What was the salt?
High-purity Sodium Chloride melts at 1473.3 degF, but unless you were using a known high-purity NaCl, you are likely to have something that melts at a lower temperature.
Potassium Chloride melts at 1420 degF, for example.
The melting point of a Sodium Chloride/Potassium Chloride Eutectic is 1215 degF.
If you are using “salt” intended for human consumption, it is likely to have added anti-caking agents and probably several other added or naturally-occurring impurities and the likelihood of it having a melting “point”, as distinct from a melting range, is slim. If it has one, your chances of finding out what it is from the supplier are probably even slimmer.
If you want to test/verify your oven, you are better off using a known good thermocouple and pyrometer or something specifically formulated to melt at known temperatures: Tempilstiks or pyrometric cones are 2 examples.
