Fox,
Im guessing you missed this, but, the engraving and metal finish displayed on the Darne in this post, were not done in France, and have nothing to do with the Darne company, or products of same.
Do try to pay attention.

Miller,
XT is a French tool steel that does allow for cyanide color case hardening. XTC is the chrome version of that steel. I seem to remember it was a through hardening tool steel, and did not allow for attractive colors in the cyanide process. The color produced on the exterior of XTC with the French grey process could be varied slightly depending on quench chemistry, and, was no more or less durable than normal case hardening colors.
Sometimes, customers asked that the grey be polished off. The guys used steel wool and gum erasers to do so. Otherwise, the parts got a coat of varnish, that does yellow with time.
I prefer the silver color left there, it is an attractive matte pewter color.



Best,
Ted