You're welcome, David! French gray is the finish obtained by removing the colored portion left at the surface by the black oxide (hot salt bluing) process. The salts used in this process form a micro-thin layer of hard, black rust at the surface when reacting in the presence of oxygen. The "bluing" penetrates into the surface just a bit beyond the color, though. We can remove the colored portion with the application of a weak acid solution (like phosphoric acid, which most people know as the acid found in Coca-Cola and toilet bowl cleaners) and retain the nearly colorless substrate immediately beneath it. This gray substrate offers nearly the same corrosion resistance as the original finish, which is why french grayed guns resist rust so much better than naked steel. At least that's how it was explained to me, and I got the info second hand from Lynton McKenzie.
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