Bluing steel is a process that protects steel from rust. There are two methods for bluing, hot and cold. In the hot process, the steel is saturated in boiling water mixed with potassium nitrate and sodium hydroxide. This submersion causes a chemical reaction that helps to prevent rusting on the steel. There are many other methods that involve hot or boiling liquid submersion, but each uses a different set of chemicals. One set includes nitrates and chromates.
Cold Bluing of Steel
Another process of bluing steel can be completed at home and is usually performed by a gun owner who wants to protect their weapon from rust. This process involves the simple application of a solution---which includes a mixture of selenium and other chemicals---onto the steel with a brush or cloth. It then reacts with the surface of the steel to blue it and protect against corrosion.
Applications and Other Considerations
Bluing steel does not alone solve the problem of rust. Even blued steel needs to be cared for to prevent oxidisation and corrosion. The simplest way to do this is to coat the steel with oil. Most gun owners will frequently disassemble their weapons, clean them, dry them and then apply oil or rust prevention spray, especially to the moving parts. This along with the added protection of a bluing agent will slow down and help to prevent rust.