#1  
Old 01-27-2009, 10:36 PM
Serf
 
Join Date: Jan 2009
Location: Dubbo, NSW, Australia
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Default thickness of hearth- thermal mass

I'm at the point now of laying the hearth bricks for my WFO and I'm wondering how thick I should make it.

I have 160mm (about 6in) of vermicrete in the base. On top of this I thought putting down left over pavers under the bricks which would make the hearth 160mm thick also. Is this too much? I'd primarily be using the oven for cooking pizza, but also the occasional roast and to bake bread also. Is one layer of brick turned on the side (110mm thick) enough. I'd just like some ideas of the amount of thermal mass needed for the oven. What have others done for their hearths?

Rod
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  #2  
Old 01-28-2009, 12:33 AM
Frances's Avatar
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Default Re: thickness of hearth- thermal mass

You'll have quite enough thermal mass with the bricks layd down flat as in the plans, no need to turn them on their sides. Check out some of the cooking threads to see how much you can bake with that floor thickness if you're still undecided. Here for instance:

PhotoPlog - Bread

A thicker floor will mean longer heat up times, will need a lot more wood, and won't hold pizza temps as easily.
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  #3  
Old 01-28-2009, 01:36 AM
Il Pizzaiolo
 
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Default Re: thickness of hearth- thermal mass

For home Pizza use in particular, you do not want a thick hearth so don't add the extra layer....IMHO

XJ
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Old 01-28-2009, 06:19 AM
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Location: Australia
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Default Re: thickness of hearth- thermal mass

Dubbo, eh. May yer god bless ya. We're near Bundaberg, and best you listen to both Frances and XJim, 'cause they both live (exist??) in snow country. (And they're both muckin good mate, I can assure you. [Apart from living in appalling climates, that is].Trust me. But maybe the chill sharpens their senses eh?)
Rod, seems that your base is well insulated: onya, good move. The rest is easy. (Strewth, did I say that?)
OK, enough of that. I firmly believe that a well-insulated masonry skin, plus two sets of doors, is the key to a pizza night that will provide useful heat for 3 or 4 nights cooking.
But to respond to your question.
I laid/layed/..used 250x250x 50 pavers on a sand base over the vermicrete. Once laid, I used the diamond blade on the 125mm angle grinder to lessen the ridges on the edges of the pavers. All good. One has cracked after 12 months ..so?
For the dome, I used edge-on solid seconds.
(Mate, our local brickworkers are familiar with Fornobravo, and were only to happy to assist. They even supplied grog (?) [ground up stuff, NOT alcohol], for use in the mortar.
Over that I basted a concrete/sawdust mix, ceramic blanket, vermiculite+ Al packages, with a final layer of water-resistant render.
My opinion? Insulate, insulate, insulate.
Onya Cpt Toddy, (perhaps we've met?),
Mundane Jeff.
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Old 01-28-2009, 09:51 AM
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Default Re: thickness of hearth- thermal mass

Primary use pizza? Lay the bricks flat over your insulation. No extra thermal mass needed. This is the consensus of experience: The FB plans started with much mass under the floor, and gradually lost it as people had trouble getting the oven up to pizza heat.

Primary use multiple bread bakes? Then you want more mass.
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Old 01-28-2009, 11:12 AM
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Default Re: thickness of hearth- thermal mass

By multiple bakes, that would mean a full load of bread with a goal of three bakes per firing. Short of that requirement, you would be better off with a thinner cooking floor -- and the advantages of the thinner floor (faster heat up, easier to hold high pizza heat, less fuel consumption).

Hope this helps.
James
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Old 01-28-2009, 09:34 PM
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Default Re: thickness of hearth- thermal mass

Quote:
Originally Posted by jeff View Post
I laid/layed/..used 250x250x 50 pavers on a sand base over the vermicrete. Once laid, I used the diamond blade on the 125mm angle grinder to lessen the ridges on the edges of the pavers. All good. One has cracked after 12 months ..so.
Jeff obviously I was worried about maintaining enough heat, and with a base three times as thick as yours I get the feeling it might have been a bit of overkill.

How does it go with the 50mm base; heat holding etc?

Rod
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Old 01-28-2009, 10:54 PM
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Default Re: thickness of hearth- thermal mass

Quote:
Originally Posted by jeff View Post
Mundane Jeff.

Hola (HiYa) Jeff, good to see you.
Hope all is well with you and yours!
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  #9  
Old 01-29-2009, 08:56 AM
Journeyman
 
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Posts: 297
Default Re: thickness of hearth- thermal mass

Rod, I struggle to load one pizza at a time.
With turn-around time at say 5 minutes, 4 shifts take 20 minutes,(yes mate, I'm a southpaw imbecile, may your god bless me. Mine refuses.hahaha - may he be censured). ahem! , and the 2" base is fine.
What I found more difficult was/is to get the dome and hearth temps in synch. With the wood you'd have in Dubbo, you'll soon get it right - and not much waste to chuck to the dogs, poor buggers.

Mate we probly skitch up a pizza once a week on average.(I've been deemed unfit for work, so the 'week' has lost most of it's meaning. Just me an' the dog these days. (Oh crapp! An' the Missus. Sorry. Been together so long, we work as one. Apologies Ladies, but I'm taken eh.)
So I should bake bread after the pizza fire, but I don't.
Next day you can do a traditional Sunday Roast, but I reckon the best bit is 2 nights after firing, a slow-cooked (120*C, [approx of course]) lump of CHEAP animal protein simply cooks itself.Give it 8 hrs - 12 hrs. All good.

Mate, I carried on a bit there, but all in fun.
Trust me, I am of sound mind: (Would an unsound mind construct such an archaic cooking structure on his own bit of turf??)

Hahaha. Now why does 'Bar be cues' present it's primitive profile?
They'll learn; maybe.
Rod, two things before I flee:
(1) Americans have problems with "modern" (hello-o-o ??) units of measurement. So 2" is near enough to 50mm, for those of us under 45.
(2) If you reckon I may be able to help, we might go via FB private channels next time.
Also, take note of 'Nissaneill' - he's no slouch.
Best in building mate.
Jeff.
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Old 01-29-2009, 09:28 AM
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Default Re: thickness of hearth- thermal mass

Quote:
Americans have problems with "modern" (hello-o-o ??) units of measurement.
You mean the French yard? We've heard of it, we even measure our soda bottles by it, we just don't like it.
(ducks)
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