In order to smelt in Terrafirmacraft, you will need to build a bloomery. This is done by placing stones in a U-shape and then adding charcoal and iron ore to the center. Once you have lit the bloomery, it will take around two hours for it to reach a high enough temperature to start smelting the iron ore. The resulting product is called pig iron and can be used to make a variety of items in Terrafirmacraft.