A study on the manufacturing method of the ancient coins, Sangpyeong Tongbo in Korea

The coin cast during the Joseon Dynasty (14th -20th ) is called Sangpyeong Tongbo, and was first minted in the 17th century. In the 18th and 19th centuries, there were significant difficulties in the supply and demand of copper, which was reflected in the material characteristics of Sangpyeong Tongbo. This study analysed the components on 134 samples of Sangpyeong Tongbo coins from the 18th and 19th centuries, and three samples were selected to analyse the manufacturing methods and materials through internal impurity observations, composition, and microstructures. An optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to observe the microstructures, and X-ray fluorescence (XRF) analysis and energy dispersive X-ray spectroscopy (EDS) were used for component analysis. The results revealed that 134 coins presented a very high lead content, and iron (Fe) was also detected in all coins. This outcome is thought to reduce the costs of coins. Inside the coins, impurities such as Cu₂S, PbS, ZnS, FeAs, and Cu₂Sb were detected, all of which seem to have originated from copper or lead ores. The poor quality of the Sangpyeong Tongbo, as documented in historical records describing the mixing of rubble stone to reduce coin quality, suggests that large amounts of lead were added to increase the weight, and that copper or lead ores were used without sufficient smelting.