Tin is an essential metal in the creation of tin bronzes and its acquisition was an important part of ancient cultures from the Bronze Age onwards. Its use began in the Middle East and the Balkans around 3000 BC. Tin is a relatively rare element in the Earth's crust, with about 2 parts per million (ppm), compared to iron with 50,000 ppm, copper with 70 ppm, lead with 16 ppm, arsenic with 5 ppm, silver with 0.1 ppm, and gold with 0.005 ppm . Ancient sources of tin were therefore rare, and the metal usually had to be traded over very long distances to meet demand in areas which lacked tin deposits.