How I built my wood-fired cob oven

Sure, let’s just get a bunch of clay and sand, maybe some straw, and build a wood-fired cob oven! I’m not afraid of a little hard work, the YouTube videos make it look like a snap. We’ll be baking by the weekend! And pizzas!

Not that simple. In this case, there was a ton of manual labour, delays with building supplies, schedules and agonizing decisions.

Mildrith, the oven, is barely two months old as of this writing. She has produced multiple loads of bread, pita and rolls, pizza, granola and cookies. It is like a miracle to pull away the door seal and see ten brown loaves sitting on the hearth. The smell is so satisfying, knowing those loaves have baked in radiant heat coming from the dense thermal cob walls. And it feels as if I have barely tapped her potential.

Many have asked me to share about how I built this cob oven and learned to use it. Lots dream of having a pizza/bread oven, but few get around to building one. My motivation was to build the Happy Monk Baking Company around this oven and sell bread at the Pender Island Farmers Market. I may or may not launch a bread subscription service on the Island, as well. This will be a small-scale production for Pender Island people, population 2,200. We won’t be slamming out hundreds of loaves daily in this operation.

Still, it had to be larger than your average backyard pizza oven to bake loaves in quantity. It also had to be fire-safe and heat-efficient. It’s got a 40-inch diameter hearth floor, six inches of thermal cob, and another six inches of clay/straw insulation. There is a further layer of Ruxol house insulation and, the coup de grâce, a plaster layer of cob and Pender Island horse manure. Yes, horse poop!

It retains heat pretty well, and even in some recent sub-zero weather, it was still warm days after a firing. I’ve taken it up to 1,000ºF for pizza and I couldn’t feel any warmth on the outside top of dome with my bare hand. I’m still learning about Mildrith’s idiosyncracies, but she has performed well, so far!

I attended two oven-building workshops and each time came away thinking it would be easy and fun. After digging out a slope on our property, moving some big rocks and pulling out some trees, I realized I was in over my head. The site needed a retaining wall. I wanted a dry-stacked rock oven base. Where was I to find the rocks and how was I to build it? And the more I pondered, building the oven out of cob would be harder than I imagined. I needed to hire someone. I needed a budget.

So it cost more than planned but along the way, I made friends with some great people, I learned a LOT, the neighbours are excited about Mildrith, the Happy Monk Baking Company and the bread that is emerging. Building the oven was a journey, like so many things. There is more to it than the final destination.

Background Reading

Kiko Denzer’s Build Your Own Earth Oven, full of pictures, illustrations and simple instruction, made the idea of building the oven seem easy. But a book can only take you so far. It’s a classic and a must-read, but once you get your hands in the mud, you’re in new territory. Especially me. I live in the “country”, but I grew up in the big city. Building stuff is a foreign concept to me

Richard Miscovich’s From the Wood-Fired Oven was also helpful. It’s more about making bread and other stuff, once you’ve built the oven. Miscovich isn’t a huge fan of cob for wood-fired ovens because its heat retention is not as good as firebrick. Perhaps he’s got a point. I’m interested in the low environmental impact of natural materials, such as cob, not to mention the sheer pleasure of pulling building material out of the very ground beneath my feet. The big value of this book for me was in using the full heat cycle of a fired oven — the highest heat for pizza and flat breads, then the declining heat for stews, soups, breads, other baked goods … even drying firewood for the next firing.

Another reference was The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans, Michael G. Smith and Linda Smiley. This provided a philosophical context for the oven project — the pleasures and practical benefits of using natural building materials. It was also useful for information on cob mixes, plasters and ways of applying them. It is aimed at those embarking on larger, more complex projects, such as houses and cottage.

Natural and local

I set my mind on cob as my primary building material because I wanted something as natural and home-grown as possible. People told me clay was everywhere on Pender Island, and indeed, as we prepared the site for the oven, there it was, right at our feet. It wasn’t quite right for the job, unfortunately, but Ron Henshaw, a neighbour up the road, had a large deposit of clay in one of his farm fields. He was happy to let us dig some of it out, which was at the edge of a pond. Fifteen five-gallon bucketfuls were not quite enough, but there was lots more available.

We found the rocks for the dry-stack oven base on North Pender Island. The contractor I hired to build the oven base and retaining wall bartered for the rocks for “future favours” for the owner of the property they came from. The contractor’s name was Peter Hughes of Oceanside Masonry, a proud stone mason who’s worked many years on the island and raised his family here. The concrete blocks for the retaining wall were a budget decision. We got them from a building supply store in Sidney, B.C. and they fit together easily without using any mortar. I would have preferred local rocks for the wall, but it would have doubled the cost for this part of the project.

