DIY Aircrete Faux Stone Facade, Part 1 — How To Make Silicone Molds For Pouring Concrete

The problem I ran into back then was that I couldn’t find any Austin stone that matched the stone on our house. I wasn’t concerned about matching the color because ours is painted. Unfortunately, our stone was painted when we bought the house, so my options were either to have it sandblasted or repaint it a color I wanted. I chose the second option because it was faster and much cheaper. And honestly, I don’t really like the color of natural Austin stone anyway. Like I said, this stone is ubiquitous in this area, and it has been for decades, so it’s not really anything special, in my humble opinion. I much prefer the light gray of our house rather than having the siding portion match the yellow/beige color of a natural Austin stone.

But the problem that I ran into is that Austin stone in the late 40s and early 50s was cut very differently than modern Austin stone. Here’s a more modern building with an Austin stone facade (with the natural color of Austin stone), and you can see that the stones are cut so that they’re pretty flat on the fronts.

Here’s a closer view of what it looks like, and this is pretty much how all Austin stone is cut these days.

And here’s a view of the stone on the front of our house. You can tell immediately, just from how the stone is cut and chiseled on the front, if it is modern or if it was built around the mid-century time period. Back then, they chiseled the stone so that the center of the face of each stone protruded much more. There aren’t many stones on our house that have a flat face on them.

So when I tried to find matching Austin stone (real or fake) to match our house, it was virtually impossible. I found some that may have worked, but they were generally pretty expensive, and I didn’t want to pay that much for something that wasn’t an exact match. So I ended up giving up on that idea and trying to find other ways to cover the skirting.

This is not my original idea. I saw this process on YouTube (I shared this video with you in post last week), and he made it look so easy. So I decided to try it out and see if this would work for my project.

To make your own silicone molds, you need two ingredients — 100% all purpose silicone caulk and cornstarch. I used a glass bowl and poured an entire package of cornstarch into the bowl. You can use the cheapest cornstarch you can find. Unfortunately, this Bob’s Red Mill was the only brand available at the store I happened to go to, and it costs about twice the price of cheap cornstarch at about $3/package. But the cheap stuff works just fine.

And then I used my caulk gun and squeezed out two entire containers of silicone caulk right into the center of the cornstarch.

And then I put some gloves on and began mixing the caulk into the cornstarch. I tried my hardest not to touch the caulk directly because that stuff is really sticky. I basically folded the cornstarch over onto the caulk, making my way around the bowl, folding the cornstarch over onto the caulk all the way around the bowl until the caulk lost most of its stickiness. I stopped when the caulk and cornstarch mixture formed a ball that felt like the consistency of Play-doh.

And then I selected the stone I wanted to use to form the first silicone mold and pressed the silicone and cornstarch ball onto the face of the stone. I really had no idea what I was doing on this first one, so this was my test.

I gave that about three hours to dry. While it was drying, I used a sieve to remove all of the caulk bits from the remaining cornstarch so that I could reuse the leftover cornstarch for the next mold.

Removing that silicone from the stone was a challenge, and as I pulled it off, it pulled some of the paint from the stone. (You can see the color that the stone was painted when we bought the house. Bleh. I certainly didn’t want an entire house painted that color.)

This is what the silicone mold (or textured mat) looked like when I removed it. I doubt that this one is actually going to be usable since concrete will probably stick to those rough areas. But this test gave me insight into what I needed to do for the next one.

I rummaged through my cabinets to see what I could find to use as a mold release, and I found this little bottle of olive oil that has been in my cabinet far too long to use for cooking.

So on the next one, before pressing the silicone to the stone, I covered the center area of the stone with a very light coat of olive oil. I just used my gloved fingers to do this. I didn’t put it all the way to the edges because I needed the silicone to grab onto something, and if the whole thing was covered with olive oil, it would have nothing to grab onto. But I really just needed a mold release on the most heavily textured areas, which is the center part of each stone.

And then I removed my gloves (I didn’t want to get olive oil mixed into the silicone) and coated my hands with cornstarch to press the silicone onto the stone. I had about 15 minutes of working time with each silicone/cornstarch ball.

This one worked out perfectly. Again, I gave it about three hours to completely dry, and when I pulled it off of the stone, it looked great! No paint pulled off of the stone with this one. So using some sort of mold release is key to getting a great result with the textured mat.

With my second test done and resulting in success, I did two more at the same time. Since my front porch skirting area is pretty short, I wanted to create a mold for a couple of shorter and smaller stones. I selected this one…

And here’s what that looked like with the silicone pressed onto it…

And then I selected this little one.

Here’s that one with the silicone…

Both of those turned out great as well. The oil used as a mold release is the key. But again, I only added the oil to the most textured center parts of the stone so that the silicone would stick to the outer areas that didn’t have any oil on them.