Lava in cauldrons doesn't burn items

[zauberparacelsus]

Tossing an item into lava normally burns and destroys it, but tossing an item into a lava-filled cauldron does nothing.

[KnightMiner]

This is intentional, tossing items into a cauldron (using a dropper for instance) is the way to automatically perform cauldron recipes, you would not want your item to be destroyed simply because you dropped it in a bit too soon.