Pompeii oven with 1/3 fire bricks - structural issues?

[dariusk]

Hey there
I've tried quite a few searches but haven't found much info on the use of third bricks as opposed to the apparent standard of half bricks for a Pompeii oven. I'm in the process of developing a plan for a build and would appreciate any thoughts on my ideas to date.

The idea is for a relatively small oven of approx 700mm (28") internal diameter using one third bricks to reduce the thermal mass of the walls, reduced firewood use and heat up times etc. I understand this will also speed up heat loss, but not sure this is a major issue... The dome and floor will be well insulated.

My questions are:

1. Will using 1/3 bricks, reducing the wall thickness to around 75mm, have a significant effect on the structural integrity of the dome? Is it more likely to fail at some point in time?

2. Will this create any other challenges with the build?

3. Is there enough thermal mass for most cooking application? Floor will be full bricks so 75mm thick also, same as the walls

I'm thinking it will be used for pizza only most of the time, but want to have the option for bread baking, roasts etc... Not multiple batches of bread though. If I need the oven heated again, I'll just light another fire?

Anyway, great site here and heaps of helpful ideas and info from reading through other peoples builds. Any thoughts and comments would be very much appreciated!

Cheers
Darius

[Tonyp]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

Hi Darius,

I can't think of any issue with using the 1/3 brick. It should be quite sturdy. You are quite right in that it will not hold as much heat for the long term to use for residual cooking but having plenty of insulation will help greatly. And as you mention you can easily just warm it up again by lighting a small fire if you need to get a bit more heat into the bricks for roasting, bread baking or whatever.

Tony

[wotavidone]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I did my 30 inch oven using 2 inch thick pavers laid on the edge. I cut trapezoids so that the vertical joins were as thin as possible.
I was a little concerned that the 2 inch thick dome that resulted might be unstable, so I gave it a coat of render with chicken wire reinforcing in it.
It hasn't fallen down yet and 50 minutes from lighting the fire to a white dome is entirely achievable, as long as I keep the thin fast burning fuel up to it.

I then did one for a mate, using three inch thick pavers. Again we laid them on edge. His is 850mm, about 33.5 inches. Again we did the render thing, but it was really just insurance.
His isn't cracking, and he does lots of bread and roasts, so obviously there is enough thermal mass.
The thread is "New build in South Oz." I helped with the dome. He did the outer arch, chimney, house, stand, etc. In that area, I am afraid looks beat functionality.
I'm quite happy with the dome, the rest - not so much .
The forno bravo plans say that an oven made from half bricks has high but acceptable thermal mass for a domestic oven, so you can feel reasonably relaxed about a 3 inch thick dome, IMO.

I did a third one for another mate, because he would not take no for an answer. That one is made from half bricks and is 900mm/36 inches.
(He's a big eater.)
I documented that one on the ukwoodfiredovens forum.
He liked the look of the freshly made brick dome so much he refused to insulate and render the dome. Needless to say, that one takes a while to heat up. I won on insulating under the floor though - you can't do that one later if you change your mind.
One note about that third oven. With no dome insulation, the temperature differential between the inside and outside of the bricks is naturally bigger. Therefore the cracks between the mortar and the bricks are bigger, as the expansion of the hot inner face attempts to push everything apart.
He just doesn't seem to care. It helps that his Greek builder mate says that all ovens crack, this one has cracked just where it ought to and any way, it's a dome so it can't fall down. Still, the crack is only about 2mm wide at full temp.

My suggestion for a thin shell oven is to do the shell at three inches, build a house around it, then fill the gap with vermicrete. You will then have a solid block of "insulating concrete foam", lined with fire brick.
The low, but real, compressive strength of the vermicrete on a concrete slab is enough to hold up a 4 inch dome and floor. So, if it is backed by a "house" with real structural strength, it will have no drama holding the dome to shape. And if it does crack - you won't be able to see it. What the eye doesn't see.....

[dariusk]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

Thanks Tony. You seem pretty confident it should hold together well, which is reassuring to hear. I'm still not decided on whether to use 1/3 or 1/2 bricks, but leaning toward 1/3 at this stage... It definitely isn't common practise to go with 1/3's and can't help wondering why this is the case. I think a lot of the prefab cast WFO's around here have around 50mm dome walls and work reasonably well. 75mm of brick would have to be better/stronger/more versatile??

Hey wotavidone, I appreciate your time with the reply. I'm hoping not to need a reinforced layer around the dome, otherwise I'd likely just use half bricks. I was thinking possibly of a 12mm or so layer of mortar over the dome once done, but not much more and not reinforced. That sounds like good advice to house it in a closed structure for added strength. I kind of like seeing the dome shape though!

Cheers guys

[ATK406]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

After building one, the dome shape really grows on you. Finishing your oven can take as long as the initial build itself and may have just as much influence on longevity too. BTW - Building a thinner wall will work out just fine. You might be raising the stakes just a little bit in terms of "geometric quality", but that is more than offset by your plan to build a smaller oven anyway. In other words, if you were planning to build a thin walled dome with a 2m diameter, you might have issues. I'm really curious how a thin walled oven performs (I suspect that most of aren't saturating our ovens in 2-3 hrs anyway), so I hope that is what you decide to do.

[UtahBeehiver]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I believe that DavidS (Aussie) has a thin wall cast oven that is smaller in diameter. He can give you feedback on smaller oven performance and pros and cons.

http://www.fornobravo.com/forum/1528-david-s/

[wotavidone]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I think a lot of the prefab cast WFO's around here have around 50mm dome walls and work reasonably well. 75mm of brick would have to be better/stronger/more versatile??

Hey wotavidone, I appreciate your time with the reply. I'm hoping not to need a reinforced layer around the dome, otherwise I'd likely just use half bricks. I was thinking possibly of a 12mm or so layer of mortar over the dome once done, but not much more and not reinforced. That sounds like good advice to house it in a closed structure for added strength. I kind of like seeing the dome shape though!

Cheers guys

Precast domes probably have quite a bit of thermal mass - that stuff is dense. A 3 inch brick dome would probably not be any stronger though - we are comparing a solid single mass with lots of little blocks stuck together with a less than perfect glue.
Still, that doesn't matter a bit.
IMO opinion a reinforcing layer is not really needed on a 28 inch dome with 3 inch walls, and about all a 12mm mortar layer would do is add a little thermal mass.
If you want the dome shape, you can still achieve all your aims.

It could go like this:

Build a 3 inch thick dome.
Give it a 4 inch layer of (say) 6:1 vermicrete, then render/stucco the outside in your igloo shape with reinforcing in the outer (say) one inch layer.
Then you will have your dome bricks pushing against the vermicrete, with the vermicrete transferring the stress to a reinforced outer shell.
No structural problems at all, I suspect.

All contained - all supported - all good.

BTW, the very lovely Artigiano series of ovens offered by our kind hosts appear to have a wall thickness of just over 3 inches, including a layer or mortar/render. I do lust after these craftsman built ovens.

Brick Oven | Brick Oven Pizza | Outdoor Brick Oven

[david s]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

dmunn built his oven using 2 1/4" thick bricks cut into various shapes to form a geodesic dome. His thread is worth reading. He did have problems with cracks in the joins which he put down to the extremely thin (for a brick oven) walls. My ovens have 2" thick walls, but I cast them in three sections using castable reinforced with stainless steel needles, so it can't really be compared to a brick oven. Anyhow check out his build here.

http://www.fornobravo.com/forum/f8/d....html#post3834

[wotavidone]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I followed the geodesic oven thread with great interest. I think the reason the oven cracked was only partly to do with the wall thickness.

I reckon it was also because:
a) the pentagons and hexagons had to be made by glueing smaller bits together, and
b) all the joins between the individual hex and pentagon shapes run full length over the dome.

In the whole dome, the mortar really did have to act as a high strength glue.

In a more conventional dome, each vertical join is ideally only one one course high, then the joint is bridged by a brick.

I do believe there is an potential issue in a thin dome, in that there is less surface area on the faces that make up the joins, but at least the individual bricks can be used to tie the other bricks together somewhat, and the mortar then has somewhat of a gap filling role rather than primarily a gluing role.

[Greenman]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

dmunn's geodesic build is an interesting read. I followed it with great interest. I am sure that there will be a MK 2 somewhere out there with some adventurous soul. The plebs among us will be happy to make a traditional functional MK 1 and dream about how much better/prettier MK 2 will be.

I am interested in a hybrid with the brick dome and cast refractory entry arch. We all need food for though.

[david s]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I followed the geodesic oven thread with great interest. I think the reason the oven cracked was only partly to do with the wall thickness.

I reckon it was also because:
a) the pentagons and hexagons had to be made by glueing smaller bits together, and
b) all the joins between the individual hex and pentagon shapes run full length over the dome.

In the whole dome, the mortar really did have to act as a high strength glue.

In a more conventional dome, each vertical join is ideally only one one course high, then the joint is bridged by a brick.

I do believe there is an potential issue in a thin dome, in that there is less surface area on the faces that make up the joins, but at least the individual bricks can be used to tie the other bricks together somewhat, and the mortar then has somewhat of a gap filling role rather than primarily a gluing role.

totally agree, that's why I cast mine.

[deejayoh]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

I think the reason that there aren't more ovens built with 1/3 bricks is because it's more work for less potential performance and minimal cost savings. You have to make more cuts, get less heat mass, and you only save the cost of maybe 30 bricks. To most builders, I am guessing that trade-off isn't worth it.

As you seem to have a specific desire for a thin-walled oven, I say go for it. I don't think you should have any structural concerns.

[dariusk]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

Thanks for all the replies, very appreciated and has help with this part of the decision making process.
I don't think I'll go down the geodesic dome pathway at this point in time... Interesting read though.
Well, while it's sounding like it's unlikely there would be major concerns with a 1/3 brick dome, I've decided to go with the proven standard half bricks and increasing the internal size slightly to 32" (800mm). Considering my lack of experience with brickwork, concrete and everything else for that matter, I thought the conservative approach would be a better bet.
Lots of reading and planning to do still and lots more questions coming too!

Thanks again
Darius

[cobblerdave]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

G'day
Don't consider lack of experience will stop you in any way from producing a full working and beautiful oven.
The minority, absolute minority of ovens in this forum where built by "professionals ".
The rest of them, full on, first timers, worried they " bitten of more than they could chew"
http://www.fornobravo.com/forum/f6/3...tml#post149565
It's even been done out of a wheelchair. Good on ya Bruce!
Plenty of pics lots of questions ..... This bunch will see you through .
Regards dave

[Greenman]

Re: Pompeii oven with 1/3 fire bricks - structural issues?

Hey Darius - If you have the motivation you will produce a perfectly operational oven with the support of the people who have done what you seek to do. Many of us had not laid a brick before we started and have ovens that work fine, most of them look good too.

Do your research, plan well and lean on the crew for what you need help with. There are people who have produced true works of art on the forum who will gladly offer advice. Mine is functional rather than beautiful but I love it anyway!

When you have your planning sorted you will just get the faintest whiff of a pizza and that gets stronger as you progress.