Selective agar products should not be re-melted. Further heating may reduce the selectivity of the medium, as many selective components are heat labile. Antibiotic supplements can be added after the base medium has been re-melted.

Can you remelt agar agar?

Agar sets very quickly, and at room temperature!

It's also stays stable when hot (up to 185° F!) and cold, unlike gelatin, which melts in heat. (You can even remelt it a few times and it will maintain its properties.)

Can you reheat solidified agar?

You can, but be mindful of evaporation.