First, because the stock of copper found in ancient China is relatively small and the stock of iron is relatively large, it is easier to express wealth with copper.

Second, the melting point of iron is relatively high, and coal was not used for metallurgy in the early days, so copper was used. The earliest use of coal in history was the Han Dynasty, and before the Han Dynasty, the Qin Dynasty had already used half a penny. This allowed later rulers to continue to use it.

Third, the chemical properties of copper are more stable than iron, and it is not easy to oxidize and rust, so it is suitable for making money. Early iron, mostly pig iron, is prone to chemical changes.

Fourth, copper is lighter because of its different weight ratio (density).