Blueing
I'd recommend a couple of books- The Complete Metal Finishing Book book by Harold Hoffman( H&P Publishing, 7174 Hoffman Rd, San Angelo, TX 76905) and it used to be sold by Brownell's. Lots of formulas including his for hot salt: 15 lbs of sodium hydroxide (flakes), 7 1/2 pounds of ammonium nitrate and 3 gallons of pure water. And, of course, the bluer's bible Firearm Blueing and Browning by RH Angier. Written in 1936 so some of the chemical names are weird.
