Introduction: Build Your Own Earth Oven
Who doesn't love delicious wood-fired oven pizza? NO ONE.
I had a dream of back yard pizza party fun times and began to do some research to make that dream a reality.
First, I looked at the materials cost for a brick oven (spoiler alert: they are prohibitively expensive to build).
I felt defeated.
Then a ray of hope when my sister-in-law described her back yard mud oven. Thus a plan was born: the cob oven.
Cob is a building material made from subsoil (lacking organic material, clay), water, and straw. I live in Virginia and our soil is downright lousy with clay so I knew that the only cost to me for the bulk of the project would be hard work.
Step 1: Make a Plan
My plan, based on some reading and research (recommend Kiko Denzer's Build Your Own Earth Oven) began to unfold. I started with a pizza peel. If I were going to be expected to put a peel in the door of the oven, the oven should be built to make that possible. I looked at various peels and based door width on what I found. once I had the width, I estimated height based on the things I wanted to cook (breads and pizza). With the height, the calculations began to dictate the remaining considerations.
The oven would consist of a bell shaped empty space (the inside, formed using sand as you will see later), a thick thermal layer to retain heat, an insulation later, and a finishing layer, all of varying composition and thickness. See the picture for my planned layers and the estimated firebrick needs for the oven floor.
Step 2: Gather Your Materials
I decided on a medium-sized oven that was not dug in below the frost line but built at ground level, I estimate the oven will last approximately 5 years. We could have built a slightly more permanent version with some additional work but figured we would learn some lessons along the way on Oven Mark 1. With my plan in place, I mapped out a materials list.
1 straw bale (2 are shown but we only used 1)
24 fire bricks (the most expensive material at $75)
1 bag perlite (in garden center)
500 lbs coarse sand (cheaper than you would think, cost me $30)
assorted glass bottles (we found some on the side of the road and used our own that we had been saving)
assorted bricks and rocks (we found our material on site, your mileage may vary)
access to water
access to dig-able clay (I estimate we used 30-35 gallons but can't be sure)
Step 3: Build a Foundation
To be clear, no one working on this build was an expert, we just jumped right in. We gathered rocks and bricks from the property (you can sub in large rocks instead of bricks) and build a circle that would contain the planned circumference. As we built it up, we filled the center with large rocks and shovelfuls of dirt. Initially, we did not plaster the foundation, though you will see us address that a little later. We wanted freedom in the build to modify if necessary.
Step 4: Insulate the Oven Floor
Once you have built the foundation to an acceptable height that you would feel okay leaning down and putting in wood to burn or things to cook, stop and fill with dirt, leaving a couple of inches for insulation. Insulating the bottom is important because you don't want the oven floor to be sucking all the heat out while trying to cook.
For insulation we used a variety of root beer bottles and beer bottles from the side of the road. We even arranged them all pretty. Fill in between the bottles with a bag of perlite and then top with sand.We used a wooden board to compact and level the sand before putting down the firebricks that comprise the oven floor.
Step 5: Build the Oven Floor
The firebricks are what your food rests on to cook; this is important because you want the firebricks to be as close as possible to one another so that they don't shift, push sand up onto the floor, or rest unevenly against each other so that your food gets caught on the edge of a brick.
Pretty much everyone will tell you to take time on this and you should. Lay one brick, kiss the edge of a second to the top edge of that brick and slide down until it sits flush. Use a level. All that.
Or at least you should. We didn't, we eyeballed it. What can I say? We wanted to make this happen. I'm happy with the oven so don't stress too much but just keep the level-ness of the floor in mind while you work.
Step 6: Form the Oven Interior
You need to have the negative space of the oven, where the food actually cooks but also need something to form all the mud/straw layers on top of. WHAT TO DO?
The answer is to soak a bunch of sand with a hose and form the interior of the oven to be dug out later. To ensure we met the planned height, we cut a nearby stick to size and formed the sand around it. The width and height should be as planned, the rest should be essentially bell shaped. If you remember from "the plan", we intended to cover this with wet newspaper prior to putting on the thermal layer. We forgot to buy a newspaper. It didn't matter (this is why I didn't put newspaper on the materials list). No worries.
