Salt-bath annealing fail!

[salfter]

I don't know if I did something wrong, bought some of the wrong stuff, or what, but after looking at the various options for annealing brass (which I wanted to get into since some of my .30-06 brass have cracked at the neck), I decided to give the molten-salt bath method a whirl. After reading what I could find (starting here and branching out from there...think there was a thread or two here on the topic), I bought a new Lee 10-lb. lead pot to dedicate to the purpose. As for the salt, I considered ordering the low-temperature mix from here, but it was going to cost nearly twice as much to ship them here as they cost in the first place!

A bit of reading suggested that Spectracide Stump Remover ought to work. It's 100% potassium nitrate in granulated form. It wasn't available locally (not much call for it here in the desert, perhaps?), but Home Depot would send it to one of their stores for pickup, so I ordered two 1-lb. bottles of the stuff.

I poured some into the lead pot, cranked up the heat, and waited for it to melt. Instead of forming the thin yellowish-looking liquid I'd seen in YouTube videos on the subject, it formed a bubbly gray sludge that threatened to overflow the pot. I pulled out some slag-like material floating on top, which quickly solidified into a hard gray material. After unplugging the pot, the rest of it is now a solid puck in the pot that won't come loose. (Firing it up again would probably melt it loose enough that I could tip the entire pot upside down to drop it out.

I had the pot turned up to full power. Did I overheat the potassium nitrate and turn it into something useless? The label on the stump-remover bottle still says "contains potassium nitrate," so I don't think it's changed to something else. I'm pretty sure you're not supposed to add water at any point, since it's going to get more than hot enough to boil off all of the water (and adding it to molten salt would produce something akin to the "tinsel fairy" who visits when water gets added to hot lead). Anybody have any idea where I might've gone wrong?

[salfter]

More details:

I fired up the pot long enough to empty it out and got this:



The two pieces at the bottom had been lifted off earlier. The liquid puddle was poured out after the puck freed up enough for me to lift it out.

[john.k]

Its likely some inerting material is added to the nitrate,so s you cant make you know what.

[dikman]

The MSDS says it's 100% potassium nitrate (so no added stuff). It also says that the melting point is 633* and the decomposition point is 752*, so a fairly narrow band to keep it liquid. When it decomposes it forms potassium nitrite and oxygen. And that's as much as I know about it.

[Handloader109]

Probably too high a temp.......

[dragon813gt]

How hot was it? I had mine over 1,000 degrees, it was pure potassium nitrate, and it didn't do that. There are better, safer, ways to anneal.

[salfter]

Probably too high a temp.......

It's a pretty good chance that was what happened. I had the pot cranked up to maximum, and it might've gone straight through from solid to useless. Might need to give it another shot, this time stepping the temperature up from minimum until it melts. I also have a PID (built it for my Lee 4-20 bottom-pour pot) that I could put on it if it turns out to need that level of temperature control.

[dragon813gt]

You will want to use a separate thermocouple if you do that. It will be useless for anything other than the salt after being submerged in it.

[JonB_in_Glencoe]

SNIP...

I poured some into the lead pot, cranked up the heat, and waited for it to melt. Instead of forming the thin yellowish-looking liquid I'd seen in YouTube videos on the subject, it formed a bubbly gray sludge that threatened to overflow the pot.

Isn't that stuff used for 'redneck style' bluing steel gun parts?
Is your Lead pot made of steel or Iron?
I bet it was just reacting to the steel and/or the lead alloy oxides on the surface