I got a set of wood letterpress blocks on a whim a while ago, and the Shapecrete Build Night at CRASH Space gave me the idea to use them to make a cement sign. I discovered while making my Concrete PotHead that a thermoplastic like Instamorph works well for making molds, so I used that to make impressions of the letter blocks.
Another option is to make individual molds for each letter and make concrete letterpress blocks. This has its own challenges, like getting the wood blocks back out without damaging the molds, but it is pretty neat.
I've included a video of me making this project in high speed on my new YouTube channel Barb Makes Things.
Step 1: Materials
- Bucket/Container for mixing
- Cups/Water bottles
- Plastic sheet
- Stick for mixing (if using bucket)
- Pot for hot water
- Needle-nosed pliers
Step 2: Making the Mold
The letterpress blocks I used are meant for decoration, not function, which you can tell as they aren't backwards. This is perfect for my purposes, as I'm not looking to make functional cement letterpress blocks either.
Heat up the thermoplastic in hot water and press into a thick pancake, then press the desired letters into the plastic, leaving a little extra around the edges. Once you have all of your letter impressions, take the excess plastic and bend it up to make walls for your mold.
Step 3: Mix and Cast
These don't take a whole lot of concrete, so only mix a little bit unless you want a big cup-shaped doorstop (which is cool too). It's best to mix it closer to the casting consistency than the clay consistency, to make sure you get all of those details in the molds, and to minimize bubbles. I mixed my concrete in a gatorade bottle, using a halved water bottle for both measuring cup and funnel, then shook it vigorously to mix. This worked pretty well and wasn't nearly as messy as using a bucket.
WARNING: Don't pour or wash any of this stuff down the drain, or it will harden in the pipes.
Scoop or pour your mix into the molds, poking, shaking, and tapping them as you go to make sure the mix gets all the way down. A couple air bubbles can turn "I Love Kate" into "I Love Kale."
After you've finished, tap the whole thing against the table a few times for good measure. You'll get more bubbles coming up, which is good. Better out than in. Cover with plastic and leave for a day or two.
Step 4: Removing the Mold
The nice thing about doing this with a thermoplastic like Instamorph is that, once it's had a day or two to dry, you can drop it into a hot pot of water, mold and all, and wait for the mold to soften up, which makes it much easier to remove without damaging the cement letters.
If you make the sign pretty thin, however, be aware that it likes to crack while removing it from the mold, and putting it in hot water apparently aids in that process. Oh well. You can always glue it back together.
Bits of cement may stay on the plastic, limiting your reuse of it, but I like to clear off as much as I can.
Step 5: Paint
You might like the letters as they are, but you can add a little more contrast by painting a bit. I've used acrylic paint to emphasize the dips around the letters and the raised sections. A little color can be fun too.