Reader Judy McCaulley wrote about a 1982 cent she thinks may be a "silver penny." She asked if there is any way to authenticate it.
Lets first note that "silver pennies" (more specifically - cents struck in error on a silver dime planchets or blanks) are more or less only possible on cents dated 1966 and earlier. 1966 was the last year the government struck 90% silver dimes (and other denominations) for general circulation. (90% silver coins produced in 1965 and 1966 bore the date 1964 due to a date- freeze.)
Of course the coin could be an ultra-rare example of a "transitional" error struck on an old silver dime planchet left over from the old days (found the during retooling or the moving of a press, etc.), but this is a very remote possibility.
On cents struck on the homogenous copper alloy planchets in the earlier part of 1982 and years previous, knowing the weight is a helpful tool in authenticating this error type. Pre-1982 Lincoln cents weigh 3.1 grams while silver dimes of the era weigh 2.5 grams. However, since McCaulley's coin is dated 1982 and since a significant number of the Lincoln cents produced that year were struck on planchets of a copper plated zinc composition that weigh the same as a silver dime blank, we cannot use weight as an indicator.
A good alternate test is to see if it feels slick or greasy. If so then it may be coated with mercury -- an alteration resulting from a common high school lab demonstration (that may or may not still be common). This slick feel can even be felt on the edge of the coin, so a heavy thumb on the face of the coin is not necessary and is potentially damaging if you leave a fingerprint behind on a genuine off-metal strike. Many coins are also plated with a silvery substance for charm bracelets and other novelties and more often than not have a slippery feel.
Another thing to look for is the size of the coin and to note if it is out-of-round. The dime is smaller in diameter than a cent, and while a dime planchet or blank will spread a bit to fill the void when struck by the larger diameter cent dies, it rarely fills the dies completely from edge to edge. This results in a coin of slightly smaller diameter than a cent. The spread is rarely even and thus the coin will not be perfectly round, though it may appear to be so at first glance. Most of the time, this spreading will also cause some excessive flow marks to appear on at least some of the characters close to the rim. Some letters (or at least their tops) will appear as though they are actually spilling over the edge of the coin when viewed under magnification.
Of course the coin in question could be struck on a foreign planchet made of silver, but this mandates a search of government records to see what was made for the year in question. I don't believe anything was made in that small a size in silver by the U.S. Mints during the 1980s, but records are available via the United States Mint's Annual Report covering that year. Earlier records may be found in the book entitled Domestic and Foreign Coins Manufactured by Mints of the United States 1793-1980 by the Department of the Treasury/Bureau of the Mint and issued by the Government Printing Office Washington in 1981. To the best of my knowledge, this book has not been updated.
A final test that should be attempted if all else fails is the "ring test". Gently drop the coin onto a hard surface, like a table, and listen for the ring of silver. You'll know it when you hear it. If the coin is plated it will not sound like silver nor will it sound exactly like a Lincoln cent. It will exhibit a duller thud. If it's coated with mercury it will sound more like a cent, but perhaps not perfectly so.
Persons not familiar or comfortable with such testing should consult error coin specialists or one of the major grading services such as the Professional Coin Grading Service, who recently began attributing major error coins.
Ken Potter is the official attributor and lister of world doubled dies for the Combined Organizations of Numismatic Error Collectors of America and for the National Collector's Association of Die Doubling. He privately lists U.S. doubled dies and other collectable variety types on both U.S. and world coins in the Variety Coin Register. For more information on either of the clubs, or on how to get a variety listed in the Variety Coin Register, send a self addressed stamped long envelope to Ken Potter, P.O. Box 760232, Lathrup Village, MI 48076-0232.