Black iron oxide is used in the foundry as a sand additive to prevent surface defects of the cast metal. Black iron oxide is an additive for cores used in steel and iron castings. Black iron oxide is added during the sand preparation process to help reduce surface defects in steel castings. Black iron oxides act as a flux to promote softening of the sand, which can then absorb more thermal stress before a crack in the core or mold occurs. Black iron oxide is very oxygen-rich, which means it can tie up gas during the casting process. In addition, they help scavenge nitrogen associated with some binder systems. They are typically used in amounts ranging from 2-5%.