210 TANNINS 



1. Those containing about 522 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 taimins 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 * proposes the following classification : — 



1. Catechin tannins, occurring in gambier, catechu and 

 Hainanielis bark. 



2. True tannins — 



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

 {h) Ellagic acid group . . . divi-divi, algarobilla and myrobalan tannins. 

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



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

 caffetannic acid and the tannins of mat6, 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, 

 myrobalans, valonia, algarobilla, oak gall, oak wood and chest- 

 nut tannins. 



These tannins have the following characteristics : — 



1 . They give with ferric salts a dark blue colour. 



2. They give no precipitate with bromine water. 



3. They produce on leather a " bloom " consisting of 



ellagic acid. 



(B) Pyrocatechol tannins, including all the pine barks, 



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



+ Procter: " The Principles of Leather Manufacture," London, 1903. 



