If your Lincoln Memorial penny has a date before 1982, it is made of 95% copper. If the date is 1983 or later, it is made of 97.5% zinc and plated with a thin copper coating.
For pennies minted in 1982, when both copper and zinc cents were made, the safest and best way to tell their composition is to weigh them. Copper pennies weigh 3.11 grams, whereas the zinc pennies weigh only 2.5 grams. Be sure to use a scale that is accurate enough to detect the tenth of a gram (0.10) or better. If you weigh a zinc penny on a scale that can only register full 1 gram increments, the penny will usually display 3 grams, since the scale rounds the 2.5 gram zinc penny upwards to 3. The wrong type of scale can be misleading when you are trying to sort copper and zinc pennies.
Drop Test for Copper and Zinc Pennies
If you don't have a tenth-gram scale handy, you can use the "drop" test. You need a hard Formica surface, a known copper penny, and a known zinc penny. Drop each one onto the table, listening to its distinctive sound. Zinc pennies have sort of a flat "clunk," whereas copper pennies have a higher-pitched, more melodious "ring" sound. Once you have a good feeling for how each type sounds, start dropping your 1982's one at a time, listening for the sound they make, and you should be able to sort them out by metal composition. Obviously, this test isn't as reliable as weighing them, but it should help you sort most of copper and zinc pennies.
Caution: Only use the drop test on circulating pennies where you are sorting copper and zinc for the bullion value only. Never drop collectible Uncirculated or Proof coins in this fashion to test them, since dropping pennies on a hard surface might cause minor damage that can make a collectible coin less valuable.Cherrypicker's Tip - Watch out for "transitional" mint errors! "Transitional" errors occurred on the Lincoln Memorial Cents when the Mint accidentally used copper blanks for 1983 pennies. These "wrong stock" pennies weigh 3.1 grams, rather than the 2.5 of the zinc cents. If you find a copper 1983, it just might be worth... a pretty penny!