[Oyeboten]

I just used Saltpeter, aka Potassium Nitrate when I blued a couple of mine back in the late 1980s.

It worked beautifully.

Used an old time two Burner Cast Iron Gas Hotplate, a Sheet Steel Box to melt the Saltpeter in, and, had a Thermometer stuck in the melted Saltpeter.

Used some long Tongs and Tweezers to put parts/framed in to or to withdraw them out from the molten 'Soup'.

 

[Oyeboten]

The few times I did a Hot BLue was some decades ago.

We used a Welded up Sheet Steel Tank, sitting on a piece of thick Steel Plate, set on to a Two Burner Gas Hot Plate.

We'd let the Salts melt of course, and, regulated the Temp by using a high heat Thermometer and manually regulating the Flame size to keep a steady constant Temperature in the molten Salts.

We then used Iron Wire or Tongs to set parts in to the Bath, and just watched them by Eye to see the colors coming up...pulling them out as the desired color was nearing. Then cooling in Hot Water and allowing the parts/frames/whatever to Air Dry, which they would to very quickly since they were pretty warm.

It was fun and worked splendidly.