Behold, the finished oven dome! Getting here from here was the slow steady work of many hands. We are grateful to those hands!
Once the foundation was finished we started laying the subfloor insulation. This layer had to be strong enough to support the entire weight of the oven but still contain air pockets to insulate the hearth. It took us several weeks, and the generous help of many local wineries and restaurants, to collect the 800-odd wine bottles we needed. The wine bottles are tightly packed in a mixture of sawdust and a little bit of “clay slip” which helps cement everything together. Clay slip is made by adding the clay/sand building soil mix to a partial bucket of water and mixing vigorously until the slip has the consistency of half-and-half. This insulating mix will also eventually insulate the dome.
Ashley, our intern, has done a great job heading up the mixing of the building soil. Our building soil is a 1:1 mixture of clay from a nearby construction site, and brickies sand. Our mix has changed as we have progressed through the project, going through a series of refinements spear-headed largely by our roommate Aaron, flute-maker and problem-solver extraodinaire. The biggest change was that we started screening the clay part-way through building the dome, and it made for a less clayey, easier to handle, easier to mix building soil.
The subfloor, which is made out of building soil, sits directly on the wine bottles. It is the heat “battery” below the hearth bricks and stores heat from the fire. Once you rake the coals out of the oven and load it with bread, it is the heat stored in this subfloor, and in the dome, that bakes the bread. The thicker it is, the longer the oven will stay hot. We decided on 7 inches, which is really just a wild guess. We arrived there by considering Kiko Denzer‘s rule of thumb that 1″ of mass takes about one hour to fire, and that we wanted an oven that held enough heat for 3-5 loads of bread. Kiko, by the way, has been the quiet guide for most of our decisions.
For the subfloor, we made the building soil a little wetter than Kiko recommends because it was more fun and easier on our bodies to sling handfuls of clay into the form, as if it were a giant brick, than to pack it in with our fists. Consolidating the floor was also fun; we surfed on pieces of plywood, shifting our weight to move the clay like a wave beneath our feet. Like water, the clay flowed to fill the low spaces, making leveling a cinch.
Next, we laid the firebrick, first as a mock-up, but then for real, on a carefully screed bed of brickies sand. We confess we were a bit doubtful that the sand would hold the bricks in place, but it worked beautifully. It was a thrill to see the oven hearth for the first time, and imagine little breads baking on it!
We insulated the subfloor with more wine and beer bottles–this is were we used all the non-standard size bottles that we had acquired. We mocked up the door and chimney vent so that we could better visualize how that would work before building the door frame out of red brick mortared with clay slip and an giant overkill angle iron. At this point the chimney is still on the worry-about-it-later list.
Now, finally, we were ready to build the dome. We hung a massive chain from the barn wall to create a cardboard catenary curve template for the oven dome. We want a low dome so that the inside of the oven stays small. This is so that steam from the wet dough hitting the hot hearth will fill the oven chamber and create beautiful crust. But because the oven is wide, there is a danger that the arc is too flat to support the weight of the dome.
In the photo, Ben is cutting the line traced by the chain on the cardboard. We placed the cardboard template upright on the firebrick near the back of the hearth and piled sand around it. The sand dome becomes the form that holds the permanent dome of building soil. After the building soil dries, we will remove the sand and the dome will do its part by not falling down.
It took nearly a yard of sand to fill the oven void. We covered the sand dome with plastic, which is plentiful on the farm, and the party started! Despite our grass-roots leanings, we opted for a top-down rather than a bottom-up dome building technique. We knew we couldn’t build the whole dome in a day, even with help, so we needed to build up layers in a way that would not weaken the dome. We had read about what we quickly dubbed the “pancake” technique in Alan Watt’s book and reasoned that even if the thin, concentric dome layers dried a little in-between, the layers could result in a laminated dome that might make the whole thing stronger, sort of like how plywood is stronger than the same thickness of wood. We have no idea, really, but it sounded good to us, so we went for it.
The technique has the additional advantage of absorbing many hands. Once the entire dome was covered with a single layer of pancakes, we whacked it with boards, slapped it with our hands, rolled it with a rolling pin (no joke, it works great!), smoothed it with a spackle knife, anything we could think of to consolidate all those pancakes into a single mass that moved as a whole. With the help of our friends, we finished three layers at the party.
Thank you to everyone who came out! At the end of the day, we relaxed with extraordinary food and drinks brought by Kevin and Sherri at the Hopkins Ordinary and our friend Conny. We also fired up our little prototype oven for pizza for the first time. The beautiful arc of fire on the inside of the dome was itself cause for celebration.