Chrome VI and leather
’Chrome Six’ is a presence in leather and an undesirable effect related to the use of chrome-tanning agents. Since tanning with chrome salts constitutes roughly 80% of the processing technology worldwide, the debate about the risk for its presence is too important to be based on hearsay alone and should, therefore, be based on facts.
What we call ’Chrome Six’ is chemically known as Cr(VI) or Cr6+. Cr(VI) is one of the possible forms of the element chromium. These different forms are expressed as oxidation states. For chromium, from now onwards to be called ‘chrome’, they are: +6 +5 +4 +3 +2 +1 0 and -1 -2. For tanners, the trivalent and hexavalent states are of importance.
Trivalent Cr3+ is used during tanning. This oxidation state is causing its green colour when the element reacts with oxygen to Cr2O3. It enables the production of green pigments and gives emeralds their incomparable colour. Cr3+ is also a trace element essential for our bodies to function.
The hexavalent Cr6+ is used as an intermediate in metallurgy and in chemical synthesis. This intermediate step strips chrome of its impurities. It's necessary before chrome can be used as ingredient for various purposes. Cr6+ has a known toxicity and is classified as CMR (Cancerogenic – Mutagenic – Reprotoxic).
The stable oxidation state in acidic conditions is trivalent. In the alkaline, medium chrome can react to hexavalent compounds. For tanners it is important to be aware of this possibility. Since the formation of Cr(VI) in leather is directly related to the use of chrome tanning agents, one needs to distinguish between the presence (in wet blue) and the risk of formation during further processing.
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