Archival sculpting mediums?
I'm working with a client that has fragile and valuable items they want to put inside a sculpted case. Some of these items are old letters they want displayed within a sculpted framework. I'm looking for suggestions on archival mediums. I know fine artists who use archival masking tape over wire frames that they then paint with acrylics but if there is a type of lightweight clay that is archival, that might be best. I'd like to be a bit more detailed. Any suggestions for a way to make a sculptural work that is fairly sturdy, can withstand some wear and will not damage paper over time I'd love to hear them!!