How Masonary Ovens Work

[enz]

This topic has been beaten before, but not to satisfaction.

I have recently read two assertions (alright, there are more, but lets start with these) that don't quite jibe with me.

The first is the repeated mention of "reflected" heat in the Forno web cookbook and elsewhere. The other is that the porosity of the brick somehow "stores" moisture that is released during cooking.

First the reflected heat: Fire brick has an emissivity of .68, so about a 1/3 of the radiation hitting it is reflected and the rest is absorbed. The 2/3 that is absorbed is re-radiated once the thermal mass is hot. So, while the reflection is important, the re-radiation is by far the dominant source of radiation to the pizzas. It may well be that there is an optimum ratio of reflected and re-radiated heat. I'm just suggesting that re-radiated heat needs to be mentioned as well, perhaps before reflection.

The next item is far more troubling. The idea that moisture is some how residing in the porosity of a firebrick at 800 deg F, or is absorbed and released as the pizza is fired, is frankly preposterous. Water at 800F is superheated steam, the vast bulk of which will have been driven out by expansion (PV=NRT and all that). On top of this, unless the brick is cooled substantially, there is no mechanism to draw any moisture liberated from the pizza or bread into the brick. Conversely, the brick would need to increase in temperature to liberate the infinitesimal bit of remaining water it contains.

The ability of the oven to retain moisture, is much more likely the result of the closed nature of the brick oven. This is true to some extent in pizza mode, but especially when you close the door for bread.

I'm expecting resistance to these statements, so let er rip.

Enz

[james]

Re: How Masonary Ovens Work

Hi Enz,

We don't really rip here.
James

[wlively]

Re: How Masonary Ovens Work

To further interesting discussion.

I haven't reviewed the technicals, but can't find fault with the reflection/radiant argument.

As for moisture, I think there are various interactions at work. I would think that most of the moisture in the oven is due to the fact that it is continually drawing in outside air (unless you have a door on), which will have a moderate to heavy moisture content. Provided you don't live in high Arizona desert. No doubt the oven traps a fair amount as well. But I don't think you can entirely discount how much may be in the bricks, the minority amount I grant you. First, the face of the brick may be 800F deg, but my thermocouples show that the center may well be below 400F at that point and takes quite a long time to elevate. I have yet to see above 600F so far. You also can't say it is 800F and therefore no moisture exists. The vacuum furnaces I work on idle at 700C and if the door is left open for any period over 3-4hrs they outgass moisture like mad when pumped down. This is of course on a process that leaves a highly porous film inside the quartz tube.

[enz]

Re: How Masonary Ovens Work

James,
That was tongue in cheek. I meant let the lively discussion rip.

wlively,
You'll notice that I was very careful not to say NO water, just very little. Anywhere above 212 F at sea level, water turns to steam and undergoes about a 1000 fold decrease in density. Any trapped moisture will undergo this phase change during the heating cycle, well before any pizza gets near it and it's gone. What little moisture is left in the brick at pizza temperature has no way out, unless the brick is heated further.

I agree that there is some moisture in the air, but much more will be carried out with the combustion gases. I still think the relatively low fire (and associated lower air flow) in pizza mode and the closed door in bread mode account for more moisture retention.

Enz

[JoeT62]

Re: How Masonary Ovens Work

Water at 800F is superheated steam, the vast bulk of which will have been driven out by expansion (PV=NRT and all that).

Enz

Ideal gas law and pizza.

Two great things that go great together!

[maver]

Re: How Masonary Ovens Work

I think the heat gradient as Wlively points out is worth some thought as to how a (albeit minor) reservoir of moisture can develop in the oven. Also, don't discount the oxygenation (burning) of wood produces many vapors, but much of it is CO2 and H2O. Drawing water vapor from the air is probably trivial compared to the vapor released during the burn (and from the wood itself). I do agree that water vapor is a more significant issue in retained heat mode (coals out, bread in), especially if we mist the oven prior to applying the door (with a wet towel hanging over the door to add more steam and help develop a tight seal)!

[james]

Re: How Masonary Ovens Work

Hey Enz,

What is the emissivity of a fully saturated brick? My thinking is that while a cold brick has the tendency to absorb most of the heat that is hitting it (the initial firing period where no cooking is going on), a fully fired brick with insulation on the other side, has no choice but reflect the heat that is bouncing again it.

If you let the fire die down when you are making pizzas, and rely on re-radiated heat contained in the brick, the oven stops cooking properly. While that might be good for many other types of brick oven cooking, it's bad for pizza.

Does that make sense? Art meets science. Pizza Napoletana meets emissivity.

On the idea that the bricks are somehow storing moisture (because they are porous), that doesn't really make sense. I hope we don't say that anywhere -- though you never know.

For pizza and bread, I think the idea is that the moist dough hits the hot brick, which immediately converts the available moisture to steam. I have always thought that was why pizza never sticks to the masonry cooking floor, and why brick oven pizza has a nice, crisp crust.

