Copper turn green because it reacts with oxygen and gas in
atmosphere giving a layer of green-blu oxides composed by copper sulfate and
other copper oxides.
This layer of oxides is also called copper patina. It's a natural process or can
be produced artificially using chemicals.
The easiest way to clean old copper pennies is to use Vinergar, or you can use also Tomato Ketchup or Lemon juice.
Tomato Ketchup and Lemon juice contain weak acids, acetic acid (vinegar) and
citric acid, that will dissolve the layer of copper oxides on the copper
To clean copper pennies just dip the pennies into Vinegar for
approx half an hour to 1 hour, and then rinse with water.
Is also possible to accelerate the process scrubbing the surfaces of the pennies
with an old soft cloth or toothbrush.