True French grey hardening is a process very similar to bone pack charcoal hardening, except the final quench occurs in a solution of potassium prussiate, instead of water. By playing with the chemistry of the quench, colors from pewter to almost a silver plate appearence, are possible.
It is no more, or less durable than bone pack case colors.
Some people blue metal parts of a gun, and cut the blue back with an acid, to get an even, silver color on the finished project, but, calling it French Grey is incorrect.
I call that process, "Tidy Bowl Grey".
Best,
Ted