Timn , I don't think I can buy into your theory that "all rust is created equal". If this were true, I can't see why gunmakers over the years bothered to concoct complicated formulas to rust blue with. Angier lists many of these in his book....which include substances such as nitric and hydrochloric acids, copper sulfate, ferric chloride, mercuric chloride, and various "tinctures" and "vitriols". If simple rust is going to always produce the same shade of black, you'd think they would have just used a cheap, simple solution of any diluted acid. And if hot bluing is just accelerated rust bluing (which it may be), how do you account for the red/plum color many guns ended up with? I doubt there was a drastic enough temperature difference in the bath to color temper the metal, so it had to be a chemical mixture variation.
Also ,in rust bluing the ferric oxide (rust) is converted to ferrous oxide but the trace elements of the solution are still in the matrix of the finish somewhere.....unless they boil off, which begs the question of why they were added in the first place.....my 2 cents
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