Just a general passive No Ouch Or Discomfort from extreme temperatures except only in one direction. The ability to wear 18 layers of clothing outside during summer, reach into the furnace to grab red-hot metal, or swallow hot coals for eventual bodyhorror reasons.
The downside is their sense of temperature will be whacked and they will serve scalding hot tea to guests without realizing.

This seemed most obviously Forge/Alchemist-themed so it's going first.