Bushveld's warnings about using eye and face protection when using molten potassium nitrate are quite valid. As stated, when heated to the melting point, the stuff is around 700 degrees F.
Another precaution I use is to place a long lag bolt near vertically into the molten potassium nitrate before it cools after use. Then, the next time you heat it, unscrew the lag bolt first. This will leave a vent hole going to the bottom of your pot that will prevent a build-up of pressure under the solidified crust when you apply heat to re-melt it. This is not my idea. I read it in Brownell's Gunsmith Kinks, and it is cheap and easy insurance. I have also read that some guys accomplish the same thing by drilling a hole into the solidified salts. But using the lag bolt is very easy, and the bolt head is there to remind you that it needs to be removed before doing anything else.
I mainly use potassium nitrate salts for spring tempering or to blue small parts like screws. I never tried it on larger items such as steel grip caps or trigger guards because I have read that it isn't a very durable bluing process for parts subject to wear. I'm curious to hear if anyone has personal experience to share on durability. I am intrigued by the higher temp charcoal process such as that used by Colt or the Carbonia process used by Smith & Wesson. Obviously, those processes were beautiful and held up well. We had a good discussion about this a few years ago:
https://doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=574458&page=1
