The two classes of tannin also display different reactions towards acqueous solution of iron salts. Furthermore, they differ in their tanning properties. Pyrogallol tannins, for example, being less astringent than the condensed class, tan more slowly and produce leather of less solidity. In addition, when extracted from the plant they generally contain smaller molecules of tannin. For a complete and even reaction with the skin or hide to take place during tannage, it is necessary to use the tanning material in the form of a liquor, i.e., an aqueous infusion of the plant material.
Modern tanneries use extracts that are concentrates of acqueous liquors.
Although tannins occur throughout the greater part of the vegetable kingdom, they are more prevalent among the Angiosperms, or higher plants, especially in certain Dicotyledon families, than they are among the lower types, such as fungi, algae, etc. The Gymnosperms also have classes in which tanning is well developed, e.g., the pines, spruces and hemlocks. Tannin may occur in almost any part of a plant, including roots, stems or trunk, hark, leaves, fruit, and even hairs. It may also be present in latex vessels and lactiferous tissue accompanied by other substances. In the living plant, tannin is present chiefly in solution in the vacuoles.