Never clean old coins, pennies, nickels, dimes, etc... Coin collectors want the natural patina. If the pennies have dirt on them then soak them in water to remove the dirt.
But don't use any detergents. Detergents can damage the surface of the coin and leave pitting lowering the value of the coin. Also do not rub the coins together to loosen the dirt, this can scratch and wear the coins and lower the value.
Coins (US Coins) will be worth face value to a non-coin collector, but if you plan to collect coins or just clean them to sell as collector pieces, then don't clean them.
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The short answer is, do not clean your coins.
WARNING - There is no way you can clean a coin without damaging the finish and reducing any collector value it may have.
Some coin catalogues suggest that you might use a soft brush on copper or bronze coins and, that there are professionals capable of cleaning coins.
Since you state that your coin is valuable, go with the short answer and do not clean your coins.
The short answer is, do not clean your coins.
WARNING - There is no way you can clean a coin without damaging the finish and reducing any collector value it may have.
Some coin catalogues suggest that you might use a soft brush on copper coins.