The Cob Oven
This has been a fun project. I really enjoy mud.
The first phase of the project was digging the hole for the foundation. That happened in 2004. We opted to use a gravel foundation, so it was just a matter of digging a hole below the frostline and backfilling it with gravel. Five strong guys had the hole dug in an afternoon. (OK, I could have dug it myself in an afternoon. But they got it done—that's the main thing.) They spread the topsoil in against the house in a place where the ground had sloped toward the foundation, so that was a plus, and they kept the subsoil in a pile, because on this acre, the subsoil is nearly pure clay. We used this clay for our cob.
Then many months went by, in which no one had time for the project.
Then summer came to us, and we felt the urge again. I got things rolling by busting up the rubble of our old concrete front steps. I'd had the rubble stacked in the backyard for, what, fifteen years? I knew I was going to use it for something, someday. I broke it up with a sledgehammer into manageable chunks. I started out trying to get nice rectangular pieces with a brick chisel, but it was no good. It took too much plinking around with the chisel to get the concrete chunks to break, and then they usually didn't break where I wanted them to anyway. So I just started mashing away with the peen of the hammer, and was not too unsatisfied with the pile of chunks I made. We used these to build a circular base for the oven.
Then it was off to the sandpit for sand, the quarry for gravel, and the builders' supply store for mortar cement.
Building the base took an afternoon. We mixed the mortar cement up with sand and water, and laid up a base of "urbanite," as cob builders call concrete rubble. It wasn't easy making even a rough cylinder out of those irregular pieces, and we didn't use a level or a plumb line; we just built by eye. We laid some heavy wire in the mortar of several of the courses as reinforcement. The final result has lots of rustic charm. Really.
Then we filled the center of the base with the gravel. We realized at that point that having built without benefit of a level, we had left ourselves an uneven and somewhat slanted surface on the top of the base, so our first venture into cob was to mix up a batch and lay it on top of the base to make a level top surface. This was done mostly by eye and by no means did we get fussy about it, but we did bring the level in for a couple of quick checks. We found that our eyes had done pretty well.
This first experience of mixing the cob was, I have to say, delightful. We mixed up the dry ingredients (3 parts sand, 1 part subsoil with a very high clay content) with a hoe in a utility tub. Then we dumped the dry mix out on a tarp and wetted it down, and used our bare feet to mix it. Damn that feels good! The feeling of that gritty mud mooshing up between your toes is a tactile satisfaction the likes of which we moderns seldom encounter anymore. Anyway, after we got it well mixed, we threw some fresh clean straw on it and trod that into it as well as we could; we found that the straw immediately took up a lot of the moisture in the mud and made it much harder to tread. Then once we had that well mixed in, we just grabbed gobs of cob and laid it out in a ring on top of the urbanite, making a nice flat solid base for the oven to come. The next day, I went out to check on it, and was impressed by how hard it had become. I picked up a small gobbet that had fallen to one side, and found that I was unable to crush it with one hand—and I have strong hands. Cool!
A few days later, when the cob ring that tops the foundation was dry, we filled the inside of it with sand to make a base for the brick floor of the oven. Then we used a four-foot board to scrape the sand level with the cob ring, and laid out the bricks for the floor. We used a total of 22 bricks: 6 x 3 for the 18"-square floor, and 4 more for the "tongue" of the floor that extends out the door. Firebrick is recommended, but we used ordinary pavers. I don't think I'll use firebrick, which is much more expensive, until I've built a few more of these ovens and have reassured myself that the design is final.
The next step was to build a dome of sand on top of the brick floor. This would act as the form for the interior of the oven. The cob dome would be built over it, and then, when that was finished, the sand would be removed. Wet sand got piled up and meticulously shaped. The outside diameter of the oven was to be about four feet. The diameter of the interior would be 18 inches, and the height of the interior would be 16 inches. So the sand dome was built 18 inches across and 16 inches high. It was well packed and smoothed, and then covered with plastic sheeting so that it wouldn't dry out overnight.
The next day, before we began to apply cob to the dome, we covered it with wet newspaper strips. In his book, Kiko Denzer recommends this so that when you go to hollow the oven out later, you can tell easily where the sand stops and the cob starts. It definitely made hollowing the dome out later a snap, and was well worth it, but we found that we had to keep wetting it as we worked, as it wanted to dry up and blow away. A small spray bottle of water kept it in line well enough. You can see the newspaper layer clearly in some of the photos below.
So! Time to make more cob! The joy of mud, I tell you. The oven was constructed in layers. The first layer, which we were building now, Mr. Denzer calls the "thermal layer." You want it to be as "massy," to soak up and then radiate as much heat as possible, so this first layer is mixed without straw. We mixed up several small batches, each about 3 buckets of sand to 1 bucket of clay subsoil. (Actually, I think I cheated a little to the clayey side.) We mixed it just as before: first the dry ingredients in the tub, then onto the tarp for wetting and treading and "burritoing." To "burrito" cob is to grab one side of the tarp and roll the cob mix up onto itself, making a sort of burrito shape. Then you tread it down well and burrito again. Tread, burrito, repeat until mixed.
We transferred each batch into a smaller tub and hauled that from the mixing area to the oven base twenty feet away. Then we just grabbed handfuls of cob and went to work, pressing it down into itself. Mix a little, cob a little, mix a little, cob a little. What a joy. Our main challenge was to keep an even thickness all around, as the two of us working on the one side tended to be a little out of sync with the two on the other. We ended up having to shave some off one side and build the other up a little. But hey, it's supposed to look rustic. As we neared the top of the dome, we were all struck by the picture it made: it looked as if someone had trepanned Jabba the Hutt. There he was, with his brain poking out the top of his head. We kept cobbing and finished his cranium, but the name stuck. Whenever it threatens rain now, someone is sure to say, "Did someone tarp Jabba?"
