Among coinage struck in the first century of the U.S. Mint's full-scale operations, beginning in 1793, it is not uncommon to encounter coins struck from dies that rusted because of improper storage.
Some coins were struck for circulation from those rusted dies. Other pieces exhibiting rust contamination were struck as restrikes at dates later than the date on the coin, using uncanceled dies sold by the Mint as scrap metal.
Read article: Coins struck from rusted dies show corrosion detail - Coin World
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