Scorch... speaking of case color... have you ever seen a 99 receiver case colored?
Yes, I have. A very finely engraved Savage 1899 in 303 Savage with premium wood and checkering.
it's case coloring... not case hardening, which is a metal treatment.
No, actually it is
color case hardening. It is carburizing, a heat treating process. Originally, CCH was done to surface harden the iron or mild steel used to make critical parts (think S&W hammers and triggers, early Winchester levers and hammers), but later it was used as a decorative embellishment. The colors come from the mix of materials the metal is packed in before heat treatment. The crucible is packed with bone charcoal, feathers, leather, and sometimes a "secret ingredient" (blood meal, cow dung, hooves, etc) to give the results a "special look". The crucible is heated to about 1,200 degrees and the metal is quenched in oxygenated water. There is also a ferrocyanide method of color case hardening, similar to the nitriding process currently used, that would give a striped colored look (early Stevens or Savage rifles used this process). Color case hardening gives a surface hardness of about 70 Rc to a depth of about .005"-.010" (as opposed to today's gas case hardening that gives surface hardnesses up to 75-80 Rc penetrates about .020"-.025").
The packing material gives the finished metal a predictable appearance. If the metal were packed in wood charcoal, the metal would be dark gray when it comes out. Mauser 98s made up to about 1950s were carburized by packing in coke and heating to about 1,100 degrees then quenched. The dark gray color of original M98s was the color as it came from the carburizing process.
Nowadays, you see coloring added to some firearms' surfaces to simulate color case hardening (Uberti SAAs, Pedersoli rifles, etc), but it is just a dye process.
Wouldn't the case color hardening still be under the bluing?
No, when the metal is polished, it goes through the case hardening to softer metal below (case hardened steel is hard to blue). The colors are just on the surface, which is why they wear off.
