Concrete Class
Lesson 4: Casting Molds With Voids
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Introduction: Casting Molds With Voids

The addition of negative space in concrete allows for the creation of all kinds of projects, like concrete lampshades to concrete cups. In this lesson we'll look at voids in concrete molds to create negative space.

For this example in lesson we are using a large mold to cast in, and some water bottles to create the negative space. At the end of this class you'll learn how to make negative space in your casts to make all kinds of projects from concrete. Ready? let's make!

Fill Mold

Fill mold about halfway with concrete, tapping the mold after a few scoops to release any trapped air before adding more concrete. Using what we learned in How To Pour Concrete, mix up enough concrete to fill about half your mold.

The first pour will be to fill the bottom of the mold until about halfway, the second pour will fill the remainder of the mold after the water bottle to create the negative space is inserted.

Add Void

To make the void in the concrete mold a plastic water bottle is a good choice. In order to prevent the bottle from floating out of the boot when filled with wet concrete we'll need to fill it with water, this is called ballast and will weigh the bottle down. Fill the bottle to the top with water then seal tight with the cap. Spray release agent all over the bottle, this will make removal much easier after the concrete is cured.

Spray the bottle with mold release and place it into the mold where you want the void to be.

Add small amounts of wet concrete mix into the boot, ensuring equal coverage between the bottle and the boot, until your mold is filled.

Because this is a deep pour, take the time to periodically tap and shake the boot mold to settle the concrete fully and release any air bubbles. Continue pouring concrete filling up your mold, but stop at the shoulder of the bottle, if you continue filling past the shoulder you'll close in the bottle after the concrete is set. Tap and vibrate the mold to remove bubbles.

You can add a small weight on top of the bottle if you experience the bottle floating out of the concrete. Leave the concrete to fully cure for 48 hours.

Removing Bottle

The concrete should be set after 48 hours and the bottle can be removed, revealing the negative space. Remove the bottle cap and lift the entire boot mold and upend it to pour the water from the bottle.

Even with the use of release agent, the bottle is likely to be stuck inside the concrete casting. Gripping the threaded neck of the bottle with pliers I twisted the bottle until it loosed from the sides of the concrete and could be pulled from the cast.

The results were a perfectly smooth negative space.

The concept of using slender plastic water bottles is a great way to make negative space in concrete molds, since the water inside will weigh them down. Since plastic water bottles are pliable, they are easily removed after they are drained of water.

How have you applied the concept of negative space or voids to your concrete project?

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