48" oven about to start, need advice about buttressing

[Kulas]

Good day everybody!

I have been planning and thinking about building a round oven for quite some time now based on the pompeii design from this site (great site to say the least!) and the progress of construction in my head as well as some on paper is taking shape already. I am planning a low dome oven 48" or around that size (depending on minimal brick cutting).

Oven diameter: 48"
Dome Height: 15-16"
Door height: 10 inches semisphere
soldier course: standing on the cooking surface so as to create more airspace for fire: 9"
Door width: 19 inches

Insulation, stand, housing, and vent, I have almost figured out in my head. I have previously built a Hand design oven and it's working just fine and makes good pizza. fire brick wall of 4.5", with cement/lime cladding of 4" with rebar. Insulation of 4" perlite on sides and on top. 6" layer of sea salt under firebrick floor of 4.5" and under that 2 inches of perlite boards. Oven internal dimension is 4' x 5'. Yes, it's big! and I use tons of wood. Fortunately, I use it everyday so I'm not using as much wood as you could imagine with an oven this big. ... a truckload of wood may go for as long as 1 month and 3 weeks if its dry. 1 month and 1 week if its not seasoned. This truck is a six wheeler by the way.

I built this oven for bread. But I mainly use it for pizza since I am only beginning to learn how to bake. And due to the mass, keeping it at pizza temps is quite easy just as long as I manage the fire well and have tons of wood. Once I have a smaller round oven I can use this for bread exclusively and for lower temp cooking such as roasts.

Anyway, with this oven I have gained a bit of understanding of how to build an oven and the principles of the design. A lingering question I have now is with the buttressing of the sides of the oven. My guess (and from what I read in the relevant threads) is that I need to buttress the sides of oven because of its higher tendency to collapse due to the low dome. I can do this with 2 inches of cement cladding with rebar, but that will drastically add to the thermal mass. Another option I have is to pour perlcrete with rebar outside the firebrick walls as a buttressing. Now, from what I see from perlcrete is that after it has dried it will be flaky. I am not sure if it will hold the pressure from the brick wall. I have also read j bek's thread and saw that he used something else for this purpose. That product he used, I do not have access to. Would a mix of 6:1 perlite and cement, 4 inches thick, effectively hold the walls in place? I would deeply appreciate it if anyone has had experience with this and would share some knowledge.

Insulation on top would be blanket and loose perlite. Only the sides will have perlcrete. Under would be board of some kind. I'm pretty sure I will be starting next month so this is getting exciting for me. Would really need some help with any or all of you guys.

-Kulas

[Kulas]

I'm almost finished locating all the materials I need for insulation: cal sil board and ceramic fiber blanket and some perlite over the insulated oven. I've also received a quote for a bag of calcium silicate insulating cement for $41 /25k bag.

I would have ordered the cal sil boards from the fb store, but the shipping was over $500 since I'm in the Philippines. So I decided to look for sources here though they may be of lesser quality.

The calcium silicate cement specs says service temp of up to 650 Celsius. bulk density 350 kgs/cubic meter. I'm thinking of using this for the buttressing of the soldier course and the angle setting brick of the dome. Do you guys think this would work as an insulator or would it be just be a heatsink wicking heat away from the oven chamber? I'm thinking of using perlcrete but this might seem as a more solid buttressing since it's cement.

Thanks in advance for any insight.

[Dutchoven]

Kulas
I might suggest that instead of buttressing as you mention....eliminate the soldier course and lay the equivalent number of courses of lets say 6" long as opposed to the 4.5"...could stand them on edge for 2 courses to get to 9"...or something else...even could miter the last one to "spring the courses for the dome
Best
Dutch

[Kulas]

Dutch,

That's very interesting. It might work, though it may be too much thermal mass on the sides? 6" on flat side four bricks is 10"... I'd angle cut 1" off to get a wall of 9"... sounds like a good idea. There may still be some possibility that the dome would push the walls out (I'm certainly guessing here ) since that wall has more strength from bottom up as opposed to side to side? But I guess that's why it should be thicker than 4.5". I definitely have to think about this first before I write further, I haven't thought of this. I'll be back.
Thanks!!

best regards,
Kulas

[dmun]

As you know from running your oven, the top of the firing chamber burns white first, and it takes a long while for the carbon to burn off the sides. I suspect that your buttressing might exacerbate this problem.

