How to Make Rocks Out of Concrete

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A stone wall.
A stone wall. (Image: common-pics)

You can make your own rocks for a stone fireplace, a wall, a garden border or a pathway through the yard. For this project you'll be creating your own mold with foam insulation, mixing up concrete and pouring it into your mold to make rocks.

Things You'll Need

  • Acetone
  • Glass bowl
  • Foam Insulation (the stiff kind)
  • Goggles
  • Natural bristle paint brush
  • Motor oil
  • concrete mix
  • bucket
  • water
  • chicken wire
  • wire cutters

Clean the surface of the object you'll be using to form the mold. If it's a rock or a brick, it may need to sit overnight before it's completely dry.

Find a well ventilated space to work in and prepare your work area with drop cloths to protect furniture or plants. Lay all your materials out on a table. Foam insulation, acetone and rocks should all be where you can easily reach them as you move from one step to the next.

Pour a very small amount of acetone (the main ingredient in nail polish remover) into a glass bowl. Dip your paintbrush into the acetone and paint the surface of the object with it. This should take only a few seconds.

Place the rock, brick or other object onto the foam. The foam will immediately begin to melt into the shape of the object. The foam insulation should be larger than the object you're copying.

Remove the object from the foam after it has sunk in far enough to create a decent mold (at least 1 or 2 inches).

Wait a few minutes. After the surface of the mold has "set up," or become firm, pour some motor oil onto a clean rag or paper towel. Wipe the mold with it, leaving behind a thin coat of oil. This will keep the mold from sticking to the cement or plaster you'll be pouring into it.

Mix your concrete according to the size of your project and the manufacturer's instructions. See "Resources" for rock-creating ideas.

Pour the concrete into the form. Press a section of chicken wire into the concrete, with wire edges sticking out slightly on the edges. This provides a point of attachment when you place your "rock" into grout or concrete.

Allow time to dry according to the instructions on your choice of concrete mixture. Repeat the process with different rock shapes to create a realistic cobblestone look.

Stain and seal the concrete rocks.

Tips & Warnings

  • You can buy sheets of stiff foam insulation at home improvement stores. Buy sheets that are at least 2 inches thick. You'll also find large scraps of foam insulation at construction sites. Be sure to ask before you take pieces of it away. The rocks for this project are for a rock facade. To make larger rocks with more depth, you'll need larger chunks of hardened foam insulation. If you want to add holes, scratches and cracks as touches of realism, remove the concrete rock from the mold before it is completely dry.

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