Additives to foundry sand cores are provided for reducing or eliminating surface defects called veins in metal castings, by reducing the propensity of sand mold and core aggregates to crack during metal casting. The additives generally include an iron oxide component such as magnetite and hematite and a glass component which is preferably free of lithium oxide. The additive may also optionally include an amount of carbon effective to reduce adhesion of sand particles to the casting.