Answer:
Caustic salts. While the actual ingredients vary from maker to maker, these are mainly sodium hydroxide (lye) and ammonium nitrate. They are mixed with water to form a solution that boils at about 290 degrees F. The mixture is very caustic to skin (eats holes in skin and clothing) and cause cause blindness if it gets into your eyes. After a part has boiled in the bluing salts until the desired color is reached, the part is placed in boiling distilled water for a few minutes to stop the bluing action,