Black oxide is a chemical conversion process used to blacken the surface of a part without adding a thick coating. A chemical conversion process uses a chemical solution to force a reaction on the surface of the part rather than a plating or coating process which adds a thicker layer on top of the base material.

Hot black oxide coating consists of a hot bath of nitrates, nitrites, and sodium hydroxide. The interaction between the ferrous material and the bath turns its surface to magnetite. Then, the material is dipped in alkaline cleaner, water, and caustic soda, making it turn black.