When I was in drafting/illustrating we used electric erasers. Luckily I still have mine. It does a fantastic job cleaning up light to moderate discoloration on coins. The eraser has enough abrasive to cut grime, but does not alter the detail. This is an "Alvin" model, but Bruning and others are good. (very quiet). You can find these all over "eBay". The eraser refills are a little more difficult to find, but they are out there. Just though I would let y'all know about it if you haven't already seen this method of cleaning our precious treasures.