I have picked up a lot of Franklin Mint Tokens from 1965-1969 and they seem to have some sort of substance on them. First of all, what is the stuff on them, second, should I clean my tokens. I use a soft silver polishing cloth and it gives them a mirror shine to them. I probably won’t sell them any time soon, but am I destroying the value of them? I have read in other post that cleaning tokens/chips/strikes doesn’t really hurt the value. Thanks for the help.