800 mm homebrew cast

[Petter]

A door 20 cm high results in a dome height of 35.5 cm to keep the ratio constant.

I thought you should cast the entire dome, are you planning on put a cast roof on soldiers?

Feel free to pick your insulation of choice, just want you to make a desition you are satisfied with in the long run.

[david s]

For neopolitan ovens with their low dome height it means the heat from the dome is closer to the pizza. This results in a faster cooking time, but has the downside of a low roof at the perimeter. Placing bread loaves close to the sides then becomes difficult and reduces loading space. To address this problem builders used a soldier course at the base which resulted in higher sides at the perimeter. Using bricks rather than a castable mix makes no difference as the resulting density, thermal conductivity and heat retention are much the same for any given thickness.
The disadvantage of brick soldiers is that they have a long vertical joint at the base of the dome. Given that the base of a dome is the weakest at that point, (think of an egg shell cut in half), and cracks in the dome will always start at the base, a tall vertical joint is introducing a structural weakness IMO. It probably doesn’t matter that much as many brick ovens are built th this way. I just don’t think you get any advantage by using soldiers then casting on top of them. Building a sand castle you can easily start it with vertical sides, then the shallow dome as you go higher.

[Linuc]

My plan is not to use bricks for the soldiers. My plan is to build the sand castle with vertical sides and then the dome. But I'm might go for an ellipse instead. Not decided yet.

A lot of useful tips, thanks! I'm going for the weep holes and probably 50 mm of perlcrete and then 50 mm of CalSil. I have nevertheless stopped counting the expenses now...

Should I just cast the perlcrete on top of the tiles? Isn't it a risk that it will clog the drainage system then? Should I just do a seperate form and then put it on top of the tiles when it's hardened?

Today I have casted the table. I have marked out where the reinforcement is so it should not be a problem to drill the weep holes.

[david s]

Yes you are correct the vermicrete mix will clog up the drain system. You can either cast the slab separately and lift it into place over the tiles or lay some garden weed mat over the tiles before filling the vermicrete mix into the space. Casting separately has the advantage of you being able to dry the slab more easily because you can turn it over or stand it on edge to dry as well as weighing it to see how the water is evaporating. Vermicrete insulating slab PDF.pdf

One huge advantage of casting is that you can easily alter the form, although departing too much from a hemisphere can reduce the structural integrity of the form. a combination of vertical sides at the dome base and a flatter lower dome height is a good combination successfully employed by many.

[Linuc]

Spread the weep hole pattern under the base of the oven's footprint. Think of it as a square (corner set of holes) just beneath & inside (25-50mm) of the projected dome perimeter (below where the oven will sit) plus one in the middle. You're simply trying to make sure any moisture running along the concrete hearth has a nearby exit hole. When building a low dome, the weight of the upper dome presses outward and down more than a "perfect" (semicircular cross-section). This excessive outward pressure may eventually cause a collapse of the structure. To compensate for this outward thrust pressure, an outside buttress or containment ring is used. We often suggest a buttress on each side of an entry arch that is low to keep it from being eventually pushed apart. Do a Google search of Low Dome Buttressing and you'll find several builds on Forno Bravo (as other architectural links) that explain low dome problems and discuss solutions.

So I should drill the weep holes something like the sketch on the right side of the picture? Why not in the corners of the insulation?

Is the buttressing still necessary when casting the dome with vertical sides at the dome base and a flatter lower dome height? I will use SS needles and PP fibre in the homebrew mix.

[sergetania]

So you think I should go for 39 cm dome height instead?

Why not use the soldiers for a cast oven? As I understand the soldiers are there for getting the right radient heat, convection and conduction. But if it won't be strong enough I guess I'm going for an ellipse.

Okey, maybe I'll have to rethink my insulation.

Sorry, but why can't you simplify everything? If the dome diameter is 800 mm make the height 400mm. Make it a half-sphere. That's what I did. Then your opening can be 250mm high.

[sergetania]

I have nevertheless stopped counting the expenses now...

Outstanding! Sooner the better! Well, not for your wallet but for the oven and hopefully your mental health! There are always excuses that a well-built oven will serve your descendants! Enjoy the build! I have tried to take pictures of every step of my build. See if they can help you with yours.

[SableSprings]

So I should drill the weep holes something like the sketch on the right side of the picture? Why not in the corners of the insulation?

