Cob oven
Dale Copeland - Assemblage artist from Puniho in New Zealand.

email me if you wish. At dale @ tart.co.nz (remove spaces)

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Friday,23th May, 2008.
I've been having such fun! Building a cob oven. Well, starting it anyway.
In case anyone else wants to have a go, and is putting together other people's experiences (successes or disasters) here's a blow-by-blow picture diary.
You can build your oven at ground level, or make a strong! sort of cave/table and use the underneath of it to store wood. But we had this concrete block (the base of the copper in an old washouse) so I'm using that.
I found enough old firebricks to make a flat base, laid it out with a brick retaining wall, then put down a layer of sand to support the firebricks.
(Mortar for sticking bricks together is about 1 part of cement to 4 parts of sand, mixed with water to make a porridge-like consistency.)
The firebricks aren't mortared down, just "kissed" into place onto the layer of sand. Making the top as flat as possible, for sliding those pizzas on.
But I had to put a ridge of mortar to support that sticking-out lip at the front.
The whole thing is filled with firebricks, and then I laid out a surround of ordinary bricks. (To rest your glass on while you wait for pizzas?)
The string is to give me an idea of the inside size of the oven.
Outside bricks mortared in place, and I started building the dome of wet sand. Very quickly realised that stacking bricks would be easier than carrying bucketfuls of sand and water, so made a little mountain inside.
Then thought that I might regret it when it comes to scooping out the mountain of sand and bricks through the little door, so put newspapers between layers of bricks, hoping that will make it easier to scoop them out.
(The mountain is too high, and soon lost its top brick)
And this is it so far. A mountain of sand (and bricks). The books said to cover with it plastic or wet newspaper to keep the sand moist so it doesn't just collapse. It's been good drying weather here, sunny and windy, and I knew it would be a few days before I could get to the clay part of the process, so I covered it with a thin layer of weak papier mache ... strips of newspaper dipped in water with a little wallpaper paste mixed in.
Once the clay is dry and I scoop out the sand, even if the paper doesn't come loose from the clay it should burn off in the first fire.

Sunday, 25th May, 2008.
For each of the last two days I've stomped a 20kg bag of clay together with 3 buckets of black beach sand.
All the books say to get a crowd of people to help and put on loud techno music to 'dance' to. A crowd would have been good, but techno music? Some sacrifices are too much. So I did it myself, to the sound of twittering birds.
You mix in the sand until a sausage shape will just bend a little and then break. If it bends right around your finger, you need more sand. And if it won't even roll without crumbling, it needs more clay.
Some people can dig their own clay, sometimes it's even sandy too. Experiment. I got the sand from our river but had to buy a whole lot of potters' clay.
That 40kg of clay wasn't enough to completely cover it, so I cut out the doorway and used that. I fear it still isn't thick enough for the first layer so I'll do another stomp tomorrow.

Wednesday, 28th May, 2008.
The first layer (about 30:70 clay:sand) is finally a more-or-less uniform thickness of about 7cm. Now for the next layer - about 70:30 clay:sand plus as much straw-like stuff as it will take. I say "straw-like" as I couldn't find any straw anywhere. There's been a most unusual long dry summer and farmers have had to feed their cows with everything they could find, including straw. I tried odd things like dog hair combings (failed miserably) then realised that the thick layer of dry pine needles lying under our trees would be just right.
You stomp the clay on a tarpaulin (I used the old trampoline cover) so you can lift the edge and roll the clay over. Any help with the stomping is greatly appreciated. This layer acts as the insulator (the first one, with all the sand, acts as the heat mass to get hot and radiate the heat back into the oven). This insulating layer should be at least 10cm thick. That framework is a rough support for a plastic covering in case of rain.


Wednesday, 4th June, 2008.
Back from Australia, back to work. Emptied the pile of sand out from inside the oven (getting the bricks out was difficult - not one of my best ideas). The clay is leather-like now. Used a kitchen knife to make the doorway a snug fit, and put handles onto the door. Now to wait while it dries. This is not a good time of year for that, so I take the occasional tin of hot coals from the woodburner and sit it inside the oven.

I've covered the door with scraps of old copper, and the whole thing is SLOWLY drying.
The columns of bricks are to hold up a temporary roof. Eventually I'll put a render over the whole surface ... a mix of cow manure & clay is recommended. I might cover it all with copper coins too. Money & muck - the thought appeals.

Thursday, 17th July, 2008.
After a couple more sessions of making pizzas for 7 people, I'm really keen on this as way of entertaining. Everyone ends up standing beside the cob oven, whether there are flames or not. There's something very fundamental about gathering around the cooking fire.
I filled the cracks with more clay/sand mix and started laying bricks. To make it easier to slide out the pizzas directly onto a brick shelf. And I'll make a brick warming enclosure too. For letting the pizza or bread dough rise.

Using recycled bricks, from the old chimney that I took out of this house a few years ago. I'm filling the middle with rubble and broken bricks and a lot of the old mortar that comes off the bricks when I clean them.

Sunday, 27th July, 2008.
The cob oven survived last night's big storm (I'm glad I've finished with that temporary roof - it would have blown to Australia in those winds!). Some of the coins came off as I brushed the final layer. Guess I should put a wishing well alongside .... the coins were probably one of my daftest ideas: they look good but will probably all fling off when I next fire it.

Anyway, the warming oven (for pizza or bread dough to rise in as the fire is heating up) is finished apart from a door, and I extended the brick construction to make a small barbecue. All great fun.

Equipment
Tarpaulin for stomping the mix on (& for rolling the mix over on top of itself)
Bricks for the oven floor - at least 22
I used 11 20kg bags of clay and about 120kg of sand. You might be lucky enough to have a sandy clay soil.
A bale of straw or similar (I used 8 big boxes of pine needles).
And a lot of time. A couple of weeks to make it, another couple waiting for it to dry. Less in summer.


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