Critical temperature varies somewhat according to the carbon content of the Steel. According to "Machinery's Hadbook" low carbon steel is normally "Carburized" at temps of from 1650-1700°F while high carbon steel is sometimes "Pack Hardened" ie packed in a carbonaceous mat'l for protection of edges etc, in which case it should be heated only to about 1400-1450°F. As I recall the technique Oscar had worked out was to heat the part in the bone/charcoal mixture to somewhere around 1600-1650°F for carbon absorption then drop the temp down around the 1400°F temp which was still above the critical temp for the case which allowed it to harden upon quenching, but below critical temp for the low carbon core. This helped to reduce the warping tendency had the frame been quenched with the entire core above it's critical temp. Length of time at the higher temp in the mix controls the depth of case, but cyanide is normally not employed for other than a case of only a few thousandths of an inch thick. When greater depths are desired the charcoal mix is normally employed.
Miller
Note that much of this applies to industrial case hardening, wherecolor is of no concern. In fact machinery's Handbook does not mention "Color Case Hardening". The same general procedure applies though for the forming of the case & hardening it. The nature of the carbon determines the color I believe.
PPS Colt & Parker did use color case hardening as both used low carbon steel for their frames. They did "invest that much time in the process", but remember around the turn of the century "Labor" was cheap.
