Myth number 6 may interest you and other bronze collectors MYTH NO. 6: A coin can't deteriorate once it is encapsulated in a slab. On the contrary, the deterioration of coins--even when housed in slabs--is a source of growing concern and represents a problem that's likely to occupy us increasingly over the next several years. NGC conducted some very intriguing age-acceleration simulations in which coins that were sonically sealed in tamper-resistant holders had their age accelerated by decades. The results proved unsatisfactory, at least in terms of copper coins: The coins actually deteriorated while they were in the holders. I have seen a number of copper coins in PCGS holders which actually broke out in spots while in the holders. There's really no way that a coin can be completely protected against environmental variables, whether it's in a slab or otherwise. We have seen a number of cases where moisture in the air permeated the holders, as well as other cases where coins made of highly susceptible and vulnerable metals such as copper were, in a sense, choking in their holders--trapped inside with airborne particulate matter which was causing the coins to deteriorate. Because copper coins are so susceptible to damage and deterioration, NGC does not guarantee the grades it assigns to them, as it does with coins produced in other metals. PCGS does guarantee the grades of copper coins--but I have seen no difference in the way these coins deteriorate while encapsulated, whether the holders came from one service or the other. This is a real problem, one I sense we'll have to address more urgently over the next several years as coins that are susceptible grow older in holders and their deterioration becomes more apparent.