So, I bought my husband THIS book a year or two ago. It’s about building a wood-fired pizza oven from mud — because he is into pizza. We have “Pizza and Piper Fridays” (see HERE and HERE). Rarely do we eat at a pizza restaurant, but a few weeks ago was one of the occasions in which we decided to go out for pizza. We found a place in our little town that had only been open for about one month. It had great reviews so we decided to try it out. When my husband went to pick up the pizza, he saw that the centerpiece of the restaurant was this huge cob wood-fired oven. This renewed his dream of building one of his own cob ovens. But, since most cob ovens have large foundations built of stone or brick and dug a few feet into the ground, he didn’t think that it was possible to make an oven at our home since we are renters. When he went back to the Earth Oven “how-to” book, he discovered a design that would work on our rented property. So then he took over the basement (formerly known as “my sewing area”).
The lighter version without a permanent foundation involved wooden sawhorses. The author of the Earth Oven made one like this that he initially thought would only last for a couple of fires, but as of the book’s printing, it had lasted five years. So we thought we’d give it a try. We’re not sure if we could load it into a trailer when we finally move, but even if we were able to move it, it probably would collapse in transport. But that’s OK, this project will be a learning experience and will help us when we make our final, more permanent version.
It’s important to find a source rich in clay — the most important component of the adobe or cob oven. Fortunately, we did not have to go very far. Just below the top soil in our yard is the good stuff. You know you have hit clay when the ground holds water like this after it rains!
Probably the most tedious part of this project is going through the soil and breaking up the clay chunks, and discarding the rocks. The book suggests throwing the soil on a 1/4″ screen to filter the rocks, but the problem with our soil is that it is so rich in clay, all the huge chunks of clay would just bounce right off the screen and we’d be left crushing them with our hands anyway!
The “foundation” of the oven consists of five 2 x 10 boards that were “Kreg-Jigged” together (see first picture), followed by concrete pavers on top. The book’s saw horse oven did not appear to use a layer of insulation, but we thought we’d put one in anyway so that as much of the heat as possible would stay in our oven. Insulation is made of clay and wood-shavings from a local cabinet maker who gave us two 55-gallon bags of pure wood shavings. Apparently the shavings are better than fine saw dust that you might get from using power saws. This recipe for insulation was the only thing in the book that I found to be way off. The book called for about one gallon of clay slip for half a wheel barrow full of wood-shavings. Of course, wheel barrows vary in size, but I found that it was closer to five gallons of clay slip per half wheel barrow full of wood-shavings.
Before we placed the firebricks, which served as the hearth, we laid some sand so that it would be easier to make sure the bricks were level. The sand we used was the grainy stuff, known as “concrete” sand that we picked up at our local quarry, thanks to my husband’s colleague at work who has a pick-up truck. Initially we bought Quickrete all purpose sand at Lowe’s but found that it was too fine for the structure of the oven.
The brick archway was amazing. We had a wooden template that was shaped into the rounded door with two 1/4″ shims underneath it. We stacked the bricks (which we chiseled in half lengthwise) around the template, then inserted a pebble between each brick to make sure it stayed at the correct angle. If you look carefully between the fourth and fifth brick on the left, you can see one of the pebbles. When you get to the keystone, it has to be wedge shaped, otherwise when it heats up, it could fall through! So the greatest technical challenge of this project was to chisel a wedge shaped brick. Easier said than done. I used a 4″ mason’s chisel (same one I used to cut the bricks in half lengthwise) to try to get the right shape. The problem is, the brick may be the right shape just at the surface or front of the brick, but underneath it can break off in an odd, jagged manner that won’t fit in the slot. It took 4-5 tries to get it just right — we feel blessed because it could have taken much longer! The coolest thing was when the keystone was placed, we removed the 1/4″ shims from below the door template which dropped down, we pulled out the door and, viola, the arch stood all by itself without any mortar! When we did put in the mortar, however, we used the finer sand because the grainier sand made it hard to pack those tight spaces.
Next up, shaping the inside of the oven (the void) with sand. It’s pretty much like making a sand castle at the beach — packed wet sand to make a dome or egg shaped void. We got the neighborhood kids to help – YAY free labor!
Then, we mixed the sand and clay to make the actual oven layer. This is supposed to be the fun part in which we play music and dance. It’s kinda neat to get the feet all muddy!
We used a 1:1 ratio of sand to clay soil. We probably could have done 2:1 or even 3:1 since our soil is so rich in clay, but we didn’t want to risk a brittle oven that might crumble. The downside is that clay doesn’t hold the heat as well as sand so we’re sacrificing some thermal mass. But then again, some people build all clay ovens which work just fine.
This part was fun too. Digging out the clay and hoping that the mud dome doesn’t collapse.
After the oven was almost completely dry, we put on a layer of insulation, about two inches thick. Then the final plaster layer went on very thinly, about one inch thick. We used a concrete stain (terra cotta coloring) to give the final appearance a deep red hue.
We are having trouble getting the last few pictures to load, and this post is already pretty long. Stay tuned for a picture of the finished product and the first bake night!
If you are local, you can just come over and see it for yourself – Susan, I’m talking to you!! =)