Intro: Make Pizza With a Plasma Cutter, a Backhoe and a Pile of Mud!
After several years of baking in North House Folk School's wood-fired brick oven, I decided to build an oven of my own. I went a little crazy with extra features (slab foundation, arches, ash dump, chimney, doors, wood storage) and decorations (limestone around the foundation), but you can make a very usable version in a weekend with salvaged materials and a couple of friends.
The place to start is Kiko Denzer's "Build Your Own Earth Oven".
I also made extensive use of Daniel Wing and Alan Scott's "The Bread Builders", which is a great resource even if you're baking bread in a regular electric oven.
Step 1: Bushwhacking, Lumberjacking and Limestoning
We have a big lawn next to the neighboring farmer's field, with a great view of Carleton College's wind turbine. What a great place for a pizza party!
The most obvious place to put the oven was in line with our garden shed, but the whole area was a dense jungle. I spent a day with our Stihl brushcutter, with a saw blade in place of the string trimmer. That thing would be awesome for fighting zombies!
I had to chainsaw out a medium-sized box elder tree as well.
Under all that jungle, I discovered an improbable trove of cut limestone left over from when they built our house in 1973. Too good! I'll use that for the keystone and the stone veneer around the foundation.
Step 2: Screeding, Floating, Edging and Brooming
I decided to start with a concrete slab foundation.
My brother-in-law spent the spring of 2007 building park structures and sidewalks with the Gulf Coast Community Design Studio in Biloxi, Mississippi, so he was fresh on all the aspects of concrete work.
Step 3: Stacking, Pinning and Filling
The North House oven has wood storage and an ash dump underneath it.
I wanted to emulate that, so I built a three-sided box out of dry-stacked cinderblocks, and my dad and I filled the four corners with concrete and rebar.
Step 4: Boxing and Dumping
I built a fireproof box out of cement board held together with angle iron and bolts.
Cement board isn't very strong, so my friend Chris kindly welded me a steel frame to span the cinderblocks underneath the cement board.
The inner box is the liner for the ash dump: It's a hole for the coals to fall through when I sweep out the oven. I will set a small grill under the hole to catch the coals.
Step 5: Hoeing and Heaving
Oh, crap! I'm building a clay oven, but our whole yard is sitting on fifteen feet of the world's best topsoil. There's no clay in there anywhere.
Chris came to the rescue once again. He used his backhoe to dig a bunch of clay-and-sand subsoil out of a hillside on his property, loaded it into a giant plastic bin on the bed of his truck, and drove it to my place. Ever tried to lever a seven-hundred-pound bin out of the bed of a giant diesel truck? Chris has.
Step 6: Chipping, Thieving, Mixing, Setting
Now that I've got my subsoil, I can fill the cement board box with insulation: glass bottles set in a mud-and-wood-chip matrix. (Chips don't hold or transmit heat, and they eventually burn out, leaving voids that also insulate.)
I made a bunch of wood chips, but the chipper threw its belt and started on fire. That was the tool gods telling me I had enough chips. Hmm, now for an instructable about replacing the belt on a chipper.
The night before recycling day, I drove all over town and stole everybody's glass bottles.
Then I tried to put it all together, but I kept running out of bottles and having to go out for more. Officer?
I left a four-inch-deep rectangular void in the center. The thermal mass will go there.
Step 7: Leveling and Laying
I poured buckets full of thick mud into the void above the bottles, and screeded the whole mess with a big 2x4.
I let the mud dry partially, and then my daughter and I set firebricks in the mud. These firebricks are the hearth-- the actual baking surface. The mud beneath the bricks holds heat from the fire. The bottles beneath the mud keep the heat from leaking out.
Step 8: Arching
I went over to Chris and Tina's, and we laid bricks on the floor in arch-shapes. We laid out two arches: One smaller rowlock arch to frame the inner door and one larger soldier arch to frame the outer door.
Tina took photos of the arches, and then traced the outline of the inside of the arches in Adobe Illustrator. Chris plotted the outlines on his big plotter, taped the plots to masonite, and bandsawed the masonite.
I took the masonite and stapled it to some short sections of 2x4, making one masonite-2x4-masonite sandwich for each arch. At this point I had two arch forms.
I set an arch forms on chopsticks on the hearth and balanced bricks on the forms. Then I took a deep breath and yanked the chopsticks. The form dropped and the arch settled but did not fall. I pushed the form out and filled the gaps in the arch bricks with mud.
Step 9: Castling
We built a sandcastle the shape of the oven void. Negative space!
We'll dig the sand out through the door when the oven is completely built and dried.
Step 10: Mudballing
We covered the sandcastle with a layer of damp newspaper as a separation layer.
Then I mixed up a power of mud by doing the mud dance.
Then I stacked mud balls all around the sandcastle, smooshing each one against the ones below.
This layer is the oven dome, which holds heat and radiates it down from above.
Step 11: Popcornballing
Having no more wood chips, I made the next insulation layer out of mud mixed with vermiculite. Had exactly the texture of popcorn balls.
I stacked popcorn balls all around the oven dome, smooshing each ball down as before.
Step 12: Adobeing
I made mud-straw ropes and used them to build a tunnel connecting the inner and outer arches. The straw made each rope velcro to the next. They didn't need to be supported from below!
I left a five-and-a-half inch gap in the top of the tunnel, took a six-inch-diameter stovepipe and wodged it into the gap, straightened the pipe by eye from the front and the sides, then built up more mud-straw ropes around the reasonably-plumb pipe. There's nothing keeping it up there but friction.
Then I covered the entire oven with mud-straw mix, for protection and final shape.
You can't believe how well mud and straw holds its shape. If you've ever built free-form shapes out of clay, working with adobe will feel like antigravity magic.
Step 13: Slamming
Tina called me up one morning to say they had some vacant space on a steel panel on its way to the CNC plasma cutter, and would I like anything cut?
Well yeah, oven doors!
Chris took the arch drawings, smoothed and enlarged them, and had them cut out of heavy-gauge steel. He added flanges and handles, and I insulated the inner door with plaster of paris mixed with vermiculite.
Thanks, man. Nice doors!
Step 14: Veneering
I dry-stacked limestone blocks around the front and sides of the oven, just for looks.
The blocks across the top of the hollow space are held up by a couple of bolted-in L-brackets. I'm surprised they hold, but they were the simplest thing that could possibly work.
Step 15: Baking
I have to fire the oven for about two hours to get it up to temp: I fire it until the soot burns off the ceiling, then for about another half an hour, and then it's hot enough for pizza-- somewhere above 800 degrees. I push the coals off to the side with a weed-whip, then mop the exposed floor with a damp rag wrapped around the weed-whip. Then we're ready to bake.