That works well,,It's another version of heat blue. A temp to give an acceptable color and something to create a zero oxygen atmosphere around the part.
Nitre does the same.
The process has been used in the arts field, likely by others. Someone told me Springfield Armory used something like it at one time,,don't know if it's true or not.
It was actually the subject of an article in a jewelry & metal arts magazine several years back as a decorative way to color steel.
A torch was an alternative way to bring the temp and color up on small pieces.
It works well, but the color just doesn't match the depth & shade of a charcoal blue (near black) that's seen on trigger guards and trigger plates, or the early charcoal blued Colts ect.
It's actually one coating of it, if done at the same temps.
Perhaps it can be used again and again on the same object to build a finish. Something to try I guess!
It would be the slow rust blueing version of it vs the quick rust version being the charcoal pit method.
The pit method is a many layered process. Charcoal (and Machine Bluing) is a buildup of layers of oxide. It's thick enough that it flakes off when it gets older and/or abused. It may flake off if the surface isn't perfectly clean before you start too!
I wouldn't doubt for a minute that some parts weren't done that way just as they can be done at high temp in nitre.
Especially handy in small shops and one-of pieces to do.
There's always more than one way to do things.
Restoration sometimes limits your choices when matching the original is all important.
But sometimes an easier to use alternative can be found and used that suits the owner just fine.
