Not a throughly scientific description, BUT, case hardening is normally done on a low carbon steel which does not have a high enough carbon content to through harden. This can be done with or without the mottled colors, primarily dependent upon the source of the carbon & oxygen in the quench. Animal charcoal & free oxygen both enhancing the colors. At the elevated tewmp, above its critical temp, or the point it becomes non-magnetic, the carbon is absorbed into the steel. The longer it remains at temp the deeper the carbon penetrates, but also the deeper it goes the longer it takes for each additional .001" of penetration. After the part has soaked in the carbon gasses for the requisite time to attain the desired depth of penetration it is given the Quench from still above its critical temp, which is necessary for it to harden. This high carbon layer which is hardened is known as "The Case". The low carbon core is left in an unhardened state. When th process is done in a manner to produce the colors it is commonly referred to as "Color Case Hardening". Realize though the "Case" still refers to the hardened area, the colors are only superficial, being microscopicaly deep. They are strictly speaking "Temper Colors". Technically there is no such creature as Case Colors, as the colors do not form the case. It is noted though these colors are formed in the "Hardening" process, colors brought on by re-heating the part hot enough to produce color is in fact a true tempering process & is drawing or removing the hardness from the case. To produce varied colors with the hot tip of a torch flame tempers out the hardness in a spotty fashion according to the color produced, leaving the case having faried hardness. The colors put on some modern guns having alloy steel frames which are through hardened are done chemically at so low a temp as to have no affect on the hardness, usually no more than about 200°F I believe. These are not temper colors, nor are they produced through the hardening process, but are simply chemical colors. These can be produced by heating a piece of steel to the range of 160°-200°F & "Painting" on the colors with a Q-tip dipped in a cold blue such as Oxpho. They will never produce the colors of those produced through the bone charcoal process of case hardening, but they do no damage to the frame & can be simply & easily sanded off.
ANYONE putting the Hot Tip of an accetylene torch to a gun frame in a manner to give it mottelty temper colors should be given a "Suspended Sentence" of thirty (30) days on the end of a "New Rope".
