Area Lamp - Part 2

Posted 12.21.2018 by Mario & Kitty Costello

Before deciding to get too much deeper into designing the concrete base for our lamp, we need to get a feel for the material we’re going to be working with. Never having worked with concrete, it’s still a mystery to us and needs a bit of research and handling to give us an idea of what we’re dealing with. It’s critical to “design to the material” you’ve chosen to work with and have a reasonably good understanding of the process first. After spending some time on the internet doing some research we found that the world of concrete countertops offered the closest parallels to the construction of our lamp base. And there are a lot of good resources out there, such as youtube videos and books, on the subject that can yield a lot of information. One of the better resources is Buddy Rhodes, who has been doing countertops for a long time and has a line of tools and materials, as well as advice on techniques and how to achieve certain finishes. One thing his website offers is a “pressed technique”, which offers a surface finish that looks similar to natural stone or travertine with voids and inconsistencies that add some visual interest. More on that shortly.

Concrete image

For this first exploration we’ll need a mold similar to what we’ll eventually be working with on our final product. Since it’s a good idea to imitate the conditions for the final effort as closely as you can, we’ll make a test mold out of the same materials to make sure they’re going to “release” from the molded part properly once it’s cured. Luckily, the final mold will be 18” tall and the materials we can purchase come in 24” lengths, giving us 6” of leftover material to play with, which is perfect for this test mold we need to build. We also built in a divider in the center of the mold so we can 1) try two different materials and 2) see how a wet concrete will bond to a dry, cured concrete. I’ll go into constructing the actual mold in more detail in the next post.

Mold image

We decided on plastic for our mold, since it’s easy to work with, relatively cheap and will resists bonding with the material it’s molding, but we’ll still put a coat or two of carnauba wax on all the surfaces to make sure the parts pull away from the concrete once it’s cured and don’t permanently become joined to the finished part. Our finished part also has a rounded corner around its edges, so we’ll want to figure out to mold one of those in at this stage to see what works best.

Mold image

The first experiment involved using a simple modeling clay that we had around the house from Crayola, which we then smoothed into a 1” radius using our hands, some water and a radius gauge. Unfortunately, we found that this particular clay tended to crack and pull away from the walls once it dried out. It would also be getting re-hydrated once we added wet concrete to it and probably wouldn’t be too dependable. After more research into various clays, we settled on a common silicone, which is easily formed and resists just about anything you can throw at it.

Clay image

The concrete comes out as a fairly fine powder with some very, very small aggregate mixed in. We mixed it up in a small container, taking care to add water sparingly to get the mixture about the consistency of cookie dough . We want the material to retain some form and stiffness on its own so it can be built up vertically on a wall and not slump down. The drier mixture will also create the air pockets and voids we’re interested in.

Mold image

We did one round with the plain concrete mixture, which comes out white and then a second one with a bit of powdered colorant added called “Smoke”. It doesn’t take much to color the mixture. We did the white section first, let it dry, removed the barrier and then did the second, colored section to see if they would bond. With concrete it’s important to keep the mixture hydrated and moist while it’s in the curing process. It loses strength if it dries out too much during this process, so we sealed the top of the mold both times with a plastic trash bag while it cured to keep the moisture from evaporating. Once cured, we found that the mold sections came off of the finished form quite easily and probably didn’t need any of the wax mold release. In the end, concrete is fairly simple and easy to work with. The really critical knowledge that was gained was understanding how wet to make the mixture so it could be used effectively in the press technique described by Rhodes.

Concrete base image
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Please allow 5 days for fabrication.