Complex sentence: The silicon in the sand then reacts with the wustite to form fayalite, which melts just below the welding temperature, producing an effective flux that shielded the metal from oxygen and helped extract oxides and impurities, leaving a pure surface that can weld readily.
Split into simpler sentences: The silicon in the sand then reacts with the wustite to form fayalite, which melts just below the welding temperature. This produced an effective flux that shielded the metal from oxygen and helped extract oxides and impurities, leaving a pure surface that can weld readily.