[UnbelievingScumOnTheOtherSide]

A lake of molten brimstone means that its temperature must be at or below the boiling point, 444.6C. (444.6C is about 832 F)

Depends on the atmospheric pressure "down there".
The critical temperature for brimstone is 1041C at 204.3 atmospheres.
But that just means that above that temp and pressure the solid sublimates rather than melts.

[El Gato]

The critical temperature for brimstone is 1041C at 204.3 atmospheres.

I wonder what the atmospheric pressure would be at the bottom of a 6,500+ km column of air? You'd have to take account of way the gravitational force changes when inside the mass of the earth. It's no longer and inverse square law, and I recall working the math to figure out what it is, but I that's been a long time ago, and I only had 4 hours of sleep last night.. for some reason...