Starrett Tool Co was a big user of Cynanide hardened parts for many of their products. I can assure you it does produce a hard skin & don't recall ever seeing a Starrett that "Peeled". I did not object to this finish on an adjustable paralell for instance, but also much prefer the bone charcoal method on a shotgun. According to Machinery's Handbook a bone process under normal methods also produces a deeper case than does the Cyanide method.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra