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

[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

[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.....

[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

[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