sources of Tannin

The selection of the appropriate blend is one of the chief problems of the tanner; according to the combination he can provide leather for any need, hard and firm, mellow and soft, light or heavy, varying in its natural color from dark reddish brown to nearly white. Two Englishmen, G. Biggin and the famous Sir Humphry Davy, were the first systematically to investigate the tannin contents of various plants. The vegetable tanning materials can be divided, roughly, into two groups each of which has somewhat different characteristics, as follows-

Pyrogallol Group. Infusions of these deposit a pale-colored sediment called 'bloom' (ellagic acid} which, if deposited in the leather, improves its solidarity, wearing properties and resistance to water. Hence they are favored for sole leather. They are also preferable for leathers intended for bookbinding, upholstery and other purposes where longevity is essential. The resultant leather is of pale color varying from creamy or :yellowish to light brown. The following are the chief materials:

Sumach	Leaves	Sicily
Chestnut	Wood	France, Italy and U.S.A
Myrabolams	Nuts	India.
Divi-divi	Pods	Central America
Oak-wood	Wood	Jugoslavia
Oak Galls		Asia Minor
Valonia	Acorn-cups	Asia Minor and Greece
*Oak-bark	Bark	Great Britain, etc.

Catechol Group. These deposit a reddish sediment known as 'reds' or phlobaphenes. 'They tend to produce a more solid leather of pink to reddish color.

Gambier	Leaves and Twigs	East Indies and Malaya
Hemlock	Bark	United States
Quebracho	Wood	Argentine and Paraguay
Mimosa (Wattle)	Bark	Australia and Natal
Mangrove	Bark	Tropical Swamps
Pine	Bark	World-wide
*Oak-bark	Bark	Great Britain, etc

*Oak-bark contains tannins of both groups

Formerly tanners prepared their own tannin infusions by steeping the ground material in warm water; this process is known as 'leaching'. The resultant liquors were weak and tanned slowly. Today the tendency is for tanners to drop leaching in favor of manufactured extracts usually prepared in factories situated near the source of the raw materials; the preparation of extracts is quite a separate and specialized business. These concentrated extracts come in the form of viscous liquors (30% tannin) or toffy-like solid extracts (60% tannin); their use, enabling the tanner to prepare stronger liquors, has greatly speeded up tanning. When the vegetable material is leached, in addition to the tannin which converts skin into leather, large amounts of other materials are extracted. These include sugars, starch, salts, acids and

1Australia was the original source of supply of wattle bark for tanning but supplies are now insufficient for her own needs and she has to import. Natal, where the black wattle was introduced from Australia, is now practically the only exporting country'.

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