I have been reading all of the info on the Forno Bravo site, as well as just about every topic on this forum.
I have also taken note about the experiences with too much thermal mass in the floor and walls (thanks paulages and dmun :) )
I am about to start building my oven and would like some comments on the following:
1. Based on the "Island Hearth" theme, but a bit different, I want to build the hearth with a bottom layer of 4" of reinforced concrete and pour 3" of perlite concrete on top of that. The firebricks I can get here are 9" by 4.5" by 3". Using these bricks, create a standard oven floor by placing the 3" thick bricks directly on the perlite concrete.
3. Then cut the bricks that make up the floor to end up with a circle (similar to what "paulages" has done) but different to the extent that I allow for an additional 3" of width (i.e cut the floor to the external diameter rather than the internal one).
2. for the dome, cut my firebricks in 3 pieces so that I ending up with a wall thickness of just under 3 inches. Place the first course on top of the oven floor and continue to build the dome as per normal.
3. Insulation: I have read that the insulation blanket is more efficient than vermiculite or perlite. Here in New Zealand, I have (so far) found Kaowool. i have also read about the "rigidizer". So, to get really good insulation, why not wrap the oven in 2 or even 3 layers of kaowool, use the "rigidizer" on the last layer, and then put a couple of coats of stucco over the top.
It would seem to me that this method of construction would result in an oven that heats up quickly, yet retains the heat for a sufficiently long time.
Any comments are appreciated.
Kiwi to build 3" thick dome made of bricks cut in thirds.
(M) Hi, Peter,
(M) Do people in New Zealand also refer to friends as "mates", like in Australia?
(M) I could have clicked on "Quote" but I' not sure if I can insert (M) into that type of Copy-Paste, so I've simply pasted your entire post, below.
(M) When reading my reply to your questions it is important to note that I am *not* and engineer, but a "Newbie" who has yet to complete his own dome. So, my responses make no pretense at being "authoritative" or correct. With those caveats in mind, let me never-the-less try to provide a "Newbie's" perspective:
(P) Hi Guys...
(P) I have been reading all of the info on the Forno Bravo site, as well as just about every topic on this forum.
(P) I have also taken note about the experiences with too much thermal mass in the floor and walls (thanks paulages and dmun :) )
(M) From the reading I've done, it seems that Pompeii oven builders do not share the passion for thermal mass to the "degree" (no pun intended) favored by rectangular footprint adherants. So you gain some things and lose others.
(P) I am about to start building my oven and would like some comments on the following:
(P) 1. Based on the "Island Hearth" theme, but a bit different, I want to build the hearth with a bottom layer of 4" of reinforced concrete and pour 3" of perlite concrete on top of that. The firebricks I can get here are 9" by 4.5" by 3". Using these bricks, create a standard oven floor by placing the 3" thick bricks directly on the perlite concrete.
(M) I suspect that 3" of reinforced concrete would be ample for strength, and that 4" may be overkill. Most builders use that figure and 2.5" of "perlcrete"; although in the reverse order. Would you lay your hearth brick floor with the 3" or the 4.5" side down? My intuition tells me that because heat rises, the dome will actually get hotter than the bricks upon which your fire rests. I've read here on this forum some data that supports that intuition but you had better check that with someone who is an expert. In any case, you may be fine with having the thinner area of 3" as a base for your floor bricks; in the same orientation as your dome bricks.
(P) Then cut the bricks that make up the floor to end up with a circle (similar to what "paulages" has done) but different to the extent that I allow for an additional 3" of width (i.e cut the floor to the external diameter rather than the internal one).
(M) In that case, I'd save myself the work of cutting the outside circle- circumference, period! Unless appearance is important to you, that is a lot of work, wear on your diamond blade, and a waste of some thermal mass; but again, if you're trying to reduce thermal mass, go for the cutting. My oven will probably be covered with a Gable House so I don't care what the exterior of my dome looks like.
(P) 2. for the dome, cut my firebricks in 3 pieces so that I ending up with a wall thickness of just under 3 inches. Place the first course on top of the oven floor and continue to build the dome as per normal.
