Others have detailed the process of bone charcoal case hardening which imparts colors to the surface as well. Over the years, gunsmithing books have told of the various mixes of charcoal, charred leather, and such that were added to the crucible. Some say that some gunsmiths urinated in their secret charcoal mix to get different colors. I suppose anything is possible, but it wouldn't be pleasant. Actually, it seems the quench process has more to do with getting vibrant colors than the pack, but it's probably both... and the steel... and the temperature... and the time... The folks who do it well are often pretty secretive about their process.
When I was an apprentice electrician, I spent several months working in a blast furnace. The restrooms were some distance away from the cast house, and as a prank, occasionally someone would urinate in an iron runner between heats. When the furnace was tapped and the molten iron hit the urine soaked coke dust lining the runner, it would flash to steam, often with an impressive explosion. The smell would fill the cast house, and it was nasty. Very nasty! Even though the cast house had no sheeting on the sides, and the cold winter winds were blowing through, the aroma was disgusting. Superheated human piss smells far worse than any skunk. I can't imagine that anyone would intentionally heat urine with a torch to put fake case colors on a gun. But people do some strange stuff. It sure wouldn't make your wife or neighbors happy. Maybe Ed will chime in.
