800 mm homebrew cast

[Linuc]

Okey, instead of starting new threads with different questions I'll try to document my journey building an neapolitan oven with homebrew cast. I have never built anything in my entire life and I have already encountered more challenges than I initially thought. It's exciting!

Location, foundation and stand:
I initially started digging for my foundation at a place where the ground was very, very wet. I had to change location and it was better - but there were lots of roots from a birch and stones so it was very challenging. I dug for a total of probably 15 hours. My foundation - which I did for a couple of weeks ago - is 1200 mm x 1400 mm. I made the pit almost 2200 x 2400 mm and fixed drainaged pipes and isolated with cellplast/cellular plastic. The foundation is 100 mm thick aerated concrete with reinforcement. The pit is now filled with gravel. I had to use drainage and cellular plastic because of the ground frost (not sure of the translation here...) and I don't want the foundation to lift when the cold weather hits.

The stand is made with 30 pieces of 150x190x590 mm blocks.

This weekends project is to make the concrete table. I'll use fine concrete, reinforcement and the plan is to make it 1200 mm x 1400 mm (or slightly bigger) and 70 mm thich. The reinforcement is only 5 mm thick so I will also use a couple of rebar (unknown size at the moment, but a bit thicker then the mesh).

Does this sound like an ok plan? Will the concrete table, foundation and stand be strong enough for the oven?


Oven plans:
800 mm ID
50 mm cal sil board directly on the concrete table
30-50 mm mix of sand, ash and salt to lay the floor on (napotalean style)
32 mm firebricks (biscotto bricks from Italy)
260-290 mm dome height inside (I've read that n
eapolitan ovens have a lower dome height)
50 mm homebrew casted on sand dome with SS needles and PP fibre
25 mm or maybe 50 mm ceramic blanket (the blanket is 25 mm but I'll try to make two layers)
50 mm perlite/portland

Chimney: 150 mm diameter. 1000 mm height.

I'll attach a couple of picture from my work so far.

Input is welcome!

[Kris S]

Hi, I'm very new to building a pizza oven myself, but here are my 2 cents of thought:

• 32mm firebricks: even though 32mm is not really thin, I think 5 to 6cm thick bricks are more the 'norm'. I guess thicker bricks mean more heat retaining mass, which could be usefull to keep the oven hot longer. a thinner brick might heat up faster and could be ideal if you're mainly interested in pizza's?
• 25 mm or maybe 50 mm ceramic blanket: from what I've read you can't overdo insulation, so go for the 50mm
• 50 mm perlite/portland: I'm not sure but I think 50mm is a bit much for a render layer, 2-3cm is more normal.

I could be wrong on all three . Waiting for the experienced posters to chime in.

looking forward to your build.

[Linuc]

Hi, I'm very new to building a pizza oven myself, but here are my 2 cents of thought:

• 32mm firebricks: even though 32mm is not really thin, I think 5 to 6cm thick bricks are more the 'norm'. I guess thicker bricks mean more heat retaining mass, which could be usefull to keep the oven hot longer. a thinner brick might heat up faster and could be ideal if you're mainly interested in pizza's?
• 25 mm or maybe 50 mm ceramic blanket: from what I've read you can't overdo insulation, so go for the 50mm
• 50 mm perlite/portland: I'm not sure but I think 50mm is a bit much for a render layer, 2-3cm is more normal.

I could be wrong on all three . Waiting for the experienced posters to chime in.

looking forward to your build.

Thanks Kris.

Yes, I'm mainly interested in pizza's and my research has shown that these biscotto bricks is the same as they use in Neapel. Unfortunately I couldn't find thicker bricks than 32 mm.

My plan is to use two layers of ceramic blanket but I'm not sure how to calculate if I have enough. I guess we'll see. The blanket is 600 mm wide and 7200 mm long. Hopefully it's enough for two layers.

Ok, maybe I'll go with 2-3 cm instead. Thanks.

[Kris S]

A 80cm i.d. oven with 5cm thick walls gives a sphere with a radius of 45cm and a surface of 2.54m², divide this by 2 and you'd get +/- 1.27m² as the surface of your oven.

Your blanket is 0.6 * 7.2m = 4.32m² with which you could fit about 3 layers!

[Linuc]

A 80cm i.d. oven with 5cm thick walls gives a sphere with a radius of 45cm and a surface of 2.54m², divide this by 2 and you'd get +/- 1.27m² as the surface of your oven.

Your blanket is 0.6 * 7.2m = 4.32m² with which you could fit about 3 layers!

Genius. Thanks. Numbers not my thing!

[SableSprings]

Linuc, you need to put your ceramic board directly under the cooking floor bricks. Sand, ash, & salt are not as good for insulation under the board as 50 mm of 5:1 (perlite:cement) will be. Also, by laying your board directly on your hearth it will absorb moisture...you need to separate the board from seepage or wicking from the perimeter of the oven. The best base would have some weep holes through the concrete top slab, some porcelain or glass mosaic tiles (to create pathways for moisture to reach the weep holes), insulation board, thin layer of sand & clay (for leveling the cooking floor), then your oven. The entire oven (cooking floor & cast dome) must be isolated & insulated. Because of the cost of insulation board (CalSil), many builders use that 5:1 perlcrete as a layer between the drain tiles & board to increase base insulation at a lower cost. Because you are only interested in pizza, the 50 mm of CalSil should be fine, but 50-100 mm of perlcrete underneath it would be much better.

Again, being in a wet climate, your main problem is going to be keeping your insulation dry. Most insulation board, perlcrete, & insulating blanket materials readily absorb water. If your insulation gets wet, it will be very difficult to heat your oven.

Good Luck, we're all looking forward to your build!

[SableSprings]

Linuc, here's a link to a pretty well illustrated oven build that shows the weep holes (these were cast in the hearth, but you can drill them just as well) & use of porcelain tile sheets under the cooking floor & oven.

https://community.fornobravo.com/for...andpoint-idaho

[Petter]

Lots of good advises, I will add that (SF for example) Neapolitan ovens have a r22 cm semi circle opening and about 39 cm dome height. With such a small oven, you could make the door 20 cm high, but keep the width at 43-44 cm for oven access. That would result in a dome height of 35.5 cm to keep the ratio constant.

If 32 mm biscotto is what you can get, it is enough with 1-2 cm sand/fireclay mix. Don't use salt in an outdoor oven. It will dissolve when wet. The ash is of no use either, just leave it. My biscotto were very uneaven, 35-45 mm thick and I turned them upside down and casted them with refractory cement and sand to a totalt thickness of 50 mm. It was fuzzy but worked well.

Use at least 100 mm insulation all around. It will increase the oven versitality besides pizza.

[Linuc]

Linuc, you need to put your ceramic board directly under the cooking floor bricks. Sand, ash, & salt are not as good for insulation under the board as 50 mm of 5:1 (perlite:cement) will be. Also, by laying your board directly on your hearth it will absorb moisture...you need to separate the board from seepage or wicking from the perimeter of the oven. The best base would have some weep holes through the concrete top slab, some porcelain or glass mosaic tiles (to create pathways for moisture to reach the weep holes), insulation board, thin layer of sand & clay (for leveling the cooking floor), then your oven. The entire oven (cooking floor & cast dome) must be isolated & insulated. Because of the cost of insulation board (CalSil), many builders use that 5:1 perlcrete as a layer between the drain tiles & board to increase base insulation at a lower cost. Because you are only interested in pizza, the 50 mm of CalSil should be fine, but 50-100 mm of perlcrete underneath it would be much better.

Again, being in a wet climate, your main problem is going to be keeping your insulation dry. Most insulation board, perlcrete, & insulating blanket materials readily absorb water. If your insulation gets wet, it will be very difficult to heat your oven.

Good Luck, we're all looking forward to your build!

Thank's for the advice. The sand, ash & salt-mix was for leveling the floor and making a stable surface for the biscotto. Something I read in a book about neapolitan ovens. But it makes sense not using salt in an outdoor oven as Petter pointed out.
My first plan was to only have 100 mm of perlcrete underneath but I read that the curing takes a couple of weeks and I wanted to save some time so I bought a CalSil board. But maybe I should use perlcrete anyway. Problem is I only have 200 litres of perlite and that should be enough for 10:1-isolation over the oven. I guess it's not enough for both over and under?

I've seen the weep hole and tile solution. Thought it wasn't really necessery, but it makes sense. How many and how big weep holes in the concrete is enough? I guess the tiles needs to be thin?

[Linuc]

Lots of good advises, I will add that (SF for example) Neapolitan ovens have a r22 cm semi circle opening and about 39 cm dome height. With such a small oven, you could make the door 20 cm high, but keep the width at 43-44 cm for oven access. That would result in a dome height of 35.5 cm to keep the ratio constant.

If 32 mm biscotto is what you can get, it is enough with 1-2 cm sand/fireclay mix. Don't use salt in an outdoor oven. It will dissolve when wet. The ash is of no use either, just leave it. My biscotto were very uneaven, 35-45 mm thick and I turned them upside down and casted them with refractory cement and sand to a totalt thickness of 50 mm. It was fuzzy but worked well.

Use at least 100 mm insulation all around. It will increase the oven versitality besides pizza.

