An alloy of copper and tin that is harder than copper. Bronze was made before 3000 BC, though it was not used in tools and weapons for some time. Tin added to copper made casting easier and the edges of tools and weapons harder. The proportions of copper and tin varied widely (67-95 percent copper in surviving artifacts) and the addition of zinc, nickel, lead, arsenic, or antimony is also known. Adding tin to copper makes casting easier and the edges of tools and weapons harder. The main disadvantage was the comparative scarcity of tin. A higher percentage of tin produces potin or speculum. The Bronze Age of the Three Age System began in Eurasia when it replaced copper as the main material for tools and weapons. It was then replaced by the more common and efficient iron, but was still used for decorative purposes. Modern bronze also contains zinc and lead.