Stopping something from rotting
How Does Skin Become Leather?
Untreated animal skin goes stiff, then rots. Tanning changes it into something that lasts for decades.
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Step 1 of 5
Skin is a mat of tangled fibres.
Under the surface, skin is a dense tangle of protein fibres running in every direction. That tangle is why it is tough and why it stretches a little whichever way you pull.
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Step 2 of 5
Left alone, bacteria eat it.
Those fibres are food. Bacteria get in and break them down, which is exactly what rotting is — so raw skin has to be treated within days or there is nothing left to use.
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Step 3 of 5
Tanning locks the fibres together.
Tanning soaks the skin in chemicals that clamp the fibres to each other. Bacteria can no longer take them apart, and the fibres can still slide, so the leather stays bendy.
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Step 4 of 5
The old way used tree bark.
For thousands of years the chemical came from crushed oak bark steeped in water. It works, it smells strong, and it takes months — the pits had to be tended the whole time.
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Step 5 of 5
It wears in rather than out.
As leather is used, body heat and movement work the fibres loose in the places that bend. That is why a worn boot or belt fits better than a new one, and why creases stay where they form.
Leather has to breathe. Sealing it completely in a plastic bag traps moisture inside and lets mould grow on it.
The short version
Leather is animal skin whose fibres have been chemically clamped together so bacteria cannot eat them.
Try it yourself
Compare an old leather shoe with a new one. The creases are exactly where it bent most — the fibres loosened there.
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