Bluing

steel temp

Bluing is a passivation process (‘passive’ in relation to being less affected by environmental factors such as air or water) in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. It is the result of an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron. By comparison, rust is the red oxide of iron (Fe2O3). Bluing is most commonly used by gunsmiths 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. It is also used for providing coloring for steel parts of fine clocks and other fine metalwork, such as by machinists, who protected and beautified tools made for their own use.

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