I had also planned a concrete slab as the floor for the oven shelter, thinking it would be a good fire prevention measure. In bread production, all the coals and ash are removed from the oven once it reaches baking temperature. If a cinder fell from the shovel during this process, it would be unlikely to ignite on a clean concrete surface and cause fire. A concrete slab would have been costly, but I was not in favour of putting more cement into the earth. There was already plenty of concrete in the house foundations and basement. We went instead with pea gravel. It is still fire preventative, and I hope eventually to tamp it down with fine sand to make it more stable.

The value of cob oven workshops

At one of the workshops I attended in Metchosin, B.C., the participants built a large part of a big cob oven for the Victoria baker, Byron Fry, of Fry’s Bakery, in Victoria West. One of the big takeaways from that workshop for me was the steel door assembly Byron had had made by a Metchosin welder and blacksmith, Ryan Fogarty. It was designed by Byron and his friend Bryce Ehrecke of Cré Natural Building, a talented craftsman and natural builder. Byron and Bryce were really helpful to me after the workshop, with great advice and follow-through.

Incidentally, From the Wood-Fired Oven (see above) has a one-page profile Byron Fry, who, Richard Miscovich says, is a “creative young baker providing the all-important field testing of the next generation of bread ovens,” and who “lives on the filigreed coast of British Columbia, one of the world’s wood-fire oven hot-spots, like Vermont and the mountains of North Carolina.”

The oven door

The door assembly created a beautiful entry point to the oven dome, with its gentle arch and two steel doors that latch together. There was a flange just inside the doors to position a secondary heat seal for the baking chamber. It is a pleasing, functional “user interface” for the baker that aids the building the fire and retaining heat inside the oven, and includes two adjustable ventilation holes for air intake. The assembly also has a chimney flange, a nice design feature that positions the chimney away from the dome, allowing unfettered circulation of the interior heat circulation.

It also looked skookum! I decided to splurge on this door assembly for my own oven, and I’m glad I did. Ryan Fogarty was a real pleasure to deal with and we had some great conversations over the phone, sharing stories and laughs. I didn’t actually meet Ryan until Jennifer and I went to pick up the door assembly at The Forge, in Metchosin, where he works.

(Ryan works with a guy called Jake James, a British born blacksmith. His trade is an ancient, honoured profession that still thrives in this technological age. Jake is also a great visual artist and teacher. You should definitely check out his website if not for the beautiful photography, but the poetic descriptions of his craft.)

the natural builder

And then there was Jacques! Pender Island has several “natural builders,” carpenters/framers who use environmentally sound building materials for their projects. Tracey Calvert, another neigbour, is a reknowned natural builder who led the first cob oven building workshop I attended, four years earlier. She is out of the business, now, but she recommended Jacques Marmen, a young builder, to make the shelter and workspace I had in mind. Tracey was also an invaluble sounding board for some of my ideas and came to the site a few times for problem-solving sessions.

Jennifer and I invited Jacques to the house to chat about the project. It was a beautiful summer evening and we sat out front, sharing a bottle of wine and some homemade soup, watching the sun set, the gulls and ravens over the water, and the San Juan Islands in the fading light. Jacques has worked years in the roofing and timber framing business. He comes from just outside St. John, New Brunswick, where I used to visit my grandparents on summer holidays. His mother language is French, but you wouldn’t know this speaking with him. He has hitch-hiked back and forth across Canada multiple times and is the self-described “black sheep” of his family.

I wanted Jacques to build the shelter with Pender Island wood and rock with a simple metal roof to keep the oven and me out of the rain when I was baking in the middle of the night. Jacques was happy to take on the job. He had one question: “Are you comfortable building the cob oven yourself?”

“Yes I am,” I said, putting on a brave face. How difficult could it really be? I’d been to two wood-fired cob oven workshops! It’s easy!

Jacques did not reply. He’d taken the measure of me and decided to play the waiting game.

the worksite

The shelter went up quickly, though there were some difficulties setting the foundations for the posts. Jacques bounded around the site, hoisting logs and rocks with brute strength and showing great patience with me and my tender back. Jacques filled the site with his music playlists, which ranged from broody to buoyant depending on his mood. He introduced me to the majestic Majical Cloudz, a Montreal-based band that is now, alas, defunct!

Jacques got the logs for the shelter posts and beams from a wood lot on North Pender, called Winter Creek Farm. It’s operated by Al Wilmott, a great bear of a man, and his wife, Julie Wilmott. Later, Al delivered a truckload of milled cedar boards for the roof rafters. Al has also sold us a few cords of firewood over the years.

I had already picked up a load of firebricks from Slegg’s Lumber in Sidney for the oven hearth. I also bought two large bales of straw from White House Stables, a farm feed store in Saanich, Vancouver Island. Two yards of stucco sand — for mixing with the clay — were delivered by Barney Simpson, a Pender Island delivery trucker. We were getting closer to the real work!

stomp, stomp, stomp!

