From what I know many bubble wraps are designed to break down over time (biodegrade). I am sure they contain plastic softeners which are disasterous to silver coins -less so to gold. The hotter they get the more chemical is transfered to the metal. The PM sells 100oz bars in bubble wrap - I take them out. I do recall leving one for a few years - did no harm to the silver but crumbled into small fragments (degraded).