A few years back, I accumulated various coins of different conditions. When my first daughter was getting married, I cleaned some coins by simple washing and brushing with soap and water. Luckily many were easily cleaned and I gave the Thirteen pieces which were in beautiful condition for the wedding. When my second daughter got married, we also used the silver pesos for the arras. Some of the coins were easily cleaned while others were not. Some coins were sent to the jewelry shop for cleaning of the stubborn stains. It was returned very clean and shiny and upon inquiry, I was told that they to have to burn it. Some of the coins got ruined while others show strange colors after more than a year. But some of the ruined coins are now of lesser value. I realized now that the the stains were some kind of epoxy or metal glue used by previous owners to paste the coins in their homes (some glued it to doors, stairs, beds, etc..). Lesson learned: Simple cleaning is enough, if it cannot be easily cleaned just sell it and buy another coin.