After the Stone Age, human society first entered the Bronze Age and then the Iron Age. Bronze, which contains tin, could be used to make ritual vessels, storage containers, and farming tools; iron, which is harder than bronze, was mainly used to make tools and weapons. China's Bronze Age began at least in the Shang Dynasty around 1600 BC, while iron became widely used in the Spring and Autumn period around 500 BC, with a time gap of over 1,000 years. However, the iron content in the Earth's crust is about 5.6%, while copper's content is much lower, only 0.006%. It is said that "rarity makes things valuable". Even today, copper is more expensive. So why did ancient workers first use bronze tools and then iron tools?
There are at least two reasons for this historical situation. First, the melting point of materials. Smelting first requires melting. The melting point of copper is about 1083°C, much lower than the melting point of iron at 1535°C, and the melting point of bronze alloys is even lower. Obtaining high temperatures was very difficult in ancient times, and even today it requires a lot of energy. The flame temperature of ordinary coal is difficult to reach the melting point of iron without the help of bellows, so the smelting of iron in ancient times was inseparable from the invention of bellows. According to historical records, the widespread use of bellows in China was around 500 BC, which was also the era when iron became widely used.
Second, the reactivity of metals. Reactivity is a chemical property of metals. Metals that exist in the form of elements in nature are usually not very reactive, such as gold, which is a very inactive metal; conversely, metals that exist in the form of compounds are considered reactive, such as sodium in table salt (sodium chloride). Copper is also a not very reactive metal and is relatively easy to be reduced. However, iron is much more reactive than copper, so the reduction of iron is more difficult than that of copper. Reduction means restoring the original state of metal elements, that is, extracting pure metals from metal compounds.
Perhaps, copper was originally "hidden" in the form of compounds in malachite. Ancient people accidentally used malachite to build stoves, and then added some firewood to it. As a result, the earliest metal copper was produced from the stone stove. However, the smelting of iron was not so easy.