An old friend, in discussing national cuisines, described British cooking as ‘medieval’, especially compared to French cooking. I was thinking this the other day as I began to binge-watch The Naked Chef, Jamie Oliver’s signature show begun in 1999 when he was just 23. In his first two shows Jamie cooked entirely with the oven; nothing in a skillet or pan. I think that, without saying it, he was out to prove several things one of which is that creativity does not end at the oven door. We’re used to two apposite images, I suggest, respecting cooking; one the picture of our Mom checking the Sunday roast, absolutely unchanging and formulaic. The only creativity might come with the vegetables – roast or mashed potatoes? Green beens or carrots? The other image is of the (usually male) French chef, his hands blurred by activity as he tosses ingredients into his sauté pan and flicks the pan midst a leaping flame.
But I agree with Jamie’s hidden message: ovens and their use can be creative. And what more creative step than to create the oven itself? That is the task that Tess, our daughter, and I set ourselves. We conceived of the project in two stages: design & construction, and use. (Ed: Each will be a separate post – D is famously long-winded. But he’s including a lot of detail for those of you who might be interested in building your own. And my job is to add the pictures, which is what everyone likes to see. To see the use of the oven, go to my other blog, This Green Life.)
Gail had forged ahead with her patio this past summer and, while the oven did not have to be build in tandem with it, the planning and construction of the patio (especially while trades were about) was a real spur to action. We had a pretty good idea of where we wanted the oven, so to plot its location I slapped together a 5′ x 6′ frame of 1″x 4″ scrap (seen at the right in the photo below.)
We then moved the frame around to see how much space the oven would take and how accessible it would be. The exercise was very useful, as we realized that the most optimal use of space was to push the oven back from the patio itself, build its platform behind our 3′ retaining wall, and use the wall for support for the slab. Our carpenters agreed that that made sense and minimized the construction of further support walls. Short walls protruding into the patio were necessary, so we used them to create a wood / implement storage area beneath the slab.
Once the frame for the slab was built we hand-mixed the concrete. There is online discussion about slabs of course; some suggest using vermiculite for both insulation and weight-saving purposes. We didn’t think that necessary, as most of the slab is sitting on packed earth. Since it will be subject to serious heating, however, it’s important to properly reinforce it with rebar. For this we relied on the expertise of Jonas, our carpenter. The only feature we included was a hole for an ash-drop. Not everyone includes this; many simply shovel or sweep their wood ashes out from time to time.
Once the slab was done, Tess and I decided on an oven design. Again, there is lots of online information available, but designs boil down to two: “quonset” and “igloo”. The igloo design is, as the name suggests, a round dome-shaped oven. I suppose it offers the advantage of the most evenly-distributed heat, but it also seemed much more complex to build. The quonset, on the other hand, suggested a much simpler build.
The plan was to use it to support our bricks and, once the oven was complete, to simply burn the frame out. I had seen very simple and basic oven building methods that involved using just clay/mud, heaped and packed over a rounded pile of branches and brush; again, once the clay had hardened, the brush was simply burned out and the oven was ready for use. Our frame had 4 hemispherical plywood forms, tied together with very thin strips, 3/4″ wide. We simply stapled them onto the plywood forms and they proved very effective in holding the entire form together. The forms weren’t truly hemispherical, actually: we knew we needed to lay one row of bricks on the slab before we began our arc, so we calculated the diameter of the dome and then added 4″ i.e. the thickness of a brick. We also cut little ‘doors’ into each of the interior two forms, realizing that we’d need access to the back of the oven/form when we wanted to burn it out.
It’s amazing how much room the oven takes up: the slab, once poured, looked vast. The oven’s burning chamber was planned at 32″ x 36″. That seemed, in the mind’s eye, to leave a lot of unused slab. But we knew that that 8″ would be taken up with two 4″ brick walls, and another 6″ minimum with concrete covering the dome (i.e. 3″, but on both sides of the oven). Still, calculating is one thing but visualizing is quite another. (When the oven was complete there was only about 6″ left on each side of the slab.)
