Question about fire-brick wall thickness

[Akse]

Trying to find a good balance between a pizza oven and an oven suitable for baking and slow roasting.

In Norway we typical build our traditional ovens from 4.5" (110mm) fire bricks covered with a 5" (130-150mm) layer of sand used as heat absorbing mass. And isolation thereafter.

I planning to use not only one layer but two layers of fire bricks that will give me a total dome wall thickness of 9" (220mm). And isolation thereafter. I will not use any sand our cladding for heat absorbing.

The floor will consist of two layers as well. Bottom layer 4.5" (110mm) fire bricks and on top 1.3" (33mm) layer of Saputo Biscotto Pizza Stones .

Anyone with experience from using ovens with thicker walls ? Or anyone with expertise that can advice me? My concerns will be that it will take too long time to heat up this oven?

Thank you for any answers.

[david s]

Most brick ovens are around 4” thick. If the bricks are laid on their edge the walls would be 3” thick. Many builders attempting this have found that there is insufficient strength in the mortar with thinner walls leading to structural problems (more cracks). Cast ovens however are usually much thinner typically having walls of 2” thickness. They have proved adequate in heat storage (thermal mass) for retained heat cooking as well as providing acceptable heat up times and low fuel consumption. Obviously the more thermal mass you have the higher the fuel consumption and the longer time required to reach temperature. If you plan on baking for the village every day where the oven never really cools down, a double brick could be a consideration. However, for occasional use thinner walls and better insulation is a far better choice. The floor thickness should be the same as the wall thickness. Sand is a poor insulator. Pumice, vermiculite or perlite are far superior and a couple of layers of 25 mm ceramic fibre blanket against the hot surface first even better again. One layer if money is tight.

[SableSprings]

Welcome to the forum Akse! As David noted above, the majority of ovens here have wall and cooking floor thicknesses of 2-4" (5-10 cm). I bake mostly bread in my oven and can easily do 30 loaves on a single firing. My oven is a variation of the Pompeii dome and I like to load 6 - 8 baguettes or 5-6 round loaves at a time. The main issue for me is not the temperature as much as it is access to the loaves through the oven opening.

Again, as David noted, the thicker walls are more common in community/commercial ovens that are used every day and never cool completely. A well insulated, 1 m diameter oven (ceramic blanket or insulating concrete -- NOT SAND) with the 2"- 4" thick refractory can be heated to pizza temperatures in a couple of hours. I put a load of wood in my 1 m oven at night and damp down the fire. By morning, my oven is fully saturated with heat and cleared (+700F or ~370C). I work my dough in the morning and start the bake when the oven has cooled down to 575F (about 300C). I normally bake 20 - 25 loaves by doing 6-8 loaves per load. Plenty of heat left in the oven to roast meats and vegetables after baking bread. Many members report their ovens retaining cooking heat of 350-450F (175-230C) for several days after a pizza party. The best investment you can make is the insulation above and below your firebrick or cast oven shell.

I guess the real question we need to ask is a bit more detail about your intended use for the oven. Just for you and your friends a couple times a week at most...or everyday, preparing bread and meals for "the village"?

Hope our answers help your oven design decisions.

[Gulf]

Hi Akse, welome to the forum.

Your question reminds me of Mexman 's adventure. His first oven was 8" thick with extremely long heat up times. His goal for the perfect oven was for a pizza oven (short heat up times). A baker who wants to make multiple batches of bread with the retained heat may want 9" thick walls. I guess that it all boils down to is, what is the main purpose that your oven is intended?

Here is a link to his first oven Should I tear the dome of my FB WFO off?

Here is a link to his third attempt. 36 inch FB Pompeii with tile dome.

[Akse]

Thank you for answers, interesting reading.


I will try to clarify my question,


I plan to build an oven for long time roasting, also good for pizza.



Option 1,

Fire-bricks 4-4.5 inch.

5 inch sand our cladding for heat absorbing.

Heat insulation



Option 2

Double layer fire bricks, total 8-9 inch

Heat insulation.



