Nitre bluing usually refers to a molten bath of potassium nitrate or saltpeter, as Mike Hunter and damascus have pointed out. The working temperatures are very high, and extreme caution must be used. Even simply heating a pot of solidified potassium nitrate can cause an eruption of extremely hot molten salts as pressure builds up beneath the still solid surface. It is recommended to leave a large screw or bolt in the bath to then later remove when reheating in order to provide a vent hole.
Saltpeter used to be available in grocery stores in the spice section, because it was used in pickling and food preservation. Some stump removers do use a high concentration of potassium nitrate, but you have to read the label.
In case the nitre bluing salts you were inquiring about were for regular hot salts or hot tank bluing, here is a recipe for homemade bluing salts. The Brownell's "Gunsmith Kinks" books have a lot of very useful information on all types and methods of bluing.
5 pounds 100% sodium Hydroxide (100% Lye)
4 pounds Ammonium Nitrate (Sodium Nitrate may be used instead)
1 gal. Distilled water
This is a very caustic mix that is used in a steel tank only.
The Lye must be pure 100% like "Red Devil" or the kind used for soapmaking
It is for steel bluing only and it will not Blue Winchester 94 Receivers.It works Identical to Brownells Bluing salts.
Temperature range is around 260 to 275 deg F for the boil to blue the parts. Hot caustic bluing is also very dangerous as the solution will cause very nasty chemical burns or blindness if it splatters onto your skin or eyes. Learn all you can before trying to do it, and wear appropriate safety gear.
