Potassium Nitrate should work for this.
I've used it for 'regular' Nitre blue and spring tempering.
We even did some coloring in a certain shop I worked in using Ammonium Nitrate fertilizer melted at high temps to try and mimic AH Fox cyanide case colors.
Not completely successful!, but not a total failure either as far as getting some different colors to appear.
I suspect some other chemical salts were dumped into the mix as well. Shade Tree Science.
It's just plain scary to work around that large of a batch of any of this stuff,,at least for me it is. Too many things can go wrong,,and fast.
Disposing of the entire super heated batch while still liquid was the biggest trick of that operation.
Calling the experiment a failure, but trying to save the large heating pot before the stuff cooled and became a solid mass inside of it.
Very lucky no one got hurt on that one.
It was decided to continue to use Heinzelmann's after that circus act.
As far as getting any moisture off of and out of any parts before they go into the Nitre,,that's something you have to be absolutely sure of. That there IS no moisture involved.
Doesn't matter if you're are working with the stuff at 650F or 850. The results will be the same and not pleasant.
Slowing warming them is the best after a wipe down. All this after a thorough degreasing.
Nitre will not degrease the parts for you. Just like regular bluing, any oil, grease, fingerprints will show in the new blue.
Silicone oil is particularly hard to get rid of. Usually shows up as tiny white specks.
Water...If you warm the steel parts too quickly to make sure they are dry of all moisture you risk having them flash rust in front of your eyes.
Those areas of that very slight rust will not blue or will be off colored reddish, brown or even grey.
The water may also bead up as any excess moisture in a high humidity environment evaporates and as the last tiny bead of water leaves the surface,,a small speck of mineral deposit is left behind.
There's a blemish spot in the new bluing.
Plus usually a halo blemish around it where the water evaporated.
Dry environment, wipe down carefully and inspect.
Then quickly into the Nitre. Don't let parts set around more than an hour or so after final POLISHING as they will start to form an oxide though you can't see it. It will effect the blue and all your work. If they have to sit,,oil them.
When de-oiling and cleaning for bluing,,do it right before the parts go in the Nitre.
When the hot parts come out, they probably will have some nitre on them in places and it will harden to a white coating.
Leave it alone and let the part cool off with the coating on it.
It's still way too hot to put under even the hottest tap water you might have.
There's a real possibility of the stuff splattering yet if hit by water in trying to rinse the excess Nitre from the surfaces of the parts.
Once you do rinse them and inspect them, if you find some blemishes you can SOMETIMES touch up the areas and place them back into the Nitre after cleaning and making sure that all the WATER is once again removed.
Sometimes this re-Nitre attempt doesn't work and you are forced to let them cool,,strip the old finish, repolish, touch up and engraving if necessary and then go again thru the whole operation. Best to do all your parts one time thru and then decide if one or more need any re-bluing
This strip, re-polish and nitre again usually means shutting the Nitre pot down while you do the prep work if you have more than a part or 2 to do.
So a re-start of the pot once again with all the necessary work and time involved.
Like a lead pot,,don't leave the thing un-attended or open to unauthorized persons from possibly getting to it while you are not around such as during it's cool down phase.
Happy Nitre Day
