WFO heat balancing and related issues (split)

[scott r]

I know this sounds crazy, but I have two different friends with imported ovens from naples that are having a hell of a time with them.  Im not sure if this is the case with every low dome oven from naples... interestingly they both have the same brand of oven.    Both of them are having the same issue. With a 700ish degree floor the pies are pale on the bottom compared to the top. The first 10-15 pies after the initial firing are ok, but once the heat in the floor is used up, it doesn't seem to easily come back without a break from making pies to tend to the fire and get things evened out again.    I sometimes hear about the opposite problem with high dome ovens when people are trying to do neapolitan (the bottom burns before the top is done necessitating lots of doming), but this was a surprise to me.   I guess what this goes to show us is that at least in a commercial situation you really have to have the right oven for your pizza style.   Something I find interesting is that one of these people also has a custom built cast oven with an ultra low neapolitan height dome.  Now, it is macked out with 4.5 inch cast walls and tons of extra insulation, but that oven bakes totally even pies at any speed, maintains both high and low temps easily, and has no hot and cold spots other than the first few inches right in front of the fire (which obviously is normal). Interestingly this cast oven has the vent going back over the dome like a neapolitan oven.  Apparently even at high speeds the neapolitan oven has hot and cold spots.  What this tells me is that its not just the low dome or type of vent that is the issue, there are other factors involved as well (the type of refractory used maybe?).  I am just putting this in my home, so im not really as concerned, but I still want to make sure i'll have no problem with pies at both speeds (if possible) when I entertain large groups of 30-40 people.

Im very interested to hear from anyone with a wood burning oven about their experiences with both high and low temp pizzas.  John, your really nice to offer that!  

Robyn B, yes I would go with a bread flour.   Better for bread is a good one to test with, but any malted flour would work (king arthur, guistos etc.)  You might want to up the hydration a bit to compensate for the longer bake and the absorption rating of your flour, but its not absolutely necessary.  (p.s. you can use normal aged mozzarella too at these temps)  

Jeff.  Im not surprised that your oven is performing so well.   Those NY pizzas looked very evenly baked.   I know your probably a very busy guy, but I was honestly thinking about making you an offer to come here and build me an oven.  My father in law is a welder, and a good friend is a mason, so you would have lots of helpers and people with resources to get tools and materials! I am very interested in your findings with the pale bottom at 650 degree issue, as your design is probably the most neapolitan of the designs I have been looking at.    

Right now I have a custom built oven from bricks, this four grand mere, and the blazing oven on my radar, but im open to any other suggestions for ovens that will meet my goal.    Its funny that even though I was entertaining the possibility of selling off some of my prized vintage guitars and amps to get an actual imported neapolitan oven, that is now off of my list.        

[scott123]

Scott, I believe that one of your friends is also a friend of mine  so I've had plenty of time to think about this issue.

I think that part of the problem here is that IR output isn't directly proportional to temperature.  For instance, 800 deg. doesn't pump out twice as much IR as 400.  The way I understand it, you basically have a temp where a material almost glows and that's some IR, but once you hit glowing, the IR goes through the roof. A 700ish Neapolitan oven setting has a cooler dome than an 850ish Neapolitan setting. That cooler dome is putting out a fraction of the IR. Without the IR, the hearth is using up it's stored heat from the conductive-rich initial firing (embers in direct contact), but not being replenished.

This doesn't help people with existing ovens, but I think thermal mass can help- to an extent. Your buddy with 4.5" walls- what I'd really like to know is the thickness of his hearth. A thicker hearth is going to store more heat, and thus lose less of it's overall energy with every pie baked. You'll still have the same deficit in IR replenishment, but... you'll bake a lot more pies before you start getting into trouble.

Bobino and Tampa Dave came up with a clad concept.  A thin-ish firebrick floor with a layer of steel underneath.  The steel would help to draw heat from the fire area, as well as help to even out the temp of the hearth overall, much like the advantage of an aluminum core in a stainless frying pan. If there's enough hearth real estate, I came up with the idea of targeted cladding. Splitting the fire area floor into half traditional firebrick and half steel with the same steel plate running under the rest of the floor.  You start with the fire on the firebrick area, and, if the floor temp starts to drop, you move the fire to the steel area and feed heat faster to the floor. Into this mix, you could even add a third mode.

