I made our casts from these molds. The Dayton Museum of Natural History fashioned the molds from fossils that they excavated in Darke County, western Ohio. The inner mold is made of flexible rubber; its purpose is to pick up the detailed texture of the original bone. It was made by applying a liquid rubber to the bone. Once cured, the rubber mold became a solid, but pliable mirror image (a negative) of the original fossil. Because it is flexible, it can be peeled off of the bone. The outer mold is hard and inflexible (in this case it is made of plaster); its purpose is to support the flexible inner mold.