Tanning
Cross-linking the collagen in a hide so it will not rot — one of the oldest manufacturing processes there is, and still one of the dirtiest.
A raw hide dries to something like board and wets to something that rots within days. Tanning converts it permanently: a tanning agent binds between the collagen molecules and cross-links them, so the protein can no longer be attacked by bacteria or enzymes and no longer stiffens as it dries.
The result keeps the hide's structure — a dense three-dimensional weave of fibres that is why leather is simultaneously strong, flexible and breathable, and why no synthetic substitute has quite reproduced it.
Processing
Most of the work happens before any tanning agent is added. The hide is soaked, limed to loosen the hair, unhaired, and split to thickness; the alkali is then neutralised and the remaining non-collagen protein digested away. Only then is the hide ready to take a tan.
Vegetable tanning uses tannins extracted from bark, wood and leaves, which bind to collagen by hydrogen bonding. It takes weeks in a sequence of pits of increasing strength, and gives a firm leather that darkens with age.
Chrome tanning uses basic chromium sulfate, which forms much stronger coordinate bonds with the protein and does the job in a day. It gives a softer, paler, more water-resistant and far more consistent product, and it is around eighty per cent of world production. The chromium remains in the finished leather.
Economic significance
Tanning has been a nuisance industry for its whole history. It is water-intensive, it smells, and the effluent carries hair, lime, protein, sulfides and — since the nineteenth century — chromium, which is why tanneries were traditionally sited downwind and downstream of everywhere else.
That has not stopped being true. Where regulation is weak the discharge is the classic example of an industry exporting its costs to a watercourse, and the chromium is the part that persists. The trivalent chromium used is comparatively benign; the concern is volume and the possibility of oxidation to the hexavalent form.
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.
produces
- Leather — material · the process that makes a hide into a material — everything before it is preparation and everything after it is finishing
takes as input
- Chromium — element · as basic chromium sulfate, which cross-links collagen in a day where vegetable tannins take weeks, and stays in the finished leather
- Wood — material · as bark rather than timber: vegetable tanning uses tannins extracted from bark, wood and leaves, and it is the older of the two routes by several thousand years
Sources
- Material WorldOur own writing