Success at making a leaf casting from a large rhubarb leaf starts with assembling the mold. The mold consists of sand in a large container, making it simple to readjust the mold for multiple leaf castings of different sizes and shapes. Placing plastic wrap over the sand allows you to mold the sand. A finished leaf casting works well as a birdbath, bird feeder or garden stone.
Fill a 2 by 4-foot container with 4 inches of sand. Large cement mixing containers work well for making leaf molds.
Form a sand mound in the middle of the container so the rhubarb leaf lies naturally in the sand. Make sure the sand is at least 2 inches deep around the edge of the sand mold.
Cover the surface of the sand and mound with plastic wrap to prevent the concrete from getting sand in the grooves.
Choose a rhubarb leaf that has a minimum 2-inch stem and no tears or holes. Place the leaf on the mound and adjust the sand, if needed. Place the center of the rhubarb leaf over the highest part of the sand mound.
Spray cooking spray over the rhubarb leaf after placing it on top of the plastic wrap sand. This will make it easier to remove the leaf after applying concrete.
Apply concrete over top of the sprayed leaf. Taper the thickness by making the center 3/4 inch thick and the edges 1/4 inch thick. Let the concrete dry for 48 hours before removing from the mold.
Remove the leaf casting from the mold and remove all leaf particles. Cure the casting for seven days by letting it rest in a protected area. Seal the surface of the casting with concrete sealer before using it outdoors.