Note in the ad the statement "Low Heat & Chemical". There is a process I have described here on this board on several occasions, taken directly from an Ithaca service manual, which they recommended for use on the Perazzis they once sold. These were I believe made from an alloy steel not suitable to regular case hardening but given a "Case Color" finish. The process consisted of heating to a temp hot enough to "Sizzle Water" (about 165°F) & then daubing &/or streaking with touch-up bluing on a Q-tip. This temp is less than boiling water & does absolutely no harm to the metal. The finish produced is actually rather well wearing & can be simply polished off if it doesn't suit. The question here is exactly what process did he use. If the colors were in fact produced my heating hot enough to produce "Temper" colors then they Do affect the original case. Yes Temper is the correct term. In heat treating the part is hardened to it's max hardness & then "Tempered (Drawn) back to desired hardness. In the old days, prior to a lot of sophisticated equipment this tempering was quite often done by "Color". The color is produced by oxidation at that temp, not by it's carburization, thus any "Temper" color produced has done so at the expense of hardness. Colors produced by "Spot" heating of a gun frame should in my opinion be avoided like the plague. The case of a case hardened/colored gun frame will normally be left at max hardness. Loss of color does not mean loss of hardness only that the oxidized layer has worn away.
Thus "Hardening" is the term which puts in the hardness & is done by heating a part above it's critical temp & quenching;
Tempering is a drawing of the max hardness back to a desired lower hardness & is done by re-heating a part to a predetermined temp which will obtain the desired hardness, but will always be less than that required for hardening.
Annealing is a process where the metal is heated to a high enough temp to remove all traces of the hardening & it will be allowed to cool slowly so no "Quenching" effect can occur to create a hardening effect.
There is no way one could, with a torch, heat an area hot enough to produce temper color without the case at that point being heated completely through, thus it will indeed affect the metallurgy of the case & it is being done non-uniformally. The original case was "Hardened" in a uniform manner, the random colors come from non-uniform oxidation, not from non-uniform hardening.
