This classic finish is found on the receivers of Colt Single Actions and many shotguns. The brilliant, swirling blues, grays and golds are the result of packing steel parts in a mixture of wood and bone charcoal, heating them to roughly 1400° F. then quenching in water. The surface of the steel is also hardened in the process. Because of the high temperatures, warping can be a problem and parts where a specific hardness is critical should not be color case hardened. Application requires a heat-treating furnace, a steel crucible to hold the parts, plus high temperature handling gear and a 55-gallon drum of quenching water.
