by 
nike15
ANSWERS: 3
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The best commercial product that I have found to clean copper/nickel is CLR.
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I've put them in vinegar before, but its been to long for me to remember how well it worked.
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Materials Dull-Dirty Pennies 1/4 cup white vinegar (dilute acetic acid) 1 teaspoon salt (NaCl) 1 shallow, clear glass or plastic bowl (not metal) 1-2 clean steel screws or nails water measuring spoons paper towels Shiny Clean Pennies Pour the salt and vinegar into the bowl. Stir until the salt dissolves. Dip a penny halfway into the liquid and hold it there for 10-20 seconds. Remove the penny from the liquid. Dump the rest of the pennies into the liquid. The cleaning action will be visible for several seconds. Leave the pennies in the liquid for 5 minutes. Pennies get dull over time because the copper in the pennies slowly reacts with air to form copper oxide. Pure copper metal is bright and shiny, but the oxide is dull and greenish. When you place the pennies in the salt and vinegar solution, the acetic acid from the vinegar dissolves the copper oxide, leaving behind shiny clean pennies. The copper from the copper oxide stays in the liquid. You could use other acids instead of vinegar, like lemon juice.
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