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Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Pizza Oven Physics
« on: June 21, 2004, 01:53:52 PM »
As you know, I've been researching the physics of a pizza oven. Specifically, the claims of Lombardi's and their 900°F coal-fired oven.

In all actuality, I became rather obsessed in trying to make a "coal-fired" oven out of my electric home oven. I tried the "self-clean" and other tricks to coax the temperature inside my oven to 600°F and beyond. I then stumbled upon the Pizza Bella oven, a small 10-inch table top unit with heating elements above and below the stone. This puppy easily reached 900°F, as read with my non-contact infrared (IR) thermometer.

I soon discovered that pizza crust burns quickly at high temperatures! As soon as the raw dough hit the deck, smoke started billowing and the intense smell of burning crust soon followed.

What was I doing wrong? I had finally achieved high temperatures, but now everything was burning to a cinder.

Another person on this board complained, too, that his pizza bottom was burning when cooked on his stone (his stone was placed directly on the floor of his home gas oven.)

So, I've been busy researching.

Here's some valuable information that has taught me a lot. I think everyone will find it enlightening (especially item #6).

1. You cook your pizzas in a wood-burning brick oven. While it seems that the oven would be most energy efficient if it were perfectly sealed and didn't exchange any air with the rest of the apartment, the fire would go out if you sealed it. Why?

Answer: The trapped air would run out of oxygen and the reaction (that is, the fire) would stop.

Why: As you learned in grade school, a fire needs both fuel and oxygen to proceed. Without a fresh supply of oxygen, the fire will go out.

2. To make sure the fire doesn't go out, the oven has an arch-like opening on one side and a chimney coming out of its top and passing up through the roof. When the fire is burning, the oven develops a natural draft: air enters the side opening and exits through the chimney. What force or imbalance in forces propels this draft?

Answer: The heated air above the fire experiences two forces: its weight downward and a buoyant force upward. The buoyant force is the stronger of the two forces, so the air experiences a net upward force.

Why: The draft is propelled by the imbalance between the heated air's weight downward and the buoyant force it experiences upward. Because heated air is less dense than cold air, the heated air weighs less than the cool air it displaces and it rises. In effect, this heated air is a hot air balloon without the actual balloon. It rises anyway.

3. Brick is a wise choice for the oven walls because the outside of the oven remains cool to the touch even when the inside surface of the oven is quite hot. If you had used metal oven walls, you'd burn your fingers on the outside of the oven. Explain briefly what physical difference between brick and metal makes brick the better choice for the oven.

Answer: Bricks have no mobile electrons and therefore do not conduct heat well. Metal has mobile electrons and is a better conductor of heat.

Why: Brick walls trap the thermal energy inside the oven, whereas metal walls would convey heat quickly to their outside surfaces. A metal-walled oven would leak heat like crazy and be a burn hazard.

4. The brick walls are dark in color, particularly in the infrared. What heat transfer mechanisms convey heat from the burning wood to the oven walls?

Why: The brick walls are above the fire, so rising hot air conveys heat to them via convection. And because the bricks are dark in color (meaning that they interact well with light), thermal radiation from the hot coals conveys heat to the bricks via radiation.

5. Both the burning wood and the pizzas rest on opposite sides of the oven's flat bottom surface. Why don't convection or conduction directly heat the pizzas?

Answer: Hot air from the fire rises up the chimney and never visits the low-lying pizza. And the bricks are such terrible conductors of heat that very little heat flows through them from the fire to the pizza.

Why: This cooking arrangement: pizza and fire at the same height and widely separated by bricks, frustrates both conduction and convection. The hot air rising from the fire just can't get to the pizza and there are no good conductors of heat connecting the fire and the pizza.

6. This cooking system rarely burns the bottom of the pizza crust. Why?

Answer: Very little heat flows into the pizza from below, so the crust is the last thing to cook, not the first.

Why: With conduction and convection out of the picture, the only thing heating the pizza significantly is radiation. Since that comes from above, it cooks the pizza from above. The bottom of the pizza is cooked mostly by heat flowing downward from the top of the pizza, so the crust doesn't burn on the bottom easily.

