Bronze

Bronze is a metallic alloy made by combining the elemental metals copper and tin.

Bronze in "The Horse of Bronze"
Both Centaurs and Sphinxes forged weapons made of bronze when the two contingent metals were available to them. These had the advantage of being much harder than weapons forged from unalloyed copper. A disruption to the centaurs' tin supply, caused by loss of contact with their trading partners the Nuggies, led to the centaurs returning to pure copper weapons, which in turn led to a major defeat at the hands of the bronze-armed sphinx army.