Try latex....follow the directions. You can find it in most major craft stores.Another idea is to use a silicon mold that you'd use for cooking. I've heard they are easy to find in thrift stores becuase many people don't like the way they clean up or move with a semi solid in them.

Hi Minnamouse - Carol is correct in using the new silicone bakeware molds. I've not done it, but I did hear they work great for cement molds.

Please go to the Garden Junk forum in here and type in mosaics in the search box at the (pretty much) top of the page - some of the gals used molds for their mosaics and I think you can find some the answers you're looking for.

Sal