Gallic Acid is found most abundantly in the vegetable substance galls, whence it derives its name; but most astringent vegetable matter contains it. The simplest mode of obtaining it is probably that recommended by Mr. Fiedler. An ounce of powdered galls is to be boiled in sixteen ounces of water until reduced to eight, and then strained. Dissolve two ounces of alum in water, precipitate the alumina by carbonate of potash; and after edulcorating it completely by repeated ablutions, add to it the decoction, frequently stirring the mixture with a glass rod; the next day filter the mixture; wash the precipitate with warm water till this will no longer blacken sulphate of iron; mix the washings with the filtered liquor, evaporate, and the gallic acid will be obtained in fine needle crystals. The gallic acid, placed on a red-hot iron, burns with flame, and emits an aromatic smell like that of benzoic acid. It is soluble in twenty parts of cold water, and in three parts at a boiling heat. It is more soluble in alcohol, which takes up an equal weight if heated, and one-fourth of its weight cold. The distinguishing characteristic of gallic acid is its great affinity for metallic oxides, so as, when combined with tannin, to take them from the powerful acids.
The more readily the metallic oxides part with their oxygen, the more they are alterable by the gallic acid. To a solution of gold it imparts a green hue; mercury it precipitates of an orange yellow; copper, brown; bismuth, a pale yellow; lead, white; and in iron, a black; whence its use in making ink, and in the operations of the dyer in making various shades of black, and in improving or fixing other colours.