"...leather used to be treated with boiling oil to make it harder..."
Actually that is something of a myth. Boiling in oil ruins the leather.
While the oil wouldn't hurt the leather, the heat destroys it.
Me and a friend of mine made leather armor as kids to sell at Ren Faires.
The correct method is to heat a pan of wax on a double boiler until it is
just molten and dip the leather in the wax. While the leather is still warm
it can be bent into shape (won't be able to after it cools). Excess wax is
polished off after the leather cools.
Didn't know that, thanks Jared for the explanation!