The cyanide process was known for a long time. It came into heavy industrial use during and after WWI. If you have ever been around a commercial facility that used bone charcoal, you quickly realize how dirty it is and how difficult it is to handle all the charcoal.

Here are a couple of early texts that talk about the process. There were several ways to implement cyanide case hardening. A molten dip, powder, placing some cyanide into the vessel and thereby use the gas method.

http://books.google.com/books?id=GT8LAAA...ing&f=false

http://books.google.com/books?pg=PA94&am...amp;output=text

Case color does not always result from for case hardening. It is the impurities that are introduced by bone or cyanide that result in the color.

Pete

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