Still here,,
This Charcoal Blue /Open Hearth method is what Colt and others used before the American GAs Furnace Co. method came on the scene.
Colt blued all pistols including the first year Model 1911's with Charcoal Blue.
Then the switch to the Amer Gas Furnace Co. rotating furnace took place. Somewhere in the 1912 time period the change took place.
A mint condition 1911 mfg'd Model 1911 and a 1912 mfg'd Mod 1911 placed side by side will show the very slight but yet noticable difference in the tone/color.
Any other Colts blued on either side of the change in operation will do as well.
Rifle Bbls were always rust blued at colt.
IIRC the only rifle bbl that was charcoal blued in a factory was the Henry rifle bbl. I can't think of another but there probably were others.
S&W changed over to the Amer Gas Furnace system about the same time as Colt. S&W kept theirs running much longer than Colt according to Roy Jinks their historian.
Winchester, who also used the AmerGasFurnace stopped using it and went to HotSalt DuLite just before WW2
The first of the Win21's were blued using the Amer Gas Furnace method. That method of bluing is also using a temp in the 830/850F range.
I've engraved/recut a number of the early 21's that were blued using that method and they seemed to be as tough on the chisels as the later hot blued 21's which didn't see those temps in bluing.
Winchester went to hotsalt blue (300F+ temps) around 1939)
I can't recall engraving any casehardened parts that had been thru this charcoal blue process.
Cutting case hardened steel is kind of odd. Since the 'case' can vary from nearly nothing to a few .000, cutting thru it is more like punching thru it, Getting underneath the hard surface and into the soft steel, the graver goes along quite nicely. Carbide gravers are a near necessity though. HS will just nub over getting through even thin case.
The problems are that the cut you make as you chisel merrily along looks somewhat like plowed furrow.
It doesn't cut cleanly on the surface. The soft steel underneath responds just fine, but that thin glass hard case breaks up in tiny fragments as the upper portion of the tip of your chisel point pushes it's way through.
Also when you come to the end of a cut, you have to dive through the casehardening once again to begin the next one. You do that hundreds if not thousands of times on a pattern. It gets to be laborious trying to get through the case hardening and it's various thicknesses.
Then there is the issue of cutting fine lines. There is almost no way to do that. A light tough such as shading is impossible as you can't get through the case. You can get a scratched surface as the tool skips off the hard case, but that's about it.
Any punch work and you just batter the punches and flatten them to useless spindles.
