I submitted this picture to my Mint Rep to get the mint's response to the apparent Alchemist ability of my sealed closet. My coins were all sealed in zip lock bags on a shelf in a cupboard that was rarely opened for 10 or 12 years. In the dark.
I laid them out for a random picture. All coins are 'Fine' silver.
"'The effect of ageing is a natural occurrence that is inherent in the coining business in that over time effects such as tarnish, toning or discolouration will appear. These effects are inevitable but normally slow due to process enhancements that we have developed at the Royal Canadian Mint
Now, over time we have worked with and changed lacquer supplier as some of the older finished coins were not as UV resistant. Keep in mind this can still affect coins stored in the dark away from oxygen and lighting that could speed up the process.
We have attempted to rework some coins in the past for customers, however, the outcome in most cases was not a successful one because the process of stripping, polishing and re-lacquering runs the chance of destroying the coin which cannot be replaced for the customer.