Ron,
  Without knowing the exact composition of the rubber mat it is "let the
sheller beware"!  There may be substances such as curing agents and other
chemicals that may affect shells.  Since some forms of shell deterioration
are temperature and humidity dependent, you may use a product for several
years with no visible adverse effects.  Move to a warmer place or one with
a higher humidity and then the fun begins :(
  My advice would be to stick with archival products.  Try to get some
polyethylene foam.  Put down a layer and then another layer which has
holes cut to fit your shells.
  Good luck.

Regards,
Charlie
******************************************************************************
Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine

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