I've been wondering something for a while…

I make and use my own hot caustic bluing salts out of caustic soda, sodium nitrate, and sodium nitirite.
All that gets mixed up with water (gets REALLY hot when the water's added) and then heated further.

What I'm wondering is that the salts seems to heat evenly up till about 250° F, and then it "sticks." The temperature will hold there for a few minutes and then rise rapidly to bluing temperature (298° F), at which point the salts boil.

Any idea on what reaction is going on that makes the temp stick and then rise so fast?

Hmmmm.  Like buffering, only with heat.

Some chemists can chime in but I'm pretty sure it's due to the micronauts who live on those tiny molecule worlds turning on their A/Cs because its getting so damn hot.

Seems to me I remember this is a common occurrence when heating just about anything up.

I just use the Brownells bluing mix.

I have a hard time with boil off and keeping temp level.

I've been thinking of going to an electric heating element for consistency.

I'm using a propane camp stove.

You have to tend it, and keep adding water. Otherwise, you can end up with a 500° pot of molten salt.