I was getting nervous about the next stage — mixing the cob, laying the hearth and building the oven dome. I lack confidence in “handyman” projects, and was inspired by Jacques’ work ethic and knowledge of natural building. He was pleased I asked him if he would continue with the oven build. He wasn’t in the least bit smug that I wanted him involved after all, despite my ridiculous claim that I could build the oven myself.

There was some preparation of the oven site before the major work began. We needed to install a drainage ditch to prevent rain water run-off from the slope compromising the oven structure. Jacques assigned that to me while he started prepping the oven base for the hearth. He smoothed a thick layer of stucco sand on top of the base, then a layer of wine and liquor bottles (all of which had been carefully emptied by me) for insulation. That was compacted with more sand, followed by a layer of thermal cob.

Welcome to the world of mixing cob! One part Pender Island clay, two parts stucco sand … throw it on a tarp (in our case, discarded lumber wrap from from the local hardware), add some water and begin mixing. Get those boots on (or, in warm weather, take off your socks and shoes) and get stomping. Stomp, stomp, stomp! Pull the tarp corners back to further combine the ingredients, then stomp, stomp, stomp again. Maybe add a little more water. Keep going until it’s just right! For the duration of the project, we were mixing cob at least 50 percent of the time. Clay and sand and sand and clay, stomp, stomp, stomp.

When is the cob “ready?” It’s kinda like determining when is bread dough ready to go in the oven. You want enough structure, but you also want it to be moist and pliable. It takes experience to know when it’s ready. Jacques knew when it was ready. “I make the best cob on the block,” he said.

the oven takes shape

Our next task was laying the hearth bricks, making sure they were flat and level. It was a painstaking process, but important to get it right. You don’t want bread or pizza peels bumping in to unevenly laid fire bricks.

We then built a stucco sand form in the shape of what would become the oven dome. The dome was covered with paper (I used cut up flour and grain sacks, but it might have been better to use newspaper). On top of the paper went the first layer of thermal cob. It was hard to determine how much cob to make, and near the end of applying this layer, I went to make another batch while Jacques worked away.

We may have spent over a week completing the first several layers of the oven. For those interested, here is a relatively complete list of the steps, including those of the hearth and final finish:

Hearth

• Sand layer applied to top of base
• Layer of glass bottles laid for insulation (see the Kiko Denzer book for this idea)
• Buried and packed sand over the bottles and base leveled
• Layer of thermal cob applied
• Sand layer applied and leveled
• Outer perimeter ring of cobapplied
• Fire bricks laid flat inside ring
• Filledperimeter ring with more thermal cob, level with fire bricks
• Placed oven door assembly at the oven entry
• Laid a half-circle of fire bricks at the back of hearth area to prevent fire pokers and bread peels from damaging the cob in the back of the oven

Dome

• Sand shaped and smoothed to form inside of dome. We used a cardboard ring to support the sand at the base
• Paper layer applied over sand dome (in our case, used 20kg flour and grain bags) to separate cob from sand)
• Six inch layer of termal cob applied over sand and paper
• Ruxol house insulation covered thermal cob and contained with burlap
• Burlap painted with a thin layer of clay slip (red potter’s clay, in our case)
• Six inch layer of insulating cob (thermal cob with the addition of straw for strength and insulation)

Finishing

• Drying of cob. At least two weeks in our case.
• We dug the sand out of the oven to reveal the oven “void.” (a big moment, as the interior oven is revealed!)
• The chimey built up through the metal roof, attachment of the damper and spark arrestor at the top
• Thin plaster layer of cob mixed with horse manure.
• Several firings inside the oven to increasingly hotter temperatures to condition and further dry the cob.
• Construct a secondary oven door to provide a further heat seal inside the door assembly
• Make pizza and bread!
• Have a celebration!

standing back from it all

Jacques and I would stand back, at the end of the day, and admire the progress — the gradual evolution of the oven shape, plans for the next day, how great it was looking, or what gender the oven might be, if any. We agreed it was definitely female, but don’t ask how we knew. We just agreed. (Find out how we eventually named her Mildrith, here). I was also concerned about what the island’s fire department would say about the oven. There is no building code that applies to outdoor fireplaces, but there are rules about open fires during the summer. Mildrith is a closed oven with a spark arrestor, so I don’t expect problems from the fire department. I’m more concerned about our neighbours worrying about the possibility of fire.

final steps

After applying the last layer of thermal cob, I thought we were almost done. We were, but there was a long period of waiting for the cob to dry. In summer, drying would be a matter of days, but in the late fall, it was more like weeks. This coincided with Jacques having to leave the project to start another job, though he came back on weekends to help with the final stages.

