cleaning old copper pennies
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 Tomato Ketchup of 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 pennies surface.
To clean copper pennies just dip the pennies into Tomato or Lemon Juice 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.
Other source on cleaning old copper pennies:
http://www.australwright.com.au/datasheets/advice/Copper%20&%20Brass/Cleaning%20Copper%20&%20alloys.pdf
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cleaning old copper pennies?