Final answer:
Zinc is added to gold casting alloy as a scavenger for oxygen during the casting process.
Step-by-step explanation:
Zinc is added primarily to act as a scavenger for oxygen during the gold casting process. It is used to prevent the formation of oxides on the surface of the molten metal, which can cause defects in the casting. Zinc reacts with oxygen to form zinc oxide, which is volatile and vaporizes during the casting process.