I agree that there is nothing inherently "moist" about a hot brick, but rather that the moisture from dough, cooking food, wood fire and water from a sprayer all swirl around inside a brick oven. And that is very good for pizza, bread and cooking.

It's interesting trying to explain why things are the way they are. Inquiring minds....

James

[enz]

Re: How Masonary Ovens Work

Enz: James has a fascinating point about absorption after the brick is at full temp.

James: What is the emissivity of a fully saturated brick? My thinking is that while a cold brick has the tendency to absorb most of the heat that is hitting it (the initial firing period where no cooking is going on), a fully fired brick with insulation on the other side, has no choice but reflect the heat that is bouncing again it.

Enz: As far as I know the emissivity remains relatively constant with temperature (absorbs ~ 2/3). But, what happens when the brick is fully saturated? I will have to consult the heat transfer texts one night soon and hopefully find an answer. This will have to wait a few days, as I'm planning to cast my solid mizzou castable volto basso this weekend! I'm going to pour the base Friday night, polish it Saturday and if all goes well set up the forms and pour the front half of the oven Sunday.

James: If you let the fire die down when you are making pizzas, and rely on re-radiated heat contained in the brick, the oven stops cooking properly. While that might be good for many other types of brick oven cooking, it's bad for pizza.

Enz: I think this tells us three things.
One, you are constantly recharging the brick while cooking with a fire in the oven.
Two, you are still getting quite a bit of cooking from convection. I know this contradicts what I've said recently, but item three will explain my change of heart.
Three, I'm convinced Maver is spot on in that the fire itself is the source of moisture in "pizza mode", as well as the pizza itself. Water is a combustion product regardless if you are burning wood or gas. So, even though you are losing quite a bit of moisture out the flu, it is constantly being replenished by the fire.

James: Does that make sense? Art meets science. Pizza Napoletana meets emissivity.

Enz: Reflection + Re Radiation + Convection + Combustion Products = Napoletana. I'm redoubling my efforts on my cast oven!

On the idea that the bricks are somehow storing moisture (because they are porous), that doesn't really make sense. I hope we don't say that anywhere -- though you never know.

James: For pizza and bread, I think the idea is that the moist dough hits the hot brick, which immediately converts the available moisture to steam. I have always thought that was why pizza never sticks to the masonry cooking floor, and why brick oven pizza has a nice, crisp crust.

I agree that there is nothing inherently "moist" about a hot brick, but rather that the moisture from dough, cooking food, wood fire and water from a sprayer all swirl around inside a brick oven. And that is very good for pizza, bread and cooking.

It's interesting trying to explain why things are the way they are. Inquiring minds....


Enz: Indeed. Thanks for the lively discussion.

[Archena]

Re: How Masonary Ovens Work

Um, isn't spalling related to the moisture content of the brick? My (tiny as in minuscule) understanding is that bricks are very much like bread - they have an outer crust. That crust is the fully cured/hardened part of the brick which grows from the outside in over time. Damage to that outer crust results in spalling of exterior brick (this is why pressure washing brick is a bad idea).

It would seem likely that serious disparity in the moisture levels of the interior and exterior - the exterior being able to rid itself of moisture faster than the interior - would result in damage to the outer crust when heated as the interior moisture expanded. I dunno, but it doesn't sound likely to me that brick could be a significant source of water without being damaged by said same.


Just wondering...

[Unofornaio]

Re: How Masonary Ovens Work

Um, isn't spalling related to the moisture content of the brick? My (tiny as in minuscule) understanding is that bricks are very much like bread - they have an outer crust. That crust is the fully cured/hardened part of the brick which grows from the outside in over time. Damage to that outer crust results in spalling of exterior brick (this is why pressure washing brick is a bad idea).
Just wondering...

Depending on your definition of "spalling" because there are many out there spalling (flaking of surface material) usually occurs on bricks that have been subjected to repeated moisture and dry cycles like were the sprinkler hits the house or a damaged down spout.
Its the moisture that causes the spalling, its not so much that the brick has a weak "crust" its just thats what we usually see. Moisture works on the inherent weakness of the clay mixture, clay by its nature is absorbent and even after firing, what looks like a solid brick, under a microscope it really looks more like coral or a natural sponge (this becomes clear when dipping in water). the water gets in these holes and crevices and the minerals in the water react with the minerals in the clay and you start the process of deterioration, Efflorescence is another good example of this water reaction, those of you that have basements or a dark moist part of your brick or block house have seen a white "fur" or "fuzz" growing, this is the result of moisture as well. If you can imagine this going on "in" the bricks cells over time it acts as a wedge and you have spalling. Same thing is true for concrete but spalling in concrete can be caused by other things as well which include improper mix hydration, over troweling and surface irritants such as salt or chemicals (for snow) as well as soil conditions such as alkali.
As to pressure washing this is not a bad thing for "the brick" you can hit it all day with a pressure washer (a modern brick) its the mortar or "joint" that is the weak spot and should not be concentrated on. Older softer bricks I'm talking 40s-50s and older yes you need to use caution with both the brick and the joints.
Hope this helps