Is the buttressing still necessary when casting the dome with vertical sides at the dome base and a flatter lower dome height? I will use SS needles and PP fibre in the homebrew mix.

Your pattern on the right is what I'd recommend. It's the oven base insulation that you're keeping dry and underneath it is the most difficult to dry if it gets wet & the moisture has no place to exit.. Most people actually trim the board to be just a couple inches outside the oven footprint. Leaving enough material on the perimeter so the outer batting "connects" & completely isolates the oven.

I believe there is less lateral/outward pressure on a cast dome simply because it is thinner/lighter than a brick low dome. There still is the outward thrust to that vertical, outer wall- if it were me I'd plan to Include buttress support for the oven. Obviously size matters here, but I'm not experienced in casting. Hopefully david s will respond to the need for buttressing in your design.

[Linuc]

Sorry, but why can't you simplify everything? If the dome diameter is 800 mm make the height 400mm. Make it a half-sphere. That's what I did. Then your opening can be 250mm high.

Yepp, I probably make it unnecessarily complicated. But I want an oven specialized for neapolitan pizza so I'm going for a low dome. I have not decided about the vertical sides yet. Don't know how to fix the buttress support so I might go for an ellipse instead.

[Linuc]

I can't find mosaic tiles for a reasonable prize. Can't I just smash some bigger tiles with a hammar?

[Petter]

Yes, Lilla Napoli did that with red tiles. I think that is a better idea than to use mesh mosaic since any flaw or shape discontinuity in the oven will be far less visibility due to the smashed tiles inherrent irregularity.

[SableSprings]

I can't find mosaic tiles for a reasonable prize. Can't I just smash some bigger tiles with a hammar?

The reason I recommend mosaic tiles in ~1 ft mesh back squares is that you turn them over (mesh side up) on your concrete hearth slab and either pour perlcrete/vermicrete or lay ceramic board on top. The mesh keeps the "channels" to your weep holes open. Since the tiles are under your oven, you don't care about matching or pattern. Most stores that carry these mesh backed mosaics have odd sheets or discontinued patterns (or old sample sheets) that they want to get rid of for cheap or free. Also ask/check around since almost anyone who has done work with mosaic tiles ends up with extras.

If you can't find any sheets, then you can use smaller single tiles or break up larger tiles. The key is that you want ceramic/porcelain tiles so they won't wick or absorb water & then you need to lay garden cloth over them to keep your "channels" clear to the weep holes (if you plan on using an insulated cement pour as an insulation base). I hope that helps.

[sergetania]

Yepp, I probably make it unnecessarily complicated. But I want an oven specialized for neapolitan pizza so I'm going for a low dome. I have not decided about the vertical sides yet. Don't know how to fix the buttress support so I might go for an ellipse instead.

I believe my oven is very suitable for neapolitan pizza. When you bake neapolitan pizza you are supposed to have flame from logs on one side going across the dome. With the dome height of only 400 mm you really don't want the flame any closer to the top of the pizza. Now, if your pizza oven is much larger then yes, you want to make the dome lower to get the fire closer. With 400 mm high dome? Nope, the pizza top may burn too quickly.

My oven is 750mm in diameter and the dome height is half that. When I have flame across it cooks pizza very quickly at 800-900F (if interested check the what-I-cooked-last-night section of the forum where I posted a few pics, you can even see a pizza inside). Just sharing my observations and thoughts on the matter.

[david s]

With any oven you need a flame on the side to cook the top faster, but also to maintain the high oven temperature required for 2 min pizzas, whilst cooking with the door off.

"With the dome height of only 400 mm you really don't want the flame any closer to the top of the pizza." I'm not sure how you come to that conclusion. My experience with my hemispherical ovens which have a dome height of only 270mm is that the fire on the side with flame travelling to the top of the dome, works perfectly. Remember that a smaller dome requires a smaller fire. Some folk believe that a smaller oven chamber will heat up quicker than a large one, however in practice they're much the same because a smaller chamber requires a smaller fire. In the case of heat up it is the thickness of the floor and oven walls that is a more determining factor for heat up times.

[Petter]

Yes, Lilla Napoli did that with red tiles. I think that is a better idea than to use mesh mosaic since any flaw or shape discontinuity in the oven will be far less visibility due to the smashed tiles inherrent irregularity.

I now realize you were not refering to the outside. Sorry for the confusing answer.