[mrerick]

Actually, Bluing is a transformation / electrochemical conversion of the surface layer of steel molecules. It's not really the addition of anything but oxygen. There is a good article here: http://en.wikipedia.org/wiki/Bluing_%28steel%29

Iron oxide (steel being iron and carbon) occurs in several different molecular forms. The form we call "Bluing" is magnetite, Fe3O4, the black oxide form of iron oxide. Magnetite is denser and thus harder than steel,

Rust is the Fe2O3 form of iron oxide. Rust is soft because it increases in volume many times during it's chemical conversion.

Bluing works to protect steel from rusting because the top surface is already oxidized into magnetite. This is why you can see red oxide rust deep in the surface of old blued guns. The layer just beneath the top layer isn't magnetite, and as steel can continue to oxidize into red Fe2O3 oxide over the years.

Marc