There were only a few things left to do. The most exciting was the removal of the sand from inside the oven to reveal the interior dome. That day, Jacques had brought his friend, Claire, along and we were all quite giddy, pulling the sand out into buckets. It came easily and we could soon see the layer of paper that had been applied to sand form. Wow! It was a real oven, now!

Horse poop!

Earlier the same day, Jennifer and I had driven up island to the house of our friends, Mae and Lester, who keep a horse on their property. They gave us four bucketfuls of fresh horse manure for the plaster layer of the oven. “How about four loaves of bread for four buckets of poop,” I said to Lester, shaking his hand. Lester had a huge grin. “Sounds good to me,” he said. Maybe he hadn’t expected such good fortune, but he didn’t refuse the offer.

The plaster layer is applied to protect the cob from the elements while allowing it to breathe. Plaster is finer than cob, though it is made of the same materials (clay, sand and water), with the addition of manure. Tracey Calvert had recommended cow manure over horse manure because it is finer (with their many stomachs) and more durable when it dries. Horse manure was easier to source on Pender, however, and we thought it would work fine in our rainforest climate, here in British Columbia.

Jacques mixed some of the last cob we would need, while Claire and I sorted through the manure to get the freshest poop we could find. Live enzymes make livelier plaster, while in older stuff, the bacteria will have already digested the fibre, making it less effective protecting the cob. Claire and I reflected on how our life paths had ever brought us to sorting through clumps of horse poop to make plaster. It was no problem, in case you’re wondering. We smelled a faint, sweet, milky aroma during this task, and there was almost no smell once the plaster was applied to the oven

I ran a space heater inside the oven for much of the next week, drying the clay from the inside. I could see the drying progress peering into the dome with a flashlight. The darker areas were moist, the lighter areas were dry. I was able to pull most of the paper off the inside walls. The paper that stuck would eventually be burned away when the first fires were lit. I set several fires over that next week, taking the temperature to hotter levels each time. I measured surface temperature on the interior walls with an infra-red temperature gun.

the heat plug

Before I could bake bread, I needed a secondary door to fit inside the door assembly pushed right up against the flanges that Ryan had welded to the back. Before the bread dough goes in, the ashes and coals are swept out, the floor is mopped clean, then the oven is sealed for an hour or so for the temperature to equalize (so there are no hot or cool spots in the hearth). The oven is also sealed while the bread bakes. Without this secondary door, there would be too much heat loss, and the bread may not bake properly.

Any wood-fired bread oven needs one of these doors … let’s call it a heat plug. In my case, the heat plug is a secondary door, because there is already a set of doors on the front of the door assembly I had made. Without the outer doors on my oven, the heat plug would the primary way of sealing the oven to conserve the heat you’ve so diligently built up inside, storing all that radiant heat in the thermal cob layer. If your heat plug is made of wood — and most are — it eventually becomes burned and degraded from the relatively high oven temperatures. It will have to be replaced, eventually. That did not interest me at all, building a new heat plug every few months or weeks.

Having a custom-built oven door assembly, with its own set of dimensions, meant I had to have a custom-built heat plug. Jacques and I discussed a number of different ideas, including the possibility of using a Pender Island-grown version of a Ironwood, originally imported from the Brazilian rainforest. The Ironwood would have extended the lifespan of the heat plug, but would eventually have to be replaced, anyway.

Jacques’ solution was ideal. He made a drawing with the exact dimensions of the door assembly and created a wooden template for the heat plug. He gave the template to Pender Island stainless steel fabricator, Callum McKay, who made a steel backing for the plug that would face in towards the oven. Jacques then built a Pender Island fir-wood backing, with two handles to push the plug in and pull it out. The wood itself was charred with a blow-torch, which increases heat resistance and minimizes possible degradation and eventual replacement. The charred fir was then screwed to the stainless steel face, with a 2.5-inch thickness of Ruxol house insulation inserted inside.

The heat plug weighs close to 25 pounds, but it sure does the trick — it seals the oven and keeps it at temperature for the duration of the bread bake. I have yet to try a second consecutive bake without re-firing, but I can report that the next day, the ambient temperature inside the oven is still above 300ºF, if the bread was baked around 500ºF the day before. I’ll update this post when I’ve tried a second consecutive bake.

perfecting

In fact, Jennifer and I made pizza in the oven before any bread was attempted. It was easy to bring the wall temperatures up over 1,000ºF and maintain heat just below to bake a pizza in less than two minutes. That’s not to say I’ve perfected pizza. I have a bit of experimenting to do before I settle on a perfect dough recipe and a few other factors.

There is more to learn about Mildrith than making the perfect pizza. I’ve had a bit of success, so far, but I know this oven is pretty close to the one I was looking for when I first took a shovel to the earth and rock just outside our Pender Island home. It has also introduced me to some remarkable people in the Pender Island community, and in that respect, the project is more than I could have expected.

Stay tuned!