Tannin, Tannic Acid, or DIGALLIC ACID

Tannin, Tannic Acid, or DIGALLIC ACID, is an astringent substance found in gall-nuts, and hence called gallotannic acid. The name tannin has, however, been applied to a number of different astringent compounds existing in the bark or leaves of most trees. These compounds, though not intimately related to each other, yet agree in giving blue or green-black compounds with salts of iron, and in producing leather by their action on animal skins. The true tannic acid is a colourless amorphous body, obtainable in fine scales or vitreous masses by the evaporation of its solution. A yellowish colour soon develops in the acid even when excluded from light. It is readily soluble in water and alcohol, but insoluble in chloroform. It reddens litmus, and has a powerful astringent taste and local action. On this account it is employed for relaxed throats as a gargle, or in the form of lozenge. As a styptic it is of much value. In the arts it is employed in the manufacture of ink, although most makers employ the crude gall-nuts in preference to the purified acid. The various tanning substances containing the other varieties of tannin are more employed in leather-making and Dyeing (q.v.).