Oxidation, or more specifically sulphurization is contact with the atmosphere over time. This means that the coin was not protected properly or stored to the standards that most of us adhere to. It is technically not "mint state" if it is tarnished in any way, although people do collect tarnished coins. You'd also be yelled at if you "cleaned" a tarnished coin, although technically it is not "dirty". Overall, the coin did not leave the mint with sulphurization.



















