Area Lamp - Part 4

Posted 12.30.2018 by Mario & Kitty Costello

With the mold finally constructed and the some experiments with the concrete completed, if feels like we have enough to finally start making the real thing. There’s always a chance that things are going to go south during the process and that tends to lead to holding off and giving yourself reasons to delay things, but eventually you just have to dive in and get started. The mold is re-usable, so the worst case scenario is we end up having to re-make some components and find a place to put a large lump of concrete.

Cast image

Here’s the mold put together with all the internal components in place . You can see the long vertical bar that will create the negative space for the large extension arm mentioned earlier to anchor to the base and the three threaded inserts that are attached to it. The bar, made from aluminum, has screws running through it to hold the threaded inserts in position and also to hold the entire sub-assembly in place. Once the form cures, the vertical bar will be removed, leaving the threaded inserts in place. It’s been coated with wax to insure it releases and also been drilled to provide threaded holes, which will accept a screw to help push it out of the cured form.

Also visible in the picture is the “switch container” that will contain the on/off toggle switch and the tube that contains the wiring. These will be molded into the base and become part of it once it is cured.

Container image

I used a larger container this time for mixing since we’re going to be dealing with larger quantities of concrete. I also bought a spade for mixing. I was very careful this time around to add water very slowly to the dry mixture to insure it didn’t get too wet. Again, it needs to have some form so that it can built up along the walls, which you can see is fairly high. I had some concerns about whether or not it would adhere to the surface as it was built up, but there were no problems. We just had to keep it dry. Each clump, when pressed to the wall, flattened out to about 1/2” thickness.

Cast image

You cans see that we also added a brass tube that pierces the center and will remain as part of the mold once it is cured. This was a bit of a nod to the original Arco base design that had a hole drilled through it, the idea being that two people could put a broomstick through the hole to re-position or move the base. We wanted to avoid copying the original design too much, but realized that it was a practical decision that made sense. Trying to move a 150 lb. piece of stone with no clear handholds would be a problem that could lead to pinched fingers or worse.

Cast image

Originally we were concerned about filling the plastic mold up with wet cement and having the mold come apart under the internal pressure by that volume of cement, so creating that layer of reinforcement, a shell of cured concrete, went a long way towards insuring the integrity of the mold once we start to fill the main volume with wetter, looser mixture of cement. But we actually filled the mold in three stages: a first stage to create the shell, a second stage where we filled it up halfway and let it cure and then a final stage to top it off and completely fill the mold. We did this so we could check the weight of the mold on a scale before adding the final layer, because we weren’t sure we would get to 150 lbs. of total weight. We clocked in at about 80 lbs, after the first fill, which meant we need quite a bit more weight than simply filling the final half with cement would give us, so we found two blocks of steel weighing about 20 lbs a piece and put those in the bottom and then we filled cement around them. Once the mixture got to the top, we put a plastic cover over the top that had more threaded inserts bolted into it (image_5.jpg) in a specific pattern and then left it alone to cure for several days. Those inserts would receive the rubber leveling feet that would go between the cement base and the floor.

Again- cement likes to remain moist while curing, so having the plastic cover over the top helped retain that moisture, but we covered the whole thing with a plastic trash bag to be certain.

Finished image
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Please allow 5 days for fabrication.