The coveted backyard oven completes any permaculurist's garden much like a gazing ball counters the bleakness of a properly manicured lawn. The oven melds the flavors of a garden the same way a gazing ball reflects the finely shorn grass upon which it sits. Crisp and defined, preferably with a thin crust and lots of cheese.
Designing one of these creations is a contentious and debated topic, much like choosing the color of your gazing ball. Blue or green? In either case, neither ornament is completely permanent. I like to keep a large selection of colors in my garage to reflect the type of day I am having. In the case of the oven, none of the materials are permanently fixed, so they are quite easy to take apart and rebuild should problems arise. This being said, we built a permanent base to hold the weight of the oven and any future modifications. It is important to choose a location that fits the site. Our oven is located far enough away from the house avoid smoke sparks becoming a hazard to anyone inside, yet close enough to the kitchen door to provide easy access to a cook carrying food to and from the house. We also faced the oven door away from the prevailing winds so the oven will not lose too much heat when the door is opened.
We began building by excavating the lawn under the oven. Then four concrete posts, scavenged from a pile left in the back corner of the lot, were placed into the excavated rectangle, squared up, and leveled.
On top of the posts, we placed a frame of treated wood, covered with a sheet of old roofing iron. Many people build foundations out of stone or bricks to resist heat and insulate the oven. So far the wood frame has held up fine, and the space under the oven is a great place to store wood, fire tools, and a tarp to keep the rain off. After making the frame, we were ready to begin creating the oven.My quick Google search led me to the conclusion that earth oven construction seems to be plagued by many ailments. Cracking and collapse are common, and problems that affect one builder do not affect another. Also, it is difficult to find a standard mud recipe, let alone recommended shapes and dimensions. In our search to find the perfect design, we decided to use an old wood burner as an interior frame for the oven. Although boxy, the iron structure provided a solid the frame that mud lacks, including a tight fitting door for efficiency. The wood burner required that the exterior paneling and air intake be stripped off before the iron firebox could be used.
The firebox was set into a layer of mud on top of the iron, stabilizing and insulating the box from the bottom. The firebox was then surrounded by a layer of bricks to provide thermal mass.
Two layers of mud were placed around the bricks. This layer held everything together and insulated the bricks and firebox. The first layer of mud we decided to use was a mixture of three parts sand to one part clay with a small amount of chopped straw added to form a cob mixture. We mixed the mud with our feet in a tarp, adding water until the mud reached a formable consistency and could still hold its shape.
The first layer of mud was formed into soft cubes about 100mm wide and stacked tightly around the bricks. We tried to apply the mud evenly around the oven, allowing areas to set up a little before continuing to build upward. We surrounded the entire oven with this layer except for the front, to allow the door to function properly.
The second layer of mud was made up of three parts sand to one part clay, with a lot more water added to form a plaster-like consistency. This layer was applied in a 20mm think layer over the first, to even the first layer and provide a smooth, aesthetically pleasing finish.
At this point the oven was smoothed over with a trowel, covered with a tarp, and allowed to dry slowly over the next three days.
By Saturday, the oven was dry enough to start a small fire in the morning to finish the drying process. After some time, we added more wood to heat the oven up for pizzas in the afternoon. Everything seemed to work fine.
We saved some mud mix from the main part of the build to repair a few cracks as well as build the chimney a little taller and reduce its diameter.
-John the Intern