Tanning is the process of preparing or processing skins/ hides into leather using tannic acid. The raw collagen fibres of the pelt are transformed into a stable material that will not rot. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard, inflexible material that when re-wetted (or wetted back) putrefies, while tanned material dries out to a flexible form that does not become putrid when wetted back. The tanning process significantly improves the natural qualities of the leather such as its dimensional stability, abrasion resistance, chemical and heat resistance, its resistance to repeated cycles of wetting and drying.
Importance of Tanning
1. It protects the leather from being dehydrated- The tanning processes always ensure that the leather maintains its inner moisture.
2. It protects the leather from decaying when subjected to water- Chemical treatment of leather which is part of the tanning process prevents the leather from going bad as a result of rotting.
3. It makes the leather porous- Working on the leather through the tanning processes opens up the leather so that it becomes airy and absorbent.
4. It greatly improves the tensile strength of the leather- Tanning builds up resilience in the leather. This makes the leather resist all kinds of weather conditions.
5. It enhances the flexibility of the leather- Tanning makes the leather supple and soft improving its workability and moulding qualities. This makes it easy to be utilized in the production of leather articles.
Kinds of Tanning Processes
1. Vegetable-tanning: This tanning process involves the use of tannins and other ingredients found in vegetable matter derived from wood and plants. Examples include chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle (acacia), and myrobalan. It is supple and brown in colour, with the exact shade depending on the mixture of chemicals and the colour of the skin. It is the only form of leather suitable for use in leather carving or stamping.
Vegetable-tanned leather is not stable in water; it tends to discolour, and if left to soak and then dried will cause it to shrink, render it less supple, and harder. In hot water, it will shrink drastically and partly gelatinize, becoming rigid and eventually brittle.
2. Chrome-tanning: This tanning process was invented in 1858. It is the most widely used tanning process today. It involves the use of chromium sulfate and other salts ofchromium. It is more supple and pliable than vegetable-tanned leather and does not discolour or lose shape as drastically in water as vegetable-tanned. It is also known as wet-blue for its colour derived from the chromium. More esoteric colours are possible using chrome tanning.
3. Mineral Tanning: In mineral tanning, the pelts are soaked in mineral substances usually the salts of chromium, aluminum and zinconium.
4. Oil Tanning: In this tanning process, the pelts are soaked in certain fish oils which tend to produce a very supple, soft and pliable leather like chamois.
5. Combination tanning: This is a tanning technique that combines two or more of the above tanning techniques discussed. Mostly, it is a combination of vegetable and chemical tanning. The pelts are first tanned using the chrome tanning technique and is later re-tanned using the vegetable tanning process. A blend of two tanning techniques is deliberately done to achieve a very supple leather. Also, leather that is to receive a finishing technique because of its final use sometimes goes through the combination tanning process.