Jonty,
I am hardly an expert.
I seem to remember a thread about this with Dr Oscar Gaddy. This was shortly after he did an electron microscope scan of one of his jobs. The carbon bonding goes relatively deep. The case color is another story. It is relatively thin. At least that is my memory. Orginally the process had nothing to do with cosmetics. It was meant to harden relatively soft metal by building up a skin of tougher (carbon rich) steel.
I do not believe you would want to have the hardening go all the way through the frame. It would make it too brittle.
Pete
