Bluing is most commonly used by gun manufacturers, gunsmiths and gun owners to improve the cosmetic appearance of, and provide a measure of corrosion resistance to, their firearms. Bluing also helps to maintain the metal finish by resisting tangential scratching, and also helps to reduce glare to the eyes of the shooter when looking down the barrel of the gun. All blued parts still need to be properly oiled to prevent rust. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 2.5 micrometers (0.0001 inches). For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.
Bluing only works on steel or stainless steel parts for protecting against corrosion. Because it changes the Fe into Fe3O4, it does not work on non-ferrous material. Aluminum and polymer parts are largely unaffected by bluing; no protection against corrosion is provide by bluing processes on them, although uneven staining of the aluminum and polymer parts can be caused by attempts at bluing.
Holster wear will remove hot bluing over long periods of use; it will remove cold bluing over relatively short periods of use, from any wear areas that are “touched up” with a cold bluing solution.
Some prefer to call thin coatings of black oxide by the name gun bluing, and to call heavier coatings by the name black oxide, but they are both the same chemical conversion process for providing true gun bluing.
Browning is controlled red rust Fe2O3 and is also known as pluming or plum brown. One can generally use the same solution to brown as to blue. The difference is immersion in boiling water for bluing. The rust then turns to black-blue FE3O4. Many older browning and bluing formulae are based on corrosive solutions (necessary to cause metal to rust), and often contain cyanide solutions that are especially toxic to humans.