During the era of circulating silver coins in the US, the US Mint obtained the silver it needed for coin production via purchase in the open market or by melting uncurrent silver coins that were taken out of circulation and held by the Treasury.
(Note: "Uncurrent" is not a chronological metric, it refers to US coins that are whole and recognizable, but reduced in weight below legal specifications - the reduction is typically the result of heavy circulation wear.)Among the US commemorative coins of the classic era, two had language in their authorizing legislation that specified the silver to be used for them was to be from "uncurrent silver coins now in the Treasury." The two coins? The coins created in support of the 1892-93 World's Columbian Exposition: The Christopher Columbus Half Dollar and the Queen
Isabella Quarter Dollar.
As the coins were part of the appropriations being made to the World's Columbian Exposition Company and the Board of Lady Managers pf the World's Columbian Exposition, respectively, Congress sought to ensure the production of the coins was as cost-effective and resource-efficient as possible. What better way to make an appropriation than to use "money" that was already on hand and didn't require dipping into taxpayer revenue?
While it's a certainty that some of the commemorative silver coins that followed in the classic series were also struck using silver from previously-minted coins that were melted down - for example, there are known cases of commemorative coins from a given year being returned to the Mint by their sponsors to be melted for striking into coins of a different date/design - the Columbian and Isabella coins are the only coins to include the specified use of "uncurrent silver" in their enabling legislation.
1892 World's Columbian Exposition - Christopher Columbus Half Dollar
1893 World's Columbian Exposition - Queen Isabella Quarter Dollar

For more of my topics on commemorative coins and medals, including more on the history and designs of the World's Columbian Exposition coins discussed above, see:
Commems Collection.