vitruvius

Firstly, my qualifications ... None at all. I have been given a plastic box full of metal discs. Most of them are coins; most are U.K. or Commonwealth(pre and post decimal). Some are foreign ... and some are filling station 'medallions'. I'm not after a valuation, nor do I want to sell them or clean them. It's just scientific curiosity The mystery is this. The 'silver' coins have become coated in copper and the 'copper' coins are coated in silver. (Not all of them and not all completely; thus my harlequin allusion) Can anyone explain how this happens?