Marco Polo considered the use of paper currency in the Mongol Empire one of the marvels of the world. Paper currency wasn't used in the entire empire. The Chinese silver ingot was accepted universally as currency throughout the empire, while local coins were also used in some western areas, such as the modern day Iran. Paper currency was used in China, continuing the practice established by the Chinese several hundred years before. The Chinese had mastered the technology of printmaking and therefore it was relatively simple for them to print bills. Paper currency was used in China since 960 A.D., when the Song dynasty started replacing their copper coinage with paper currency. When the Mongols invaded Song China they started issuing their own Mongolian bills in 1227. This first attempt by the Mongols did not last long because the notes issued expired after several years and were inconsistent throughout the parts of the Mongol Empire that issued them. In 1260, Kublai Khan created the Yuan Mongol's first unified paper currency with notes that did not have any expiration date. To validate the currency, it was made fully exchangeable to silver and gold and was accepted as tax payments. Currency distribution was small at first, but the war against the southern Song dramatically increased circulation. With the defeat of the Song, their currency was taken out of circulation and could be exchanged with Mongol currency at a relatively high exchange rate. Regardless of persistent inflation after 1272, paper currency backed by limited releases of coins remained as the standard means of currency until 1345. Around 1345, rebellions, economic crisis, and financial mismanagement of paper currency destroyed the public's confidence in the bills.