How to Cast Concrete Shapes and Forms
If you want a special shape for a solid object in your garden, like a natural looking rock, a heavy bowl or pot, stepping stones, or even an animal shape or bench, concrete may be your best medium. Concrete is heavy, but that can be an advantage outdoors. It isn’t difficult to cast concrete into paving stones, basins, sculptures or other shapes. If you are artistic, you can sculpt forms yourself, otherwise here are some fundamentals on how to cast concrete shapes.
The form you build is what will give your object shape. The interior of the form is where the cement will be poured and that will define what you get. You can buy forms from craft suppliers or industrial shops. Or you can make your own.
Materials for forms can be as simple as a wooden frame or as detailed as impressions made in foam, plastic, silicone or urethane. Your form material does not have to look attractive on the outside, but it needs to be a perfect imprint on the inside. The concept is simple: pour wet concrete into the mold, let it harden, then remove the outside mold to leave your final construction. Little imperfections can then be chiseled or sanded and any finishes can be applied if desired.
If you are building your own mold, you might want to sketch out what you want on paper first. For a rough structure like a basin or a bench that will have a thick coating applied later, you can do a simple mold like creating a container with wood. Once the wood is removed and the basic cement form remains, the surface coatings will create the finished effect.
For more detailed objects, choose the piece you want to reproduce. Then coat it with a thick painted or poured layer of foam, plastic, silicone or urethane. When it is dry slice the coating smoothly and peel it off the object beneath. The impression inside will now be the form you will refill with cement to produce your own version.
Whether you buy or make your own flexible mold, clean it before using and apply a light coating of oil (like 5-30 motor oil) to the inside surface. Mix up Portland cement, mortar — or for a more delicate structure – plaster into a thick paste. Pour it into your mold a little at a time, tapping or lightly shaking it to release air bubbles before adding more.
Let your creation set for twenty-four hours before gently removing the mold. Concrete will set best if you occasionally mist it lightly with water for the next few days to allow for a slow-drying cure. Then clean off any unwanted pieces and finish your concrete shape with textures or paint.
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