Leather
Animal hide stabilised so it will not rot — one of the oldest manufactured materials, and where a great deal of the world's chromium ends up.
A raw hide is collagen, and collagen rots. Tanning cross-links the protein so it can no longer be digested by bacteria or enzymes, and the result is a material that stays flexible, resists water, and lasts for centuries rather than weeks.
The process is genuinely ancient — worked hides predate agriculture — and the chemistry was understood empirically for millennia before anyone could describe what the tannins were doing.
Processing
Vegetable tanning uses plant tannins, takes weeks to months, and gives a firm leather that darkens and develops a patina — the traditional method and now the minority one.
Around eighty per cent of leather is chrome-tanned instead. Basic chromium sulfate cross-links the collagen in a day rather than a season and gives a softer, more water-resistant, more consistent product, and the chromium stays in the finished material at a few per cent. That makes leather a significant destination for chromium, and it makes tannery effluent a serious pollution problem where it is not controlled — the chromium used is trivalent and comparatively benign, but the volumes are large and oxidation to the toxic hexavalent form is possible.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Chromium — element · at a few per cent in chrome-tanned leather, which is around eighty per cent of what is made — a significant destination for the metal, and the reason tannery effluent is a pollution problem
- Nitrogen — element · collagen is a protein, and tanning cross-links it rather than replacing it
is produced by
- Tanning — process · the process that makes a hide into a material — everything before it is preparation and everything after it is finishing
is an alternative to
- Polyvinyl chloride — material · as synthetic leather, which is mostly PVC or polyurethane over a fabric backing — cheaper, uniform, and it does not develop a patina because there is no collagen to age
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing