2024 Neapolitan oven build

[daidensacha]

david s This is an image of the anticipated vent chamber from the inside bottom of the arch up to where the flue plate would fit. The vent opening as stated in my previous post would be 373x82mm, to match the area in size of a cross section of the 200mm flue.

The lower 116mm high edition with vertical sides would be the part inside the brick arch. The upper chamber with angled sides that transition to a 255mm square section would be above the brick arch.

The concept you gave of the wider shallower opening like the head of a vacuum cleaner is why I changed the dimensions to 373x82mm.
Alteranive possibilities would be 245x128mm (which would include the 12mm heat break.

Question is if the wider would be better to catch smoke coming out of the arch. My arch has a radius of 383mm.



In this image I have calculated the section of the inner arch that would relate to the width of the vent opening. Yellow if it were to be 245mm wide, and green if it were to be 373mm wide. Obviously the 373 wide would be more efficient when the fire is fresh and smoking more.

[daidensacha]

1. As the oven floor is always cooler near the oven mouth, insulating under the entry bricks will also help reduce this temperature difference and heat being drawn from the floor to the supporting slab. Some builders include a small gap between the floor bricks of the oven and the floor of the entry to reduce the difference and the heat sink effect.

2/3. I’m not really sure what you plan with the flue gallery when you say you plan to cast it, yet the drawing shows brick construction. The area that gathers the smoke should be at least double the area of the cross sectional area of the 6” pipe. Ie wide but shallow like the head of a vacuum cleaner. This allows you to make the gallery quite shallow to make the oven easy to work.
Regarding using standard bricks on the front I can’t see that being a problem as there will be no flame impingement there.

Thanks David. I was reading about putting a heat break in the floor as well. Although I haven’t come across any details yet how to do it, namely whether to leave it as a gap, or to fill it with vermicrete or such. Per your previous advice, I was planning on filling the heat break gap between the oven and vent arch with vermicrete.

WIth regards to casting the gallery, i’ve attached some image which as a visual aid make it more clear. I need to transition from the vent gallery opening in the arch (387x128mm) to a cast flu with diameter of 200mm. To do this, I will cast directly on the arch a piece that will transition from the “387x128mm” to a square of 255x255mm. This is specifically so I can fit the cast adapter that is made to mount the flu in. See image attached.

That is what I mean by casting the gallery. I’m not fluent in the terminology, so when I say vent gallery, what I am meaning is the area of the vent opening from the inside of the arch up to where the gallery meets the flu adapter plate.

The area of a cross section of the 200mm diameter flu is 314cm2, and compared to the gallery opening which is 495cm2. The vent gallery opening at the planned size if 57% larger than the cross section of the flu. Is it to big? If I imagine by your analogy of the head of a vacuum cleaner (brilliant way to describe it by the way, can image with the draw of the flu if it is the correct size it would be like the vacuum), at the present width I could possibly recuse the depth of the vent opening to 8.2cm. At this depth the vent opening would be 317cm2. Is this a good idea? While the opening would be 387x82mm, it would open up to meet the flu adapter plate where it would be 25.5x25.5cm, 650cm2. Double that at the vent opening. A good space for smoke to gather before going up the 200mm flu?
If I did reduce the depth of the vent opening to 82mm, it means I could reduce the depth of the vent landing by 46mm.

I attached 2 images of potential arch layouts to incorporate the above. In one, the idea was to slice regular bricks to 40 thick, and to mortar those on the front of the firebrick arch. Then I read your comment, so in the second image changed the front of the outer arch (in from of the vent opening) to incorporate regular clay bricks. I will buy some 100 year old bricks here in the next days. That would allow me to have my decorative arch in brick. The other alternative I had been considering is to tile around the arch. I see you did that was well.

[david s]

1. As the oven floor is always cooler near the oven mouth, insulating under the entry bricks will also help reduce this temperature difference and heat being drawn from the floor to the supporting slab. Some builders include a small gap between the floor bricks of the oven and the floor of the entry to reduce the difference and the heat sink effect.

2/3. I’m not really sure what you plan with the flue gallery when you say you plan to cast it, yet the drawing shows brick construction. The area that gathers the smoke should be at least double the area of the cross sectional area of the 6” pipe. Ie wide but shallow like the head of a vacuum cleaner. This allows you to make the gallery quite shallow to make the oven easy to work.
Regarding using standard bricks on the front I can’t see that being a problem as there will be no flame impingement there.

[daidensacha]

Points of confusion, needing clarification
As I come closer to starting my project, there are some points that are clear to me that I need to clarify. Any advice on the following points would be really appreciated.

1. Insulation under vent landing/ Heat break size/location

Vent area insulation

From Forno Bravo plans.
If you will be building a landing in front of your oven opening, it is not necessary to pour vermiculite concrete all the way to the front of the hearth. Rather, you can end the form where your oven vent will begin, and only pour vermiculite directly under the oven chamber and vent area.
Fill the form under the landing in front of the oven with standard concrete

What I’m not clear on here is where the oven vent begins. The components of the opening as I know them, are the inner arch, the vent landing, then the outer arch. I will have an oven landing in front of the oven, which will be plain concrete.

Q. Up to which point is it best to insulate under the vent area?
Q. Would it be up to and including the outer arch?
Q. Should this insulation be only under the vent area, or also under the arch walls?

Heat Break
I also want to integrate a heat break, although I am unsure where the better place to put it is. I read some put it between the inner arch and the vent gallery, and others put it between the vent gallery and the outer arch. David also has explained to me the difference between heat loss/transference through conduction, and radiation, and the benefit the heat break has with reducing to a degree heat transference through conduction, but not radiation.
By David’s logic, it seems practical to have the heat break between the vent gallery and the outer arch, to prevent the heat transference by conduction to the outer arch, and prevent possible cracking from expansion to the face that is exposed to the outside conditions.

I was planning on 10mm heat break filled with vermicrete insulation. Inner arch, vent area and outer arch come to 460mm (18.11”). This is without adding a decorative arch in front. How do others do this if you want to add a decorative arch in front? Can you replace the outer arch firebricks with normal bricks? I have used recycled bricks a lot on difference projects, and they do break down over time. If I were to use them as the front arch instead of firebrick, i’m concerned what the heat will do to them over time.
Q. Is it better to plan for and add a separate decorative arch in front of the vent area?

2. Vent Gallery Construction

I am working on formwork to cast my vent area out of castable concrete directly on the brick arch and vent. It will be 50mm thick, and wrapped in heat blanket before bricking up around it leaving an air gap as well. My reasoning is to prevent heat transference to the outer wall of the chimney.

The planned inside of the vent gallery was to be be 373mm wide x 228mm deep x 235 high (from inner brick arch to top of vent). I have flipped back and forward between half brick, and full brick, uncertain if the half brick width will be enough to create sufficient draw.
Q. Can I reduce the depth of the vent opening by half a brick to 114mm and will it be big enough to draw smoke up and out the 200mm flu??

3. Recommended Flu size
I read that for 40” oven, mine is 1020mm close enough to 40”, the recommended flu size is 200mm. It will be 835mm from the inner arch, to the top of my oven chimney. The oven will be in the corner of an outdoor kitchen covered with a terrace roof. I planned to put a double wall flu on top of the chimney and through the roof within will extend the height.
Q. What would the recommended height be to creat a sufficient draw?

4. Gallery Landing Depth
I have little experience with creating a chimney, and the relationship between the oven opening size, vent gallery size, and flu size (width and height). Any input on my plan, or the correct sizes and dimensions so it functions well would be appreciated.

Image below: My arch plan with the heat break between the vent gallery and the front arch. Also with a 373 x 114mm vent gallery opening. I’m struggling to understand how to reduce the depth of the gallery landing as in this image, it is 460mm including the decorative arch.

Options in Table columns
Half bricks: If I use half bricks with a 10mm heat break between the vent landing/gallery and the separate decorative arch.
Option 1: Reduce half bricks either side of vent opening to 80mm, with a vent opening of 114mm, and heat break of 10mm between vent gallery and decorative arch.
Option 2: Reduce half bricks either side of the vent opening, and the decorative arch to 80mm, vents opening 114mm, heat break 10mm between vent gallery and decorative arch.
Option 3: Per option 2, but removing the decorative arch and placing heat gap between inner arch and the vent landing/ gallery.
Option 4: Per option 2, but increasing outer arch to half bricks(114mm) and and placing heat gap between inner arch and the vent landing/ gallery.

	Half bricks	Option 1	Option 2	Option3	Option 4
Inner arch	114 mm	80 mm	80 mm	80 mm	114 mm
Vent Opening	114x373 mm	114x373 mm	114x373 mm	114x373 mm	114x373 mm
Front of arch	114 mm	80 mm	80 mm	80mm	114 mm
Heat break	10mm	10 mm	10 mm	10 mm	10 mm
Separate Decorative Arch	114 mm	114 mm	80 mm	None	None
Outer vent landing	352 mm	318 mm	284 mm	204 mm	238 mm
Total vent landing	466 mm	398 mm	364 mm	284 mm	352 mm

** The options listed in this table based on the assumption that 114mm x 373mm vent opening will create sufficient draw.
** Outer vent landing is from the outside of the inner arch to the end of the vent landing where it meets the oven landing in front of the oven.
** Total vent landing is from the inside of the inner arch to the end of the vent landing where it meets the oven landing in front of the oven.

[daidensacha]

Apart from the water resistant calcium silicate board which I’ve never seen or used, the standard cal sil, 5:1 vermicrete will suck up any water into itself just as much as blanket will. The good news is that when any of them are dry they just go back to their original condition. The one that I’ve found that does not is mill board. It turns to mush and is very weak when it’s dried.

Two pallets with all my stuff for the oven was delivered Friday and is now sitting in the garage waiting for me to put it together. It’s good to see and feel the stuff, because until now my only sense of how it is has been from images and reading about it.

The calcium silicate board is quite solid, and not what I was expecting. Much harder than I anticipated, and I see why you say it wont have issues holding the oven weight. I don’t really want to give it a water test, although that might happen when I have a left over offcut. I guess as I have enough I’ll just go ahead and cut it 75-100mm larger than the outside of the dome. Just means I’ll need to take care not to damage it or get it wet when doing the bricks.

[david s]

Apart from the water resistant calcium silicate board which I’ve never seen or used, the standard cal sil, 5:1 vermicrete will suck up any water into itself just as much as blanket will. The good news is that when any of them are dry they just go back to their original condition. The one that I’ve found that does not is mill board. It turns to mush and is very weak when it’s dried.

[daidensacha]

Calcium Silicate floor insulation coverage

I was watching another youtube build of a precast Forno Bravo oven, and the guy mentioned it is good to extend the calcium silicate floor insulation so that the insulation blanket covering the dome comes down and stops on top of it. If there is water this will prevent it wicking water up the dome.

Is it necessary? In nearly all the builds I have seen where the soldier course is laid on the calcium silicate board, it is cut inline its where the outside of the dome meets it. Then the blanket and vermicrete is added, but I haven’t heard this mentioned or seen before.

I do have enough calcium silicate board, if I am crafty and stick the off cuts here and there. I’m wondering how others who make ovens do it?

Example of the calcium silicate board extending past the outside of the dome under the fire blanket covering the dome.

[daidensacha]

2 x 3000x50x1.5mm V2A 1.4301 Stainless steel bands delivered yesterday, along with bricks, insulation, mortar, plaster. Moving on with the original plan to attach the stainless steel bands with bolts and angle.

Thank David for the info. Spoke with my wife’s brother who is an engineer, he concurs with your opinion on not needing springs. I’m not sad about that.

My brick cutting jig is ready to weld together, all the parts are there and prepared. Once they are together I can measure for fixing holes that will fix the angle of the brick for the individual courses. Those holes will be the pivot from which the jig will move the brick to make left and right cuts and the precise angle for the course radius.



Once this is done I’ll complete the adaption for the wet cutting saw platform.

[david s]

As I’ve said previously, The expansion issue is probably not something you need to worry about and the amount is around half that of a kiln. You are correct in concluding that the bricks will expand tight against the band and then as it heats and expands the pressure is relieved and the reverse probably occurs on cooling. However relieving this variable tension with springs, although in principle may sound like an ideal solution, could be introducing further problems. The use of steel rods and nuts is the common method for kilns and I’ve never seen springs used although I’m sure if it were a problem someone would have tried it and you’d see it as a common method. Most electric kilns just have an outer sheet steel skin to hold the bricks in position. Cheaper ones that don’t have stainless often rust out well before the bricks are worn out. I’ve got plenty of expensive insulating fire bricks from old rusted kilns.
The expansion at 500c is not really a big problem, only around 3mm so probably not worth worrying over. Using springs may work, but I wonder how they’d look in 10 years time, particularly as they won’t be stainless?
The first oven I built had a problem with the door. I thought I’d be clever and made the door first with a wooden face and a cast insulating panel(using crushed insulating fire brick as the aggregate for the mix. In order for the door to release from the opening I made the door with a slight conical edge so it would fit and seal nicely against the oven mouth.placing some plastic around the door edge so the dome casting mix would release nicely when the dome casting was placed over the door. The door ofcourse removed beautifully, but on firing if the cool door was fitted to the hot kiln, on cooling it jammed so tight that the door became impossible to remove easily without damage to either it or the oven mouth. A few millimeters can create a big problem. Obviously I abandoned this type of door for future ovens. With the thousand of years oven oven building history and probably millions of variations and solutions to problems I should have realised that this solution must have been tried many times before and like mine, found to be unsuitable.

[daidensacha]

I found some examples of springs, one takes 200kg, the other 300kg. They would be attached to angle at the arch, with insulation between them and the dome, so protected from any significant heat. Question is what sort of pressure would be sufficient force to reinforce the dome? That would allow expansion/ contraction, and the pressure would remain pretty constant.

[daidensacha]

david s The feedback is good, and appreciated. It can only help me to understand and pushes me to research. It’s a rabbit hole I hadn’t anticipated, but I’m committed so let’s go.

To come to the right answers I need the right questions.

Firebrick expansion

How much do firebricks expand when heated to 500C?

• The first obstacle to getting relevant accurate info is that it depends on the type, brand, and materials used in the firebricks.
• Searching through information via Google, i came across a white paper with an interesting statistic. The thermal expansion of a 150mm firebrick at 500C was .254mm. At this rate, with a dome that approaches a circumference of 3 meters, it extrapolates to 5mm expansion.
• The unknowable for me is the temperature distribution in the brick, from the inside exposed to the fire which would be hottest, to the outside of the dome which would presumably be less.

V2A 1.4301 Stainless Steel

• Some data on its qualities. Austenitic (non-magnetic), low thermal conductivity (heat resistant), corrosion resistant.
• Has a high module of elasticity at 200 GPa. (see Elastic Modulus: Definition, Values, and Examples for explanation of module of elasticity)
• Coefficient of thermal expansion 16 K-1 · 10-6 (this was not in my schooling. doing my head in calculating it for 3000mm long strip, get varying result from 12-25mm @460C)

From reading, it’s clear both the bricks and stainless expand, but to different degrees and at different times due to the differing rates of exposure to the heat.
If my calculation is anywhere near the ball, then the stainless steel would expand slightly more than the firebrick over a 3 meter length. Enough that when the oven is heated, there would be less reinforcing tension translated from the stainless band to the oven dome.

Open to any input if someone has more education about these terms and what it means.

It did come to me as a possibility to add a high tensile spring into the band mechanism. Then enough tension could be added that the spring would allow for expansion and contraction, albeit the reinforcing strength would then also differ, it would possibly be a way to eliminate any slack in the band when the oven is at full heat. Thoughts?

[david s]

It is common for brick kilns to have some steel bracing. During firing thr refractory material (bricks) will expand from the heat well before any steel bracing, but as the temperature climbs and the steel begins to expand, sometimes there needs to be some adjustments by tightening the bracing. As the kiln cools the steel bracing starts its shrinking and should be loosened again to the original position. Kilns generally reach more than double the temperature of a WFO, and therefore double the thermal expansion. , but the same principles apply.
For most steel braced ovens the adjustment is not possible because the adjusting nuts are probably deeply embedded in insulation.
This difficulty does not present itself for an oven built as a hemisphere, because its form is far more stable and self supporting. The other big advantage of a hemisphere is of course its superiority as being the most efficient chamber for even circulation and therefore economy. Kilns built in a cube formation are much better to load wares into, but end up with cool spots in the corners. Bread ovens with a square or recatangular chamber with a low roof hold more bread and are easier to load than a circular or hemispherical dome.These are usually considerations for commercial ovens rather than those for home owners.