(M) "Normal" varies, even in the instructions and photographs. Some images show the first course as "soldiers" standing on their 2.5" edge; other show the first course as also being on their 2.5" edge but cut to 4.5" and called "splits". I have let my split soldiers "sleep" and have read no objection to my system; but who cares what a "Newbie" writes ;-)
(M) You are of course correct that the lesser thickness reduces thermal mass, and if that's what you specifically want, I'm sure it would work. You'd also gain by using fewer bricks and probably have an easier time laying up the bricks as the angle of each brick along the "hemishphere" would be less acute.
(P) 3. Insulation: I have read that the insulation blanket is more efficient than vermiculite or perlite. Here in New Zealand, I have (so far) found Kaowool. i have also read about the "rigidizer". So, to get really good insulation, why not wrap the oven in 2 or even 3 layers of kaowool, use the "rigidizer" on the last layer, and then put a couple of coats of stucco over the top.
(M) Why not? Because those blankets are expensive, mate! But if you are independently wealthy, that should work fine.
(P) It would seem to me that this method of construction would result in an oven that heats up quickly, yet retains the heat for a sufficiently long time.
(M) What is "sufficient". Only time will tell ;-)
(P) Any comments are appreciated.
(M) Well, you got my admittedly amateur's view point.
Just two comments: Three inch fire bricks layed on the perlite concrete seems just fine to me as far as thermal mass, particularly considering that you will be laying it on top of a layer of fire clay or refractory mortar. I'm not convinced that an additional thermal mass under the floor is necessary in a home oven. Multiple batch bread bakers might disagree.
Paulages used three inch firebricks to finish the upper part of his dome, and it created larger gaps to be filled with mortar. On the other hand it will lay up more quickly, maybe one fewer rings. This image of his shows the difference between the 2 1/2 inch bricks in the lower three layers and the 3 inch ones above
Good luck with your project. We like to see pictures, so that people like me who haven't built an oven yet can learn from other's experience.
Firebrick is heavy!
(M) I hope that this post will not look picayune but when David wrote:
(D) "I'm not convinced that an additional thermal mass under the floor is necessary in a home oven."
(M) I wanted to make the point that there is a structural reason beyond the thermal for a steel reinforced "thermal layer". When I brought home and started laying up my firebricks I was glad of that "thermal" layer because of the weight of all those firebricks.
(M) When you start to mix your "perlcrete" you will see how porous it is. Somewhere I saw a posting of it's relative strength ( I don't think it specified shear or compressive) but it is not strong enough to give me confidence in using it without a regular => 3,000 psi concrete layer. Perhaps with imbedded re-bar perlcrete may be sufficiently strong but I would not want to take that chance.
On another theme, Pete, I hope that the fire brick you will be using has the correct proportion of Alumina. I think it needs to be about 30%. I used Clayburn (from Canada) "Flame-Low Duty Firebrick". The term "Low Duty" is unfortunate because it can connote low quality. I think it means it is not for industrial use. Clayburn sent me the Spec. sheets on only 5 of their most common refractory bricks. Their "Low Duty" had only between 22% and 26%; somewhat less than recommended, but their CB-70D had 68-72% and would get so hot that you'd burn the bottom of any food on it's surface before baking the center. The other major component is Silica and that should be around 60%. Mine are between 62-66%.
i have a hard time believing even with the high-duty firebricks, you could possibly get the floor too hot for pizza nepolitana. maybe without the extra thermal mass beneath them, and i REALLY blazing fire? my floor bricks are a medium duty, as are some of my dome bricks. if i point my thermometer at a low, and then a medium duty brick right next to it, i can't tell any significant difference other than "damn those bricks are hot, and the other ones are ...hot".
my number one complaint about the ratings of my bricks was price. towards the end, there weren't any low-duties to be had, and i had to shell out a bit more$ than i'd liked. not to discredit what you're saying though marcel, i'm just saying that in practical experience, i've yet to be able to tell a difference.
and by the way, i don't like it when i stick my head in the oven and see those large gaps in the dome from using the 3" bricks, but i don't believe it affects performance or structural stability at all. if anything, it's less thermal mass and therefore heats up more quickly, but we're talking about the focal point of all of the fire here. it gets hot no matter what. byut i do hate it when i stick my head in the oven and see those large gaps...or see pictures of it on the internet. DOH! just kidding...
