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Best material to use for oven

 
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Hello, Would it be possible to use old roof shingles to form an arch or keep the heat in an outdoor oven please? They're over a hundred years old, yellow and cream in colour. I live in Catalonia nd they're all over the place here.Thought perhaps I could stack them up top form the sides of the dome, and stick them together in place with local clay and builders' sand, they're the curved ones.
People use them here to cook cebollas in, so guessed they could possibly be suitable.
I also found old terracotta tiles and bricks, the bricks are yellow too. Thought the tiles might be suitable for the floor of the pizza oven instead of fire bricks I can't afford.
Could I test them by sticking them in somebody's log burner first or is that a dangerous idea?
I'd prefer to build the oven with what's lying around in the garden,there's loads of building rubble here. The new bricks I'm finding are obvioulsy useless they have holes in and shatter easy.
I have masses of rocks, but have read that they could explode.
I'm quite keen not to explode myself whilst making the oven.  Thank you.
Staff note (thomas rubino):

Hi Rachel, I hope you don't mind but I moved your post to the rocket oven forum.

 
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Hi Rachel;  Welcome to Permies!

I'm not totally sure about your roof shingles , but as an oven  roof, set with sand and clay. I think they will work fine for you.
The terracotta brick and tile as a floor, could crack out with the high heat in an oven.  But might last a good while before they do.

Stones are not safe, they can explode.  All cob for your oven bottom might work out ok as well.

Use what you have, if they fail over time you will have a better idea what material to use for the next one.
 
Rachel McGill
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Thanks for moving my post Thomas and Hello.
Thank you for the advice too, greatly appreciated.
What is "all cob" though please?
 
thomas rubino
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Hi Rachel;   All cob is just that.  Clay and sand and some wood ash. Mix it with water , all into thick goop.  Apply and let harden.  If you wish it to be insulating then you add short pieces of straw to the mix.
This would be the same mix you use to form the roof with your tiles.  

Cob has been being used longer than any other building material.  Think clay huts in Africa, or adobe bricks.  Clay, sand and wood ash was one of the earliest building materials.
 
Rachel McGill
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Thanks Thomas.
Would I just pack that mix down flat as the oven bed floor instead of layering fire bricks then?
I'm making an oven to cook pizzas, meat and bake bread in ideally.
I am clueless on the ratio required, 2 :1, sand to clay?
Do you happen to know the ratio of wood ash and straw I would need to add to this please Thomas?
 
thomas rubino
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Hi Rachel;
To start, I should tell you I have never built a cob oven (although I would like to) all my experiance with cob has been building rocket mass heaters. That said, I will give you my opinion, just don't take them as oven building facts. Experianced builders may have a better idea.
Sand to clay ratio depends on how "good" your clay is.  My clay was very good so I needed a 3 part sand to 1 part clay.  Test clay by rolling into a snake and seeing how well it bends.
Ash is added to "taste" … say 1 part ash with 1 clay and 3 sand.  I will mention that very fine sand (beach sand) is not a good choice. Course sand is preferred. Sometimes called builders sand.  But can be collected from streams and rivers.
Straw is cut down to 2-3" pieces before mixing. It is used as an insulator. It also works as a binder in lower heat areas to help control cracking.  I do not know what part of an oven you would want that in.


Hopefully an experienced builder will give you better direction.
 
Rachel McGill
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Thank you Thomas.
That's a great help.
I have the rock base developing now, looking good, just need to add an insulation layer of glass bottles and sand, then decide what materials I have that will work best for the refractory layer.
I'll find out where to get the straw and follow your advice.
I normally make sculptures out of clay, not functional objects.
I'm enjoying this a lot though.
Got my builder friend coming to have a look today, she'll help me on teh refractory layer I hope.
Definitely going for the old roof shingles on the outside after the two layers of cob have been added to weatherproof it.
Have a lovely week, will post a photo of it when I'm done if you'd like to see what you've helped with.
 
thomas rubino
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Hi Rachel;  Yes please!  Send photo's of the whole project.  While building and finished!
 
Rachel McGill
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Will do. I'll take one today before it starts to grow.
My friend just left, I have the right flat red bricks in the building rubble I dug up in the garden, so that's a bonus.
 
Rachel McGill
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Hello Thomas,
    I made the cob oven.
   I'm not sure if it'll work yet, as it's only just finished.
 Took the sand out and now it's drying out.
Made a unit next to it for plates and just becausemy daughter suggested the idea, so she's happy now.
Decided to make a 15% trial maquette oven first, before I do a bigger one.
My parents had just dug up an old pool at their house, tested their clay and it's great for making the cob.
Very exciting taking the sand out of it and seeing it still standing.
Not quite finished shaping the outside, building the base out at the front and widening the chimney too I think. Got a bit more bricklaying to do with the cob yet and building up the left of the unit.
I think I can easily make sculptures with this stuff and paint it, it's fantastic. I've never  used it before, great find. Thanks for your advice Thomas.
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Nothing? Or something? Like this tiny ad:
dry stack step
https://permies.com/t/125100/dry-stack-step
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