Step 7: Build the Thermal Layer
The oven's thermal layer holds the heat from your fire long after it is gone so that you can cook delicious pizza (among other things). For the thermal layer, we needed 1 part clay, 3 parts sand, and enough water to make it easy to form but not so much that it was wet. The test for the right consistency was forming a golf ball sized bit of it and dropping it from wait height without seeing cracks (too dry) or it splatting (too wet). The mixture may change based on the clay you obtain. The subsoil/clay that we dug up on the property really only needed about a 1:1 mixture of clay:sand to achieve the golf ball test. Experiment by digging up the clay, putting some shovelfuls on a tarp, throwing some sand and water on it and dancing all over with your feet (if you want it slightly less dirty, fold the tarp in half over the mix and dance on the tarp. Think grape smashing). Once you pass that golf ball test, get to layering.
Our thermal layer was about 3" thick. We essentially formed mud bricks and smashed them together, then compressed with a board (shown in image). Now your oven needs time to think about things. We gave our Jabba the hut a week to dry out a little (may need a loose tarp if you are building in a wet environment).
Step 8: Let Dry
We waited a week before returning to our little Jabba the Oven.
Step 9: Cut a Door
Be sure to stick to your planned measurements as best you can! The door height:interior dome height is important so air can be drawn in for the fire to get hot. The door should be approximately 63% of the height of the inner dome (the top of your wet sand form).
Step 10: Build the Insulation Layer
The insulation layer makes the oven cool slooooowly so you can cook stuff. The insulation layer is just clay and straw, with enough water so that it all sticks together. We found forming little handsized bricks and building up to be useful. This layer should be about 4-5" thick and ends up looking a little ridiculous. You can also see that it was about this point in construction when we finally dug out the sand from the interior. We waited because, as we added layers, we were compressing and didn't want to compress our way into a pile of mud. The second photo shows the sand out of the oven and the doorway edges with smoothed out by hand with a little assistance from some water.
Step 11: Coat With a Finishing Layer
Our oven was a bit...straw-y. The initial plan for the finishing layer advised adding straw but we chose to forgo and use a sand/clay mixture to smooth everything out. We poked little fingertip bumps in it because we were unsure if we wanted to do a plaster layer after it dried and wanted the plaster to have something to hold onto just in case.
Step 12: Let Dry
We waited three weeks before returning to the oven. If necessary, you can build a series of small fires over time to assist in drying it out faster. As we built the oven on vacation land quite a distance from our home, that wasn't the preferred method for us. Instead, we covered the oven loosely with a tarp to protect it from the elements and let time go to work on the drying.
When we came back to the oven, we removed the tarp and heated it up using small kindling and fallen tree limbs that had dried out (honestly, the smaller bits worked better in drawing in air than the larger kindling). We heated the oven up with constant fire for about 3 1/2 hours.
For the test run, we decided to have a pizza party, making dough and readying toppings.
Once enough time had passed and the oven was hot to the touch (on the outside), we swept out the embers and got to cooking. I recommend using an infrared thermometer to judge temperature as we went through a bit of trial and error (pizza cooked slowly at first so we went back to the fire for about 20 minutes, then pizzas cooked in about 3 minutes apiece). You can see that we swept out much of the embers but also moved some to the sides in the last image.
Step 13: Bonus Step! Coat With Lime Plaster
The front image of the oven shows what it looked like after we obtained two 5 gallon buckets of lime plaster and coated it. We wanted slightly more protection than a tarp without having to build a roof for the oven and this met our needs. We didn't make the plaster ourselves, merely obtained it and smashed it all over the oven. Looks pretty, right?
We anticipate the oven to maintain functionality for anywhere from 3-6 years, perhaps longer with care. We plan on taking the lessons from this oven to build Mark 2, which will be dug into the earth for more stability.