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
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
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
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
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
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
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 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
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'll dig the sand out through the door when the oven is completely built and dried.
Step 10: Mudballing
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
I stacked popcorn balls all around the oven dome, smooshing each ball down as before.
Step 12: Adobeing
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
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
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.