Usually Sodium Nitrate but Potassium Nitrate works as well.
Melt and use at the temp that gives you the desired color or effect (spring temper for example).
Use a good quality high temp thermometer. Don't get any water/moisture in the stuff or on the part(s) before you dip them into the molten salt.
As with molten lead, it will explode and you will be wearing the molten salt and it will burn through everything and cause critical burns.
The old Winchester recipe noted that they added some Manganese Dioxide (10% ??) to the molten Sodium Nitrate.
That allowed the salt to used at a lower temp and remain liquid it says.
Would be helpful if you were going after the Straw colors and others that are gotten at lower temps than the salt(s) normally melt at (just less than 600F).
I generally use Sodium Nitrate for Nitre Blue color. But molten lead will do
Springs hardened and and then tempered in molten lead at 735F come out a nice Spring Temper Blue color,,,and why wouldn't they.
It's all just Heat Temper Coloring
For Straw color I stick with careful use of a torch. Seems to work OK for that.
