Realcent.org

The Coinage Act of 1857 was an act of the United States Congress which forbade the use of foreign coins as legal tender, repealing all acts "authorizing the currency of foreign gold or silver coins". Specific coins would be exchanged at the Treasury and re-coined. The act, which is four paragraphs long, was approved on February 21, 1857.

If you think about most US silver coins came from melted foreign coins so no telling how old the silver is in that junk silver is or what coin it was from Wikipedia--

Coinage Act of 1857 was an act of the United States Congress which forbade the use of foreign coins as legal tender, repealing all acts "authorizing the currency of foreign gold or silver coins". Specific coins would be exchanged at the Treasury and recoined. The act was approved on February 21, 1857.
Before this Act, foreign coins, such as the Spanish dollar, were widely used[1] and allowed as legal tender by the Act of 1806.[2]
In addition to foreign coins being banned, the Coinage Act of 1857 also discontinued the American half-cent.

And see--http://memory.loc.gov/cgi-bin/ampage?collId=llsl&fileName=011/llsl011.db&recNum=184

henrysmedford wrote:OK Roman Debasement see--
http://en.wikipedia.org/wiki/Roman_currency#Debasement_of_the_currency