give a rather flat leather, 

 though possibly firm. They yield no bloom or gallic acid, 

 but have associated with this other characteristic substances. 

 Of these the catechins are the most typical, and have been 

 considered as the parent substances of the catechol tans. 



The catechins are white crystalline substances, ap- 

 parently isomers with the general formula C lfl H 14 O 6 . They 

 have different melting-points, and varying amounts of water 

 of crystallization, but are otherwise exceedingly similar in 

 properties. They are sparingly soluble in cold water, but 

 freely in hot, and in alcohol and ether. They are pre- 

 cipitated by lead acetate, mercuric chloride and albumin, 

 but not by gelatin, tartar emetic or alkaloids. In gambier 

 liquors they are especially strong, and sometimes crystallize 



