Tannins characterisation in new and historic vegetable tanned leathers fibres by spot tests

This paper describes the adaptation and evaluation of three chemical tests for tannins characterisation in vegetable tanned leathers. Tests were performed on fibres of new leathers tanned with different known vegetable tannins and historic leathers. Rhodanine test, nitrous acid test and acid butanol test, developed to identify, respectively, gallotannins, ellagitannins and condensed tannins, are described. Ferric test and vanillin test, the two traditional tests used for vegetable tanned leathers characterisation, were also performed and their usefulness discussed. Gallic acid, ellagic acid and catechin, structural constituents of the different classes of tannins were also tested. Results of the developed methodology allowed the identification of tannins’ chemical class in new and historic leather samples studied. Data obtained permitted to verify the information on tanning materials used in new leathers. Vegetable tanning technology was confirmed in historic samples and tannins were characterised. This study shows that these tests are useful and can be a valuable source of information to evaluate new vegetable tanned leathers quality for conservation and restoration purposes as well as historic leathers tanning technology.