Proof coinage
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Proof coinage means special early samples of a coin issue, historically made for checking the dies and for archival purposes, but nowadays often struck in greater numbers specially for coin collectors (numismatists). They can usually be distinguished from normal circulation coins by their sharper rims and design, as well as much smoother fields.
Preparation of a proof striking usually involved polishing of the dies. Modern US proof coins are often treated with chemicals to make certain parts of the design take on a frosted appearance, and the fields taking on a mirror finish. Several other methods have been used in the past to achieve this effect, including sand blasting the dies, and matte proofs. Proof coins of the early 1800s even appear to be scratched, but it was part of the production process.
United_States_penny,_obverse,_2002.jpg
Most proof coins are double struck. This does not normally result in doubling that is readily observable, but does result in the devices being struck fully.
See also Commemorative Coins