Ackster was no longer sure what was happening with the fight and his own body. As he slurped down more and more of Viztar's fire, he could feel himself heat up from deep within. It didn't feel like a fever or his body temperature rising. It was more like his soul was growing hotter.
'I guess that makes sense.'
The skills were related to his soul, so for Smelting Furnace to affect his soul rather than his body was reasonable. It also explained how he could store so much inorganic and organic material inside himself using Smelting Furnace. It was a space connected to his soul the same way his Inventory was.
That space was now being overtaken by green fire that smelted down and refined everything Ackster had stuffed in there, withdrawing everything useful and rebuilding Ackster's body with it while incinerating and disposing of anything that might remain.