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Old 09-03-2011, 06:02 AM
pyg pyg is offline
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Default thermal mass ratio hearth:dome

Not really sure where to post this, but this forum topic has been one of the most interesting/helpful to me so far. If I'm reading the Pompeii V2.0 pdf correctly it looks like there is about 2.5 inches of thermal mass in the hearth and 4.5 in the dome. I am in process of constructing a 36" WFO and am wondering if anyone here has contrasted and compared different thermal mass ratios. My intuition says that with this ratio the hearth will hit higher temperatures than the dome initially but give it up rather quickly as well. Most oven designs I have studied have a more balanced ratio ranging in thickness depending on use. Has anyone added material to the hearth to balance the ratio? Obviously thinning the dome is not really an option with a firebrick Pompeii, but ~3 inches of thermal mass all around seem about perfect for a pizza specific oven. Anyone using an oven with more hearth than dome? Thanks in advance.

Last edited by pyg; 09-09-2011 at 06:15 AM. Reason: spelling
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Old 09-08-2011, 07:06 PM
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Default Re: thermal mass ratio hearth:dome

Had to correct myself here as I mis-read things. Dome is 4.5" and hearth is 2.5. I have taken readings and the dome seems hotter but it is also not touching the pizza. All I know it the ratio works as the top of the pizza and the bottom come out evenly cooked for me.

Last edited by gt40; 09-08-2011 at 09:29 PM.
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Old 09-08-2011, 09:18 PM
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Default Re: thermal mass ratio hearth:dome

Interesting point. I have seen a build or two on this site where the floor bricks were set on edge, providing a consistent 4.5 inches of thermal mass all around...
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Old 09-09-2011, 12:32 AM
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Default Re: thermal mass ratio hearth:dome

My hearth and dome are the same thickness, I dont see the issue?
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Old 09-09-2011, 06:43 AM
pyg pyg is offline
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Default Re: thermal mass ratio hearth:dome

Quote:
Originally Posted by brickie in oz View Post
My hearth and dome are the same thickness, I dont see the issue?
I didn't mean to suggest a problem with the Pompeii design. Mostly I was hoping to hear from others experiences with different ovens of different thermal mass ratios. In particular has anybody used one of the [light] precast ovens and a Pompeii? Has anyone used an AS/barrel type as well as a Pompeii or something else? If so was the AS/barrel hearth slab above or below the insulated slab?

I'm currently working on a ~36 inch WFO very much like the Pompeii but oriented more toward bread. I am currently planning to balancing the hearth:dome ratio as well as making it 4.5-6 inches thick. I have used two other WFOs in the past and am basing my choices on those experiences but would like to know how it works before I commit to a particular mass thickness and ratio (impossible I know). One of them was a tiny prototype that had a lot of problems because it's hearth was just firebricks flat on top of concrete blocks laid sideways; dome was ~4 inches. The complete lack of insulation and relatively thin hearth meant that no matter how I fired it I could never get enough heat out of the hearth to cook well. This oven certainly taught me the value of insulation and my current build will reflect this to excess.
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Old 09-09-2011, 09:43 AM
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Default Re: thermal mass ratio hearth:dome

Quote:
My hearth and dome are the same thickness, I dont see the issue?
I think Al's oven is the exception since virtually all ovens are built according to the FB hearth/dome thickness specs. Your inquiry has surfaced previously and in the absence of data that support otherwise, the findings were that:

1. a 2.5" hearth is sufficient for pizza
2. No one can support evidence that a 2.5" hearth under-performs in residual-heat cooking
3. A thicker 4.5" hearth is superior in heat-retention for multiple-batch bread baking
4. A thinner hearth (and dome) heats up quicker/loses heat quicker than a thicker hearth and vice versa

I have seen only a handful of owners who have two ovens with different hearth thicknesses, so a comparison of the performance differences of the two is hard to come by.

With no data to support the benefits of relative hearth thicknesses, it comes down to what you plan to do with your oven. I questioned the 2.5" hearth when designing my
39" brick oven and ended up with a 2.5" firebrick 'sub-floor' and a 1.25" (3cm) soapstone oven floor 'heating element' for a total thickness of 3.75". The thought here was that the seamless soapstone should be superior for pizza and flatbreads while a thicker overall floor should enhance residual-heat cooking (and bread) nicely.

John
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Old 09-10-2011, 05:12 AM
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Default Re: thermal mass ratio hearth:dome

Quote:
Originally Posted by GianniFocaccia View Post
I questioned the 2.5" hearth when designing my
39" brick oven and ended up with a 2.5" firebrick 'sub-floor' and a 1.25" (3cm) soapstone oven floor 'heating element' for a total thickness of 3.75". The thought here was that the seamless soapstone should be superior for pizza and flatbreads while a thicker overall floor should enhance residual-heat cooking (and bread) nicely.

John
Hi John,
So how has your oven performed? Does it respond in the way you thought it would with the extra hearth mass. What is your routine for using the oven? How long to heat for Pizza? How much longer for oven saturation and how long does it hold temps for bread and then slow cooking?
I saw your soapstone and I am very envious! Love your entire oven!
Thanks
John
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Old 09-10-2011, 09:27 AM
pyg pyg is offline
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Default Re: thermal mass ratio hearth:dome

Quote:
Originally Posted by GianniFocaccia View Post
I have seen only a handful of owners who have two ovens with different hearth thicknesses, so a comparison of the performance differences of the two is hard to come by.
Yes, that was my point in starting this thread. I was hoping to get experiential data about thermal mass ratios although I did expect multiple WFO experience to be rare. Your enumerated points are pretty much exactly what I've concluded from research on this forum and elsewhere. Thanks.
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Old 09-10-2011, 09:31 AM
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Default Re: thermal mass ratio hearth:dome

Quote:
So how has your oven performed? Does it respond in the way you thought it would with the extra hearth mass.
Thanks John. I was very fortunate to acquire the soapstone so easily. Unfortunately, I don't have any results to report since the oven is not finished yet. I had hoped to close the dome up in July or August, but during this time I was on multiple weekend road trips and an entire week at Stanford U. at my kid's swim meets.

I have pondered each of your questions many times. The only observations I can report are when I once lifted the center SS slab up to investigate the temp of the firebrick sub-floor. It was late afternoon and my half-built oven had been in shadow about 45min after being in the sun all day. The SS was still hot (about 110F if I remember), but what was surprising was how hot the firebricks below were, all from conduction. I'm wondering if the soapstone will increase or decrease the time it takes to fully saturate the floor during firing but anticipate the increase of floor mass (33%) will contribute to increased heat-retention over time.

My plans are to make pizzas on friday evenings, bread and roasts on saturdays and slow-cooked dutch-oven dishes on sundays, but who knows? The oven's coast curve won't be observable until I get it finished!
John
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Old 09-10-2011, 09:38 AM
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Default Re: thermal mass ratio hearth:dome

Quote:
I was hoping to get experiential data about thermal mass ratios
pyg,

I hear ya. At this point, the best we can do is build our ovens to the specs we think will give us what we want and report the results for future builders. You're on the right track.
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