This process of tanning leather with the use of chromium salts was invented in 1858. During this process the hide is subjected to treatment using chromium(III) compounds which are significantly less toxic than hexavalent chromium. Chromium sulphate and other salts of chromium are used to increase the space between the protein chains of collagen.
Before tanning with chromium salts the pH of the hide must be significantly increased in order to improve the efficacy of penetration by the tanning agent. This ensures that the chromium complexes are small enough to fit between the fibres and residues of the collagen.
Once the optimal penetration has been achieved, the pH of the hide is raised again by a process called basification that allows the molecules of chromium to cross-link with the protein chains of the collagen. The chromium then forms extremely stable bridge-bonds which is why it is considered one of the more efficient tanning compounds. The bridge bonds provide hydro-thermal stability and increase the leather’s resistance to shrinkage on exposure to hot water.
In their raw state, chromium tanned hides are bluish in colour and this is referred to as ‘wet-blue’. More esoteric colours are also achievable using chromium as the tanning agent. The process of tanning using chemicals is also time-efficient since it takes just one day.
The leather that results from this process is stretchable and pliant and is excellent in the manufacture of leather garments and handbags.