The Metal Age began around 4,500 BC and marked the last period of Prehistory. It was divided into the Copper Age, Bronze Age, and Iron Age. During the Copper Age, copper and gold were the first metals used by humans and were hammered into tools and weapons. The Bronze Age followed, using an alloy of copper and tin to make stronger weapons, tools, and utensils for agriculture and domestic use. Finally, in the Iron Age, iron became popular as it was even stronger than bronze, and new techniques like casting in molds were developed. Trade expanded, agriculture continued, and social stratification increased during this time.