It's actually quite easy. I use pure charcoal from Brownells and a heat treatment furnace, the type isn't important as long as it can hold 900 degree temperatures and you can control it. The parts are given a high polish to at least 600 grit and than I degrease them. The parts are buried in the charcoal in a container that can be sealed, placed in the furnace and brought to 900-950 degrees and held at temp for 30 minutes to an hour. After this soak turn the furnace off and leave the parts inside until they oool down to about 200 degrees. At this point I generally take the parts out and degrease them again (this was probably the stage when they were traditionally rubbed with lime) repacked in the charcoal, resealed, and placed back in the furnace and reheated to the 900-950 degree range for another hour or so. Lastly the furnace is shut off and the parts are cooled down to 200 degrees and than rubbed with oil (I generally just use any heavy oil on hand, sometimes motor oil). The parts are left alone over night and the color tends to get deeper and more even during this time. Not hard, but you need to have a furnace that you can control for good results, also the charcoal from Brownells is the only thing that has given consistant results in my tests, same for color case hardening.
