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Concrete Class
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Lesson 6: Casting Unusual Shapes
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In previous lessons, we've covered using standard box-like items to make our concrete molds. But there's so much more concrete can do besides square edges.

Concrete will take the shape of any mold you use, and you can make some fantastic shapes with everyday objects to give concrete an organic and unique shape. In this lesson we'll explore using a balloon to make a rounded mold with concrete and create a candle holder no one will believe is made from just concrete.


Setup

As discussed in Lesson 1, aggregate is added to cement to add strength. As with the drawer pulls we made in Lesson 3, we won't need this concrete project to have compressive strength (as the void in the middle and the thin walls would defeat any material strength the aggregate could give). Sieve the concrete to remove all the large and medium aggregate, leaving the cement and small aggregate.

The only other supply you'll need for this easy project any type of balloons.


To hold the balloon as we work on it we'll use the saved aggregate that was sieved earlier in a cup to act as ballast.

I made a few concrete balloons so I portioned the aggregate into 3 cups. I find that disposable cups are best for messy projects like concrete, that way there's no fuss with cleanup.


Inflate a balloon as large as you like and tie off the end.

In order to cover the balloon with concrete it will need to be secured so it doesn't move around. Orient the balloon so the tied end sits inside the cup with ballast prepared in the previous step, then secure it in place with a small piece of tape from the balloon to the cup.


Apply Concrete to Mold

The easiest way to work with concrete and unusual molds is to use your hands. There's no tool that can match the dexterity and familiarity of your fingers, just make sure to wear gloves, as concrete will burn skin.

Gently plop gobs of cement on the top of the balloon, then tap the balloon to settle the concrete and spread it out.

Continue adding more concrete to the top and tapping until the concrete starts spilling over the sides and down towards the cup.

If any small sections of concrete start creeping faster than others you can carefully push it back upwards to prevent it breaking off. However, there really is no wrong way to do this, so let your design take whatever shape you like.

After concrete application let cure overnight.


Remove From Mold

Unlike the other molds in this class, this unusual mold is so thin and has a large surface area so the concrete cured much faster, in only 24 hours (probably less).

The balloon has pulled away a little from the cured concrete casting, and you may be able to pull the balloon out. However, it's easier to remove and less likely to break the fragile casting if you just pop the balloon.

Wear a face mask and eye protection, as the balloon can spray fine particles when it pops.


After removing the balloon carefully clean the inside of the concrete candle holder with a damp cloth.

Be gentle when handling the thin concrete castings, even though they are concrete they are still fragile. Even if a small portion of the candle holder breaks off it won't really matter, the irregular sides and organic shape of the concrete lends itself to minor mishaps.


Paint

Painting the insides is an optional step, but will really elevate the look of the piece. From a functional perspective, lighter colors will help reflect light and make the concrete candle holders look even brighter.

I like the idea of glowing orbs in my garden, so I chose a metallic brush-on acrylic paint for my candle holders. Spray paint would also work. Any metallic or glitter paint would be a great choice for the interior paint

I had a few color choices on hand, so decided to try gold and bronze colors.


The last step is to add tea lights to each candle holder, depending on how large your casting is you might be able to fit 2 or even 3 tea lights inside and really boost the brightness.

Though these candle holders are fun any time of the day, they really stand out when the sun goes down.


Wrap-Up

The skill of using a balloon of the mold is deceptively simple, yet has a very big payoff. This is true for a lot of unusual molds for concrete, and this is just one example of how you can use everyday items as a mold to hold and shape your concrete as it cures.

Did you make your own concrete candle holders from this class?


Up next, we'll tackle how to finish concrete to expose the aggregate and make it mirror smooth.

CLASS PROJECT

Share a photo of your finished project with the class!

Nice work! You've completed the class project