On Gallic and Tannic Acids in Dyeing. 265 



On the Action of Gallic and Tannic Acids in Dyeing. By 

 F. Crace Calvert, F.C.S., M.RA. of Turin, Professor of 

 Chemistry at the Royal Institution, Manchester. 



Persoz, in his Traite de Vimpression des Tissus (vol. i. 

 page 262), remarks, that " It is desirable, as much for the in- 

 terest of the manufacturer as for that of science, that we 

 should know positively whether it is gallic or tannic acid which 

 plays the most important part in dyeing with gall-nuts." This 

 statement of Persoz, together with the knowledge of the fact 

 that the manufacturers of extracts of colouring matters, were 

 prevented from preparing extracts of tannin masses, by the 

 rapid change which these extracts undergo, induced me to 

 make the following researches, with the hope of throwing some 

 light on the subject. 



The first experiments were made with the view of ascertain- 

 ing the action of gallic and tannic acids in the dyebeck. For 

 this purpose, I dipped 100 square inches of iron-mordanted 

 cloth into baths composed of 20 grains of these acids, and 1J 

 pint of water ; and the dyeing was allowed to go on in the cold 

 for 24 hours. It was found that the gallic acid rapidly dyed 

 the iron mordant, but the colour soon disappeared, whilst with 

 the tannic acid, although the black was slower in forming, it 

 remained permanent. Similar trials were then repeated, but 

 gradually raising the temperature of the bath during 1J hours 

 to 180° Fahr., and then for half an hour to 212°. The general 

 results were similar, the only difference being, that the black 

 at first formed with the gallic acid, more rapidly and com- 

 pletely disappeared than in the experiments done at natural 

 temperature. 



These facts led me to believe that the gallic acid acted as a 

 reducing agent on the hydrate of peroxide of iron fixed in the 

 cloth as a mordant. To substantiate this view, I took a por- 

 tion of the liquor from the bath in which the dyeing process 

 had been conducted, and on examination found it to contain a 

 large quantity of protoxide of iron in solution ; whilst in the 

 case of the tannic acid liquor, no reduction of the oxide of iron 

 had taken place. I also added a small quantity of hypochlo- 



