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Your leather shoe soles are tanned using bark - the word 'tan' being derived by the principal of using tannin (from tree bark) in the process of preserving skins.
You can tan using tea leaves...

Another way is brain tan them - I did a few like that years ago and they're still good.

The American indians did it, and if you want them waterproof you smoke them.

I used sheeps brain I got...