 The Coinage Act of 1792 , the legislation which provided for the establishment of the Mint of the United States ( today the United States Mint ) , authorized coinage of multiple denominations of gold , silver and copper coins . According to the act , the dollar , or " unit " , was to " be of the value of a Spanish milled dollar as the same is now current , and to contain three hundred and seventy @-@ one grains and four sixteenth parts of a grain of pure , or four hundred and sixteen grains of standard silver " . The act went on to state that the coin would be struck in an alloy consisting of 89 @.@ 2 percent silver and 10 @.@ 8 percent copper . The purity and weight standards outlined in the Act were based on the mean of several assays conducted on Spanish milled dollars . However , the dollars were mandated by Spanish law to contain 90 @.@ 2 percent silver , and most of the unworn examples in circulation in the United States at the time contained approximately 1 @.@ 75 grains ( 0 @.@ 113 g ) more than the silver dollars authorized by the Act . In 1793 , President George Washington signed into law a bill which declared Spanish milled dollars legal tender , provided that they weighed no less than 415 grains ( 26 @.@ 9 g ) , which meant that at the lowest weight allowed by law , the Spanish dollars would contain approximately 0 @.@ 5 percent less silver than the United States dollar coins . As a result , the United States silver dollars and unworn Spanish dollars were largely forced out of circulation in accordance with Gresham 's law ; the lighter Spanish dollars were shipped in quantity for circulation in the United States , while the heavier pieces would be turned in to the Philadelphia Mint to be recoined into United States coinage to take advantage of the discrepancy in weight . At that time , silver bullion was supplied to the Mint exclusively by private depositors , who , according to the Coinage Act of 1792 , had the right to have their bullion coined free of charge . As large silver coins were a preferred method of commerce throughout the world , especially China , a considerable number of the United States dollars requested by silver depositors were exported to satisfy that demand . 
