24" Interior Diameter

[Petter]

Petter,
The downsides are of course, vastly reduced strength and durability, leaving the insulating firebricks vulnerable to knocks and abrasion damage (forget using it as a mobile oven).
I presume you used dense fire brick for the floor. The lack of thermal mass there would leave the oven pretty unworkable otherwise.

Sure, time will tell how well it holds up. I have no intentions to move it and I coated the inside with a slurry of Sodium Silicate and fireclay just in case. It protects the bricks from wear and dusting. Since the oven is straight, you are nerver near the walls anyway. Remember, this was only a mock-up...

The cooking floor is dense, it would wear very fast otherwise and make some gritty pizzas...

As a side note about its heat retention, if I go and eat for 30-60 minutes without refueling, it takes about 3-4 minutes with flame to get up to cooking temperature again. Not as good as a dense brick oven, but far better than a steel oven.

[Timothy_W]

Chrismonty, I built a 24 for a friend who wanted a smaller oven. It’s not finished yet still curing going to 650 tonight, but I think it’s going to work fine for a single pizza at a time, a Dutch oven will fit, as would a loaf of bread. Can’t say how it’s going to cook exactly yet but I’ll post how it goes. Fits easy on a 4x4 square. I put a thermal break where the floor meets the tunnel and it will have a door that blocks the vent, hopefully it will have enough mass to retain heat for a while.

Cool oven. Not a bad option for home use. But if you need an oven in which you can cook a variety of dishes or bake at least 2 pizzas at the same time, you need more options. When the whole family gathers with us, you need to cook a lot at once. If I bake one small pizza, they will eat me faster.

[david s]

Petter,
The downsides are of course, vastly reduced strength and durability, leaving the insulating firebricks vulnerable to knocks and abrasion damage (forget using it as a mobile oven).
I presume you used dense fire brick for the floor. The lack of thermal mass there would leave the oven pretty unworkable otherwise.

[Mahone]

Chrismonty, I built a 24 for a friend who wanted a smaller oven. It’s not finished yet still curing going to 650 tonight, but I think it’s going to work fine for a single pizza at a time, a Dutch oven will fit, as would a loaf of bread. Can’t say how it’s going to cook exactly yet but I’ll post how it goes. Fits easy on a 4x4 square. I put a thermal break where the floor meets the tunnel and it will have a door that blocks the vent, hopefully it will have enough mass to retain heat for a while.

[Petter]

Hi David, I agree with what you say and I believe what you are describing is actually the thermal diffusivity. Insulative bricks transport heat slow, but their density is also lower. Thus less heat is required to warm them up. This is explained by the 'rate of temperature movement' = thermal diffusivity = TC/(density * Cp) which is constant since Cp also is constant for bricks. This is why the heat up time too is constant (or only depends on material thickness as long as fire heat is in abundance).

I fire only one log at the time during heat up, I.e. as little as I can without babysitting the fire. Since the heat transport into the bricks is slow, I don't need a big fire, only time.

As a side note, the reason I built the oven with insulative firebricks was that every one said it was a very bad Idea. I wanted to know how bad it really was. I also could make the entire oven in the garage with only clay sand mortar and a hand held saw during a couple of evenings and then lift it out. The whole oven costed me about $10 since I worn out the saw. The rest was free-materlial from different place. After using it some 7 times and about 80 pizzas I feel comfortable with its performance.

Pros are easy lightning and fire management. This gives surpriceingly consistent results. Cooking time varies typically +- 10 during an evening's bake.

Cons are pizza size limitation to 30 cm and that I have to insert the turning peel from the most uncooked side of the pizza, increaseing the risk of bottom holes. I don't think it is as much of an issue that I don't see the fire side of the pizza as I first though. Turning after 30 second works fine.

I'm still going to build the larger oven with bricks and proper insulation. I wish to have the ability to make and cook other type of foods and I like the visual Impression of a traditional oven. This mock-up was fun, fast and enabled many rememberable food memories from early summer.

[Timothy_W]

With a 24-inch stove, you will be limited in your options. This is extremely small. Try to allocate more space for the oven, make it at least 30-32 inches and it will be easier for you to work.

[david s]

Hi Petter,

I think you are getting confused between thermal conductivity and thermal mass.

Thermal conductivity being the degree to which a material conducts heat while thermal mass is the heat capacity of a material per unit mass.

The density of Insulating firebrick is around 500 kg/m3 or 0.5kg/Litre- thanks metric.
The density of water 1000kg/m3 (1kg/Litre)
solid reds around 2000kg/m3 (2kg/Litre)
firebrick around 2300kg/m3 (2.3kg/Litre)
steel 8000kg/m3 (8kg/Litre)

The reason your nsulating firebrick oven took the same amount of time to heat to temperature as a dense brick or cast refractory oven has more to do with the materials inability to both conduct and store heat. It's low conductivity means most of the heat is being reflected and going up the flue. This situation leads to fast heat up of the oven interior because that heat is being absorbed at a slow rate because of its low thermal conductivity. Given long enough, the heat will eventually penetrate the material while its low thermal mass does not have the ability to store much heat. The insulative qualities of the material slow down any heat loss but lack of thermal mass leads to poor heat storage so there's little heat to save.

