My daughter's preschool class cleaned really old dark pennies and I was amazed how well they came out! What they did was put the pennies in a little dish, covered them with table salt and then poured vinegar on them to cover. They let them sit a while and then rinsed and wiped them and they looked great. Some need more time then others in the solution and you can use your finger to rub the salt a bit to take off tough spots.