joeb
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Posts: 4
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Post by joeb on May 22, 2016 18:50:47 GMT
ive just got to a critical point on the build of my River Cottage style pizza oven.
Ive built a base out of aerated blocks then layer some lintels on top and four 600 x 600mm slabs in a square on top. i am going to lay fire bricks on top to create the oven floor. My question is will the the bricks protect the slabs from breaking under the heat?
cheers
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Post by downunderdave on May 22, 2016 19:19:15 GMT
I don't know what a "River cottage design" is (some pics would help), but you need insulation between your floor fire bricks and the concrete slabs under them. Otherwise heat will be lost from the fire bricks to the concrete via conduction resulting in a floor that is unable to hold pizza cooking temperature. A heavy thermal mass floor is sometimes used for bread ovens, but not for cooking pizzas which need double the temperature. Remember the old saying "the higher the temperature the greater the heat loss".
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joeb
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Post by joeb on May 22, 2016 19:57:57 GMT
What would you recommend as the insulator? Is there some sort of heat board?
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Post by downunderdave on May 23, 2016 1:54:15 GMT
There are many insulating boards available from insulating or refractory suppliers, but you need to select something with enough compressive strength to cope with the weight. Most builders use (in order of expensive to cheap) cal sil board, foam glass, vermicrete. You can make your own vermicrete from 5:1 vermiculite and cement and simply cast it in place.
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Post by sparkles on May 23, 2016 4:36:54 GMT
Hi Joeb Downunderdave is of course correct. Heat travels from a hot body to a cold one until they are in equilibrium. So if you don't insulate then all you will do is heat the slabs up which will lose heat probably slightly slower than you can burn wood so you'll have a good patio heater. I too am not familiar with the River Cottage design but I guess it's a cob oven which unfortunately I have no experience of. You said you built the base out of aerated blocks. Do you mean aggregate blocks? If you did build it from aerated blocks and you have any left you can use these under the brick hearth as they are fire resistant and thermally insulating you can get them from Wickes www.wickes.co.uk/Forterra-Thermalite-Aerated-Block-440-x-215-x-100mm-3-6N/p/120190
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Post by sparkles on May 23, 2016 4:55:56 GMT
Having just watched a YouTube video of this type of build I can confirm it is a cob oven and it appears that insulation isn't a consideration. I personally would still insulate under the hearth as I'm not sure how concrete slabs would react at such temperatures.
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Post by oblertone on May 23, 2016 7:56:20 GMT
What would you recommend as the insulator? Is there some sort of heat board? It's not the cheapest option but you can get vermiculite board in fairly large sheets (look on eBay); my own oven has one 25mm layer under the hearth bricks and works just fine. As an added bonus it gives you a flat surface to work with, hence my hearth bricks are laid 'dry'.
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Post by cobblerdave on May 23, 2016 9:26:16 GMT
G'day Or you can use thermolite block ( aerated concrete block ) as you have used in your stand. Regards dave
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joeb
New Member
Posts: 4
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Post by joeb on May 23, 2016 19:57:23 GMT
Thanks guys, so I've got loads of the aerated blocks left over, I'll use those and put the firebricks on top. Ace!
For my next question! I've been in touch with the clay man(!) and they have said that I can just use the clay directly (terracotta red grogged) as its already puddled. Do you think I should still mix it with sand?
Thanks all!
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Post by gracie on May 23, 2016 20:06:28 GMT
Thanks guys, so I've got loads of the aerated blocks left over, I'll use those and put the firebricks on top. Ace! For my next question! I've been in touch with the clay man(!) and they have said that I can just use the clay directly (terracotta red grogged) as its already puddled. Do you think I should still mix it with sand? Thanks all! Hi, I used pure "economy" terracotta and added sand. If you use the grogged clay, you'll end up spending a fortune as it's normally more expensive than straight terracotta, plus you don't know if it's the right proportion of grog and you might get shrinkage. At least adding your own sand you know the proportions and you only need to pay £3 or so for 25kg sand!
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joeb
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Post by joeb on May 24, 2016 20:37:36 GMT
So I've read this on delivery! Best I start tramping the sand then!
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Post by gracie on May 25, 2016 7:00:49 GMT
So is your clay grogged already? Do you know the % of grog? The ideal mix is 1 clay to 2 sand/grog so just add sand to make up the difference. Top tip when "puddling"...use a pair of wellies or shoes that are easy to remove and have another clean pair of shoes or sandals to slip on once you're done tramping. That way you can leave the clay packed shoes on the tarp to prevent treading clay all over your site. My patio was a lovely red brown colour by the time I'd finished!
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Post by nannynina on May 13, 2018 21:31:06 GMT
Hi all, it's been a while since this was started, but was wondering if you could help me? For an average cob oven, started on a metre sq. base, approx. how much clay, and how much sand did you use? Thank you
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