From my internet notes...
"Some color case hardening is done in the Vat method, Stevens and others used this method. It is called Cyanide. Cyanide is a salt that when heated up to about 1600 degrees or so anneals what ever is put into it and then hardenens it. It goes from the cyanide bath to a water quench. The Stevens got there ripple effect from a chain that was used to lower the parts into the bath and pull them up, supposedly the cog wheel the chain ran in helped make the pattern as it stopped momentarily at each cog. There is a company in Wyoming that uses this method with nice results. Not something you can do at home."
"Signal Mountain Gun Works uses an abbreviated potassium-cyanide based process for color case hardening. Browning/Winchester uses the company for their firearms since the receivers are already sufficiently hard (SAE 4140 steel heat-treated to a hardness of RC 38-42) and case colors are the main goal. Since case colors are the main objective (not a thick hardened case) the potassium-cyanide bath is heated to 1215 to 1225 degrees and the parts are left in only 5 to 10 minutes. I understand that a much thicker case can be achieved using the same process, but the temperature is increased to the range of 1400 to 1650 degrees and the parts are “soaked” for at least an hour, usually longer."
So to sum up they use heat and Potassium-Cyanide salts. I am sure the fumes are vented most likely after some type of conversion to harmless state by a catalytic conversion process and all workers around must wear breathing hoods and chemical barrier type clothing. Plus I suspect the EPA might become your best neighbor and constant companion. Even skilled amateur should let the pros handle this one.
