Slag is a secondary output of the Thermal ExpansionInduction Smelter. The slag can be cooked in a furnace to make Rockwool which is non-flammable wool. Slag can be used to make a florb. Florbs are a item that can be used to make a holdable liquids, not using a bucket. The liquids vary from hot Lava , to Gelid Cryotheum.