Finally, a question I can relate to. Ancient dung? As in fossilized
dinosaur poop? I think it’s called coprolite? Casting it in polyester
resin, is the way to go. You want the clear mass casting polyester
resin, available from Mr. Fiberglass. Mass casting is for thick
sections, that need to cure slower, to reduce shrinkage and exotherm.
You can purchase the paperweight sized molds easy enough. Just
google, paperweight molds. Many shapes and sizes available.
The tricky part is working with that amount of resin. Your biggest
problem will be air bubbles. You will need to vacuum the resin before
pouring. And or, cure the piece overnight in a pressure pot. Either
way, you can achieve a bubble free casting. The polyester resin is
easy to use, but you only get about 15 minutes working time before it
starts to gel.
The fossilized dinosaur poop is basically a rock. That means it will
sink to the bottom of the mold. You will have to do a 2 stage pour.
Pour half the mold, let it cure a bit, then insert the rock and pour
the rest of the mold.
Hope this helps.
I have resin tutorials on my website, that might be helpful. Night
Sky Jewelry - Resin Jewelry Tutorials