You probably won't see the toning because it happens so slowly, and is so subtle, that your memory will not even notice it. And I'll wager your coins are not stored in flips where the air can get to them either. Store one in a flip. However, a couple years from now, take your coins stored in flips into a coin shop. Then compare them with some freshly minted coins of that year. Then you'll see the difference. You'll see that your coins have indeed toned.