Is it an ancient coin? If so, be sure not to wash it. There is also a famous saying in the collection circle: "Wash it and gold turns to sand."

If necessary, you can use a needle made of metal softer than copper to pick (such as silver). Needles, gold needles) Because the hardness of gold and silver is softer than copper, this can be done without damaging the money itself. Do not use dilute hydrochloric acid, etc.