So many of us coin collectors, dealers, hobbiest, etc take great care in the handling of coins. Many hold them by the edges, use cotton gloves, etc. Today I was looking at some of my coin Albums, the kind with plastic slides. It made me remember back when I was a kid and convertible cars had plastic rear windows. Everyone was warned not to use an oily cloth on them. Also, be carefull not to use you hands on them due to the acids and oils on your hands. So just for the fun of it I removed all the plastic slides from an album and putting them all together I noticed they became very dark and yellowish. So I put them all in a sink, filled with water and dishsoap, spashed them around a bit, rinsed and dried them. FANTASTIC. They all looked new again and even all together, still lots of light passed through.
The thing is we all take care with the coins but seldom pay attention to the holders. Those plastic slides in our albums have been touched many times over the years and even possibly by the people that put them together. Think about all the oils and acids just sitting there on those plastic sheets waiting for a coin. I'm now in the process of cleaning all the ones I have and there are a lot of them. Try it. You may be shocked at how dirty they are.