Creating a sand mold for concrete is a quick and inexpensive method of reproducing ornamental objects in concrete form. The process uses fine wet sand to hold the image of the casting model. The sand is just strong enough to avoid crumbling as you pour concrete into the depression left by the mold. After a short wait for the concrete to harden, you can pull the cast replica of your modeled object free and enjoy a beautifully cast concrete ornament suitable for display.
Create a box form large enough to hold the cast. Build a box using 3/4-inch plywood as the base and sides. Use a table saw to cut the plywood to the dimensions needed, allowing about 4 extra inches to the width, length and depth of the box to allow space for the sand to shift when imprinted with the model. Join the box sides with wood screws.
Fill the box to the top using fine mason's sand. The finer the sand used the greater the detail you'll get in the mold to carry over to the concrete.
Dampen the sand with water to make it moldable.
Press the model of the object that you wish to reproduce into the wet sand, dislodging the sand in the process. Pull the model carefully from the sand, straight up to disturb as little of the sand as possible, leaving an imprint of the model behind as a sand mold.
Repair any parts of the imprint that crumbles with your fingers or with a small putty knife. Moisten the sand occasionally with a spray bottle filled with water to keep the mold fresh.
Attach a paddle but to an electric drill and use the paddle to mix the concrete mix with water in a bucket. Create a smooth, pourable mix, about the same consistency as oatmeal.
Pour the concrete into the mold, filling the imprint completely. Wait for the concrete to set. Setting times depend on the concrete used, check the manufacturer's instructions for the time required.
Pull the concrete from the mold and brush off any clinging sand. You should have an object the same shape and size as the model.