I would think that most parts like hammer, breech blocks, trigger need to be case hardened before being continually put into use.  Many of these parts were blued or blacked.  We all have seen case hardened parts that were blued over and now have a purple cast to them.  There may be more ways to blue but what I’m familiar with is modern hot blue, cold blueing, nitre and rust blue.  I’m guessing the modern hot blueing is what is giving the purple to the parts but please correct me if I’m incorrect.  So my question is what was used back in the day when these old guns were made?  Since rust blueing has been around a long time I assume that is the answer but something like nitre may have been used for efficiency reasons. I’m interested and would like to know for a project I currently have going.  I’m looking for the darker black blue without the purple.
Thanks, Bob

Thanks Vall, just the answer I was looking for.

Dave, great info on fixing the purple.  When you say “blue wears off case hardening easily” are you referring to just the hot blue or all types of blues?