We have figured that out Pineknot, but the case layer does have enough carbon in it to make the temperature important. What nobody seems to know is how much carbon content is typically found in the case, and thus the min quench temp to ensure hardening. There is also a strong correlation between case depth and temp/time. So if one is casehardening fresh metal, a higher temp and longer soak are necessary to get the carbon absorption in the first place; if recoloring, less time and a lower temp should be OK provided one starts above the recalescence point of the case layer...which nobody seems to know because we don't know the carbon content of the case. It seems that 1450F is a safe quench temp but a lower temp may work (and is desirable if it does to get better colors and less distortion) if you are coming off a higher temp. How low nobody seems to know. A charcoal fire can be hot or cool depending on how much air is forced through it. So knowing that this was once done on an open charcoal fire doesn't tell us much about how to do it today in a proper furnace.
FWIW I successfully recolored some parts from three different guns this evening...ramped up to 1525-1550F, held it there for 40 mins, then came down to 1300-1325F, held it there for 15 mins, then immediately quenched in water with some KNO3 in it. Parts have hard surface and strong color. If anything too dark and too much brown but that's probably a function of the charcoal mix. I used bone:vegetable charcoal in the ratio of 1:2. Not sure how much carbon was left from the original case hardening but I am presuming a lot of it.
