The next step in the leather-making process after pickling is tanning-or the conversion of putrescible animal skins or hides into a stable commercial product-leather. When a protein is altered in chemical composition so as to become more resistant to heat, hydrolysis and micro-organism, it is considered to have been tanned and the chief engine of change is called a tanning agent These vehicles are vegetable tannins, basic chrome salts, zirconium, aldehydes, syntan and unsaturated oil.

In turning back the pages of time we find that tanning was one of the earlist crafts known to mankind. Before the tanning process pessaged from the crafts to the science, man turned to mother nature to provide him with his materials-leaves, twigs, braches, barks, wood, roots and fruits of her many plants which yielded substance dissimilar in composition and nature but possessing tanning properties. Triggered by the development of modern techniques, the tanning methodsput on the white robe of a chemist, experimented in the world"s laboratories and come up with some revolutionary processes of which the principal one is chrome tannage.

Chrome tanning finds fovour over the other tanning methods chiefly for two reasons: a)it can be acdompanied in a much shorter time and b) a produces a leather that combines to the best advantage most of the chemical and physical propeties desired by the leather users.

Though the actual chemistry involved in chrome tanning is quite complex, an endeavour to simplify it follows. It should howver be noted that the formulae are general and broad and the reader is recommended to refer to standard books for greater details.

Chromium salts like chromium sulphate (Cr₂(SO₄)₃) or chromium chloride (CrCl₃) have no tanning properties. To have any tanning property the chrome salt must have hydroxyl(OH) group in the complex directly attached to chromiumatpm. Such chromium compounds are called basic chrome compound. When they are dissolved in water, they hydrolyse and yield extremely acid solution.

• Cr₂(SO₄)₃ + 2 H₂O = 2Cr(OH)SO₄ + H₂SO₄
• CrCl₃ + H₂0 = Cr(OH)Cl₂ + HCl

Chromium belongs toa group of elements known as trivalent represented as Cr each stroke representing one valency linkage. The above equation then can be written as follows:

It will be oberved from the above equation that in the formation of basic chromium salt, a hydroxyl(OH) group is introduced in the molecule. By adding an increasing amount of caustic alkali it is possible however to have two or all the acid radicals replaced by hydroxyl gruops(Basic groups). The compound thus formed are described as basic salts. Addition of an increasing amount of alkali to the chrome saltsolution progressively increasesit astringency (power of tanning). When al the acid groups are replaced by hydroxyl groups chromium hydroxide(Cr(OH)₃) is formed which is insoluble in water and precipitates. This can illustrated as follows:

The collagen fiber of hides and skins can only fix basic Chromium slats. Since the number of basic (OH) groups held by the basic Chromium salt is decivise factor in the tanning action, the tanning power(Basicity) of the chrome compound is expressed as "The percentage valencies of Chromium salt attached to hydroxyl group".

Continued