What is "Slow Rust Bluing"?

Slow rust bluing is called such because it is, well... slow.

Rust bluing (sometimes spelled "blueing") was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The traditional process is to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water or steam to convert the red oxide (rust) to black oxide, which forms a more protective, stable coating than the red oxide; the boiling water or steam also removes any remaining residue from the applied acid solution (often nitric acid and hydrochloric acid diluted in water). The loose oxide is then carded (scrubbed) off, using a carding brush – a wire brush with soft, thin (usually about 0.002 in (0.051 mm) thick) wires – or wheel.

This process is repeated (each rusting "cycle" can take several hours to a day, depending on the specific solution used and humidity) until the desired depth of colour is achieved or the metal simply does not colour further. This is one of the reasons rust bluing is generally more rust-resistant than other methods. At the completion of the bluing process the parts are then oiled and allowed to stand overnight. This process leaves a very attractive, gun metal grey to deep blue-black finish.