7. Since the wood and pizzas are side-by-side on the oven bottom, they don't “see” one another very well. How does heat flow from the wood to pizzas? How are the oven walls involved?

Answer: Heat flows from fire to pizza mostly indirectly: heat flows from the fire to the oven walls via convection and radiation and then heat flows from the oven walls to the pizza via radiation.

Why: The brick oven cooks via radiant heat. The walls of the oven become very hot and it is their thermal radiation that cooks the pizzas lying on the hearth.

8. If you were to install a big fan inside the oven to circulate the air artificially, the cooking process would change significantly. Why?

Answer: By forcing convection, you would bring hot air in contact with the pizza and the pizza would cook via both radiation (as before) and convection (something new).

Why: Convectional doesn't heat the pizza when the oven has no fan. But with the fan circulating the air vigorously, convection becomes a significant conveyer of heat to the pizza. How this affects the taste of the finished pizza, I don't know. But the absence of fans in most brick ovens implies that it isn't desirable.

The above information was found at:
http://rabi.phys.virginia.edu/105/2003/ps8s.html

So, while Lombardi's claims to have a 900°F coal-fired oven, the fact of the matter is that the deck of their oven is nowhere close to that temperature. The deck is most likely conducting heat at approximately 650°F, the walls are probably radiating heat at 750°F, and the oven ceiling is probably radiating heat at 900°F.

The problem with laying tiles directly on the floor of your gas oven is that the flame heats the floor directly since the flame is in direct contact with the floor. As a result, the stones superheat to the temperature of the flame (using heat conduction), which makes it very hot! A regular gas oven heats the compartment by convection and radiation. So, the flame heats the oven floor, the super hot oven floor radiates heat upwards and heats the air inside the oven. The hot air is what's measured by the oven's thermostat.

This is easily fixed by placing the pizza stone (or tiles) on the bottom-most rack in the oven. This provides an air space between the super hot oven floor and the bottom of the stone. In this situation, the stone will reach the proper temperature.

The next hurdle to overcome is how to get the edges and top of the crust to cook at the same speed as the bottom. The bottom cooks via heat conduction, but the sides and top must cook via heat radiation. Since heat radiates from the surface of hot objects, and because a home oven is so big (top to bottom), the surface of the pizza is very far away from the radiated heat (if cooked on the bottom-most rack).

So, it makes sense that the secret is to move the stone to the top-most rack. Plenty of radiated heat up there, but now the stone is too far away from the bottom heating element to heat up and maintain temperature.

So, I think I have the ultimate solution... a solution that I suggested many years ago (do a Google search for "Steve Zinski two stone method"). I suggested using two pizza stones to cook pizza, one stone on the bottom-most rack, the other on the top-most rack. The oven is preheated and the pizza is initially cooked on the bottom stone. The bottom stone is hotter because it's closer to the bottom heating element. About half way though the cooking process, the pizza is transferred to the top stone and the oven is set to "Broil". The top stone continues to cook the bottom of the pizza, but the broiler kicks in and provides additional radiated heat to cook the sides and top of the pizza.

I am a little embarrassed that I came up with this method years ago, but this is when I had first started out and I moved on to other ideas in trying to create a hotter oven. Now it seems that I have come full circle!

Another idea that I have is to put the upper stone directly above the the lower stone to see if the radiated heat from the upper stone will help cook the top of the pizza (i.e., stone #1 goes on the bottom-most rack, stone #2 goes on the second from the bottom rack). Or, perhaps I could line the upper rack with foil to reflect the heat back down onto the pizza.

As you can see, more experimentation is needed. But I think I'm on the right track now.

The moral of the story: A hotter oven is not the answer! A 550°F - 650°F oven is all that's needed to cook the bottom of the pizza... It's getting the top of the pizza to cook faster so that when the bottom's done, the top's done too.
« Last Edit: June 21, 2004, 01:59:46 PM by Steve »

cory

• Guest
Re:Pizza Oven Physics
« Reply #1 on: June 21, 2004, 04:35:40 PM »
Genius!  Alton Brown would be proud.  I think I'm going to try a version of this method Tues or Wed Night.  I'm going to put foil on my top rack in the middle of the oven, and put the stones on the bottom rack, making sure to give myself enough clearance.  I think this combination of heating will work wonders.

