When the metal is hot enough, but below the welding temperature, the smith sprinkles some sand onto the metal, which reacts with the wustite to form fayalite, which melts just below the welding temperature.
When the metal is hot enough, but below the welding temperature, the smith sprinkles some sand onto the metal. The silicon in the sand reacts with the wustite to form fayalite, which melts just below the welding temperature.