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 dated 1982, when both copper and zinc cents were made, and best way to determine their composition is to weigh them. Solid copper pennies weigh 3.11 grams (+/- 0.130 g.), whereas the copper plated zinc pennies weigh only 2.5 grams (+/- 0.100 g.).
Back in the early 1970s, the rising price of copper was pushing the cost to make a penny over its face value of one cent. Fortunately, the price of copper dropped and production continued. Unfortunately, the increasing price of copper in the early 1980s forced The United States Mint to change the composition of the penny permanently. This was to prevent a melt off of pennies. It has been the past experience in the United States that when the melt value of a coin exceeds its face value, people will melt the coins in order to sell the raw metal and make a profit.
In an effort to thwart a melt off of pennies in 1982, the United States Mint made half of the pennies out of solid copper and the other half out of copper plated zinc. Although it is illegal to melt pennies and sell the raw metal, people still pull the solid copper pennies out of circulation in order to save them for their copper value.
The Best Way to Tell the Difference
Be sure to use a scale that is accurate enough to detect a tenth of a gram (0.1 g.) 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 or granite countertop 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.
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.
Watch out for "transitional" mint errors! "Transitional" errors occurred in the Lincoln Memorial Cents series when the Mint accidentally used solid copper blanks to make a few pennies in 1983.These "wrong stock" pennies weigh 3.11 grams, rather than the 2.5 of the copper-plated zinc cents. If you find a solid copper 1983, it just might be worth... a pretty penny! Take it to a trusted local coin dealer to verify your findings.