Pompeii oven - too thick?

[dmun]

Ok, I'm gonna say something that may be completely wrong, but I'd like some feedback from people who have practical experience. In my field (clockmaking) there's nothing more irritating than some amateur, who has no experience or feel for the traditions of the craft, get on some list and say that something that has been done for hundreds of years is completely wrong. So here goes.

I think that for the purposes of a home oven that the pompeii oven is too thick, or has too much thermal mass. For my purposes, I want an oven that will heat up quickly and efficiently to make some pizza, and retain enough heat to bake one batch of bread, or at a stretch, bake the Thanksgiving turkey.

For my purposes, I don't think the oven needs to be much thicker than the commercialy made refractory pizza ovens. It seems to me that that with three simple changes, I'd get an oven that's much more fuel efficient, and is ready to bake more quickly:

1) Lay the oven floor with the fire clay slurry directly on the insulating layer of the hearth, without that addtitonal slug of concrete for thermal mass.

2) Make the first, or soldier course, of the oven with wide side of the half bricks facing in, instead of the narrow side.

3) Build the dome with fire bricks cut in thirds rather than half, reducing the thickness of the dome from 4-and-change to two-and-a-half.

Now I know that cutting the fire bricks in thirds will leave some bricks without a factory edge, and a cut face looks different than a factory face, but I could use these middle sections on either side of the opening where they are less visible. It also means that each brick needs full cuts rather than score-and-break treatment, but I'm inclined to get the big brick saw anyway.

I don't mean this to be critical of the people who have designed and built the pompeii oven. I know the project was designed to built by beginners in masonry, who could split the bricks with an inexpensive brick set. I just think that these few changes could make it a better home oven.

Any comments?

[james]

Dmun,

I think this has a great deal of merit. It's also interesting (ironic?) that you would want to develop this theme, when early on in the development of the Pompeii oven there were many people who had read the Breadbuilders book and thought the oven had too little mass. Of course that isn't right, and your posting touches on some interesting ideas. Don't forget that the barrel vault oven has something like 9 1/2" thick walls (4 1/2" brick and 5" concrete) -- so you can see the irony.

As an aside, I have fired my barrel vault oven a couple of times now, and am finding it as challenging as I feared. More on that later.

To your points:

1. Yes, you can definitely lay the cooking floor directly on the insulating layer. Many of the precast oven installation guides say that you should do that.

2. I will be looking forward to seeing responses from previous builders on whether they think the effort of the extra cutting is worth the return of having a lighter dome. From a practical perspective, the real issues are heat up time and heat retention time. You want the oven to heat up as quickly as possible, while still holding enough heat to cook lots of pizza (and the oven not give up), one batch of bread and a turkey (with veggies), which might be the most challenging. The FB precast ovens are designed specifically to do just that, and it would make sense that a site built oven could be tuned to the builder's specific requirements.

Thanks for the thought provoking posting.

James

[paulages]

i thought about this same concept when building my pompeii, but didn't quite have a total grasp on what that would actually mean-- in terms of the building process and what that would change. so i followed the instructions gratefully and now have a well-functioning oven.

i can see the benefits of an oven with less thermal mass, and especially a floor with less thermal mass. i would like to see how this would affect overall temperatures. i ended up cutting all of my bricks with a 10" bricksaw anyway, so cutting them into thirds would have been almost as easy.

to keep my oven as hot as i like it for pizza, i have lots of coals and at least a couple of large logs really putting out a large flame. as soon as my flame is not licking all of the way to the top of the dome and a bit over, the floor begins to cool down below 600 deg F, and upper dome below 850 F. 90 second pizzas become impossible, and the crust starts to suffer and toughen up.

i can't help but wonder if less thermal mass and more insulation would help?

[fmed]

Hi All,

(...new here)

I thought I'd bump this thread since I'm in the research phase on building my own pizza oven. My goals are to build an oven specifically for pizza (and if I can bake a few loaves of bread afterwards -- bonus!)

The Pompeii page on this site asserts that this oven heats up quicker than a barrel vault oven. Is this really true? I can see that this is true for an Alan Scott oven with the thick thermal mass. Can you not build a well-insulated barrel vaulted oven with less mass (or no mass) which will heat up just as fast?

I'm currently leaning towards a rectangular oven with walls and floor of regular firebrick. The vault will be cast refractory done in sections. For insulation I'm thinking of using dense mineral wool (maybe 5" thick?). The outside may be clad with cement board on steel studs. Then stucco for the final surface.

Here is a reference:
http://mha-net.org/msb/html/bakeov17.htm



Looking for feedback,
-fernando

[james]

Ciao Fernando,

Take a look at the "why round" page -- it has some good information on why round ovens are a good idea for pizza.

http://fornobravo.com/pompeii_oven/why_round.html

In general, regardless of mass, round ovens do a better job of storing and reflecting heat for pizza than rectangular ovens. I think you can make the case that the rectangular foot print is more efficient for a commercial bakery producing lots of bread. I think this is why virtually every pizza oven in Italy is round.

