Clay Oven Construction and Deconstruction, Part 3

It is time to close the books on our first wood-fired oven, which we built ourselves last spring. The oven lived a happy life — it baked about 2000 loaves of bread for us in 2014 — but it has baked its last loaf! Rather than letting our first oven go quietly into the night, however, we want to share what we learned by examining some of the problems we experienced. You may want to check out part one and part two of our oven building series for more explanation and photos of the process.

Ben mixing insulation

Building up insulation

After completing the clay dome in May 2014, we insulated the dome with a mix of watered down clay combined with sawdust that we got by the trailer-load from a nearby sawmill. Ideally, after the oven has been fired repeatedly, the sawdust near the dome partially burns out, leaving a “clay foam” in the words of Kiko Denzer. We believe that we may not have used enough clay in our insulation mix, however, because we experienced smoldering fires in the insulation layer for weeks, and eventually had to redo much of the insulation.

 

Starship sourdough!

Note in the picture that the insulation at the top of the dome (from the final day’s work) seems to have had the correct proportion of clay and sawdust — it didn’t burn out! We replaced all of the insulation that we could reach, but we could not get under the hearth to the subfloor insulation, which also ignited. The smoke found its way up through cracks in the hearth and the oven smoked eerily for about a week as the subfloor insulation smoldered. It stressed us out quite a bit in early July as we waited to see what would happen. Would the dome collapse as the subfloor insulation turned to ash? It did not, and we continued to use the oven until Thanksgiving, when we roasted our turkey and other dishes in the dome. But by then we knew the oven was doomed.

Oven layers starting to delaminate

Every week through the summer, more and more small chunks of clay were dropping from the ceiling of the dome onto the hearth. The dome was built in layers, and it was essentially delaminating, peeling back from the door to the highest point of the dome. But don’t be discouraged if you’re dreaming of building your own clay oven. We believe that we could build a better oven if we did it again, and we want to share some of our “lessons learned,” even though we have chosen to hire professional masons to build us a new brick oven (which we’ll be posting about later this week). The professionals can build so much quicker and better than we ever could, which frees us up to do what we do professionally: bake bread and grow produce.

Unmortared foundation wall bowing outward

The insulation had separated from the subfloor

So, things we would do differently: (1) We would not dry-stack concrete block for the foundation. We did see a working clay oven that used dry-stacked block as its foundation, but it was about half the size of ours. That smaller oven performed fine, but there are intense forces at work with such an extreme heating and cooling cycle. We think the materials’ expansion when heated, plus the weight of the dome, caused the dry-stacked walls of the foundation to spread out slightly, thus causing much of the cracking we saw on the inside of the dome. The gradual spreading of the dome could perhaps have been halted by a rigid support or buttress, such as an I-beam running down each side, bolted together in the front and back of the oven.  We also should have built the subfloor wide enough to fully support the downward pressure of the dome walls.  Amazingly, the glass wine bottles held up under all that pressure!

Rather than one catenary curve, we could have used several

(2) We used one catenary curve to shape the sand form when building the clay dome, but we should have used several. In the picture, you can see the cardboard cutout of the catenary curve that represented the highest point of the dome. This part of the dome had no cracks in it, and in fact, some of the cracks veered away from it! But the lower part of the dome, near the door, was flatter than a catenary curve, and thus was not as strong. If we built such a large dome over again, we would make several cardboard cutouts, descending in size, to guide the sand form from the highest point of the dome down to the door.

clay ‘pancakes’ did not consistently meld to form a single mass

And finally, (3) Our “pancake” method of building the clay dome did not work as well as we had hoped. During the demolition of the old oven in January 2015, we could see that many of the pancake layers never bonded, which caused the dome to delaminate.  You can see how easily the dome comes apart in this video.

Ideally, the dome would have been built all at once, but this was such a large dome that it was built over the course of three days, and some of the layers partially dried before new ones could be added. And so, the dome did not behave as a single, solid block of clay, but had internal weaknesses that accelerated the cracking.

The wine bottles holding up the oven were still intact!

 

One thing we were happy to see upon deconstruction is that the wine bottles that we used to insulate the subfloor were all intact.

So if you are thinking about building a clay oven for yourself, we would be happy to talk with you to share even more technical details and tips and know-how. We also want to give a big thanks to everyone in the brick oven Yahoo group, and Kiko Denzer in particular. Without the advice of the helpful builders there, our oven would never have lasted as long as it did.

Building a wood-fired oven, part 2

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.

dancing the sand into the clay

flipping the mix

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.

The first layer of clay

There she is, slinging mud again!

The finished subfloor

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!

Phew, it fits!

Ashley and Heather, ready to lay some bricks

A classy joint

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.

The hearth and more insulation

Don’t worry, the door will be taller than this!

[short] door & chimney vent mockup

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.

