How to Make Concrete and Stone Molds

Concrete and liquid simulated stone products are often poured into molds to make decorations, pavers, building mediums and other items. These molds are important because they ensure that every single item is identical, which is often vital when erecting structures or laying out a pattern. While many materials might be difficult to remove from rigid finishes like concrete or stone, rubber is flexible enough to accommodate them. Remember to use release oil on your molds before each pour to prevent sticking.

Instructions

1 Work a lump of clay into a round with your hands on the surface of a plastic cutting board. Use a rolling pin to roll and shape the clay into a shape that resembles a square and is a half inch thick. Center the item you are copying for your mold and place it on the surface of the clay.

2 Roll up balls of new clay and press them around the perimeter of the mold item and into any abscesses. Smooth the surfaces of the clay balls and push them together with a palette knife to produce a consistent clay border. Make straight cuts with a clay chisel to remove excess clay that extends more than 4 or 5 inches past the mold object.

3 Find the thickest edge of the clay border and use this side for the fill hole. Place a small funnel at this edge so that the lip of the wide end is aligned with the edge of the clay and the narrow end of the funnel points inward; push down gently to settle the funnel in place. Cut down the clay border around the item and the funnel to 3/4 inch with the clay chisel.

4 Trim each piece of foam board to make it just one-half inch taller than the cast mold. Attach these pieces in a straight line along a pallet board with hot glue; space them about one inch apart. Fold the palette board between each piece of foam board to form a circle.

5 Wrap the wall of foam board around the outside of the casting mold. Arrange wooden popsicle sticks across the top of the wall from one side to the other in both directions so that they form a grid. Glue the sticks into place to prevent the wall from collapsing inward.

6 Brush a layer of form release oil onto the surface of the clay and all over the molded object. Carefully pour the liquid rubber over the lowest area of the cast mold, letting it rise up over the higher areas and the mold object. Allow the rubber to dry undisturbed 12 to 24 hours.

7 Lift away the foam board wall and remove the clay and funnel gently from the mold. Clean off any pieces of clay or rubber from the mold item. Repeat steps 1 through 6 and cast a mold of the opposite side of the item.

8 Apply form release oil to the interior and edges of both mold halves. Clamp the halves together in alignment; place squares of plywood between the clamps and the mold to cushion the pressure points. Turn the pour hole upward and pour prepared concrete or liquid stone product into to fill the mold; let the masonry dry 2 days and remove the mold.