CLASSIFICATION 207 



THE CLASSIFICATION OF TANNINS. 



With the present incomplete state of our knowledge con- 

 cerning the chemical constitution of the tannins, it is difficult 

 to make a proper chemical classification of these substances. 



According to Trimble * the tannins may be divided into 

 two main groups : 



1. Those containing about 52-2 per cent carbon. 



This group includes the tannins contained in oak 

 galls (gallotannic acid), chestnut (wood and bark), 

 sumach, pomegranate, etc. 



2. Those containing about 59-60 per cent carbon. 



This group includes tannins of oak bark, kino, can- 



aigre, ratanhia, catechu. 



He points out that the fact of similar percentage composi- 

 tion would not itself be sufficient to justify this classification, 

 but he finds that the classification still holds when the reactions 

 towards certain reagents are compared as under : 



Group i. Group 2. 



Ferric salts. Blue colour and precipi- Green colour and pre- 



tate. cipitate. 



Lime water. White precipitate be- Light pink precipitate 



coming blue. becoming red and 



brown. 



Bromine water. No precipitate. Yellow precipitate be- 



coming brown. 



Dekker f proposes the following classification : 



1. Catechin tannins, occurring in gambier, catechu and 

 Hamamelis bark. 



2. True tannins 



(a) Gallic acid group . . . gallotannic acid ; tea and sumach tannins. 



(b) Ellagic acid group . . . divi-divi, algarobilla and myrobalan tannins. 



(c) Oak bark group . . . the majority of red-producing tannins. 



3. Pseudo tannins (which do not form leather with hide), 

 caffetannic acid and the tannins of mate, hops, etc. 



Perhaps the best classification is the one given by Procter,;]; 

 who divides tannins into two main groups : 



(A) Pyrogallol tannins, including divi-divi, galls, sumach, 



* Trimble: loc. cit., vol. ii. p. 132. 



f Dekker : " De Looistoffen," Amsterdam, 1906. 



Procter; "The Principles of Leather Manufacture," London, 1903. 



