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Concrete Class
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Lesson 5: Casting Molds With Voids
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The addition of negative space in concrete allows for the creation of all kinds of projects, like concrete lampshades to concrete cups. In this class we'll learn how to make negative space in concrete by making a planter pot using an old boot as the mold.

For this class we'll need 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!


Prepare Mold

To make a planter out of this boot the first thing we need to think about is drainage. Without proper drainage, water would accumulate in the heel of the boot and rot the roots of whatever is planted inside.

Fold down the top of the boot to get access to the bottom of the inside. We'll be drilling an opening through the heel to make a drain hole. To preserve the hole while the concrete is curing I'll be using a pencil. A drill bit was sized to match the diameter of the pencil, then a hole was drilled straight through the heel.

The pencil was inserted into the opening with the remainder left inside the boot cavity.


Fill Mold

Unfold the top of the boot to prepare the inside for filling with concrete.

Using what we learned in Lesson 2: How To Pour Concrete, mix up enough concrete to fill your mold.

For this project you can break the mixes into two pours if that makes you feel more comfortable. The first pour will be to fill the bottom of the boot until the top of the pencil, the second pour will fill the remainder of the boot after the bottle is inserted.

Make sure to tap and shake the boot to settle the concrete into the toes of the mold and release as many air bubbles as possible before moving on to the rest of the pour.


Add Void

After filling the boot partway with concrete we can focus on making the cavity for the planter. To make the cavity which will hold the plant you'll need a long slender bottle with a mostly uniform shape along the body of the bottle.

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.

Insert the bottle right side up into the boot and rest the bottom on the top of the pencil, then carefully add small amounts of wet concrete mix into the boot ensuring equal coverage between the bottle and the boot.

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 the boot, 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.

After pouring ensure the bottle is fully seated on top of the pencil by gently pushing down on the bottle, and make sure it's centered in the middle of the boot. Then, leave the concrete to fully cure for 48 hours.


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 with the top of the pencil visible at the bottom.


Drill out the pencil from the rubber boot and concrete by using a drill bit slightly smaller than the diameter of the pencil. If you used the same size bit you may catch the bit on the sides of the concrete and prematurely dull your bit.

It's easiest to drill the pencil out from the bottom of the boot.

With the pencil removed, there's a clear passage for excess water to drain from inside the planter.


This concrete planter is functionally complete, but don't really showcase the concrete since it's hidden inside the boot. The boot siding needs to be cut away to reveal this planter's secret identity.


A sharp hobby knife was used to selectively cut away the boot siding, leaving the bottom in tact. If we were to remove the entire boot from the concrete cast we'd have a rounded foot shape, the inside of the boot, rather than the chunky boot sole. Leaving the bottom portion of the boot alone we get a nice mix of old and new. The knife was traced around the decorative edges of the boot around the ankle, then a slice was made up the calf of the boot to the top lip.

The boot sides were then easily peeled from the concrete form.


Light Finishing

The inside of this rubber boot had a cotton weave which unexpectedly ended up sticking to the concrete cast. Your boot may not have this, but if it does I'm not sure it could have been removed prior to pouring concrete. Regardless, this cotton weave would need to be removed also to show the concrete underneath.

The weave can sometimes be peeled from the concrete, but often breaks apart in frustratingly small sections.

Instead, a random orbital sander with an 80 grit sanding pad was used to quickly remove the cotton weave and expose the concrete underneath. In the image above you can see the difference between the two sides.

With the concrete showing the planter is almost finished.


Prepare for Foliage

Before any plants can go inside the negative space in the concrete cast some care is needed to ensure the drainage opening stays unplugged.

Small metal mesh was inserted into the bottom of the planter to keep rocks and dirt clear of the drain hole. I used an inexpensive mesh stationary cup I found that had a tight mesh.

The mesh was cut into a circle shape and inserted into the bottom of my planter boot.


After lining the bottom of the negative space with mesh, I used pea gravel aggregate that I saved from the drawer pulls in Lesson 3 as the drainage rocks on top of the mesh.

Then, a layer of soil was added before adding my small plant.


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 and because 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?


In the next lesson we'll move away from standard rigid molds to create unusual forms and explore more organic shapes that are possible with concrete.

CLASS PROJECT

Share a photo of your finished project with the class!

Nice work! You've completed the class project