Got the cyanide info from a textbook on steel, and I used to add bits of burnt leather to the mix for the nitrogen. The weird thing is, the Low Wall receiver that I case colored back around 13 years ago came out too dark, I thought from a hairline crack in the crucible/pipe. But now it's looking a lot nicer!
Molten sodium cyanide immersion is one method of carburizing mild steel, it has nothing to do with the charcoal pack method.
Bone charcoal and wood charcoal heated in a crucible will produce mainly carbon monoxide. Wood charcoal alone (that has been heated to the point that all volatiles are consumed) will produce nothing, it's pure carbon.
What function does the "nitrogen from the burnt leather" serve?
Case-hardening is not magic and the chemical reactions that yield the hardened surface and the colors are quite well understood. I have no idea why people persist with witchcraft-like case-hardening recipes/methods.
