The process to cast a resin stone is time-consuming. A mould is made of the desired rock shape prior to casting the resin rock. The easiest mould to make is one using a previously found stone. The stone is used as the master in the mould-making process. A homemade stone is possible by using spray foam and carving the desired stone shape. A split box is needed to place the stone master in to create the rubber mould.
Lay a large garbage bag on a flat, stable surface. Spray a mound of spray foam in the centre of the garbage bag to form a 12-inch high by 13-inch round glob of foam. Allow the foam to expand and dry for 15 minutes.
Carve the mound of spray foam with the utility knife to the desired rock shape. The height of the rock should be 9 1/2-inches tall. This height is needed to make the proper mould from the 8-inch tall wooden frames. Dispose of the excess.
Place a large plastic garbage bag on a flat stable surface. Place one section of the wooden box frame in the centre of the garbage bag. Seal around the inside edge of the frame with a 1-inch round coil of modelling clay. Place the clay along the seam and push slightly with your fingers. Press the clay into the seam; it should be tight against the bottom surface and the outside edges.
Place the canning funnel in the centre of the box frame. The small 1-1/2 to 2-inch funnel end should be pointing up.
Shake a thin coating of baby powder over the garbage bag, funnel, modelling clay, sides of the wooden box frame, and the surface of the foam rock.
Mix a bag of plaster in a 5-gallon bucket. Mix according to the instructions on the package.
Pour 2 inches of plaster in the wooden box frame. Be careful not to move the funnel from the centre of the frame.
Center and sit the foam rock on the funnel. Carefully fill the remainder of the box with plaster until it reaches the top edge of the box. Leave the box in place. Let the plaster dry for 24 hours.
Center and place the second box frame exactly on top of the first frame. Seal the seam with a 1-inch coil of modelling clay. Push the clay into the seam, onto the top of the plaster, and onto the sides of the box.
Sprinkle baby powder over the surface of the foam rock, the set plaster, and the edges of the wooden box.
Mix a second batch of plaster according to the directions on the box. Slowly pour the plaster into the top portion of the mould. Fill the mould to the top edge of the box. Allow the plaster to dry for 24 hours.
Lift the top section of the mould off the foam rock. The baby powder will help release the plaster from the rock. Tap the outside of the wooden mould with a hammer. Shake gently and the plaster mould will come loose from the wooden sides. Place the mould on a level surface so the indentation of the rock is on the top.
Lift the foam rock from the bottom form. Tap the sides of the wooden box to release the plaster mould. Turn the mould over and place it on top of the other plaster mould. Make sure to match the rock shape before sitting the mould into place.
Remove the canning funnel from the top of the mould.
Mix the resin according to the directions on the box. Pour the resin through the funnel hole. Add enough resin to fill the void. Allow the resin rock to dry 24 hours before removing it from the mould.