I hope to start an option 2 discussion.



My idea is to replace heat absorbing «sand our cladding» with an extra layer of bricks, option 2.



To clarify, the only perpouse for layer two will be for replacing heat absorbing layer.



My theory is that extra layer with bricks will be more efficient caused from high density and better Thermal Conductivity.

High density and better Thermal Conductivity will be faster to heat and less energy consuming?



Double layer fire- bricks totally isolated from any other concrete product should give med benefits as less structural cracking over time.




Roger,

[mbarrett5076]

I am relatively new... ok just new... I have fired up my 1 meter oven several times. It is built with 1 layer of brick the walls are 4", ceiling 6" with high temp concrete/fire clay mix. The floor is 1 layer flat (3 inches thick) on top of a 3.5 inch thick concrete/fire clay mix, on top of 7 inches of std concrete. (all w/ rebar). The walls and ceiling are insulated w/ 3" of ceramic wool blanket. If I had to do over again, I would have built it on a more insulated substrate vs. the concrete/fire clay.

Most recently, fired it to 850/875f (454 C) for an evening of pizza about 2 hours at this temperature (fire burning during the cooking). Closed it up at 9 pm, 6 am next morning was 425f (220 C). I lost 10 degrees f ever 60 minutes down to 280 (138 C) and then it was more like 5 degrees f an hour. it was 185 f (85c) the next morning at 8 am. at which time I stopped measuring. The key to this schedule was getting the single layer of bricks fully heat saturated. The insulation may be more effective in retaining heat than a 2nd layer.

The oven door is sheet metal w/ 1" vermiculite insulation and doesn't fit as good as it it will soon. It fits well, but certainly isn't airtight... yet.

I measured the ceiling, walls and floor. It was clear the floor was absorbing the radiant heat (once the fire is out, the heat radiates in all directions, not just up) and was cooler than the walls oven until it the temp got down to 225 f or so. Then heat loss slowed way down. I think the floor had reached some level of relative saturation.

Small physics moment - heat transfers most efficiently with the greatest differential in heat. The insulation creates the environment where there is little difference in energy/heat. the greater mass will continue to pull heat away from the oven for a much longer time, until it reaches the temp.

The key to avoid structural cracking is very thin mortar joints and plenty of them. There is a wonderful artisan who cuts every brick and lays the brick lengthwise toward the oven. This creates the maximum number of very thin mortar joints w/ the expansion forces being only 3" by 4" in cross section before a mortar joint vs. 9" until the next mortar joint is reached.

Lastly, the oven heats reasonably quickly with only a single layer

[david s]

Hi Roger,

Modern insulating materials have largely replaced the need to increase thermal mass to extend cooking time. A good example of this is the widespread use of ceramic fibre, low mass kilns replacing old fashioned double brick kilns. The savings in fuel being enormous because the energy input just goes to heat the wares and kiln furniture rather than adding a heavy kiln structure as well. For ovens this means a lower thermal mass, but far more efficient insulation to hold escaping heat is also a better option. This advantage well outweighs any benefit you might gain from increased structural integrity from the double brick. Two layers of brick also double the cost of materials and labour of the dome. The mantra for this forum which attracts universal agreement from members is “insulate, insulate, insulate”

Cheers,
Dave

[Akse]

Thank you so much for advising me.

I am not convinced that a single layer is ideal for my new oven. And two layer can be too much as you suggested.

Therefore I started to consider using one and half layers?

To find out best possible solution, I will run some test.

I have the possibility to build the oven with one layer first using mortar. Wrap oven with ceramic fiber insulation blankets. Then run my tests.

Next step will be to build second brick layer without mortar, stack them dry outside first layer. Wrap oven with ceramic fiber insulation blankets. Then run my tests.

Then I can compare results with one and two layers, posible ending up using one and half layer.

All bricks are ordered with an angle to fit my dome.

See attached photo.