I'm not trying to frighten you away from your plan, but just be aware of the extra work you have to go to as well as overcoming some problems that may occur.

Check back here

https://community.fornobravo.com/for...137#post457137

[daidensacha]

I gave up the dream of building a true Naples brick oven, but I'm still interested in seeing others do it. My questions is how the stainless band(s) that hold/buttress the solider course deal with the expansion can comes during firing. I once saw a precast oven on a trailer where the builder had accidentally made the metal straps that crossed the oven to keep it from sliding off the trailer too tight. You could actually see the oven dome trying to expand upward and then being squished outward then it was fired. Or at least that was what it looked like.

How do builders deal with that.

And congratulation on the great work so far. It's all very thoughtful.

Thank you. All my work to date has been in sketch-up, and google. But it’s been a great process, refining, changing, building it virtually over and over again. It’s helped me to understand what’s needed, and how to tackle details that had me stumped, like the arch. This still evolves and changes, and when I make one change often it cascades to other aspects so I need to alter other parts of my plans.

I first came across Forno Bravo plans around 9 years ago, and it’s been my dream since then to build an oven. I decided this year to take the jump and “do it”. I’m no expert, far from it. There are many others in this forum who have far greater knowledge and experience with these ovens.

I did read about expansion of the firebricks. If I remember correctly, nature of the material is such that it is minimal. The greater stress I am worried about is the weight of the dome on the soldier course. I am laying them directly on the insulation board, without cementing under them as that isn’t really possible on the insulation board. I also read it shouldn’t be anyway, to allow movement and expansion, which will stop the reduce the possibility of cracks.

I don’t imagine the reinforcement strap should be tensioned with a view to stopping expansion, but from my view a potential explosion of the soldier course outwards from the downward pressure of the weight of the dome. My brother in law is an engineer, so i’ll lean on him for advice about the the required tension. It’s a good point you raised though, and i’ll be mindful when it comes to tightening the bands. Still a little way off, will start excavating a 9000x5000mm are for a terrace come outdoor kitchen. The pizza oven will be in one corner, with 2 meter bench on the left, and 3 meter bench on the right with a fireplace and grill. It’s still cold, too cold to lay the slab, but I can prepare the foundation and formwork so I’m ready as soon as there is a good patch of warmer weather.

[totalnewbie]

I gave up the dream of building a true Naples brick oven, but I'm still interested in seeing others do it. My questions is how the stainless band(s) that hold/buttress the solider course deal with the expansion can comes during firing. I once saw a precast oven on a trailer where the builder had accidentally made the metal straps that crossed the oven to keep it from sliding off the trailer too tight. You could actually see the oven dome trying to expand upward and then being squished outward then it was fired. Or at least that was what it looked like.

How do builders deal with that.

And congratulation on the great work so far. It's all very thoughtful.

[daidensacha]

rsandler Thanks, I’m moving forward with my plan to build the brace reinforcement out of V2A Stainless steel.

I picked up 4 x 100mm lengths of x50x3mm flat V2A stainless steel, and 4 x 40mm lengths of V2A 25x4mm stainless tube yesterday. Called into a local engineering workshop today and they were more than happy to weld them together for me.


All tthe other pieces are on the way.
1 meter M16 A2 stainless threaded rod, with washers and nuts. ( I will cut this into 4 x 250mm lengths, or shorter, once I know what’s needed.)
2 x 3000x50x1.5 V2A stainless sheet strips.

This will last my lifetime.

All I have to do is drill the holes, and join the bands with solid stainless rivets. But I’ll leave that until when I have the dome and arch up, and know the exact measurement I need to fix them. Having a plan with the details worked out puts me at ease.