Black oxide

[frugalweaver]

Black oxide is achieved using special salts that create a tightly adherent magnetite (Fe3O4?) on the part. It is a good protective surface.

I read this on another forum. Could anybody give a more complete explanation?

Could this be done to an existing blade without rehardening?

 

[son_of_bluegrass]

That black oxide is the same patina that develops on carbon steel blades. It is a form of rust. I've heard letting rust form then boiling will produce it and exposure to mild acid (vinegar, lemon juice etc) will do it.

It protects against further rusting but will wear off.