How to Make a Cob Oven (and Avoid the Common Pitfalls)

Courtesy of Helen Atkinson in Baltimore
 
Now for something a little different. Something a little fun. Not that Helen’s cob oven is particularly ‘little’. Au contraire, this wonderful project is quite the cob oven piazza:) Ah, there’s so much you can do when you learn the art of cob and earth plaster. Your imagination is the only limit.
 
As soon as Helen joined The Mud Home Facebook group last year, she began sharing her oven progress. What I loved about her posts was how she clearly enjoyed experimenting. This is Helen’s third cob oven now, and each time they get better and better. There's a lot of very good information coming here, no matter how much you've researched cob ovens, so do read it all.

Helen’s cob oven piazza. Imagination is the only limit here.

 Why Three Ovens?
 
“The first one had way too many deep cracks in it,” explained Helen, “So I tore it down a few days after the build and rebuilt. The second one included a four-inch layer of insulation and was great. We had some lovely pizza parties, but when I forgot to put the tarp on and it rained, there was some definite erosion. Now the third iteration has a whole new (flat) floor under it and roof.”
 
Do You Need A Roof?
A roof over a cob oven is pretty much always recommended, unless perhaps you are in the desert. Rain will indeed erode that cob dome. But I love how Helen kept improving her design like this. Cob ovens routinely fail, for many reasons. I know when people experiment and tinker and redesign, they have learned deeply what works and what doesn’t. Not being one to miss an opportunity like that, I asked Helen for all her tips. This oven is very well done.
 
Many thanks to Helen for this very detailed explanation. It will be useful for anyone wanting to give this a go.
 
How Helen Made Her Cob Oven

The cob oven base.

1. First she built the foundation out of stacked prefab stones from the hardware store. “The ones that are used to build round fire pits,” says Helen.
 
2. Next she filled the empty round base (which was almost hip height) with ‘urbanite’ from a roadworks nearby. "We fetched it in wheelbarrows," said Helen. "And then added sand to fill the tiny gaps. After that I put a border around the base made of large and small river rock collected from a park nearby.”

Glass bottles for the insulation layer.

3. It’s advisable to add a decent insulation layer underneath the actual oven. This helps the oven heat up fast (otherwise all the heat is sucked down into the base). A good way to do this is with a layer of recycled glass bottles. Helen embedded the bottles in sand and then about 3-4 inches (10 cm) of cob.

From front to back. The firebrick base (turquoise arrow) with firebrick doorway, and flat stone at the front acting as a lip to drag out the ashes (green arrow).

4. The oven base is formed out of fire bricks. “I put a 3-4 inch layer of cob on top of the insulation layer, so that the outer edges would meet up with the oven sides and be like a chocolate covered peanut. Next I put some sand down in which to arrange the fire bricks, laid the fire bricks carefully in the centre in rows and a large flat stone to stick out the front so I have a lip to drag the ashes out into a bucket,” says Helen.

Firebrick base with first part of the cob oven now on top of the firebricks.

5. The dome of the oven is made from cob. Here’s how she made the cob dome:
i) Helen shaped a sand pile into a round mound on top of the firebrick base (16 inches high).
ii) She covered the sand pile with wet newspaper (makes it easier to remove the sand at the end).
iii) She covered the sand mound in 4 inches of cob. This is the layer that is built to take the heat. Her tip here was to press the cob down and around the sand dome in a spiral (rather than patting it against the sand dome itself, which moves the sand around in the middle.
iv) Once that was completely dry, she carefully dug out the sand, leaving the clean oven floor (firebricks) with a cob dome on it (Sigi Koko has a great YouTube video on how to do this step).

The layer of insulating mud plaster, made with hamster bedding.

6. Next she added a 3-4 inch layer of insulating mud onto her cob dome, that included dry hamster bedding. The hamster bedding is supposed to incinerate and create pockets of air. “A friend gave me an old cast iron stove door, and I built a doorway out of fire brick with a chimney made from an old piece of ceramic pipe, and used a stiff mud plaster to cover it all and hold the chimney in place,” Helen said.

Another layer of mud plaster to cover the whole oven.

7. Now that the oven structure was built, Helen plastered the whole lot with an earthen plaster she made with deer poop added (for the record: you don’t have to plaster the base of the oven, it was an aesthetic touch).

Finishing plaster layer with sculpture beginnings.

8. Next she decorated her new oven in beautiful mosaics and sculptures. Once it had completely dried she coated it an a whopping 7 coats of linseed oil. “I think this is what really saved me,” she said. “Even though I had built the shelter to cover the oven there is enough sideways wind and rain to wash the sides away without the oil.” Take note folks!
 
So there you have it. How to make a very good cob oven that heats up fast and doesn’t collapse in the first rain of the year.
 
Of course I have to ask what will come next. Perhaps an entire cob oven complex with cob benches and picnic tables and… Just joking Helen:)

Such a stunning result.

The Top 3 Reasons Cob Ovens ‘Fail’
As I said, cob ovens do misfire quite often. That’s not really the end of the world if you’d like to just have a go. Ultimately we all learn from our mistakes a lot faster than we do from online commentators like me:) But for the record, here are the most common pitfalls:
 
1. The oven isn’t adequately protected from the weather. You really need it well covered, preferably by a roof. And if you’re in a rainy climate, some linseed oil won’t go amiss either.
 
2. Not enough air flow, so the fire struggles to get going. For the best suction, you want an entry point for air (small flue or tube, or open oven door), and an exit point for smoke (chimney or flue).
 
3. No insulation. This means it takes an age for your cob oven to heat up. If you’re baking pizzas all day this may not be such a drama. But if you want to get cooking fast, then insulate.

Related links:
Cob vs earth plaster, what's the difference?
http://www.themudhome.com/mudbuilding/earthen-plaster-and-cob-whats-the-difference

Introduction to Earth Plaster
 
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