Sure, pure neatsfoot oil (warmed), light coat, set it in sun. It'll darken up. Remember to turn your project over!
What are you making? That leather seems pretty thick?
Brains. Can't get more natural than that.
I understand that every animal has enough brains to tan its own hide and enough guts to make sausage from all the meat. Nature's cool like that.Brains. Can't get more natural than that.