Originally Posted By: Ted Schefelbein
True French gray is an actual hardening process, similar to bone pack case color hardening, with the final quench being in a solution of potassium prussate, instead of water. The "tyde bowl" method cuts back blueing to a tasteful gray color, but, provides no hardening.
Best,
Ted


Cyanide case coloring is used for French grey at least that's what this shop uses. You might want to look at this link, under Cyanide case coloring.

Ron's Gun Shop