Those black spots on the metal. What is that?

[Doug Miller]

"Controlled rusting"? Or controlled oxidation?

[CW]

The process was known as cold rusting. Rust (red) would actually be formed and would be polished off leaving the black oxide. To get a uniform surface, the rusting and polishing have to be repeated a number of times. The metal needs to be polished when the rust layer is light. If left to rust to long, pitting would occur. When I was a kid, I used to do this with knife blades. It took some time to get a uniform finish but, once you did, it was quite resistant to further rusting. I thought that I had come up with quite the process. Later I learned how old the process actually was. Reinventing the wheel, as it were.

[Doug Miller]

Interesting. Didn't know that. Thanks for the explanation.

[dingbat]

Generally known as "browning" in the UK. Still fairly common amongst gunsmiths, especially as legal UK shooters are now generally firing antique black powder kit.

[Swingman]

Very common here at one time also. My grandfather's old Damascus barreled shotguns that I grew up duck hunting with were not "blued", but "browned".