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Message:: [QUOTE="BAJJERFAN, post: 2131090, member: 74330"]Why would you think that the Mint is putting some kind of coating on the coins after they are struck? What would be the point of that? Assuming that 99.99999% of people are unaware of it, how do you explain the toning that these coins sometimes incur? Wouldn't you have to dip off the coating before you could tone it? As indicated earlier, my examination of the spotting on a single  coin from an NGC slab indicated only chlorine was present. If there was a coating there would surely be some carbon, oxygen there too. During the ESCA examination  process, the spot was zapped with a beam of argon cations which readily removed the spot. Look at the physical process of handling a large volume of blanks. This is probably done in bins, totes or baskets. In order to do an effective job of washing and rinsing them they have to be tumbled, agitated, etc. Even plates in a dishwasher need to be separated from each other if you expect them to be cleaned on both sides. A part of the problem might be simply that the wash and rinse aren't making thorough contact with the blanks. This would be akin to a bag of Morgans in storage where the ones next to the bag get toned and the inner protected ones don't.[/QUOTE]