and gold coins had already started in the eighth century, they weren't often used as a medium for exchange until later when the Japanese started importing Chinese coins, which replaced the Japanese commodity currency economy. As internal trade grew due to agricultural and handicraft developments, the people started preferring coinage to barter, leading to a growth of demand for copper coins. The Southern Song dynasty prohibited the export of its coinage in 1179 due to its problem with the outflow of currency, but shiploads of Chinese coins would still enter Japan annually through Ningbo. There is evidence to suggest that the Yuan dynasty used to extensively export Chinese cash coins to Japan for local circulation. The Sinan shipwreck, which was a ship from Ningbo to Hakata that sank off the Korean coast in the year 1323, carried some 8,000 strings of cash coins, which weighed about . Since the trade had begun with Japan, and they received payment in Chinese coins for Japanese goods, they stopped minting their own copper coinage until 1587. The Ashikaga shogunate imported Kōbu Tsūhō (洪武通寶), Eiraku Tsūhō (永樂通寳), and Katei Tsūhō (嘉靖通寶) from the Ming dynasty, which they referred to as Toraisen or Minsen (), but the high demand for copper coinage inspired local and private production of copper coins (Shichūsen, ). An example of a Shichūsen used for trade with China and the Ryukyu Kingdom would be a Kōbu Tsūhō coin minted by Satsuma domain which included the character “” (Ji) on the reverse indicating that it was minted at the town of Kajiki, while still using the inscription of the Hongwu Emperor of Ming China. Some Shichūsen would also bear the inscriptions of coins from the Song dynasty, although it was not uncommon for many coins to simply be recasts and copies of older Song and Ming dynasty coins in the form of Iutsushi (鋳写し) or by simply adding extra carvings on existing circulating Chinese coins. Bitasen () refers to the Shichūsen coinage produced in Japan by the nobility and private local mints, and not by the imperial government or before the establishment of the Tokugawa shogunate which were often poor in appearance, as well as damaged and worn out imported Chinese coins. Over time these coins would become damaged, and this made sellers more discriminating in what coins they would accept at face value, often accepting them only at ¼ of a