PS, speaking of AB, has anyone here tried to brush the outer crust with olive oil?  Is this considered cheating?  The one good thing about my first pizza attempt was that the top of the crust (non blackened part was actually pretty golden brown with various char marks on it.  It looked and tasted "real."  (if not too big)

Cory

Les

• Guest
Re:Pizza Oven Physics
« Reply #2 on: June 21, 2004, 08:11:01 PM »
I was devilishly delighted to hear someone has gone through the same hoops I have trying to recreate those great pizza's one gets from certain pizza places.  I too have tried the two-stone method, both cooking between two hot stones and lifting the pizza to the top and using the broiler.  I also tried a closed gas grill where the temp inside registered 800 degrees (I burned up the pizza and my pan in short order).  None of it gave me the results I wanted.

I've watched the pizza specials on the Food Channel where they tour the US spotlighting various pizza styles, and of course one can't help but notice the high temperatures used.  Because I rent, building a brick oven isn't an option, so recently I saved my money and invested in a Bakers Pride countertop pizza oven (the 16 incher).

It gets up to 650 degrees (about 175 degrees higher than my regular oven), and the space for baking is only about 4-5 inches inside, with the lower element closer than the upper.

I can report I am finally in pizza heaven (even though I am certain a wood-burning oven would be better), and stuffed to the gills (and so is the wife) because I can't stop making pizza!  It's taken a few tricks, but I am finally creating the kind of pizza (and in six minutes) I've been hoping for.

One trick I can recommend is spreading the sauce or tomatoes and cheeses from edge to edge on the pizza.  Not only does it make me excited to get to the crunchy, cheesy last bite, it also protects the edge from over-cooking. (Cory's wondering about oil on the edges actually will cook the top faster because the oil gets so much hotter than water.)

Probably everyone knows this, but hand stretching gives a more tender, chewy crust (and with bubbles) because rolling out the dough flattens and eliminates the air pockets.

Tonight I am attempting to use a pizza stone on top of the wire pull-out tray (I usually use a pizza screen, as recommended).  I am a little worried because it raises the pizza 5/8 inch in an already-tight area.  If anyone is interested, I'll report what happens.  Online I have found a handmade stone at 3/8 inches, so if this comes close, I might try that.

Anyone interested in a Bakers Pride countertop can check them out at Valiant Equipment at www.valiantequipment.com  My oven came damaged, and rather than make me deal with insurance etc., Bakers Pride took care of the whole thing and sent out a new oven even before I'd returned the old one.  The woman in charge of customer relations at Valiant, Camisha, was extremely customer friendly, providing tracking numbers, calling me back, etc., even though their main business is full-scale restaurants.

Les

• Guest
Re:Pizza Oven Physics
« Reply #3 on: June 21, 2004, 11:21:22 PM »
I thought I'd add that the Bakers Pride has separate switches for the upper heating element and the lower.  Tonight I tried using a pizza stone and it burned the bottom a bit.  The separate switche design is going to allow me to turn off the bottom element (at some point I've yet to figure out), and let the stone cool sufficiently to balance with the top element.  Otherwise, the stone looks like it is going to work well.

Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Re:Pizza Oven Physics
« Reply #4 on: June 22, 2004, 07:56:43 AM »
Les,

Thanks for your insight! I, too, have tried the gas grill method with mixed results. Again, the problem is that the gas burners are below the stone which superheats due to convection.

I read some of Peter Reinhart's American Pie book again last night, specifically the chapter about ovens and cooking. He pretty much summed up what I posted. Not sure if a layer of aluminum foil above the pizza will work since it's the thermal mass that's needed to radiate the heat and cook the pizza. A pizza stone seems to be the best bet.

Tonight I am going to try an experiment (pictures will be forthcoming). I plan to use one oven rack to hold my tiles and function as the deck, the other rack will be positioned one notch above the bottom and will be layered with tiles as to function as the ceiling. I will build sides out of more tiles to create the walls... you see where I'm going with this... an oven-within-an-oven!   There should be enough thermal mass surrounding the pizza as to radiate heat and cook the pizza properly.

