I have had guns restored using the bone charcoal case hardening process. As has been stated many times, it can warp parts. It is not the only case hardening process that was used. Many makers choose to use the cyanide method.
Yes the use of a torch can be a very bad thing. Which by the way appears to be what Ed and friends are using.
There is another approach that has been discussed here in the past. Essentially heating the metal to about 175F to 200F and applying cold blue or other chemicals. Some have reported excellent results with this approach.
Pete
