I think the fertilizer thoughts are referring to potassium nitrate, also easily found in stump killer. Its usually mentioned for nitre blue colors, but might benefit from checking if running up to the carbona black color. Low temperature heat treating salts are easily available and would cover that mid 800* range if the color was just temperature dependent.
Correct craigd. Ammonium nitrate would decompose explosively at a much lower temperature of 390-400 degrees F. Potassium Nitrate melts at about 633 degrees F and begins to boil at 752 degrees F.
The stump killers and fertilizers are not pure lab grade product, so these temps may not apply exactly. A pot of previously melted salts may erupt explosively when re-melted unless there is a means for pressure to escape.