I'm thinking that having brick buttresses, at intervals, with the door opening functioning as two, and maybe four others to carry the stress of the dome weight to the floor, without adding to the overall mass of the oven. Cathedrals are built like this, with stout columns bearing the load, leaving space for windows and "curtain walls" in-between.

Forget about perlcrete having any tensile strength. Even fully cured you can dig your fingernail into it like cork.

[Kulas]

Thanks dmun.

I have noticed that with century old churces made of stones. I am, however, doubtful with my skill with brick and mortar and first hand engineering knowledge to achieve something like that. Maybe I could do something more simple for my skill.

Perlcrete, as you have confirmed, has no tensile strength. But will building a round brick housing to hold the 4 to 5 inches of perlcrete around the soldiers make the perlcrete rigid so that it can support the pressure from the dome?

After laying the soldier course and the angle setting brick, I could make a brick housing 4 to 5 inches larger than the outside diameter of the oven and same height as the soldier course and angle setting brick, so I can pour perlcrete there and wait for it to dry before continuing with the dome construction.

I think that the perlcrete will be strong enough if it sets between firebrick and outer brick housing. Also, in my experience, the insulating properties of perlcrete improves several months or even a year after first use of the oven if you put straw, or even rice ears, in the mix. (I'm not sure if rice ears is the right word, this is the outer covering of rice grains taken off during milling) It will burn or disintegrate over time and insulation is better than new. So with this in mind, I think even with a bit of compression of the perlcrete from the dome, it would have enough strength IF it is also being supported by a brick housing. What do you guys think?

I am very grateful for your patience in reading my posts and for everybody's help. It is very much appreciated. Thank you.

best regards,
Kulas

[Dutchoven]

Fantastic idea here! Door jamb bricks act to buttress the dome at the front buttress at the opposite poles at the back and maybe two on the sides! Excellent should distribute the thrust perfectly! Kulas I think you should go this route!
Great thinking Dmun!
Dutch

Originally Posted by dmun

As you know from running your oven, the top of the firing chamber burns white first, and it takes a long while for the carbon to burn off the sides. I suspect that your buttressing might exacerbate this problem.

I'm thinking that having brick buttresses, at intervals, with the door opening functioning as two, and maybe four others to carry the stress of the dome weight to the floor, without adding to the overall mass of the oven. Cathedrals are built like this, with stout columns bearing the load, leaving space for windows and "curtain walls" in-between.

Forget about perlcrete having any tensile strength. Even fully cured you can dig your fingernail into it like cork.

[Kulas]

I'm off to a nearby church for site seeing!!

[gjbingham]

Kulas,
You must be cooking a ton of pizzas to want to build that size oven. Though smaller than the first, its still plenty big to feed a lot of people. Are you commercial? A truckload every 6 - 9 weeks sounds like you must be.

[Kulas]

gjbingham,

yes, I'm commercial but in a small town where wood is still available (and cheap) Dumaguete City, Negros Oriental to be exact. A ton of wood maybe, but not a ton of pizzas. I can probably use a smaller oven right now, although I hate to build again in the future if its just not big enough. Who knows, maybe in a few years there are more people here. I hope with this round oven I consume less wood for pizza and less wood for the bread oven also.

Anyway, I went to look at an old tower yesterday. It was built with stone and it had buttressing as what dmun described for old churches. I'm going back to look at it again, to have a better idea. Though its not a dome, I think the buttressing is the same. I will take pictures tomorrow of that tower and also some baguettes! Best ones I've made so far! (That's because I've just started learning bread baking, not that it's great, but its an improvement.)

best regards,
Kulas