1. Fire on steel (fastest floor feed)
2. Fire on brick (slower)
3. Fire raised on grates (slowest)

If, say, the cladding ends up drawing too much heat for 850ish high temp bakes, you can raise the fire off the hearth and remove some of the conduction from the equation.

This is all extremely theoretical, and would involve a lot of testing to get the right thickness of materials, but the aspect that appeals to me the most is the amount of control.

[scott r]

this is all very interesting!   yes, we do have the same friend.    My other friend with the killer cast oven has 6 inches of insulation under his hearth.  The floor is the forno bravo brick floor, which I think is 2 inches?  so thats 8 inches under the pie.      I guess his neapolitan oven doesnt have that much.   It makes sense because the battle with those 1 minute pies is the opposite, so they probably dont want that much stored heat, or just don't find it necessary for their 1 minute pies.             

[TXCraig1]

Scott, I believe that one of your friends is also a friend of mine   so I've had plenty of time to think about this issue.

I think that part of the problem here is that IR output isn't directly proportional to temperature.  For instance, 800 deg. doesn't pump out twice as much IR as 400.  The way I understand it, you basically have a temp where a material almost glows and that's some IR, but once you hit glowing, the IR goes through the roof.

Scott, something at 800F would emit more than twice as much Longwave IR (LWIR) compared to something at 400F (more IR period for that matter). It's Planck's law. Also, the IR radiated is a function of temperature, not a function of color. As something starts to glow, it is radiating more and more energy at shorter wavelengths - hence the visible light. Radiance at the IR wavelengths is also increasing, but there is not a step function at the LWIR (heat) wavelengths when something starts to glow.

CL

[TXCraig1]

Here is another illustration that might make it clearer. I called out the Longwave IR (heat) and visible light portions of the spectrum. You can see that at 1500F where iron starts to glow cherry red, energy is just begining to be emitted at the red end of the visible spectrum. You can see a big bump in IR vs 800F, so in that sense you were correct, however the big increase is in the NIR, SWIR, and  MWIR portions of the IR spectrum (.74 - 8.0 micrometers), not the LWIR a.k.a. "thermal IR" spectrum (8 - 15 micrometers).

[shuboyje]

I agree that a bit more thermal mass would have a positive effect on New York style pizza, but at the cost of longer firing time.  For a home oven how many 18" pies can you really need, lol.  A commercial oven is another story. 

I think how you fire the oven can also have a huge effect on cooking New York style in a low dome oven.  I've noticed a pattern in my oven that makes sense and seems to apply to this situation.  Let me give you a scenario then explain what I think happens and why it may apply.  I fire the oven for Neapolitan pizza.  I get the floor up to 900F+ and keep a roaring fire going on the left hand side  that stretches across the entire width of the oven.  Eventually I stop feeding wood and generally cook the last pie with a bit less top heat as the fire burns down.  Shortly after this the floor temperature plummets, down to the 700F range.  I haven't fired the oven long enough to totally saturate the floor with 900F heat, so without the roaring flame the temp drops.  I then push the fire to the back of the oven and let the temperature "drop" for new york style pizza.  I come back about 15 minutes later and the fire has burned down to coals with no open flame but the floor is now 1000F.  The oven then takes about an hour and a half with no more wood to drop back down to 675F or so where I cook the new york style pie. 

Here's my theory of what's going on.  It's gonna get kinda technical.  Inside a wood fired oven you have three distinct processes happening.  Direct combustion of the wood, pyrolysis of the wood, and combustion of the gases produced via pyrolysis.  Direct combustion of the wood is very effective at heating the hearth via conduction.  Combustion of the pyrolysis gases is very effective at heating the dome via convection.  For reasons I've yet to figure out, but I'm gonna guess it has to do with air to fuel ratio, when both processes are happening at the same time combustion of the pyrolysis gases seems to win out in the battle for oxygen.  Even a small amount of open flame seems to drastically reduce the intensity of the direct combustion.  So, as my Neapolitan fire dies down the slight small open flame still present keeps oxygen away from the coals.  Although the intensity of the open flame has dropped, the intensity of the combustion of the coals has not intensified to pick up the slack.  The temperature falls.  Once I push the fire to the back and let the open fire die out, all the oxygen is fed to the coals.  This leads to visibly intense combustion which pumps a huge amount of heat into the floor via conduction.  The temperature of the hearth skyrockets.  It isn't until the fuel begins to run out that the temperature drops. 