You can definitely put your cooking floor directly on the vermiculite or perlite insulating layer. I did the Florence house that way with the Casa90 cooking floor and it worked great. I also did roasts and a lot of bread, so I think you will be fine ther.
As you say, put rebar in the lower layer to give the hearth structural stablility, and you are set.
One thing to work through is that the vermiculite layer is a little crumbly -- so you might have to work a little harder on getting your floor level, and on the oven enclosure top walls -- setting it and finishing the sides. Nothing terrlbe, but you don't have the hard, smooth surface you would have with concrete.
I would also vote that Paul is right on the higher duty firebricks. I can't image the concept of "too hot" in your oven. You need to be balanced -- ie an 800F dome ando 450F floor will bake a pretty bad pizza, but Pizza Napoletana ovens are 900F top and bottom, and make a great 60-90sec pizza.
My view is the biggest risk to the venture is not getting your hearth hot enough (or your hearth giving up it cooking temperature after cooking only a few pizzas), not its getting too hot and not holding heat for the second batch of bread.
The instructions from Forno Bravo warn about using High duty fire brick.
(M) The reason I suggested choosing bricks with attention to their material proportions was because of what I read right on the Forno Bravo Site:
"High duty fire brick. These brick have very high alumina content, get very hot (1500F and up) and are designed for continual high-heat applications, such as furnaces. They are expensive, and will get too hot for some of your oven uses, such as baking bread and roasting. "
sorry, marcel. again, i wasn't trying to refute your statement, but rather was commenting curiously on this line of reasoning myself, becuase it seemed a bit different than the data i had found when researching bricks. from what i could tell, the medium and heavy duty bricks are intended to be used in higher temperature applications and are designed to bear such high temperatures, not actually aid in gaining higher temperatures. maybe i've misunderstood this point, but really was just chipping in the practical knowledge i've gained from my own oven.
maybe jim could comment on this one? haven't heard from him for a while.
No problem with the exchange. That's what Forums are for!
(M) I'm fine with your viewpoint. I didn't feel challenged or refuted. I was somewhat puzzled :confused: as I based my suggestion to Peter on what I had read, not on what I had experienced, since I'm still (working between rainy periods) trying to close the dome.
(M) I don't know if you checked your private messages here recently but I wrote that I couldn't get to your images on PhotoBucket. Is it possible that you set your preferences on PhotoBucket to exclude public viewing? ___ I'd still like to see them as I want to learn from you before we share pizza and beer.
(M) btw, Tony Gemignani will be demonstrating his "Five-Time World Champion Pizza Thrower" technique at our Gourmet market, Market of Choice on Sat., Oct. 22. He is also offering a class, with dinner and wine the preceding Friday but $55 seems a bit steep for my wallet. I hope he'll demonstrate more than just tossing pizza.
(M) I'm experimenting with balloons between my veins;-) :D
I have used them (them what don't break) to bridge the gap between my 8 vane supports. I wish I had listened to your suggestion to go with 16 vanes. Also, for any "Newbie": regardless of the # of vanes you use, be sure to take your time with symetrical cuts. I was sloppy and am seeing those sloppy results now.
(M) I will soon transfer some photos of the gravity defying bricks supported only by air, .... well, with perhaps a little help from a rubber membrane.
i'll look into the photobucket thing.. but i'll also have to download the kazillion pictures i don't already have on photobucket for you to see different pictures. most of the pictures there are the same ones on here. i do have lots of extras though, but some of them are on a computer my girlfriend sold to a friend while i was in japan. :(
$55???? i'll show you how to toss a good pettola for 55 CENTS!...or a beer, whichever comes first. i did once experiment to see how big i could possibly make a pizza, and i successfully tossed and cooked a 36" X 45" cheese pie. stretching it was the easy part. to get it in the oven, spread out two 20" wooden peels side by side, and then as i put the front end in the oven, i stretched it out as i pulled the peel out from underneath it. i've also successfully "juggle tossed" a good several rotations once with two other people.
so sign up now---dinner, wine and pizza circus for only $39.99 (i have to undercut this guy...)
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