Very nice work!!!
I think they combine the bone charcoal with the wood charcoal is so you don't use up so much of the bone charcoal? just a theory though.
~Wes~
as you say they combine it, using the wood charcoal to fill most of the box, and use the bone char around the actual parts, when i used a lot of wood char as packing i found i only got mostly blue colours, much better using all bone and leather char if you want a variety of colours, but with colour hardening your in the lap of the gods as it where, sometimes its good other time not so good, temperature is also critical, to cool no colour to hot the same, you need a temperature controlled kiln to do any good, pointless trying to do it in a forge of coal fire ect , very hit and miss, been there done that.
one more thing thats critical is the speed the parts exit the box into the water, you need to devise a way of removing the lid very quickly, if to slow the parts will become exposed to the air and oxidise, work ruined. also remember to brace your work as best you can to try and minimise warping.
there is very little information available on colour hardening, all trade secrets ect, most of what you read in gunsmithing books is usless, always critical info missing.
hope your successful, as proper bone colour hardening looks class compared to the modern cyanide hardening found on modern replicas, awful