An old (and fraudulent) trick in the gold business is to vigorously shake a bag of gold coins together, in an effort to free a few bits of the previous metal in the form of dust, which can be sold to the benefit of the owner. The devalued coins still retain their face value, so there is no loss in that regard. This is known as “sweating” gold. The U.S. Treasury engaged in the same practice, though to be sure not on a deliberate basis.
An 1893 letter from the U.S. Treasurer to the Superintendent of the Philadelphia Mint notes that bags used to store gold coins contained measurable amount of gold dust. Two bags are noted in this correspondence, one yielding 2.87 grains of gold and the other 2.16 grains (worth 12.3 cents and 9.3 cents respectively). Enos H. Nebecker, the U.S. Treasurer, notes a number of coin bags from the New York assay office had been forwarded to the Mint for harvesting the remaining gold. Presumably the number of bags was large, as the shipping costs from New York to Philadelphia would have offset any profit.
What the government took with one hand it gave back with the other. Congress made appropriations to make the country’s circulating coinage whole, so that if underweight gold pieces were returned to the Mint, they could be melted and recoined at full weight.
Link to February 27, 1893 correspondence: https://archive.org/details/extractgolddustfromoldcoinbags18930227/mode/2up
Link to U.S. Mint general correspondence on Newman Portal: https://nnp.wustl.edu/library/archivedetail/515202