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
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