Verdigris is a chemical residue that builds up on some metallic surfaces exposed to moisture. Old pennies found tucked in a drawer, beneath a couch or in the attic may be covered in this greenish substance. The copper is prone to tarnishing and the verdigris builds up if it is not cleaned. A simple pantry staple can restore the pennies to their former shine.
Pick off large areas of verdigris from the pennies using a toothpick. Use it to pry up the caked-on residue and then rub the surface of the pennies firmly with the toothpick to loosen remaining verdigris.
Submerge the coins in a bath of mild detergent and warm water. Leave them to soak for 15 minutes or until the water goes cold. Then, pick them up and rub them between your fingertips. Scrub off the residue manually, using the toothbrush to help you if necessary.
Fill a container with enough vinegar to cover all the pennies. Add the pennies, making sure to submerge them to avoid exposing the verdigris to the air. Shake the container to level out the pennies.
Allow the pennies to soak in the vinegar at least 24 hours. The acidity of the vinegar breaks down the verdigris compound, allowing easy removal. If you prefer, you can substitute hydrogen peroxide for the vinegar.
Drain the pennies from the vinegar and rinse them under warm water. Then, rub and scrub them with a toothbrush.
Finish the process by polishing the pennies with a dry, clean cloth.