Here's how I think this applies.  Most people seem to keep a small fire going for new york pizza.  Almost treating it like Neapolitan just scaled down, lower temp and smaller fire.  In a low dome oven you don't need flame to cook a 4 minute pie, the radiation off the dome and convection from the combusting coals is enough.  All the fire is doing is robbing oxygen from the combustion of the coals and therefore stealing heat from the floor.  I think the solution for cooking tons of new york pies in a low dome wood fired oven could be pretty simple.  Fire to neapolitan temp, push the fire to the back, let the temp rise then drop and finally what I'm not doing Add fuel to maintain combustion and therefore hearth temperature.  The fuel you add is the important part.  Either wood burned down to coals in an open pit, or natural lump charcoal which is essentially the same thing.

Not sure I'll ever have a need to test this, but I'm pretty confident it would work.  Maybe one of you buddies will be willing to try it out to fix their issues.

[BrickStoneOven]

Why do you push the coals to the back of the oven when making NY pies? Why not leave them on the side?

[shuboyje]

Because I make them 18" with a 18" door, lol.  Getting the peel in and around the corner so to speak would be a real pain.  Straight in the door with the fire at the back is much easier, and with a 4 minute bake seeing the side of the pie exposed to the fire is not as critical as with a 60 second bake.  I have also always been of the belief fire at the back is more thermally efficient , but this isn't even a secondary consideration in putting the fire at the back.

[scott123]

Scott, something at 800F would emit more than twice as much Longwave IR (LWIR) compared to something at 400F (more IR period for that matter). It's Planck's law. Also, the IR radiated is a function of temperature, not a function of color. As something starts to glow, it is radiating more and more energy at shorter wavelengths - hence the visible light. Radiance at the IR wavelengths is also increasing, but there is not a step function at the LWIR (heat) wavelengths when something starts to glow.

Thanks, I did not explain that well.  What I'm trying to say is that, according to Stefan's Law, as temperature increases, energy emission isn't linear- that it increases exponentially, so that IR emission doesn't scale down linearly for NY style temps. This, imo, is one of the major reasons why the hearth isn't replenishing properly.  Also, since NY style pizzas are generally larger, you have greater coverage/more blockage, but I think that's secondary.

[BrickStoneOven]

Because I make them 18" with a 18" door, lol.  Getting the peel in and around the corner so to speak would be a real pain.  Straight in the door with the fire at the back is much easier, and with a 4 minute bake seeing the side of the pie exposed to the fire is not as critical as with a 60 second bake.  I have also always been of the belief fire at the back is more thermally efficient , but this isn't even a secondary consideration in putting the fire at the back.

Gotcha, I just thought it was for some other reason. I should have pushed the coals to the back instead of leaving them on the side as well. I learned the hard way when I burned my forearm on the arch when launching a 17" pizza through a 17.4" door. Now the coals always go in the back when making NY pies.

[TXCraig1]

Thanks, I did not explain that well.  What I'm trying to say is that, according to Stefan's Law, as temperature increases, energy emission isn't linear- that it increases exponentially, so that IR emission doesn't scale down linearly for NY style temps. This, imo, is one of the major reasons why the hearth isn't replenishing properly.  Also, since NY style pizzas are generally larger, you have greater coverage/more blockage, but I think that's secondary.

Makes sense.

[Vesta Wood-Fired]

I guess his neapolitan oven doesnt have that much.   It makes sense because the battle with those 1 minute pies is the opposite, so they probably dont want that much stored heat,

Exactly right.  In a Neapolitan bake from anywhere from 800 and above you wouldnt be able to put the pie on the floor.

