Thanks junho. The salts and thermometer came from Brownell's. Like anything else, if you want to invest the time and money, anyone can learn to do it. The video only shows some of the process and as I mentioned, 600 degrees is very dangerous. The other issue is that some parts should not be nitred. No high stress parts should be heated to that temp. Rifle bolts for example should never be nitre blued.
I originally started doing it as a service to my engraving customers and after a while, offered it to the general public as well.