As some of you know I studied metallurgy, but we didn't do anything to do with colour case hardening as such, but from my observations having looked at quite a few videos, and having a fair amount of experience with other forms of heat treatment I think I have some input that May be of use.
1, When heat treating in a completely inert environment, so without any oxygen, the steel doesn't change colour at all, this sounds unlikely, but as it is a really rare event that steel is heated without any air it's hard to get a feel for it, as an excercise/practical lesson we sealed a piece of hard tool steel in a welded container that was purged of air and filled with Argon(i think) and we then heated it in a muffle furnace until the critical temperature had been reached, we were assured that there should be a colour change on the tool steel, the test piece was allowed to cool slowly and when opened the test piece was softened/normalised proving that it had been up to temperature, but it was only very slightly discoloured, it hadn't even got to very light straw.
There is little point trying to eradicate all oxygen as some is required for the colouring process to occur, the chemicals in the charcoal must help to define which colours develop, but all colouring is as a result of oxidisation and you need oxygen for that to happen.
2, When watching videos of the colour case hardening process I have noticed that when the charcoal pieces are small then the mottling of the surface is also small but when the charcoal lumps are larger so too are the patches of different colour. so I figure that this might have some effect, personally I like larger patches.
3, Charred bone is tri calcium phosphate, which contains oxygen in it's own right,
I figure that the black areas of the treated metal are those that have been in contact with the oxygen rich charred bone and the silver areas are those that have been in contact with the wood charcoal as the reducing environment of pure carbon at elevated temperature will strip oxides off the metal, it follows that the yellowy straw areas at the colour boundaries are as a result of the not quite reducing or oxidising environments between the two distinctly different types of charcoal.
In my humble opinion the mystique behind how the process works is one of those smoke and mirrors jobs that metallurgists have been doing since they were called alchemists. and I also reckon that there are probably easier materials to use than charred bone that will do pretty much the same thing but potentially in a more controlled fashion, even using plain old river gravel mixed with hardwood charcoal could give a similar effect, as long as there is enough oxygen present surrounding the first layer of charcoal, for a second or two before quenching.
Finally, Japanese sword makers use mixtures of a fine Kaolin like clay and powdered charcoal or graphite in varying quantities painted onto their blades after fine finishing and prior to final heat treatment these compounds painted on the metal act to both protect the surface from oxidisation and also to promote oxidisation in other areas in order to both give a decorative effect and also to slow the quenching process to affect the final hardness of the blade's edge. A similar trick is the use of wheat flour dough squished onto the detailed edges of punching tools to prevent scale build up during heat treatment as the carbon in the flour eats up all of the oxygen before the steel can oxidise.
So, if their is anything here of use, you're welcome to it, otherwise, Sorry for taking up space.
Best Regards
Rick
Whatever it is, do it today, Tomorrow may not be an option and regret outlasts fatigue.
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