“Husk at elske mens du tør det
Husk at leve mens du gør det”“Remember to love while you dare to
Remember to live while you do”
– Piet Hein (1905 – 1996)
After finishing the outdoor tabletop and using it for a few weeks, I started thinking about what kind of top to make for the dining table.
The orange top is 36″ in diameter (1017 square inches). This fits the space, it’s large enough for 4 people, and it weighs only 50 lbs. For a dining table, I wanted something that would fit 6 people, so I looked for ways to make a top with more surface area, keeping in mind that the table base is only 27″ wide, so something too large and heavy might tip over easily, and anything wider than about 48″ won’t fit in the hatch of my car.
I learned about Piet Hein while living in copenhagen. There is a story about how he used a Lamé Curve to solve a traffic roundabout problem in stockholm. He became known for this shape, calling it a Super Ellipse – a curve that lies between the ellipse and the rectangle.
In three dimensions, he called this a Superegg, and designed a lot of stuff based on this shape, like this beautiful piece of wood he’s sitting on:
He also designed a Super Ellipse table for Fritz Hansen, which they still produce today.
The tabletop I’m making differs slightly from those because the dimensions are different. Originally I tried to scale those dimensions down, but then it was too narrow. The real constraint was the casting surface. I had a 48″ x 48″ piece of melamine to cast on, so I kept it about that length.
I had no idea how to lay out this shape, but I thought a good way to do it would be to divide the base board into a grid, create a photoshop file with the profile overlaid on the same grid, and then draw the lines where they need to be, the same way painters or sculptors translate a scale drawing or model to full size.
Before getting too far into that, I miraculously found a website that made this whole job a lot easier. Super Ellipse | Lamé Curve Calculator, Plotter, and Template Maker. Working with this, I decided on a top that was about 48″ long by 40″ wide. This is about 1219mm x 1016mm, with the value for the ‘radius’ being n=2.5.
From this website you can print out a template, tape it together, cut it out, and then just trace around it, getting a nearly perfect curve that is otherwise a real pain in the ass to lay out unless you’re a mathematician. Forming up the mold after this was pretty easy.
Detail of the template pages after they’ve been taped together. The grid helps keep the pages lined up, and the picture with the rectangles shows the orientation of each print out.
The template is taped down to the base board and traced out with the red marker.
1.5″ foam strips are stuck to the base board with double sided carpet tape while the template is still in place. The foam will snap if bent too sharply, but if you tape the sides of the foam, you can usually bend it without it snapping. Drywall screws hold the foam down every 8″ or so. The carpet tape is strong, but this will help keep the foam from trying to come loose and straighten out.
There were a few places where the curve wasn’t perfect, especially where the the two pieces of foam butt together. In these places I used a few extra screws, or screwed another piece of wood behind the foam to help hold it in place. With the foam walls in place, a plastic liner is stuck to the inside of the foam with carpet tape and will give the concrete a smooth surface.
In the picture above you can see a place where the curve isn’t perfect. There were two places where the liner needed to be shimmed. A small piece of plastic worked well for this.
The form is caulked with silicone and tooled to create a round over on the edge of the table top, and after it cures everything is cleaned with denatured alcohol. With the table vibrators secured, it’s finally time to make cast concrete and make a big mess, which I’m very good at doing!
The last thing to think about was how thick (and heavy) this piece should be. The outdoor tabletop is about 5/8″ thick, but this is pretty thin for a piece of concrete. To keep the proportions looking right, this top needs to be thicker. The herman miller table base I’m using is sold with a granite top that is 7/8″ thick (and weighs 200 lbs.), so in trying to keep things close to what was originally intended, I cast this top about that thick.
I used 100 lbs. of mix for this tabletop, which means I’ll probably need help bringing it upstairs, but I think the thicker top will look better at this size. This is a fiber reinforced concrete mix, half dark base (grey portland cement) and half white base (white cement). It is a combination of the casting mix, and about 10 lbs. of the more fibrous backer mix, along with some 2″ long basalt fibers thrown in to help make it quite fucking strong. It is colored with charcoal pigment, and I want to polish the surface, exposing the fibers in the mix, hopefully getting a very very dark grey tabletop that will be interesting to look at.
First the liquid activator is poured in a mixing container, then the pigment is blended in, then 50 lbs. of mix is added and blended, then 25 lbs. more, and then about 16 oz. of water, and then the rest of the mix, a bit more water, and finally the basalt fiber is tossed in and it’s blended one last time.
Casting is the quickest part of a project like this. The mix is dumped into the form with the table vibrators going full blast help it to settle out. It’s really important that the tabletop is level, otherwise the piece will be thick on one side and thin on another. After the mix settles, the vibrators are turned down and allowed to do their thing for a few minutes while air bubbles escape to the surface. The piece is then covered with plastic and will cure for a day or two before I will demold and finish it up.
This tabletop (48″ x 40″) will be about 10 square feet. The circle top 36″ in diameter is about 7 square feet, and a 48″ round top is about 12.6 square feet. I think the shape of this one will fit 6 people more comfortably and look nicer than a circular or square top. I’m curious to see what it will look like after polishing, because the backing mix and basalt fiber aren’t really ever used in the surface of a piece like this.