precious metal. Bronze Age Certain metals can be recovered from their ores by simply heating the rocks in a fire: notably tin, lead and (at a higher temperature) copper. This process is known as smelting. The first evidence of this extractive metallurgy dates from the 6th and 5th millennia BC, and was found in the archaeological sites of the Vinča culture, Majdanpek, Jarmovac and Pločnik in Serbia. To date, the earliest copper smelting is found at the Belovode site; these examples include a copper axe from 5500 BC. Other signs of early metals are found from the third millennium BC in places like Palmela (Portugal), Los Millares (Spain), and Stonehenge (United Kingdom). However, as often happens in the study of prehistoric times, the ultimate beginnings cannot be clearly defined and new discoveries are ongoing. These first metals were single elements, or else combinations as naturally occurred. By combining copper and tin, a superior metal could be made, an alloy called bronze. This was a major technological shift that began the Bronze Age about 3500 BC. The Bronze Age was a period in human cultural development when the most advanced metalworking (at least in systematic and widespread use) included techniques for smelting copper and tin from naturally occurring outcroppings of copper ores, and then smelting those ores to cast bronze. These naturally occurring ores typically included arsenic as a common impurity. Copper/tin ores are rare, as reflected in the absence of tin bronzes in western Asia before 3000 BC. After the Bronze Age, the history of metallurgy was marked by armies seeking better weaponry. States in Eurasia prospered when they made the superior alloys, which, in turn, made better armor and better weapons. Significant progress in metallurgy and alchemy was made in ancient India. Iron Age The extraction of iron from its ore into a workable metal is much more difficult than copper or tin. While iron is not better suited for tools than bronze (until steel was discovered), iron ore is much more abundant and common than either copper or tin, and therefore more often available locally, with no need to trade for it. Iron working appears to have been invented by the Hittites in about 1200 BC, beginning the Iron Age. The secret of extracting and working iron was a key factor in the success of the Philistines. The Iron Age refers to the advent of iron working (ferrous