The Illinois State Museum’s Giant Ground Sloth casts are made of plastic resin that is reinforced with fiberglass cloth. The casting resin picks up the bone texture from the flexible rubber mold. The resin is applied in liquid form, but hardens as it cures. The cast will be a positive or replica of the original bone. By reinforcing resin with fiberglass, we can make our casts hollow; thereby reducing the weight of the replica.