There was a guy called Daniel Mendoza back in the pre gloved era who used to soak his fists in vinegar and use tree bark to train on , basically the tree was his punchbag.
Helped make his knuckles horned and more lethal and e was often involed in fights that lasted over 3-4 HOURS.
Crazy but true.
We used to go to a boozer in N London that had bare knuckle fights in the big room above.
2 Albanian fellas beat the absolute fuck out of each other for about 40 mins , actually turned my stomach the state their faces were in.
One of em looked like he'd been smashed in the face for about 10 mins with a cricket bat , he didnt even look human as the swelling was that bad it actually engulfed his nose and eyes etc.
Cunt wanted to keep fighting as well.
Made the one above look tame.