... Hence the cyanide bath to attempt to recreate the colors without the hard skin that isn't needed with the lower carbon. I have a late 30's Sterlingworth and if I remember my reserch they switched to cyanide for late production. Does this indicate the use of a higher carbon steel later in production?
The cyanide method is fully capable of imparting carbon into a low carbon steel and producting a casehardened surface. I also beleive that the later Fox guns were still low carbon steel frames. I could be wrong.
