I am actually now very curious to find out what sodium nitrate adds to the equation - is it just contributing that additional ion to help with oxidation? Or is something more complex going on?
I think you get better quality control with rust bluing because of the successive applications of the rusting solution. If there is an issue in the finish you are more likely to see it and to be able to correct it. In addition, it is easier to stop oxidizing once you have gotten the color you want.
Differences in color relate to the level of polish and the extent to which the surface has been oxidized. Ultimately we are talking about a variation that runs from the "royal blue" on an early 20th century Colt to the type of flat "black oxide" finish you might find on a military gun, like a S&W Victory model.
A bit of a change of subject, but has anyone found any good alternatives to nitre bluing? Nitre is a nice finish but (like color case hardening) it has just about no corrosion resistance.
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