Things You'll Need
Cardboard of wooden box
Concrete is a strong adaptive material that's often cast for use as sculpture or accent pieces. These concrete castings are commonly seen in homes and gardens, placed as centerpieces that dominate an area or as small add-ons that can be mortared to buildings, fountains or located anywhere a piece of artwork is desired. Creating a concrete casting requires that you use a mold, but for specific objects a unique mold often must be made where none is available. For one-of-a-kind casts, clay can be used to recreate an object in concrete from a model on hand with a few short steps.
Place the polymer clay in the bottom of a box. Form the clay into a block large enough to press a complete impression of the model you're creating a mold of, using the box as a mold frame. Lay the box on a stable flat surface. For smaller molds, a cardboard box is sufficient. Use wooden boxes joined by screws for larger molds to support the weight of the materials used.
Place a thin layer of petroleum jelly on the model, covering the entire surface that you're creating an impression of. The jelly will act as a releasing agent between the model and the clay so that the clay will not stick to the model or deform as you remove the model from the clay block.
Press the model into the clay block, applying firm pressure straight down into the clay. Do not wiggle the model or twist at all so that a clean, unaltered impression is made. Remove the model from the clay by raising it directly up from the clay, leaving an impression in the clay that can work as a mold.
Mix the concrete with water according to the manufacturer's instructions. Pour the concrete into the clay mold contained in the box. Shake the box to distribute the concrete evenly throughout the mold and to move bubbles in the concrete to the top in order to prevent holes from forming in the concrete casting. Fill the mold completely and then use the flat edge of a trowel to level out the concrete surface.
Allow the concrete to dry according to the manufacturer's instructions.
Disassemble the box surrounding the clay mold and then remove the clay from the concrete casting.
Remove the residue of the clay and the petroleum jelly, washing the petroleum jelly from the surface of the casting with a warm detergent and water mixture. Dry the cast concrete and then spray a layer of concrete sealer onto the stone to provide protection for the stone.