Walking the streets of Boston a few years back, I had the idea of decorating the city with colourful charms. Street Art made of simple shapes like heart, stars, crescent moon... dangling from trees and fences.

I wanted to make dozens of them! But sometimes, my ambition lacks perseverance. And even though I still want to make more, I’ve decided to do this Intructables so that people can pick-up this project, and start decorating their houses and cities with long lasting concrete charms.

Step 1: The Shape

I started with a heart as it is easy for me to sculpt by hand. Also, the overall roundness of it made the unmolding easy.

Step 2: Sculpt It!

For a sculpting medium, I went for floral foam. These super light bricks are phenolic foams that can be bought at dollar stores.

I would recommend to put gloves and mask when manipulating this foam.

To create the desire primitive volume, I glued the bricks together. I used rubber cement but I would Not use it again as it doesn’t break off as easily as the rest of the foam… so my second try would be with regular white glue.

Now Sculpt it! This material is super friable, sculpting it doesn’t require tools. Its fun and easy.

Step 3: Mold Making!

I was very excited to finally get to use some expandable urethane. Now this product is SUPER sticky and the expansion is not as predictable as I dreamed. I took some aluminum foil for a container. But it could be anything that can be cut easily.

I sprayed a foam nest and pushed the sculpture in. You need to apply some weight on top of the sculpture or it will just rise on top of the expanding foam instead of being enveloped by it. I waited until the foam was set to repeat on the opposite side. You might find areas where the foam didn’t expand to. Just apply more foam and make sure it expands to contact the sculpture.

While the expandable foam is still mushy, I regret not having put drinking straws into it. Using a small rod, I could have use these straw channels to align the mold later on…

Step 4: Cutting and Cleaning the Mold

This step is fun!

Make sure the foam is fully and deeply cured.

I used a bread knife to cut the mold into two. Making sure the cutting plane included the 2 points where my chains will come out of.

Removing the floral foam from the mold can be done with your fingers. Final touches can be done with a hard brush.

If defaults are found in the mold, now is the time to do some touch ups. Be careful as adding more expandable foam can quickly mess up your mold and working in small areas can be challenging.

Step 5: The Chains

I used 2 x 2ft metal chains bought at the hardware store. This is the most expensive item in the project :)

Step 6: Mold Features

Carve the mold with a small indentations around where you want your chains to come out of. Doing so, I did keep a small notch of foam to position a chain link while the cement will still be fluid.

Carve a 1,5 in hole on the highest point of the mold to pour in the cement and let the air out. I did 2.

Step 7: Cement Pouring

Now I’m no concrete expert so you might wanna refer to other Instructable on the subject. I used sand cement and poured it to fill half the mold. I place my chains softly on the wet cement, about 3 chains links in.

I closed the mold. That’s where the alignment straws would have come handy.

I used wrapping plastic to hold the mold togheter as I filled it to the top with more cement. Be careful as the mold is a bit fragile and the cement, super heavy.

Step 8: Wait for It!

Now that the chemical reaction in the cement has given substantial strength to it, you can unmold it. I like to unmold it as the cement is still a bit friable as you can file the mold edges off.

Again, I’m no expert, but you can then keep your cement charm wet many days to make it mature in strength even further.

Step 9: Clean It!

I like to rinse it. Then let it dry fully.

Step 10: Paint It!

I used enamel paint as it gives a super durable colourful glossy finish. I paint the back side first. Careful not to have drips that runs on the other side. You might want to mask half of it first. Wait for the paint to dry, then paint the front side.