text: In the process of bluing, an oxidizing chemical reaction on an iron surface selectively forms magnetite (FeO), the black oxide of iron (as opposed to rust, the red oxide of iron (FeO)). Black oxide provides some protection against corrosion if also treated with a water-displacing oil to reduce wetting and galvanic action. Bluing is often used with carbon steel and cast iron pans in conjunction with seasoning.