Packing in charcoal for annealing provides an oxygen free environment to prevent scaling. There are other methods. It can be done in an inert gas oven and some people use a steel wrap especially made for this. I've tried the wrap and do not care for it. I have had things hardened in a gas furnace but never annealed. I always anneal and feel it is a very necessary part of the process.
A key to using the stainless steel wrap when annealing is to put a little piece of fine steel wool in with the part. That way the steel wool will burn and use up any air left in the wrapped package this preventing any scale.