The reverse is true for a steel lined oven where thermal conductivity is high, so little heat is lost during fire up but the heat loss is very high on cool down. The middle of the road ie density and thermal conductivity of a refractory material like, or similar to firebrick, well insulated, has proved for the past couple of thousand years to be the best solution.

In modern kiln design, in an effort to reduce fuel costs hot face ceramic fibre blanket with its very low thermal mass and low thermal conductivity has replaced dense firebrick and even insulating firebrick so the energy input is largely stored in the kiln furniture and the wares being fired in the kiln which both have comparatively much higher thermal conductivity and thermal mass,

[Petter]

Sorry, I got lost in translation. I wrote "thermal diffusivity (heat transport rate)", while "heat" should be substituted to "temperature".

In short, an insulative material transport heat slower than a dense. However, this is countered by its lower mass and thus less energy is needed to heat to a specific temperature. This makes the temperature gradient propagate at a similar rate in these Ca-Si-Al-O-systems.

When you get into the, and sub 100 kg/m3- range, the micro structure starts to add its component. Fine dispersed reduce the thermal diffusivity.

[david s]

I can’t say either the theoretical evidence or my practical experience of conductive heat transfer rates of different thermal conductivities agree with you.
https://www.physicsclassroom.com/cla...-Heat-Transfer

[Petter]

I made a small oven of insulating firebricks this summer, about 34x57 cm (w d) cooking floor with fire in the back and a flame protection brick separating the coals from the pizza. It still takes 1-2 h to heat up since the thermal diffusivity (heat transport rate) is equal to dense firebricks. However, it consumes 2-4 kg to heat up and 1-2 kg/h while cooking.

It takes me about 3-4 minutes to prep and 1:30-1:50 to bake a 30 cm / 12" for me. I would not dream of doing multiple pizzas due to constant supervision and turning right now. A larger oven would give more consistent results and be easier to manage. An assistent rather than a larger oven would be the key to faster though put.

A normal bake is 8-12 pizzas. 20 max so far. If I have guests, I cut the pizzas in half or quarters for sharing (thanks david s for that!) and it works great.

A larger oven is a lot more theatric though.

[wotavidone]

I made mine 30 inches, but the next two I assisted with are bigger. I prefer the largest one (1m/39") for cooking for a crowd.
However, here in country South Australia, you can literally grow enough firewood for your oven in your back or front yard. I do. I have a eucalyptus tree in the front yard. I can't remember the species, but it is a mallee type.
i.e. it can be coppiced indefinitely.
So when the branches look like hitting the house or the power lines, out comes the saw.
A 24 inch oven will definitely be adequate for family gatherings.
It will have pros and cons over a larger oven.
A small oven doesn't necessarily heat quicker than a large oven, all other things like wall thickness and insulation being equal. They may have smaller mass, but the fire is smaller too.
However the total amount of wood required is smaller, so you can justify lighting it for only a couple of pizzas.
And yes, you just about need to be a trained and experienced professional to work more than 1 pizza at a time.
If you watch a lot of Youtube, you'll see the guys working 6 pizzas in a big commercial oven generally have someone very professional and competent making the pizzas for them.

[david s]

The reality is that for most oven owners they say the size they settle on and begin cooking with is the smallest they could cope with. However the reality of mathematics is that a mere 20% increase in internal diameter results in a massive 73% in chamber volume which is roughly proportional to fuel consumption. Also expect a very large increase in volume of materials required for construction as well as the same increase in labour.
It is no trouble to heat a small oven to cook a loaf of sourdough and a couple of baguettes a family roast or a few pizzas, something you just wouldn’t contemplate with a large oven.
The disadvantages are higher heat loss leading to quicker temperature loss, door height limiting size of big roasts and limited floor space for multiple pizzas. The first question I invariably get is how many pizzas can you fit in your oven? The answer is only one comfortably, but reality usually means even big oven owners cook one at a time as it takes longer to prepare a pizza than it does to cook it. Size all depends on how you want to use it.

[Neil.B]

Mine is 32" due to limited space. Many normal ovens in the UK fit in a 24" gap so I can't see why a 24" oven wouldn't work. Obviously the bigger you go the more options you have for various types of cooking.

Have you tried cutting out cardboard to see how it feels, remember to make allowances for your vent, landing and outer arch, as well as insulation around the outside.

[Chrismonty44]

Hello everyone!

First time builder and forum visitor! I was hoping to get some guidance.

I am planning a build for my own pizza oven and I am debating size. I am a little limited on space and trying to save some money. Has anyone built/cooked with an oven that has a 24" interior diameter? I am debating between 24" and 30".

Thanks for the help!!

Chris