My first guess would be that your gold bar isn't 100% gold, because alloying with a little copper or silver will give you more durable and practical ingot. Ifyou look at a typical gold bar they'll usually have embossed their purity. Even "pure”, 24-carat gold is probably "fineness" 998 or 999, which translates to 99.8% or 99.9% by mass.