We gave the thermal layer several days to dry, then Banks opened a small hole into the sand dome. Then we gave it a few more days, and I enlarged the hole a bit and excavated. That went quickly, and the layer of newspapers made it easy to tell where to stop digging. Then Banks enlarged the hole (the doorway) to its final dimensions—or so we thought.
The next layer is for strength and, to some extent, for insulation, so we added quite a bit of straw to it. Our first experience at mixing cob with straw in it had taught us two things: long straw makes a mass of cob that's hard to manipulate, and the straw soaks up a lot of moisture when you add it to the mix. So in preparation, we ran the (rotary blade) lawnmower over several flakes of straw to chop it up a bit. I suppose we lost some of the reinforcing effect from the straw by reducing its length, but this made the cob much easier to work with, both at the mixing stage and when it was being applied. We made the initial clay/sand mixture quite wet in anticipation of the thirsty straw, and that helped a lot too.
Then we set to slinging cob again. This stage took another afternoon. We built an arch up over and around the doorway as we went. Again, we had some minor difficulty keeping the outside symmetrical because of the uneven distribution of labor around the dome, but we stood back periodically and made the necessary adjustments: scrape some off here, slap some on there. Easy.
The next step is not necessary, but we're glad we did it. Banks did it, actually. He added a sculptural layer of cob. Jabba became a giant totemic frog. After the cob dried, he mixed up a batch of mud plaster (his proprietary recipe, but I can tell you it involves horse manure) and put on a finish coat.
We let it dry then for several days. Tt just happened in our case, but according to Denzer, it isn't really necessary. Then it was time for the first firing!
No, wait, a few more things to take care of. We needed a pile of firewood, a door, and some implements. The firewood was easy. You want small stuff, not logs, and it seems like we have an endless supply of small to medium branches falling out of the trees around here. We already had some cached in a big plywood box with a cover, so it was good and dry and ready to go. The door had to be built out of a couple of pieces of 2x6, battened together and then sawed into the shape of the doorway. We scrounged up a sawed-off old shovel for removing the coals at the end of the burn, and rigged up a rag on a stick for sweeping the ashes out once the coals had been removed. (You wet the rag first to keep it from bursting into flame.) As it turned out, we kind of forgot that we would need someplace to put those coals once we removed them, but I'll get to that in a minute.
OK, now time to fire. Banks built a fire in the front of the hole, and we were off. We found that we could get a good little blaze going in the front part of the oven, but nothing we tried would get the fire to burn back into the back half of the chamber. Dang it. But after a good three hour burn (maybe four, it's been a while now since we did this) we got it hot enough that the outside was warming up. We figured if that much cob was heated up, the inside had to be pretty hot. So, time to clear out the fire and put the calzones in the oven!
No, time to panic. Where would the coals go? They're hot, you know. Urgent scrounging session ensues. Finally, we locate a piece of sheet metal with a lip around the edges (a side panel from an old washing machine; now aren't you glad I saved it?) and position it on the ground under the door. We scrape the still-flaming coals out onto it. This is not so pleasant, because the coals heat the sheet to the point that the paint starts burning off the underside, but we drag it off downwind and rush back to the oven, anxious to get the calzones in as soon as possible while the oven is good and hot. The wet-rag-on-a-stick (there's a technical name for it but I forget at the moment) works great to clean the leftover ashes off the oven floor. The calzones get popped in with a long spatula. Door goes into place. Now, wait.
Within minutes, very good smells start leaking out. I crack the door to look in, and the calzones are puffing up nicely and starting to ooze a little. Yesss!
Result? Calzones are the best ever. The oven wasn't as hot as we would have liked, but it was hot enough to get the calzones cooked. We fired the oven again once or twice in 2005, but we were a bit discouraged that the fire could not be made to burn in the back two thirds of the oven. So, without burning motivation, we kind of let things slide for the rest of the year.
Jump ahead to 2006. Jabba, in spite of being tarped all winter—or possibly, in part, because of it—has had his features worn down quite a bit. I think it was a combination of the tarp flapping in the wind and the fact that it isn't really a waterproof tarp. Too bad, although he has a certain ruined character now that would be hard to achieve intentionally.
Anyway. Denzer's book, I recalled, mentions at some point that the best ovens all seem to share a certain proportion between the height of the internal chamber and the height of the door opening. I couldn't remember the exact proportion, but it was something very close to 3/2. So I checked Jabba, and he didn't quite conform. Fortunately, the doorway was on the short side, so tuning it would be a matter of cob removal, not cob addition. Banks grabbed his doorway shaper—an old farrier's rasp—and added 1/2" to the doorway opening. We had company for the weekend, so Banks and I fired 'er up. Yay! Happy dance! Now the fire burns all the way to the back; there's only a small corner where it refuses to burn. We fired for less than three hours and had an oven plenty hot to bake 3 pizzas in sequence. I'm convinced another hour would have made a huge difference; Jabba was just starting to get warm on his forehead.
Diane had bought me a proper "peel" for Father's Day. A peel is a huge spatula that you use to transfer bread or pizza in and out of the oven. It's a big improvment over a kitchen spatula, or even a barbecue spatula, because it's much wider, and it has a longer handle to keep your hands from getting scorched.
End saga. Project highly recommended: Fun, cheap, easy, family-friendly, fringe benefits of delicious outdoor-prepared food. The food doesn't taste like woodsmoke, but there is a subtle and delicious difference between, say, pizza baked in the kitchen and pizza baked in a wood-fired oven. The taste is indefinably but unmistakeably different, and the texture of the crust is much better—more delicate. Next stop: Chicken!