According to a book I have about neapolitan ovens the totalt height from floor to roof should be the ID divided with 3-3,2. Since I'm making an 800 mm ID oven the dome height would result in only 26,6 cm but that seems really low so I'm thinking of 30 cm height instead. I'm planning making the door 20 cm high, 44 cm wide and 13 cm deep.

Another "weird" thing with my oven is the form. Neapolitan ovens have the first row bricked with the stone standing (soldier course) and then the dome, but since I'm casting with homebrew I'll try to make standing walls and the the dome/elipse form, but still only 26-30 cm floor to roof. Make sense?

Thank's for the advice not using salt. Makes sense.

100 mm insulation. So 50 mm CalSil under is not enough? Does 50 mm ceramic blanket and 50 mm of perlcrete sounds like a good plan?

[SableSprings]

Linuc, we look at about 12-13 mm (1/2") diameter weep holes as good - a pattern of five under your stove is perfect (imho ). 50 mm of CalSil under your cooking floor is the minimum (and common builder's choice) for insulation but adding any additional insulation under the board is a good thing (again imho ). I think 50mm CalSil on top of 50mm 5:1 perlcrete would serve you well.

Also, be aware the low vault domes do require additional butressing (often strapping) because of the increased outward thrust of this design.

[Petter]

According to a book I have about neapolitan ovens the totalt height from floor to roof should be the ID divided with 3-3,2. Since I'm making an 800 mm ID oven the dome height would result in only 26,6 cm but that seems really low so I'm thinking of 30 cm height instead. I'm planning making the door 20 cm high, 44 cm wide and 13 cm deep.

Another "weird" thing with my oven is the form. Neapolitan ovens have the first row bricked with the stone standing (soldier course) and then the dome, but since I'm casting with homebrew I'll try to make standing walls and the the dome/elipse form, but still only 26-30 cm floor to roof. Make sense?

Thank's for the advice not using salt. Makes sense.

100 mm insulation. So 50 mm CalSil under is not enough? Does 50 mm ceramic blanket and 50 mm of perlcrete sounds like a good plan?

I also have read the height to diameter ratio. I think it mostly apply to large ovens. As you also have noticed, something has to give in a small oven. Your door suggestion looks very good. I would however recommend a higher dome to capture more heat. The heat will also be more eavenly distributed. A 9/16 ratio is used by SF for example.

For a cast oven you don't use the soldiers, calculate an ellipse or super ellipse instead. The super ellipse looks like a rectangle and ellipse combined. See FGM or Forno Bravo for cross section examples. The continuous transition from vertical to horizontal provides better strength than a sharp ditto.

50 mm is enough for cooking, I would expect only minor performance difference with a live fire. The 50 mm extra provides a peace of mind and the possibility to use the oven to more than pizza. I recall that If T=500 C at the floor bricks, the temperature at steady state is just below 70 C on the outer side of the 50 mm CaSi-board. Don't waste heat :-)

[Linuc]

Linuc, we look at about 12-13 mm (1/2") diameter weep holes as good - a pattern of five under your stove is perfect (imho ). 50 mm of CalSil under your cooking floor is the minimum (and common builder's choice) for insulation but adding any additional insulation under the board is a good thing (again imho ). I think 50mm CalSil on top of 50mm 5:1 perlcrete would serve you well.

Also, be aware the low vault domes do require additional butressing (often strapping) because of the increased outward thrust of this design.

Thanks. How far apart should the holes be? Should I just spread them on the area under the CalSil-board?

Not entirely convinced about the 50 mm 5:1 perlcrete yet. I of course do understand the meaning, but I have just ordered 200 litres which is enough for the on oven insulation - but not for both. My budget is already broken so my better half won't be super happy if I keep buying stuff for the oven!

I do not understand what you mean about butressing. Is this a problem when I'm casting the dome or when it's done?

[Linuc]

I also have read the height to diameter ratio. I think it mostly apply to large ovens. As you also have noticed, something has to give in a small oven. Your door suggestion looks very good. I would however recommend a higher dome to capture more heat. The heat will also be more eavenly distributed. A 9/16 ratio is used by SF for example.

For a cast oven you don't use the soldiers, calculate an ellipse or super ellipse instead. The super ellipse looks like a rectangle and ellipse combined. See FGM or Forno Bravo for cross section examples. The continuous transition from vertical to horizontal provides better strength than a sharp ditto.

50 mm is enough for cooking, I would expect only minor performance difference with a live fire. The 50 mm extra provides a peace of mind and the possibility to use the oven to more than pizza. I recall that If T=500 C at the floor bricks, the temperature at steady state is just below 70 C on the outer side of the 50 mm CaSi-board. Don't waste heat :-)

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.

[SableSprings]

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.