Plain charcoal, actually made from trees not the pitch-coke stuff you burn in the barbeque, works.
But adding some stuff, like bone charcoal makes the process go faster.
By adding some bone charcoal one may get case colors. Pretty.
Leather does nothing for pretty.
But leather can nitride the surface a bit.
Huh?
Make it harder on the very surface, better for sliding wear. If you like slick, a little leather tends to help.
P.I.T.A. Metallurgist