[david s]

That sounds like a reasonable plan, but lots of extra work. Remember also that with no mortar there will be poor heat transfer by conduction with far less points of contact. This will probably result in longer heat up times and faster heat drop off than if both layers were connected with mortar.
Your results will be interesting.

[david s]

I am relatively new... ok just new... I have fired up my 1 meter oven several times. It is built with 1 layer of brick the walls are 4", ceiling 6" with high temp concrete/fire clay mix. The floor is 1 layer flat (3 inches thick) on top of a 3.5 inch thick concrete/fire clay mix, on top of 7 inches of std concrete. (all w/ rebar). The walls and ceiling are insulated w/ 3" of ceramic wool blanket. If I had to do over again, I would have built it on a more insulated substrate vs. the concrete/fire clay.

Most recently, fired it to 850/875f (454 C) for an evening of pizza about 2 hours at this temperature (fire burning during the cooking). Closed it up at 9 pm, 6 am next morning was 425f (220 C). I lost 10 degrees f ever 60 minutes down to 280 (138 C) and then it was more like 5 degrees f an hour. it was 185 f (85c) the next morning at 8 am. at which time I stopped measuring. The key to this schedule was getting the single layer of bricks fully heat saturated. The insulation may be more effective in retaining heat than a 2nd layer.

The oven door is sheet metal w/ 1" vermiculite insulation and doesn't fit as good as it it will soon. It fits well, but certainly isn't airtight... yet.

I measured the ceiling, walls and floor. It was clear the floor was absorbing the radiant heat (once the fire is out, the heat radiates in all directions, not just up) and was cooler than the walls oven until it the temp got down to 225 f or so. Then heat loss slowed way down. I think the floor had reached some level of relative saturation.

Small physics moment - heat transfers most efficiently with the greatest differential in heat. The insulation creates the environment where there is little difference in energy/heat. the greater mass will continue to pull heat away from the oven for a much longer time, until it reaches the temp.

The key to avoid structural cracking is very thin mortar joints and plenty of them. There is a wonderful artisan who cuts every brick and lays the brick lengthwise toward the oven. This creates the maximum number of very thin mortar joints w/ the expansion forces being only 3" by 4" in cross section before a mortar joint vs. 9" until the next mortar joint is reached.

Lastly, the oven heats reasonably quickly with only a single layer

Insulating under the floor of the oven is essential. Folk think that heat rises, but that's only by convection, it doesn't care about direction when travelling via conduction. If there is no insulation under the floor then heat can easily wick away into the concrete supporting slab. You may get the dome to saturate with heat but at pizza temps the floor will drop off in temp after a few pizzas resulting in frequent floor reheating and soggy uncooked bottoms.

"...was cooler than the walls oven until it the temp got down to 225 f or so. Then heat loss slowed way down. I think the floor had reached some level of relative saturation."

That is because "the higher the temperature the greater the heat loss". For baking and roasting this presents less of a problem, because the oven is operating at around half the temp of pizza cooking, but the oven will not retain heat as well as one with under floor insulation. The problem can be difficult if not impossible to rectify, but if you have not mortared down the floor bricks and they are inside the dome rather than on top of it you could remove them and add 2" calcium silicate insulating board then replace the bricks. This will reduce the height of the oven door opening, but improve the ovens performance enormously.

[mbarrett5076]

I agree with radiant and conduction heat loss, only convection goes up, everything else is in all directions.

Unfortunately this construction is complete the pizza oven a sitting on basically 3.5 inches of rebar reinforced fire clay/cement which is on top of 7" of cement and rebar... the whole thing housed in poured cell and rebar cinder blocks. The insulation is between the oven and the cinder block. it isn't gonna go anywhere we joke that the cubby under the oven is our hurricane/tornado safe place

Haven't had an issue w/ soggy crust. I do keep a fire going on the side when cooking pizzas. When it gets down to embers, I put a small log on to light up the oven. So I can see and keep the temp up. Toward the end of the initial heat up, i spread the embers for 15/20 mins and then clear them to a side, clean the floor and start cooking. Have run the oven like this for 2+ hours making pizzas and everything was nice and crispy.