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Yes most any nitrate, except ammonium nitrate could decompose your product due to ammonia being a strong base. Both Sodium or Potassium Nitrate work fine though
Yeah a little hotter, but not much, plus MAPP is way more expensive, at least around here in Cali. Mapp in air is 3,670 °F while Propane in air is 3,596 °F. Adding Oxygen to either or even a small blower to blow in air would be the best bet to cheaply ramp up the temp to potentially over 5000 °F, which may exceed your furnaces limits so BE CAREFUL. But yeah, looks good. Reminds me of the electric Arc mini furnace Grant Thompson build on his Youtube channel.