Floor layer using weird form fire bricks

[adners]

Ok, been gathering material for my upcoming build and (perhaps stupid) bought 300 unseen fire bricks (far from home) which my relative picked up along with 100 kgs if fire clay.

Everything seemed to be fine, except that the bricks were not really rectangular but instead had a more “slightly triangular” shape going in thickness from 3 to about 2.5 inches. Stacking 2 on top of each other indeed creates a flat surface although the thickness of the brick layer would be 5.5 inch. (I also have about 1 sq metre hand made terracotta plates for the baking floor if I can’t find real biscottis.

My dream/plan is to build a neapolitan oven, but for the floor - stacking 5.5 in of bricks on the vermiculite iso layer feels like a bit to much (unnecessary) thermal mass.

But would that be recommended compared to build the floor with sand/clay and then try to “level” the triangular shaped bricks into a flat surface?

(I read that terracotta plates isolates a bit from the fire bricks but there is a bit of uncertainty here..)

Not sure that I managed to describe my issues here (I am Swedish) but please ask and I’ll try to give a better explanation.

[Gulf]

Hi Adners,

Welcome to the forum. I believe these brick were molded in that shape to fit a cylindrical furnace or kiln. They can range from very soft to extremely hard depending on the exact type kiln for which they were made. Can you get an alumina content from the seller? You may be able to research the markings on the brick yourself to find that out. You can also tell if they are remotely suitable by how easy or difficult they are to scratch or abrade by rubbing two of them together.

If a wet saw is available, there is a way to get around the shape of the brick. You could spiit he brick the long way. That would take making a jig to keep the brick oriented to the blade in a way that would keep the 3" and 2.5" sides in line with the saw blade and in equal halfs. You could then lay them on edge reversing each course.

I don't understand the reasoning for terracotta plates on top of firebrick?

[adners]

Hello, and thanks for the rapid answer and a fantastic community!

It is a good idea to split the stones, will keep that in mind.

The bricks are old (unused) fire bricks, from a stone factory in Denmark which no longer exists.. (the bricks are marked with Bornholm Hiporos 85) bought them from what I believe can be described as the Swedish version of Craigs list. The seller intended to build a oven from them but the project got stalled and he sold me the bricks and 100 kgs of clay. (And yes, this was indeed before reading the appendix in the pdf about choosing bricks..)

I found these values for the stones online, nothing about alu content though.. could I br in for a negative surprise here?

Thermal conductivity:
0,28 W/mK at 400 degrees of Celsius
0,49 W/mK at 1200C

Specific heat capacity:
978 J/kg at 400 C
1191 J/kg at 1200 C

As for the terracotta plates, two guys running a Neapolitan pizzeria here in Sweden wrote a book about the everything from building ovens to baking pizza. They recommend the Forno bravo plan but with some minor changes for the perfect neapolitan pizza.

It says that if the thermal conductivity in the fire bricks can be to heigh (perhaps not my problem I guess) and handmade terracotta has better values and a good option for the floor if there is a problem getting real Biscotti plates. (Since the terracotta is handmade there are still lots of isolating bubbles in the material compared to industrial terracotta plates.)

On the other hand the book says that instead of fire bricks below the cooking surface a traditional way is to use a mix of sand, ash, quenched lime and coarse salt.

But then again I have not built anything yet, so all I have are some qualified(?) guesses

Been experiencing with “nearlypolitan pizza” in my regular electric oven, but this summer finally started to gather material in our summer house for a WFO - making this a long term project since we soon will be heading back to the apartment where no wood fire ovens are welcomed...

BR
Anders

[Karangi Dude]

Hi Adners,

How much do those fire bricks weigh? they look porous and very light in colour they could be insulating fire bricks

Cheers Doug

[adners]

Hello,

hm.. they weigh 2 kg each, about 4 lbs?

[Karangi Dude]

Adners, I am just checking thats all, my medium duty firebricks weigh about 4kg each and have quite a smooth surface they are 230x115x75mm

[adners]

Yes, appreciate all concerns! But getting a bit scared here. according to the guide I think isolating fire bricks have half the density/weight of mine(??).

My first idea was to build the dome in vermiculite like the yoga-ball ovens I found on YouTube, perhaps that is a better idea than take a chance on these strange fire bricks.. if so I have 300 bricks for sale in Sweden

[Karangi Dude]

Don't be put off,
I am just concerned that they are not the right brick, one way to tell is to hold one brick in each hand and bang them together (not to hard) they should make a sharp clinking sound if the sound is more muffled and like a thud then I would say they are not suitable. Insulating firebricks are also quite soft you can cut them quite easy even with a hand saw

[adners]

No! I really do appreciate all help! It is better to identify weak spots now before starting to assemble the build.

I tried to bang two bricks together and definitely more of a sharp clink I think I will take the chances with these ones

[UtahBeehiver]

I worked with unusual shaped bricks, some tapered like the ones you have and others tapered the other direction. It is doable, IMHO, these will work better on the dome, inner and outer arch areas. It will be more difficult on the floor. So maybe a trade off, getting flats for the floor and using what you have for the other areas. We see builders try p or vrete cast dome ovens, but it is the general opinion that this mix should only be used for insulation purposes and not as the main oven structure. A dense castable refractory is recommended instead.

[adners]

Oh thanks for the input

I will have some time thinking of the bricks, will probably build the oven next summer. It is in our family summer place and we have just finished the ground work. Soon heading back to our city apartment where wfo are banned..

Thanks all!

[UtahBeehiver]

I am a little concerned about the base using unintegrated concrete slabs/pavers rather than a poured concreted base with reinforcing bar and wire. These ovens, all said and done are "really" heavy..

[adners]

Yepp, we tried to do the maths but as we know by now - I have been known to have a bad luck when I think

A friend of mine has a pizza oven in his lawn which weights about 600 kg, he made the ground as we did it.

We have dug down to the rock bottom, filled the hole with gravel, sand and packed it before adding the tiles. Each tile weights in for 14 kg and are almost 4 inches thick. But as you said, a concrete ground is a much more reliable option.

I think my 300 bricks weights in for 480 kgs in total, but I am not sure how many are to be used. For the base we will use light weight “leca”-blocks. A couple of years ago we build a quite big timber sauna using the same ground so I hope it will work also for the oven project.

But yes, once more I am taking a different path compared to the pdf instructions

[adners]

Time for a follow up, the oven was built using more or less the original idea from above. Turned out great! I easily get the temp to 500 degrees Celsius, can bake 60s pizzas and even during the winter in Sweden the floor temp the next day was 130 degrees plus.

here are a few insta posts from the build with the stones mentioned above:
https://www.instagram.com/p/CDKMGCUB...=1o9wmaopcmi62

also, here is the oven in action: https://www.instagram.com/p/CLzfUTTh...=1h19u75tlbwzf

My next step is to fix looks/outside, but really happy so far with the inside

[MarkJerling]

Well done Anders! Your pizzas look tasty!