I have case hardened steel for hammer or trigger sears or emergency homemade taps and dies using the old formula Kasenit, or even using plain table sugar to supply the surface carbon. But I have never done bone charcoal pack color case hardening, though I'd like to try it one day. The 1333 degree F temperature is the critical temperature for annealing steel. Hardening steel takes a higher temperature and involves a quench in air, oil, brine, or water, depending upon the steel alloy. Drawing the hardness is done at much lower temps. I have read of a wide range of temperatures that will work for carburizing or case hardening the surface of steel with time as an additional factor... lower temperatures naturally taking more time. Doug PA24 addressed this aspect earlier, and I do not know what effect time and temperature has on colors. Obviously, Doug has found a very nice combination of time, temperature, materials, and methods. I would assume that annealing steel in a pack of carbon rich charcoal or the like would not only prevent scale, but also add carbon to the surface of the steel, but would not result in colors or surface hardening without the quench.
The stainless steel foil does make a very good one-time use crucible, but as craigd notes, it is easy to puncture on a sharp edge. Speaking of sharp edges, the foil itself will cut you like a razor, so caution is required. Paper, sawdust, or the like placed inside with your parts will do a better job than steel wool of consuming any oxygen inside which would form scale because it will use up any free oxygen at a lower temperature. I've done just as well preventing scale when annealing parts or spring steel by using a capped section of pipe as a crucible with a very small vent hole drilled in one cap, and some wood or paper inside to consume the oxygen. A small plug of fire clay is used to seal the vent hole after the correct annealing temperature is reached. The large annealing furnaces at the steel mill where I did my electrical apprenticeship used inert gasses such as nitrogen and, believe it or not, hydrogen to purge out the oxygen and prevent scale. Our Salem furnaces used nothing more than dry sand to seal around the base of the furnace to keep oxygen from entering. We had a plant where we did catalytic cracking of liquid ammonia into nitrogen and hydrogen for the annealing departments.
