Casting flux is used to
Fluxes are specifically used

To remove gas or prevent its absorption into the melt.
To reduce metal loss.
To remove specific impurities and nonmetallic inclusions
To refine metallic constituents, or to lubricate and control surface structure in the semi-continuous casting of metal alloys.

Key concept
Flux powder may be sprinkled over the molten metal to reduce the oxides and increase fluidity for casting.