Vegetable tannins for the most part are uncrystallizable colloidal substances with pronounced astringent properties. They have the ability to precipitate gelatin from solution and to form insoluble compounds with gelatin-yielding tissues. which is the property which enables them to convert raw hide and skin into leather, consolidating the dermal network of the hide into firmer and drier structures of improved thermal stability, durability, and water resistance.
Because they are extremely complex substances, vegetable tannins are difficult to classify; however, they are usually considered to consist of polyphenolic systems of two types: the hydrolized tannins (the pyrogallol class), the main constituents of which are esters of glucose with acids such as chebulic, ellagic. gallic and m-digallic; and the condensed (catechol) tannins. which are based on leuco-anthocyanidins and like substances joined together in a manner not clearly understood. The pyrogallol tannins may be hydrolyzed by acids or enzymes and include the gallotannins (from plant galls) and the ellagitannins. which produceBLOOM (1) on leather, and which are characteristic of divi divi, myrabolans, sumac, tara, valonea, and other well-known tannins. The condensed tannins are not hydrolyzable and are characteristic of hemlock, mangrove, quebracho, wattle, and the like. The condensed tannins are more astringent, i.e., they tan more rapidly, than the pyrogallols have larger molecules and are less well buffered. They yield less sediment. or lose less upon standing, but the leather they produce often tends to turn a reddish color upon exposure to natural light. They also yield phlobaphenes or REDS .
The terms "condensed" and "pyrogallol," as such do not mean that the tannins contain these substances but simply indicate that dihydric and trihydric phenols are produced respectively when the materials are heated (dry distillation). Quite often the "tannin" derived from a plant material. e.g., oak bark. has characteristics of both groups and consequently is generally considered to be a mixture or compound of the two principal types.
The two classes of tannin also display different reactions towards acqueous solution of iron salts. The condensed tannins produce green-black colors while the pyrogallol class gives blue-blacks (a reaction important in the manufacture of some inks). 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. and, being better buffered. i.e., containing weak organic acids and their salts, they resist changes in pH value when acid or alkali is added.
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. the latter usually being concentrated under reduced pressure to provide highly viscous or even solid products. Other materials extracted are known as non-tannins (abbreviated non-tans) and may include: hydrolysis products of the tannins, starches, gums, hemicelluloses, poly-saccharides, hexoses, pentoses, uronic acid. organic acids (lactic and acetic), together with their salts, inorganic salts, proteins and zymoproteins (enzymes), if the temperature is not too high, as well as coloring matters such as brasilin, fisetin, and quercetin.
Vegetable tanning liquors are extremely complicated in their chemical composition, and the tannin/ non-tannin ratio, the color, and the particular substances involved have a considerable (and far from completely understood) bearing on their tanning properties and, therefore, on the quality of the leather produced.
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.
The Dicotyledons include many families in which tannin occurs quite freely, among which the most noteworthy are the Leguminosae, e.g., the black wattle; the Anacardiaceae, e.g., quebracho and sumac; the Combretaceae, e.g., myrabolans; Rhisophoraceae, e.g., mangroves; Myrtaceae, e.g., eucalyptus; and Polygonaceae, e.g., canaigre.
Tannin may occur in almost any part of a plant, including roots, stems or trunk, hark, leaves, fruit, and even hairs. It may occur either in isolated individual cells, in groups or chains of cells (the more common occurrence), or in special cavities or sacs. 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. As the cell ages and loses its protoplasmic contents, the tannin usually becomes absorbed in the cell wall; in dead plant tissue tannin often accumulates in considerable quantities. Tannins often occur in green or immature fruits, the quantity decreasing as the fruit ripens. and they may also occur in seeds. often becoming more abundant following germination. Tannin is also quite prevalent in tissues as a result of pathological conditions, such as plant galls. Certain of these galls constitute the richest sources of tannin in plants, e.g., Chinese galls, which have a tannin content ranging from 50 to 80%.
The use of vegetable tannins in the manufacture of leather probably predates recorded history, and there is creditable evidence that they were in use in Egypt as far back as 5000 B.C.
The ancient Greeks and Romans were well versed in the art of vegetable tanning and evidence indicates that vegetable tannins were used in China many thousands of years ago. (175 , 248 , 291 , 306 , 363 )