What is a blueing finish on a gun?

Answer 1

Bluing is basically a controlled oxidation, similar to rust. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron, which occupies the same volume as normal iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action.

In contrast, rust, the red oxide of iron (Fe2O3), does not occupy the same volume as iron, thereby causing the typical reddish rusting away of iron. Both "cold" and "hot" oxidizing processes are called bluing, but only the "hot" process provides any significant rust and corrosion resistance, and then only when also treated with an oiled coating.

Early gunsmiths used a chemical process that produced a brown finish---called "browning". Rust bluing was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rustuniformly, then immerse the parts in boiling water to stabilize the rusting process by removing any remaining residue from the applied acid solution. Then the rust was karded (scrubbed) off, leaving a deep blue finish. This process was later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor intensive. It is still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas.

Link: http://en.wikipedia.org/wiki/Bluing_%28steel%29