The use of silver as money was established at the very time of the Ming dynasty (1368–1644). Paper money was first issued in 1375 by the founder Hongwu Emperor amid the ban of silver as medium of exchange. But due to the increasing depreciation, the paper money became basically worthless and the ban on silver usage was finally lifted in 1436 (1st year of the Zhengtong Emperor). Meanwhile, silver was made much available through foreign trade with the Portuguese and the Spanish, in the beginning of the 16th century. The great tax reform by the statesman Zhang Juzheng in 1581 (9th year of the Wanli Emperor) simplified the taxation and required all the tax and corvee to be paid in silver. This can be seen as an indication of the firm position of silver in the monetary system of the Ming. However the reform would not have been a success or even feasible if the enormous amounts of silver had not been available through trade and imports from the Americas, mainly through the Spanish.