Chromium tanning is currently the most important and widespread way of tanning for the production of leathers for shoe uppers, upholstery, garments and fancy goods. Above all basic chromium sulphate is used which sees the metal as undaunting trivalent cation. The spectre of a later and undesirable formation of its hexavalent alter ego has always undermined the confidence of each and every tanner. It may form a problem which might affect European and World tanning production.
Recently CEN/TC 309 “Footwear” has developed a method to evaluate the tendency of footwear materials to generate chromium (VI) after weathering. The current study has been carried out on a semi-industrial scale to experiment the effects which ensue from operational variables of re-tanning stages, dyeing and fatliquoring, both regarding the formation of hexavalent chromium in the samples produced and in the following production as a result of ageing.

The tests have led us to the following conclusions:

– The type and amount of fatliquor added have an important role in the development of hexavalent chromium
– The most easily oxidised chromium can be presumed as the one that is slightly or incompletely bound within the leather
– The number of intermediate washings interferes with the final content of Cr (VI) and precisely speaking, in the case of an increase there is a drop in the amount of hexavalent chromium.
– The influence of ageing on trivalent chromium has been highly and widely shown; above all, it has been pointed out to what extent the heat factor is predominant compared with the light factor
– It has furthermore been noticed how the amount of time, between the end of ageing and tests, may influence the amount of definable hexavalent chromium .

In the end, experiments have revealed which factors can lead to the formation of Cr (VI). As a result, it has been viable to pinpoint a series of effective strategies to curb this from happening.

– use of poorly oxidised well purified fatliquors which are without an excessively high iodine index and which can be easily emulsified.
– stockpiling of leathers in moist conditions that inhibit the formation of Cr(VI) (U.R.>35%),in a cool and not airy environment;
– a greater number of washing cycles in order to eliminate possible residues of unbound Cr (III)
– should it be possible, use of complex metal dyestuffs (especially complex chromium)
– resort to specific treatment using reducing or antioxidant agents

If these strategies are used, the risk of oxidation of Cr (III) will be cut but it is not enough.
The spectre of hexavalent chromium always hovers above the tanning industry like a black drape.

After two years of research and experimental trials carried out in Osmo’s own labs, the technical staff at Osmo have developed PRODOTTO CR – an indispensable vaccine against hexavalent chromium. A liquid chemical discovery that can be added to the drum to prevent chromium oxidation during the later stages or which can be sprayed onto the leathers already exhibiting the problem.