You can't melt THRM, only set it on fire, which turns it into lava, which becomes BMTL when cooled.
TUNG remains TUNG when melted, and only upon cooling can it partially transform into BRMT(TUNG), which can be re-melted and cooled to obtain TUNG.
BMTL remains BMTL when melted and cooled.

Basically, you only get BRMT if you apply pressure to BMTL or TUNG (which can happen by accident, as LAVA creates pressure, especially noticeable when there is a lot of it), but not by melting. Furthermore, when BRMT is melted, it turns into BMTL (but not the other way around).