[ithacaartist]

David, here's a leg up for your terminology.

The term "fire blued" is usually applied to the cosmetic coloration done to certain small parts on the early Lugers. The grip screws, front sight blade, and the sear bar spring were subjected to heat in order to turn their surface molecules a bright blue color.

Rust bluing is a slow, cold process during which chemicals are wiped onto the parts, which are left to rust in a humidity box, then boiled in distilled water to turn the red rust to black/blue. In between each treatment, the gun's surfaces are "carded" with steel wool to smooth the surface. The process is repeated several times until the proper finish color is achieved. This was how Lugers made before the middle of 1937 were finished.

Mid 1937, the Mauser factory adopted "hot dip" or "salt bluing," wherein the parts were immersed in high temperature liquid chemical salt baths. Completed in one shot, it's much less labor intensive than the hand-done rust bluing, so the factory saved considerable time/money with this process.

Early 20th century guns (but not Lugers) sometimes had a finish called "charcoal bluing," during which the parts were buried in smoldering charcoal to develop a very handsome dark blue finish, albeit quite thin--somewhat akin to fire bluing.

I think you meant "rust blue" above, for Imperial or Weimar era Lugers...although some of the small parts are fire blued, or "strawed."