In the books they pretty much confirm it takes more than just heat to destroy the One Ring.

Good forges burn hotter than lava, and dragonfire even hotter still, yet Gandalf said that the biggest, baddest dragon who ever lived Ancalagon the Black (who makes Smaug look like a whimpy little butterfly in comparison) wouldn't be able to harm the One Ring.

It's the magical tie to Orodruin/Mount Doom that allows for its destruction.