oven floor brick conductivity

[hngten]

Hello. I'm preparing to build my second pizza oven, but I'm hung up on the over floor brick options. I'm hoping for some guidance in picking the proper brick type. I'm reluctant to use terms (firebrick, low/med/high/super duty. etc) because of the different regions, experiences and generality of their use. I'd like to stick to specifications if possible. Here's my story...

My first oven, a barrel type, I built 12 years ago with local materials. It worked fine for all sorts of cooking, but lacked in speed for cooking pizza (we all know why). It was strong, plenty hot, but it took 8-10 minutes for a 16" pie, and I ONLY cook pizza in it. I was unhappy with it's performance so I demolished it and starting over. Hearth is still in good shape, no spalling.

My new build will be a 36" dome style built with castable refractory with thermal breaks, the usual CF blankets and a hard mortar shell, 6" duratech chimney . CaSi over FoamGlas under the brick floor. But the floor composition has me confused. I now know more about thermal conductivity than I've ever known thanks to this forum. I've learned there's low conductivity for fast heating and cooling and high conductivity that can burn the bottom before the the top is done, resulting in the need for "doming".

So, my question is, what is considered high and what is low? I tried to use density as a gauge, but I've found brinks with a density of 135 lb/ft with a "low" conductivity of .60 W/(mK) and bricks with a density of 133 to 142 lb/ft with a "high" conductivity of 1.2 to 1.4 range. I also saw a post elsewhere that showed cordierite as 3.0 and steel as 50 W/(mK), that only confused me more. What's considered too high or low and what's the sweet spot? FB supplies a brick that has a density of 134 ib/in but doesn't mention a thermal conductivity rating. I thought may density related to conductivity? or the mineral make up?

I know I just need a recommendation on a range of brick conductivity for my PIZZA ONLY oven that gets used for a couple hours, once a month for backyard parties, but I really want to understand the science behind the choice. Thanks

[Petter]

It all depends on how long your cooking time is. People use steel (~17-50 W/mK), soapstone and corderite in home electric oves for 3+ minute pies. According to pizzamaking.com, normal medium duty firebricks (~1.2 W/mK) are good down to ~90 s, then the risk of bottom burning increases a lot. Low duty firebricks so called WG ~0.6 W/mK are good down to ~60 s. Biscotto di Sorrento/Saputo will do 35-40 s pies with ~0.4 W/mK.

It is assumed that the temperature has reached equilibrium in the dome and floor for all cases. If you search more you will get the temperatures too. Short story is that higher temperature requires a less conductive floor.

It also depends on the size if your door and geometry of dome. A Pompeii oven has a wide and high opening. It will keep the floor temperature down compared to a Neapolitan pizza oven with a 9" semi circle opening and a door/dome ratio of 0.57 instread of 0.63. Therefore, you might get away with denser firebricks than the above stated.

[hngten]

Hmm..... I guess I'm still missing something. If normal medium duty firebricks (~1.2 W/mK) are good for a ~90 s pizza cook (the slowest cook in the example), then why does the risk of bottom burning increase? I would think the slower cook time would take longer to burn the bottom? Is the temp is higher in that scenario? Is this an equilibrium issue? I was planning on the build being more of a pompeii design, so .63 ratio.

[Petter]

The cooking time depends on the top heat i.e. dome and flame.

High temperature yields short time. In the optimum design, the bottom heat is balanced to the top heat in a way that the pizza is ready simultaneously on top and bottom.

Second best is to have the bottom ready before the top. The pizza is then domed a few seconds to finish the top.

Worst scenario is if the top is finished before the bottom since it is difficult to shield top heat. This occurs if the floor is not conductive enough for the desired cooking temperature.


If you go below ~90 s with a thermal conductivity (TC) of 1.2 W/mK, the floor will be puttning too much heat into the bottom. The floor always cools slighty when the pizza is launched. This protects the bottom from burning. However, if the TC is too high, the temperature drop will not be high enough since the surrounding mass wants to equal the temperature. The temperature renovery also goes faster with higher TC. This is why you should replace the pizza on the same spot after turning.

Look at post 247:
https://www.pizzamaking.com/forum/in...opic=22618.240

(It is actually better to talk in terms of floor temperature rather than cooking time. However, cooking time is more easily understood.)

[Petter]

If you don't specifically aim to only do sub minute pizzas, go with the medium duty firebricks and be happy. See the above as good bakgrund information.

[hngten]

Interesting thread, it kinda worked through the problem I'm trying to avoid. So, it seems with an oven temp of 400c/750f, the 1.5 TC brick was too hot. If I went with a 1.2 TC @ 400c/750f, is that gonna be enough of a drop in TC? I looked up the WG brick you mentioned, it looks to be around .6 TC. I can get a .6 TC brick locally (although they call it a superduty), density of 135 lb/in. I'm afraid a .6 might me too low TC? I think I make take the extra time to make the floor removable just in case. Cook time is less important for me, but poor dome/floor equilibrium was the problem with my last pizza oven. I'm not going through that again!

.6 TC @ 135 lb/in -or- 1.2 TC ranging anywhere from 130 to 142 lb/in ? Which would you choose?

[Petter]

I believe you should use the 1.2-bricks since the whole Pompeii wfo is optimized for that.

To get the most out a low-TC brick, you would have to re-work the dome and opening as well. Otherwise, you would struggle to get the floor temperature up. That would mean a complete different oven.

I made a small oven last summer for testing and I really enjoyed the soft- and freshness of short baking. Sourcing and planning for a larger at the moment while waiting for the temperature to go above 0 °C. I, however did buy the Biscotto di Sorrento floor since I aim for a short bake.

[hngten]

Ok. Is there a Alumina% or Silica% that is optimal? I have found Alumina% in the range of 30-45% and Silica in the 50-63% range. I don't know what the Alumina and Silica do to influence behavior of the oven.

[Petter]

In that range, the phases in the material will be unchanged, only more or less of either. They will therefore just affect the density where higher Al2O3 increases the ditto. So, pick your density and the composition will be given. For further information, have a read in the Forno Bravo Pompeii E-book about a suitable value.

[hngten]

Ebook said seek 38% Alumina if it’s in the budget, so I guess it’s time to brick shop. thanks for everything, I’m off and running!

[hngten]

I though I was done asking question, but then I looked at some tech data sheets! Check out these two very similar bricks with drastically different Thermal conductivity.

......................Example 1 ..........Example 2
Alumina % ........ 40 ........................38
Silica %. ........... 55 .........................56
Ferric oxide%.... 1.5 .......................1.69
Density ............. 135 ......................133
TC W/mK. ...........60 @ 660 f .........1.2 @ 600 f (Twice the TC!)
Max service
Temp. ................3000 f ....................2850 f


How can this be? Does anyone know how Thermal conductivity is built into bricks? I thought it was a by-product of composition and/or density, but this appears to disagree with my understanding.

[Petter]

Example 1 seems very strange. Most likeky an error. If not, I would expect a very very fine structure in the (sub)micro meter range. That I highly doubt. The brick would then appear like a CaSi-bord in brick shape...

get the 2...