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Genesis

Building a cob pizza oven

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I have been thinking about getting a wood fired oven for the backyard for a while now but the nature of my block means I can't install a prebuilt one (plus who has a few grand to drop on an oven that will only be used a handful of times a year?) and I have zero building skills so that rules out a brick oven.

After a bit of searching I came across cob ovens, built with clay, sand and straw. Seems like a good route to take, cheap, easy and effective way to build but I had a few questions and thought a few of you guys would have for sure already had a crack at this.

Any one had any experience with cob building? Tips and tricks?

My soil has ZERO clay and I don't have access to an area with clay heavy soil, is there an alternative? Like can you purchase a clay additive or bags of dried clay powder from a building supply store? Google searches have been zero help.

Also anyone that can offer advice is invite to a pizza party in the very distant future lol

Cheers

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I've never built anything with cob construction, so I can't help much there.

Maybe if you cruise a few websites relating to foundry work and melting metals etc you might get a few ideas on a refractory mix that you can use to line the inside of a cob oven. They have some good mixes that use fireclay (which you can buy powdered) and ordinary portland cement. Look into the refractory mix they use for the furnace, not the crucible.

I think that would possibly be the cheapest route but could have issues with cracking and durability.

I saw an excellent pizza oven that was made from two old beer kegs, the bottom keg was the firebox and the top keg was the oven. It was good for pizzas, legs of lamb and bread etc. Kegs are stainless though, so it's tough on tools. You might be able to get any welding done at a local car exhaust shop for a reasonable price.

Edited by Sallubrious

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Firstly on clay,

Clay deposits in the lows of the soil layers, if you want to find clay you have to dig through the topsoil, which can be deep,shallow, or non existent. The clay may also be deep, shallow, non existent, but that doesn't mean it isn't 20 cm's away or a meter etc. If not go to a river and dig around, clay is every where..

The refractory capability of clays is your next concern, but not really a big concern, the lighter the color of the clay the more refractory, but as you said its not going to be used too often, so build it, use it, look at lining did it check or crack etc

The sand you use, I would go with a coarser sand, river sand is good and adds to refractory you could also add some finer beach sand to the final lining but wash it first.

The straw, I would leave the straw out totally, better to use just sand at 80% clay 20% maybe mould bricks, build over a wooden form, and burn the form out when fully built and dry, or mould your oven again a form but if you want a dome??? Do it all on site.

Reasoning straw is proven to cause more cracking than it is believed to prevent, sand will not dramatically change volume I.e. expand / contract so less cracking.

Inner lining, using same clay + sand + perlite or similar mix up a mortar that when your oven is built dried and the form is burnt out render the inner wall, this will give your strong she'll a softer but higher refractory lining. This is cheaper than tubs of fireclay I.e. the commercial render you use to repair fire places, and will definitely do for a pizza oven.

Refractory is the ability for a material to reflect heat, keep its strength and shape, so it will keep the heat in the oven space.

Pm if you want more details.

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Thanks for that man, heaps of info.

You know anything about buying clay? Transporting clay from the closest river into my backyard isn't really feasible as my block is aggressively stepped with multiple retaining walls meaning even mowing grass is a mammoth under taking. Would be easier so carry cement-esque bags of clay and run a hose down there.

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Aerated concrete is very light and easy to transport and can be easily cut to shape with a cheap handsaw, maybe you could build a domed igloo type structure into one of the terraces and line it with a refractory mix like Dworx suggested. The blocks wouldn't be the main refractory system just a light easy construction base with the refractory lining doing most of the work at the hot face.

I wouldn't go for full size hebel blocks, I'd cut them down so that if they cracked the cracking would be over a small area and be less likely to collapse than a whole block. Being an igloo shape with tapered blocks the shape of the structure should hold everything in place if cracks did develop.

You'd probably do best with a pre-mix commercial refractory mix, but it is not cheap. But you can mix your own if you take care not to get the mix too wet and cure it properly and then fire it the right way a few times before you burn it at full temp for a cook.

Here's a few links to some info about DIY refarctory mixes

http://powerityourself.com/gasifier-build/how-to-make-high-temperature-ceramic-refractory-cement-hearths/

http://www.traditionaloven.com/tutorials/concrete.html

http://www.traditionaloven.com/building/details/refractorymortar.shtml

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could it be any easier or cheaper than this?

 

also look into rocket stoves and using a rocket stove in conjunction with a cob oven

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