Color case hardening finish .vs Boiling...

[Eaglelord17]

Jim is correct. The oxidation that creates those swirls is very delicate. Boiling would most likely greatly diminish if not remove what is left of it, carding would erase any trace it was ever there. Most new factory color-cased finishes will add a clear coat over top to protect them (e.g. uberti reproductions). If you want them to last, keep solvents away, and keep the surface protected. Oxygen will slowly degrade it and make the colors less vibrant over time.

as for restoring, the only way is to redo to he color case hardening process. There is a video on YouTube somewhere of the guys at Turnbull restoration restoring the color case hardening on some old lever guns. They pack the actions in bone black or charcoal mix, heat in an electric furncace to a specific temp for specific time, then pull out and quickly water quench. Also, it's worth noting that the colors only develop in an oxygen depleted environment. So what I described above is really the only way to do it.
I've read of someone having limited success on small parts with an improvised setup of a tin can, and a charcoal BBQ, but the colors are never as vibrant as doing it a more controlled procedure. There is another video I saw somewhere of a guy doing this improvised method to fabricate some leaf springs he needed for an old rifle. worked great for functionally to temper the parts.

I am not a expert by any means on colour case hardening, but I recall reading most (basically all) colour case hardening finishes on modern reproductions aren't actual case hardening resulting in that colour, rather a chemical solution they apply to the metal resulting in that pattern instead. That could also have a fair bit as to why they use clear coats as the chemical reaction is only on the very surface and likely isn't as deep as a classic case hardening procedure.