Kevin, Sulfur is an anion material, and in high enough amounts, can cause a corrosive reaction. Gold should be impervious to this effect, but silver is a metal that corrodes easily. You can get cardboards that are minimum in sulfur, or sulfur-free, but they are more expensive than regular cardboard. I don't know if you can get carbon impregnated sulfur-free cardboard. Doug Pauls Contamination Studies Labs [log in to unmask]