Soluble iron compounds catalyse the oxidation of a large group of organic substances related both to the tannins and to hide substances. Oxygen is taken up on the surface of wet fibres and produces hydrogen peroxide as a result of the catalytic effect of the surface in the presence of strong acid (as it is observed in the case of activated carbon and sulphuric acid). Certain tannins may oxidize directly with production of hydrogen peroxide. The peroxide so formed is decomposed by the catalytic effect of traces of iron and the oxygen soliberated brings about the direct oxidation of the hide-substance which loses its fibrous structure as a result. When an active dry surface, thus, reacts with molecular oxygen, peroxygen is formed. The latter reacts with sulphur dioxide producing sulphurous acid, which, in the presence of water will be converted to sulphuric acid. Conditions imposed upon a tannage in order to achieve the highest resistance to aging are: (1) the pH of the liquors in the tannage should not fall below 4.0; (2) the salt content of all the liquors must exceed 0.4 N; (3) the liquors should contain enough oxalate in solution to form complexes with any iron present; (4) after tannage, the wet drained leather should be drummed with successive additions of a soluble oxalate and of a soluble calcium salt to form that quantity of calcium oxalate which experience shows is required to protect such a leather (this may vary from 1-10%). After washing and fatliquoring, the leather may be processed further or dried in the usual way.