Abstract: During the Tang and Song Dynasties,copper coins underwent a complete transformation from weak supply to abundant supply.This process follows the two basic principles of making profits when casting coins and being determined by the state,but its effectiveness is constrained by factors such as the supply and demand relationship of copper coins, the development of mining and metallargy,and the private money market.The revenue of official coinage decreases with the increase of coinage,and the cost of coinage decreases with the increase of coinage.When the revenue of coinage equals the cost of coinage,a “critical point of coinage” is formed,which is the limit of the scale of official coinage.During the mid to late Tang Dynasty,the high cost of minting coins due to inefficient mining capabilities led to the early arrival of the critical point for minting coins,resulting in a sustained lack of supply of official copper coins.The government was trapped in the dilemma of increasing minting losses and reducing minting power,and private money continuously squeezed out official copper coins.During the Five Dynasties period,the northern regime attempted to increase the actual purchasing power of official copper coins by banning private money and admitting a customary rule among the people.The southern regime adopted the practice of casting lead iron coins and enforced the equal circulation of lead iron coins and copper coins in an attempt to reduce the cost of casting coins,but none of them were able to reverse the decline of official copper coins.After the establishment of the Northern Song Dynasty,the increasing scale of fiscal and market monetization stimulated the demand for copper coins and boosted their purchasing power.The Song court also established a series of effective mining governance systems,stabilized the supply of mineral materials,and vigorously rectified private money through financial hubs,achieving the combination of copper coin system and credit system.This not only laid the foundation for large-scale copper coin minting in the early and middle stages,but also greatly delayed the arrival of the critical point for official copper coin minting.

Key words: copper coins, profitability principle, state monopoly over currency, coinage threshold, mining policy capacity