Bluing (steel) - Terminology

Terminology

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.