Is molten potassium nitrate possible? Is it possible to melt potassium nitrate without it decomposing into potassium nitrite and oxygen? If so when it solidifies does it keep its oxidizing properties?
https://en.wikipedia.org/wiki/Potassium_nitrate gives a melting point of $\pu{334°C}%edit$ without decomposition, but then decomposition sets in at $\pu{400°C}$ so you have a narrow liquid range.  

If you avoid decomposition the oxidizing property still works.