How leather is made

A hide is rotting the moment it comes off the animal, and tanning permanently locks its protein so it never rots.

stations: 6
things upstream: 26
raw sources: 14
steps from the ground: 6

The short version

Leather is animal hide whose protein has been permanently chemically changed. Hides are salted, soaked, limed to loosen hair, fleshed, and bathed in a tanning agent, mostly chromium salts. Afterward the hide is split, dyed, oiled and dried, and it no longer decays.

Salt the hides

Fresh hides are cured with salt as soon as possible. Salt draws out water by osmosis and stops bacteria from eating the collagen before the hides reach the tannery.

Goes in
Cattle hide Rock salt
Soak them soft

In a rotating drum, clean water washes the salt out and puts the water back so the hide can be treated.

Goes in
Water
Lime off the hair

Milk of lime makes the hide swell, loosens the hair and breaks the bonds that hold it in place. Machines then strip the hair and scrape off fat and flesh from the underside.

Goes in
Quicklime
Bate and pickle

The lime is washed out, enzymes soften the hide and a salt and acid bath lowers its pH so the tanning agent can get in.

Goes in
Rock salt
Tan with chromium

Chromium(III) sulfate is added to the drum. It forms bridges between the collagen chains, and the bath is then made less acidic so the bonds lock in. The pale blue result is called wet blue. Chrome tanning takes about a day and makes most of the world's leather.

about 1 day

Goes in
Chromium
Crust, dye and finish

The tanned hide is split to thickness, shaved, dyed, oiled, then dried and softened. A surface finish can be added, including buffing flaws away and embossing a grain.

End of the lineLeather Next it goes into mechanical wristwatch, baseball, basketball and 12 more.

Worth repeating

Chrome tanning accounts for 85 to 90 percent of modern leather making.
Traditional tanning stank so badly that tanneries were pushed to the outskirts of towns.