When standing on top of a flaming krissure as Willow, she doesn't seems to gain any sanity from it, even though it's a source of fire.
1. Select Willow in a Shipwrecked world.
2. Lower your sanity a bit.
3. Dig up magma piles until you find a Krissure.
4. Stand on top of the Krissure during the day and observe the sanity not rising when it is spewing flames.