Bone charcoal case colors can be "tweaked" to a lighter or darker overall impact by varying the variety and amount of specific ingredients. Over time these individual ingredients have been the basic wood and animal bone charcoal, with a plethora of weird components including animal feces and urine, diatomaceous earth, bits and scraps of leather, creosoted rope, and scraps of bark from various species of trees.
Probably the most important part of the process is the preparation: degreasing, polishing to the correct finishing grit, fastidious separation of individual parts and their placement inside the crucible as the vessel is packed with the mixture prior to putting it into the furnace. If parts are packed too close to the walls of the crucible, excess heat may warp them. Packed too loosely in the mixture, and the overall coverage and pattern of the carbon deposited on the surface appears "washed out" and inconsistent.
Needless to say, the exact formulas used by the best case coloring artists are very closely guarded secrets. When the "sweet spot" is hit upon quenching and the final colors are closest to original, careful records are kept for the next go-round.
There are a number of very good videos on YouTube under the heading, "Case coloring of firearms." They give a good idea of the wide spectrum of final results and, depending on the make of firearm, show what comes out when a "one size fits all" approach to recoloring is done.
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