210 TANNINS 



that contained in oak galls. This substance is, however, 

 better named gallotannic acid, as it is customary to name the 

 tannins after the source from which they are obtained ; thus 

 quercitannic acid indicates the tannin of oak bark, sumac- 

 tannin that derived from sumac, and so on. 



PYROGALLOL TANNINS. 



GALLOTANNIC ACID. 



(Syn. Tannic acid, or merely " Tannin ".) 



The two chief commercial sources of gallotannic acid are : 



1. Turkish or Aleppo galls, produced by the gall wasp 

 Cynips gallce, which lays its eggs in the buds of Quercus infec- 

 toria. These contain from 50 to 60 per cent of gallotannic acid. 



2. Chinese galls, produced by the burrowing of Aphis 

 chinensis in the leaf-stalks of young twigs of Rhus semialata. 

 These galls may contain up to 70 per cent of gallotannic acid. 



Gallotannic also occurs in sumach (Rhus Coriaria), in tea, 

 and in many other plants. 



Extraction of Gallotannic Acid. 



Gallotannic acid is best prepared by extracting finely-, 

 powdered gall nuts with a mixture of twelve parts of ether 

 with three parts of alcohol ; twelve parts of water are then 

 added and, after shaking, the lower aqueous layer is run off 

 from below and evaporated. The resulting tannic acid may 

 be decolorized by boiling with animal charcoal. 



Pelouze recommends the following method : The pow- 

 dered material is heated under a reflux condenser with a 

 mixture of thirty parts of ether, five parts of water, and two 

 parts of alcohol. On cooling three layers of liquid are formed, 

 of which the lowest contains 33 per cent, the middle 8 per 

 cent, and the top 2 per cent of the tannic acid present in the 

 substance. 



Gallotannic acid forms an amorphous powder-]- which, 

 when pure, is almost colourless ; it is readily soluble in water, 

 forming a solution with an astringent taste and which reacts 



* What is known as " Crystal tannin " in commerce is not really crystalline; 

 it is made by drawing a syrupy solution into threads and breaking these up aftet 

 drying. 