[scott123]

Once I push the fire to the back and let the open fire die out, all the oxygen is fed to the coals.  This leads to visibly intense combustion which pumps a huge amount of heat into the floor via conduction.  The temperature of the hearth skyrockets.  It isn't until the fuel begins to run out that the temperature drops.

I could be wrong here, but I think you might be overestimating the power of conduction in this system.  How far is the pizza from the coals? 12"?  That's a long way for energy to travel through a poor conductor like firebrick. If it's a comparison between conductive energy traveling through 12" of firebrick vs. a 9-12" journey of IR (through air), my money is on the IR having the lion's share of the impact.

[shuboyje]

I'm pretty confident it is conduction.  You can actually measure the temperature drop(a few degrees) at each brick, which I get a real kick out of, lol.  Radiant heat from the dome would not produce those results.  It could be radiant from the coals but I still think my money is on conduction.  The coals are known to conduct a lot of heat to the hearth, some will even rake the coals across the hearth to quickly recharge it.

[scott123]

Actually, with the inverse square law, the further away from the heat source, the less IR impact, so radiant heat could produce a successive drop in brick temp as you move away from the flame.

This wouldn't be that hard to test.  Confine the fire to a platform and see what kind of hearth temps you get to test IR's impact and put in a false ceiling over the embers to test conduction. I wouldn't bet a lot of money on the outcome, but I'd still lean towards IR.  I can see that energy travel maybe 6" during a typical bake, but 12", I don't know.

If conduction were the star player, 2 feet from the flame- pretty prime pizzamaking real estate, would be considerably colder than 1 foot, because of the additional journey the energy has to take, but we know that 2 feet and 1 feet don't have dramatic differences in temp.

[shuboyje]

I thought about that too scott, but my fear is effecting airflow.  I think tsting would be harder then it seems for that reason.

Do you not think the temperature changing exactly at the joints between bricks in a gradient as you move away from the coals shows the heat is conducting from brick to brick?

[scott123]

Jeff, are you saying that each brick is a fairly uniform temp, but, as you cross the joint to the next brick, that's where the drop occurs?  I guess you do have some air between the bricks that might create some kind of break, but I don't see it being that dramatic.

The false ceiling would probably mess with air flow, but a platform should be fine from an air perspective.  A large enough piece of steel, even just 1/8" off the hearth would pretty much remove conduction from the equation. Ideally, it would be best if you could sit the platform on insulating brick, but I think even resting the platform on regular brick wouldn't be the end of the world, as long as the brick would contact the plate minimally (such as resting on an edge).

What I'm picturing when I think of conduction is a 12" long firebrick column with a torch blasting away at one end.  How long before the other end gets hot? How long for a 24" column? Whenever anyone puts firebrick in a home oven, the pre-heat times are outrageous. I know that a home oven element is by no means a torch, but you're still talking travel time.

[shuboyje]

Jeff, are you saying that each brick is a fairly uniform temp, but, as you cross the joint to the next brick, that's where the drop occurs?

Exactly.  It is exactly at the joint.  I don't know anything else that would cause it but conduction from one brick to the next.

I hear what you are saying about heat conducting across a couple feet of material, but at the same time I've never found the radiant heat to be a major player in wood fired ovens, other then cooking the side of the pie and causing leoparding.  For instance the dome directly above the coals does not experience a temperature spike like the hearth does.  If the effect is from radiation it should.

[scott123]

If conduction is a major player here, could spreading out the glowing embers to all non baking areas of the oven (not just the back) make a difference?

Also, the approach you initially recommended- would a simpler description be "less flame, more glow?"

Until this can be tested, I'm not completely jumping on board the conduction train, but, out of all the potential workarounds for our NY style in a Neapolitan oven baking friends, this, by far, would be the easiest to try. I hope they give it shot.

[TXCraig1]

I have a hard time imagining heat leaving the bottom of the coal pile moving horizontally through the fire brick and up into the bottom of the pizza. I'm sure some does this, but it's hard to believe it is a meaningful amount.

CL