Here we are, about to start the oven:
We had a pretty good idea of the building process. We laid one course of bricks on the slab to establish the footprint of the oven:
Then we ‘arc’ the bricks to form the dome. Tess did all the calculations: she calculated the total number of bricks required (she was right within six bricks) and the small angle we would have to maintain between each course to create the arc.
Materials are relatively expensive, certainly more so than conventional brick masonry. This is because both the bricks and the mortar must be ‘refractory’; the bricks are made of ceramic material, for example, so that they can resist extremely high temperatures while offering low thermal conductivity (i.e. you want to keep the heat in the oven. When we consulted the personnel at the brickyard they recommended a single tub (50 lbs.) of mortar, but that turned out to vastly underestimate the required amount. That was because we did not explain ourselves properly. Refractory bricks use a very thin layer of mortar; probably less than 1/8″ in, say, a fireplace wall. But to ‘arc’ them meant filing a wedge-shaped gap and that consumed, eventually, four tubs (at $69 each!) We could have economized by using bits of gravel or some other suitable infill, but we valued speed over economy.
Again Tess’ calculations were spot on: the last (‘key’) course at the top of the dome fit just about as it should. Because the pre-mixed mortar has the consistency of peanut butter we only laid three or four courses at a time, so that the accumulating weight would not squeeze the mortar out. We were surprised that, even days later, the mortar still seemed damp and pliable. We thought about that and concluded (rightly, I think) that, being refractory mortar, it required heat to eventually set completely. Tess did almost all the brick laying (aided by the occasional other helper) and I did all the support. For the front and back walls she carefully marked bricks for cutting and they proved very easy to trim to dimension with an angle-grinder fitted with a masonry blade.
We were of two minds about the chimney; a masonry one, I thought, would be disproportionately large and would spoil the design. I also wanted it to draw well. So we decided on a 4″ simple pipe design. Tess found a 4′ length at a local online supplier for $30. It’s basic steel and already has a sheen of rust on it, but that’s fine; we think that will add to the look, and the steel is so thick that it will be a long time before the rust has any serious effect. I was worried that the chimney might be unstable, so I drilled four holes in its base and set four screws into it, protruding about 1 1/2″ to act as anchors in the concrete that would cover the bricks. We supported the chimney during installation with a scrap piece of lumber.
Once the basic shell of refractory bricks was complete we covered it first with a layer of wax paper, then a layer of cardboard. This ‘break’ between bricks and concrete is essential to allow for expansion of the bricks. Without it you risk cracking the concrete. We then attacked the concrete. Again, online advice was almost unanimous: if you could arrange it, vermiculite is the best approach, replacing much of the sand/gravel mix with the vastly lighter volcanic product. We sourced it at our local garden shop but, again, it does increase the cost (we spent $200 for 11 bags, eventually). The mix is hard to describe, probably more like a granular oatmeal than concrete. Our proportions were as follows: 6 shovels vermiculite, 1 shovel masonry sand, 1.5 shovels type S cement, 4 litres (quarts) water. It was a bit finicky and touchy to apply and required someone of Tess’ patience to succeed. Anticipating that it wouldn’t adhere very well to the cardboard, we quickly laid expanded steel lath over the oven, stapling it into place.
There is a lot of moisture left in the bricks, mortar, and concrete, so no heat should be applied to the oven for a minimum of a week. After that a process of small fires helps to drive the moisture out carefully and ‘cure’ the oven. Our schedule meant that we couldn’t get to the curing stage for several weeks, so that was a good thing. To ensure a gentle, steady heat the first time I followed (yet more) internet advice and lit a 10 kg bag of charcoal, creating a burn that lasted for several hours but never got too hot in any one spot. After that there was no putting it off: the form had to be burned. I would have preferred to light several much smaller fires, but it seemed unavoidable that the form be burned all at once. So I did it. I don’t think the oven suffered any damage by the burn; it was very hot, but it was also very quick.
After the form was burned out I lit fires on each successive day, each day a larger and longer fire than the last. After four or five days I judged it done. We still had no door, so I simply blocked the door up with extra firebricks.
The cost for all the materials was very close to $1000. The labour was free (well, it might have cost a neck rub and a pizza dinner or two!)
To see how we fire up the oven and cooked the pizza, as well as a Christmas “peel”, see This Green Life’s post.