Pyroflux

Pyroflux is a "smelty hot sauce, essence of lava".

Can be produced in small quantities on Nauvis by core mining, but requires vulcanite blocks to produce in bulk.

The primary use case of pyroflux is for smelting ores, which produce molten ores that can be turned into ingots. This requires quite a bit of set up, but is far more efficient particularly for steel, which can be smelted directly from iron ore and coal instead of the usual two-step ore to iron to steel path.