Copper-based alloys were used primarily for official cash coinage for over two millennia in China, and coinage dating from the Song Dynasty (960-1279) is no exception. Cash coins were cast on a huge scale during the Song Dynasty due to rapid commercial growth, especially during the Northern Song (960-1127), and were the mainstay of the currency system, partly because cloth – which had been used as a form of currency in the past in Imperial China – was now considered a commodity, and partly due to the opening of new copper mines which resulted in the high availability of raw materials. However, coin production slowed considerably during the Southern Song Dynasty (1127-1279) with the arrival of paper money.
As previously stated, copper alloy coins were the main form of currency during the Song Dynasty. While this was the case, iron coins started to be used extensively from the beginning of the dynasty. Although iron coins have been known to have been cast as early as 24 AD, their use became much more common during the Song. This is possibly because despite the opening of new copper mines, the commercial expansion resulting in increased demand for coinage exceeded this new influx of raw materials, and copper and tin were now in short supply. Iron coins were mainly cast and circulated in Sichuan and Shaanxi provinces where copper was in particularly short supply.
Iron was used for several reasons. Iron ore was not in short supply, and iron casting was relatively cheap in comparison to bronze casting. The infrastructure for iron production was already in place in China, and the technology used for casting iron coins was the same as that used for bronze casting. All these factors made iron coinage a practical choice. Despite this, the vulnerability of iron to corrosion and problems with getting a fine impression on iron, ensured the continued use of the copper-based cash as the main form of coinage.