Saturday morning we got up early and decided to tackle taking apart the clay oven (Sunny). The plan was to remove the clay dome and hopefully save the clay to reuse in the new oven. I wasn't sure this was going to work because the inner shell of the oven had been exposed to some pretty big, very hot fires, many burning for four to five hours. I wasn't sure whether the inside of the oven had reached a temperature high enough to vitrify the clay, which would prevent us from softening it and reusing it. I had a suspicion we'd be dealing with a large, turtle-shell-shaped BRICK. Greg headed to Menard's (like Home Depot) for some supplies we'd need, and I set to work demolishing Sunny.
I took a few photos during the process and used my rock hammer, which has been collecting rust on Greg's workbench for years, and started pounding on the oven. After several hard whacks, I started making slow progress. I headed out to the barn to look for the sledge hammer thinking that was the right tool for the job, but couldn't find it. Anyway, as I broke through the outer layer of "cob" a mixture of clay, sand and chopped straw, I realized it was coming apart in a distinct layer, separate from the inner clay-sand layer, exactly as I had built it.
I threw the outer layer pieces into the wheelbarrow, and as I broke through the dense inner layer, I threw those pieces into a big black tub that normally serves as a sheep water trough. The demo took about an hour and when I was done, I removed the fire bricks and set them aside, leaving the cinderblock foundation and the sand bed that had held the fire bricks.
Greg and I came up with a plan and got busy building the wood and melamine forms to serve as the outer shell of the concrete countertop, which we were going to pour on top of the cinder blocks. We made an outer frame and an inner frame to keep the concrete from covering the sand bed.
Once the framing was done, Greg rounded up some rebar and some pieces of a cattle panel to serve as structural support for the concrete. We debated how we wanted to finish the edge of the countertop and just decided we'd leave a sharp edge and then use a sanding block or his orbital sander to round the edges over once everything is finished. Greg picked up some concrete stain at Menard's that can be sprayed on after the concrete dries, so it isn't just plain old gray. Since the clay is a warm tan/brown color, it should blend in well. We plan to stucco the vertical surfaces of the cinder blocks with a surface-bonding cement and then paint that the color of the trim on the house so it should blend with the brown of the concrete pretty well.
Greg borrowed an electric concrete mixer from someone he works with and he'd calculated how much concrete we'd need so once we'd formed up the frames we quit for the day and planned to start early Sunday morning pouring the concrete.
Before we cleaned up our mess, I filled the buckets and water trough containing the remains of Sunny with water from the hose. I figured we'd leave it overnight and hopefully by the following morning the water would have softened the clay enough to determine if we'd be able to reuse it.
On Sunday, we got up early so we could get going on the concrete pour. I checked the buckets and sure enough, everything had become a mushy, gritty goo. Perfect. All I'd need to do is let it settle and skim off the water layer on top of each bucket, add a little extra sand and clay and we'd have plenty of material to build a bigger, thicker-walled oven.
Greg set up the rebar inside the frame and we fired up the mixer. Greg ran the mixer and I shoveled the wet concrete into the frame. We used seven 80-lb bags of concrete to fill the frame, and once we were done, Greg used a 2 x 4 to smooth the mix and level it, then we left it to set up. After it had set up somewhat, Greg worked on finishing the surface as smoothly as possible. We may need to polish it with the sander before we stain and seal it. Maybe not though. It looks pretty good to me.
We wet it down with a hose and covered it with a tarp to slow down the drying process to allow it to cure.
Once we had rinsed out the concrete mixture, we decided to try using it to homogenize the recycled cob. We processed each bucket as it's own batch, and ran the mixer for several minutes on each batch. This worked really well and managed to break up most of the chunks of clay. Once we get to the point of rebuilding the actual oven, things should go pretty fast since I won't be starting from scratch. It takes quite a bit of time to manually mix the ingredients for the oven. Having the mixer was a huge time saver.
One thing I'm going to do differently this time is put a sheet of aluminum on top of the sand layer and below the fire bricks. This should help reduce the heat loss through the bricks into the sand layer. Another thing I'm going to try is a one to two inch insulation layer of clay/sand/straw and vermiculite on top of the clay/sand thermal layer. I hope to really cut down on the heat loss through the walls of the oven by incorporating the vermiculite as an insulator. I'm also going to increase the wall thickness of the thermal layer to 5 inches. It will take a little longer to heat the oven up to temperature but it should stay hot much longer by doing all of these things. This will potentially allow several hours of cooking, starting with pizza and breads, which require very high temperature but cook very fast, followed by slow roasting of vegetables and chicken as the oven slowly cools. Finally, by loading the still-warm oven with a stack of firewood, the residual heat will help to dry it out so it will be ready for the next firing.
A well fitting door and possibly a chimney with a damper are also design features that I did not use in the old oven but may use with Sunny 2.0. By putting in a chimney above the door, I hope to be able to get more complete combustion inside the oven, cut down on the soot around the door (which gets all over you and doesn't look particularly attractive). A tight fitting door will be a lot more functional and safer than stacking bricks in the opening, which is how we closed up the old oven. This oven will definitely be a lot more user friendly.
We wet it down twice a day on Monday, and again on Tuesday morning, and once Greg got home from work, we took the forms off. Here's what it looks like thus far. The hole in the middle is exactly the size of the fire bricks. We will install them after we wet down and tamp the sand bed. We planned everything out so once we put the bricks back in the hole, they will be at the same height as the surface of the concrete and everything will be flush and smooth, so the pizza peel will slide in and out easily.
Here's how it looked today (Tuesday) when we took off the frame.
Here's a link to my inspiration, a 9 foot tall oven called "Maya" in a restaurant in Corvallis, Oregon built by Kiko Denzer...