 

Our hi-tech architectural template

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.

Filling in around the template

So much sand!

making the sand form beautiful

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.

clay pancakes with a 7″ story stick

Love the handprints!

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.

The first pizza!

Winter is vacation time on a farm, right?

One might think winter is vacation time on the farm. Not much grows at arctic temperatures, the soil is frozen, and most farmers’ markets are on break. But as we’re discovering, farming is still a full-time job in the winter-time! Here’s a peek at what we’ve been up to.

We have hired a full-time intern, Ashley Wright, who will start the first week of April.  In May, she will be joined by a full-time volunteer Brenda Fisher.  We are excited to welcome these two gals to our farm!

Yes, its winter, but we are trying out growing lettuce in the high tunnel, which is a passive structure that collects heat from the sun during the day and protects the plants from icy nighttime temperatures.  Winter lettuce leaves are thick, because its growing so slowly, and their sugars are concentrated to help them survive freezing.  This is especially exciting because this is the first crop we have harvested that we planted ourselves!

Ben and Heather with their first harvest

Pretty little lettuces

We have assembled most of the materials for our wood-fired oven: firebricks for the hearth, red brick for the door, concrete block and gravel for the foundation, masonry sand, clay, sawdust, wood chips and, yes, plenty of beer bottles for hearth insulation.

The first block!

red brick for the door, and firebrick for the hearth

our clay from a nearby construction site

masonry sand, and lots of it

gravel, for the foundation

wood chips for the insulation, from a local arborist

We are nearly done with the oven foundation!  We’ve laid concrete block and have been filling it with a mix of rock that has been pulled from our fields where it likely landed in the 1969 mudslide, and gravel. The arctic weather delayed this work somewhat, but we still feel on-track for having the oven finished by April markets.

We are also building a smaller clay oven for the house, and to gain experience working with our materials.

the first layer of subfloor insulation: beer bottles in a mix of clay slip, sawdust and wood chips

We bought a tractor!  Its a 1982 Ford, 40 HP, with a loader, which has already been put to good use moving heavy stuff for the oven around!

our new baby

We have planned our CSA and placed our seed order! This involves zillions of decisions–which crops? which varieties? how much space does it need? how much space do we have? how many transplants? how many plantings? when do we plant them so that we have vegetables when we want them? Yes, we have become best friends with Excel.

We are selling bread and vegetables in Charlottesville and Lexington.

raisin rye, cooling by the window

Our CSA is about 60% sold out!  Thank you to everyone who has signed up so far!

We are planning Thorny-O, an oldtime music and dance festival that we are organizing and hosting on the Valentine’s Day weekend.  We’re looking forward to welcoming new and old friends to our lovely farm!

Whew, I think that’s it.

Building a wood-fired oven, part 1

Here, finally, is the first oven-building update.  We have some plans, we have some clay, and we have a hole in the ground.

The plans aren’t perfect–the hearth, for example, needs to be level with the top of the foundation–but I feel comfortable that we’ve thought as much as we can about the design.  Our plans are the result of oven visits, lots of reading, consulting with the brick-oven yahoo group, and conversations with Felix Addison and Kiko Denzer.  And I expect they will continue to change as we start the build!  If you would like to have a closer look at our plans, contact us and we’ll send you a copy.  And we welcome any feedback you may have!

Front elevation through highest part of the dome (top) and subfloor plan (bottom)

Floor plan

Front and side views of the door

We are going with a 4.5′ by 5.5′ egg-shaped hearth and a seven inch thick clay dome in the Quebecois style.  I like this style because the entire oven interior is rounded, which should help prevent eddies (read: cold spots).  Also, the highest, widest point of the oven is in the back, with the dome sloping down- and inward towards the door.  This will (hopefully) help support the dome at its widest point and encourage the heat to circulate fully through the oven before it exits the chimney, which is right in front of the door, making the oven more efficient. The oven dome will be insulated with twelve inches of sawdust held together with clay slip, while the firebrick hearth and clay subfloor will be insulated with a foot of beer bottles–of which we have a lot–embedded in the same clay-sawdust mixture.

The clay is a grey marine clay that contains quite a lot of white silt.  Ben found the clay at a nearby building site, where it is being excavated for a foundation.  We’ve tested the clay on some bricks, and a 1:1 mixture of clay and masonry sand seems to give us the best compromise between strength and shrinkage.  The next step is to use it to try to build a quick little oven like the ones Kiko describes in his book, also known as our oven bible.

bricks made with different clay:sand ratio s

Our grey clay turned alien green by the light coming through the tarp

So with plans in hand we have started the foundation.  We finished excavating and tamping down the 18-inch-deep hole in the ground on Christmas Eve.  Most of the rocks in the top 8 inches or so.  Its tempting to speculate that they came down the mountain with the 1969 mudslide that covered the property.