Ron's Gunsmithing and Williams Gunsite both offer Cyanide Case Color hardening, there are others. (1600-1750 F) The idea is simple. Add carbon to the steel, the same as the bone charcoal method.

http://www.ronsgunshop.com/pricing.html

http://www.williamsgunsight.com/gunsmith_service_michigan.htm#METAL%20WORK

Old methods of hardening required heating the metal red hot and quenching in linseed oil, see the video Master Gunsmith of Colonial Williamsburg for an example. This usually produced a yellow tinted case color.

Products like Kasenit and Cherry Red are used today for hardening without coloring. They both require the part to be heated to 1000 F.

http://www.dixiegunworks.com/product_info.php?products_id=6465

http://www.midwayusa.com/viewProduct/?productNumber=119479

http://en.wikipedia.org/wiki/Case_hardening#Cyaniding

Pete

http://www.damascus-barrels.com/Movie.html