The bark and young cones afford a tanning material, inferior indeed to oakbark, and hardly equal to that of the larch, but of value in countries where substances more rich in tannin are not abundant.
An excess of the precipitant is shown by a drop of the solution imparting a yellow colour to a solution of tannin, prepared by dissolving one part of tannin in 300 of water; drops of this solution are placed on a white porcelain plate, and as the precipitant is added to the lead solution a drop of the latter is removed from time to time on a glass stirring-rod and added to one of the drops on the porcelain plate.
The commercial value appears to depend on the essential oil and aroma, not on the amount of caffeine, tannin or extract.
In British pharmacy gall-nuts are used in the preparation of the two astringent ointments unguentum gallae and unguentum gallae cum opio, and of the tinctura gallae, and also as a source of tannin and of gallic acid.
Owing to the quantity of tannin they contain, no harmful miasma exhales from the Irish bogs.
The character of the acidity, however, changes, the free tartaric acid gradually disappearing, forming bitartrate of potash and being otherwise broken up. On the other hand, the free malic acid increases and the tannin decreases.
The proteid matter combines with a part of the tannin in the wine, forming an insoluble tannate, and this gradually subsides to the bottom of the cask, dragging with it the mechanically suspended matters which are the main cause of the wine's turbidity.
The principal chemicals in tea are: caffeine, tannin and essential oil.
It may be obtained by extracting powdered gall-nuts with a mixture of ether and alcohol, whereupon the tannin is taken up in the lower layer, which on separation and evaporation yields the acid.
Frequent doses of a teaspoonful of tannin dissolved ' in water should be administered, together with strong tea and coffee and mucilaginous fluids.