This is Colts bluing process back before WWII, produced absolutely deep beautiful blue. Guess S&W had similar.
That's the American Gas Furnace system. The 'rotary drum furnace'.
It used the proprietary oil Carbona, and thats where the term Carbona Bluing comes from.
Truth is the system will produce bluing with use of most any type of oil, even plain Linseed which Winchester used with the process when bluing no critical parts like small hardward for the guns.
The small amt of oil used is mixed into a small size 'char' which was at the time bone char.
Said to be the for the best results though again some have used certain wood char in part and bluing results as well.
It's a temper blue (850+F) so the rest of the ingredients are what produces the small but sometimes brilliant differences betw one and anothers work.
The underlying metal polish is of primariy importance.
This took the place of Charcoal Bluing which was done on open hearth burning wood charcoal.
Nearly the same look, they can be compared side by side and the difference can often be seen.
Just about all of the US gun industry switched to Carbona Blue system just before WW1.
Colt around 1911
Winchester used it,,S&W of course.
H&R, Savage, etc.
A great time saver and cut labor costs.
Rust blue was continued to be used for bbls, mag tubes and the like. SxS soldered assemblied certainly can't stand up to the temps of charcoal or carbona bluing.
About the only production rifle bbl I can think of that was Charcoal blued was the Henry rifle. There are likely others.
One-of hand built (muzzle loader) rifles were sometimes charcoal blued along with their steel fittings.
The polish given to the metal was and still it the real secret to the fine final finishes of the day.
Hot Salt Bluing came around in the industry in the late 1930's. for the most part.
Sloppy polishing gradually took the place of the fine finishes starting in the 1970's IMO.
The public has accepted the lesser quality, so there was no reason to offer anything better. Much the same in
most any factory made product.
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