Also, the outward thrust of the dome of the barrel vault forces you to use a certain amount of concrete (or an engineer butress system ala Notre Dame), which drives up the mass beyond where you want it.

I have a thinner mass barrel vault oven (split firebricks set on the thin side), that still has a lot of concrete around the dome. It really doesn't cook pizza nearly as well as our Forno Bravo precast ovens, or a Pompeii oven -- in terms of heat, heat up and even, consistent heat on the floor. Also, the rectangular footprint is a pain when you are trying to do lots of pizza.

My two cents.

Have fun and share your progress however you decide to go ahead.

James

[fmed]

Thanks for the quick reply James. I have read that very informative Why Round page. Thanks for writing that.

If the vault is a pre-cast piece, would there still be the outward thrust issue? If so, I would imagine some steelwork could alleviate this without adding thermal mass.

Decisions, decisions....

[Robert Musa]

thinner might be better. here's my (amateur ) reasoning.

assuming all other aspects of the oven are identical (except brick thickness);

1. it would take less fuel to heat the smaller mass to a given heat (or the mass could be heated in a shorter period of time given the same amount of fuel). thin wins...

2. the surface area of the domes (both interior and exterior) would be roughly the same (exterior surface of thick oven would be somewhat greater). so one would expect the number of btu's lost through radiation over any given period of time (the bleed rate) to be about the same. thus, once the oven reached pizza temp, both ovens would require about the same amount of fuel to replenish the lost btu's. maybe a tie - maybe advantage thin ....

3. because the thinner oven stores less btu's, once the fire is removed (or becomes smaller than the amount required to replace lost btu's) the thicker oven would require a longer period of time to return to ambinent temp than the thin oven. thick wins ...

question: would it be more difficult to build a structurally sound thin oven for those of us with stubby fingers?

like paulages, i find that maintaining a "super hot" oven requires attention. in our last pizza go around i adopted the previously discussed method of lots of wood right up front and was able to get the dome over 1000 (f) within 90 minutes, however the bleed rate is a pretty steep curve at that temp.

i have a theory that the height of the dome may have more to do with getting 90 second pizza than anything else. (mine is too high). then again, we're just guilding the lily here. my experience is that anything which comes out of our oven is better than anything we can buy in a restaurant by a factor of 10. i would be surprised if tweaking this a little one way or another will result in dramatic differences.

[fmed]

Originally Posted by Robert Musa

question: would it be more difficult to build a structurally sound thin oven for those of us with stubby fingers?

This is my rationale behind designing a rectangular oven with a precast vault. It looks easier to build. I'd love to have the time and space to build a Pompeii style dome, but I am frankly intimidated by the brickwork.

[Robert Musa]

Originally Posted by fmed

but I am frankly intimidated by the brickwork.

ha!! once you build the base and the hearth form you become imbued with magical brick laying powers. i actually found building the dome to be the "zen" stage of the oven construction as opposed to the "man with a hoe" aspects of the other parts...

[Marcel]

#16

(M) "fmed" wrote:

(F) "
This is my rationale behind designing a rectangular oven with a precast vault. It looks easier to build. I'd love to have the time and space to build a Pompeii style dome, but I am frankly intimidated by the brickwork."

================================================== ====

(M) I was in your position but compelling arguments by Jim, James, Paul and Robert Musa finds me back to "circle one" ;-)

(M) It is not too late for me to change my mind, Fred, but I would need to read even more compelling arguments than those offered by my "oven gurus". I have read virtually every post in this Forum I could find.

(M) The following are some of the reasons I returned to the Pompeii design:

1- There has been such detailed and thorough analysis here that I have a lot of confidence in getting guidance. If you look at the "vault" sites, I don't believe you'll find this kind of cyber support.

2- Robert Musa convinced me that the dome is stronger; even when built to less than rigorous tolerances.

3- Fred di Napoli turned me on to a template that comes with a CD and is available at Home Depot (mine sells them for $16.95) that does all the Math. for not just the number of bricks per ring, but the angle of the wedge shapes required. The movable template will let you project those angles onto your bricks. A cheap ($85) tile saw that cuts 1-3/8" will let me either cut about 1/2 way and split, or, for visible surfaces, turn the brick and let me cut all the way through. The "Angleizer" is the name of the template and CD Combo.

4- "Paulages" (don't we all, Paul?) has contributed a great many terrific photos of his process. He indicates where he would have done things differently and makes excellent recommendations.

(M) These are not the only really active members here. There are David, Luis, Hope, etc. .... all of whom are available to help you. Yes, the comraderie of a community oven building outdoor workshop is enticing but we have a cyber community that is open to exploration. They do not build only by formula. There is terrific variety here.

(M) Note that I have said nothing of mass because like many, I believe either foot print can be utilized with greater or lesser thermal mass.

(M) Ultimately the decision as to footprint is yours and I'm sure your views would still be welcome here if you chose not to go with the Pompeii.

(M) Good Luck irrespective of your decision!

Ciao,

Marcel