thermal mass ratio hearth:dome

[pyg]

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.

[gt40]

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.

[ggoose]

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...

[brickie in oz]

Re: thermal mass ratio hearth:dome

My hearth and dome are the same thickness, I dont see the issue?

[pyg]

Re: thermal mass ratio hearth:dome

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.

[GianniFocaccia]

Re: thermal mass ratio hearth:dome

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

[Aegis]

Re: thermal mass ratio hearth:dome

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

[pyg]

Re: thermal mass ratio hearth:dome

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.

[GianniFocaccia]

Re: thermal mass ratio hearth:dome

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

[GianniFocaccia]

Re: thermal mass ratio hearth:dome

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.

[dmun]

Re: thermal mass ratio hearth:dome

I built an oven with the dome bricks sideways (2.5") for a uniform brick thickness. It works fine, except for a lot of cracking, which is not a problem, except right around the door opening, which I have had to patch, and will again.

The 4.5 inch dome thickness is a lot more stable. As for the thickness of the floor, the matching 4.5 inch thickness is overkill, and hard to heat to pizza temperatures, although the additional thermal mass may be useful if your main goal is to bake repeated batches of bread on a single firing.

[Aegis]

Re: thermal mass ratio hearth:dome

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

Thanks for the reply John,
This is what I am aiming for myself, as soon as my second round of curing fires are over (had to repair huge crack) I'll be getting some good data to share.
Thanks
John

[pyg]

Re: thermal mass ratio hearth:dome

Well, I cranked out 12 12" pies in 35 minutes on Thursday evening as my oven nears the end of curing/drying out. For the purposes of this thread I choose to build a very well insulated ~6" dome and ~6" hearth as I'm actually more into bread than pizza. The interior shape and design in general is very close to the 36" Pompeii except in addition to the extra thermal mass I have a clay dome with one course of firebricks on edge to protect from tool and fuel damage. It's still a little early to draw conclusions other than it's awesome! It's cooling off faster than I expected but I attribute that to curing.

I'm having problems with the hearth temperature exceeding the dome for baking bread which I expect is from two different sources: 1) I've been a little too casual with fuel wood selection and preparation. Some of the wood I've used has been too large or not dry enough with less bright fires. 2) The dome is still pushing moisture out of the insulation layer. Should do better when it's fully dry. I also can have this problem during cooking pizza when I don't have a good bright fire.

I hope to post some thermal performance conclusions and photos in a couple of months.

[pyg]

Re: thermal mass ratio hearth:dome

Well after quite a bit of [TOTALLY AWESOME] use, I can report that although my oven has some issues it performs very well. As I stated in previous posts, my dome and hearth are ~6" and in terms of thermal performance I can report the following:

1. This amount of thermal mass takes a lot of fire over a long time to saturate the whole thing. I don't have exact data but getting to triple load bread baking temperature I usually fire the oven to pizza temperature followed by pizza of course. If the oven started cold I fire a bit more after pizza. The next morning I make a wild guess about how much fire will bring my dough and temp together at a reasonable time in the afternoon.

2. For bread I find when the outside of the mass reads ~400F and the inside is ~500-550F I can bake 18kg of dough in three loads very nicely, with the final temperature ~400F and rising. It's possible I could do more with a rest in between, but I haven't tried that yet. Sorry to mix Imperial and Metric, but I simply can't imagine doing bakers math in Imperial units.

3. As I noted in prior posts the hearth seems to be a little hotter than the dome for bread. It is less of an issue than it was initially, but I still get more color on the bottom. I'm still not certain of the reasons, but if I felt it was a big enough problem to address I would look into the thermal characteristics of the sand/clay dome versus the fire brick hearth or even less likely the height of the dome.

The 4.5 inch dome thickness is a lot more stable. As for the thickness of the floor, the matching 4.5 inch thickness is overkill, and hard to heat to pizza temperatures, although the additional thermal mass may be useful if your main goal is to bake repeated batches of bread on a single firing.

4. While intuitively I want to agree with this, in practice, for the rather small amount of pies I tend to make at a time (<20), even from a cold start the hearth heat doesn't *saturate away* fast enough to make a big difference. If I start in the upper 700s, it will still be over 650 by the time I'm done. If I was planning on making a lot of pizza I would of course have to preheat or switch around my live fire and cooking space, or some such.

[pyg]

Re: thermal mass ratio hearth:dome

For pizza the 36" hearth WFO I talked about easily got too hot on the hearth, which I can't explain based on all the replies, but perhaps a Pompeii ratio that has a bit of time to cool/normalize would come out right.

I'm completely unapologetic for the thread necro. Does anyone have new experiential data to submit?