Sentence:  […] can vary in different positions in the furnace and during the <WSD>smelt</WSD>. / Furnaces are unlikely to survive the <WSD>smelts</WSD>; all that often remains on metal production sites is just furnace bases and broken fragments of furnaces […] 
Meaning of the word marked with <WSD> tags:  Any of the various liquids or semi-molten solids produced and used during the course of such production.