Zircon is a naturally occurring refractory mineral and is therefore subject to variability. It is chemically inert and is stable to very high temperatures. Zircon has a very high thermal conductivity and acts as a chill to eliminate metal mold reactions and burn-in, resulting in a smooth cast surface. Zircon is available in both flour and granular form.
Zircon is almost exclusively used in the primary slurry and as a primary stucco. Because of its high density, Zircon provides excellent flow characteristics in a primary slurry. The round grains do not cause defects such as rough casting surfaces.