Wish me luck!!
« Last Edit: June 22, 2004, 07:58:01 AM by Steve »

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Randy

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• Pizza, a great Lycopene source
Re:Pizza Oven Physics
« Reply #5 on: June 22, 2004, 09:13:57 AM »
Steve, unfortunately you are spinning your wheels here a bit.  The top tile will be very close to ambient temperature of your oven and there will be no substantial gain from radiant heat.
The only way I know to accomplish what you want is to preheat the top stone or tile to a higher temperature than oven-ambient like you do the lower stone using the radiant heat from an element or burner.
Have you tried preheating your bottom stone then turn the broiler on when you slide the pizza in.?
Randy

Pizzaholic

• Registered User
• Posts: 164
Re:Pizza Oven Physics
« Reply #6 on: June 22, 2004, 09:22:43 AM »
Has anyone used a hearthkit??? It is an insert that goes in an oven.

I have an OOOOLD gas grill that I replaced the innerds a couple of times. This last time I bought a perferated plate that goes across the burner. It holds the bricketts. I notice that most of the heat is going around the edges on the left and right sides. To the poilt of being red/white hot, with little heat in the middle.
Would I be wise to put my stone in the middle, and let the convection heat around the edges cook the top of a pie??
Pizzaholic

I am thinking about getting a convection/microwave oven in the future for above the stove, has anyone tried to do pizza in one of these??

Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Re:Pizza Oven Physics
« Reply #7 on: June 22, 2004, 09:40:59 AM »
Steve, unfortunately you are spinning your wheels here a bit.  The top tile will be very close to ambient temperature of your oven and there will be no substantial gain from radiant heat.
The only way I know to accomplish what you want is to preheat the top stone or tile to a higher temperature than oven-ambient like you do the lower stone using the radiant heat from an element or burner.
Have you tried preheating your bottom stone then turn the broiler on when you slide the pizza in.?
Randy

Randy,

The purpose of the top stone is to prevent temperature swings. When the door to the oven is opened and the pizza slid in, the oven temperature will drop 100°F instantly, mostly from the top of the oven chamber. So, this alone puts the pizza at a disadvantage. The top stone will still be radiating heat and will help minimize the temperature swing. I might try building my own "HearthMaster" by putting the tiles on the top shelf and adding sides with tiles. Then, when the pizza goes in the oven, crank on the broiler to provide direct radiant heat.

Les

• Guest
Re:Pizza Oven Physics
« Reply #8 on: June 22, 2004, 12:00:06 PM »
Les,
Thanks for your insight! I, too, have tried the gas grill method with mixed results. Again, the problem is that the gas burners are below the stone which superheats due to convection.
I read some of Peter Reinhart's American Pie book again last night, specifically the chapter about ovens and cooking. He pretty much summed up what I posted. Not sure if a layer of aluminum foil above the pizza will work since it's the thermal mass that's needed to radiate the heat and cook the pizza. A pizza stone seems to be the best bet.
Tonight I am going to try an experiment (pictures will be forthcoming). I plan to use one oven rack to hold my tiles and function as the deck, the other rack will be positioned one notch above the bottom and will be layered with tiles as to function as the ceiling. I will build sides out of more tiles to create the walls... you see where I'm going with this... an oven-within-an-oven!   There should be enough thermal mass surrounding the pizza as to radiate heat and cook the pizza properly.
Wish me luck!

I definitely wish you luck. You are going further trying to manipulate a conventional oven than I tried.  If all else fails, and you can justify spending \$490 (or \$440 for 14 inch size), I again highly recommend Bakers Pride countertop pizza oven (there are other brands, but they cost about the same).  It works.

Probably lots of people know this already, but I also just got a big 525 watt Kitchenaid mixer.  OMG, it kneaded the dough in five minutes.  I love stuff that works!

Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Re:Pizza Oven Physics
« Reply #9 on: June 22, 2004, 12:27:58 PM »
I just wrote to the folks at Hearth Kitchen (http://www.hearthkitchen.com), the makers of the HearthKit oven insert. I asked for a review sample so that I can compare their product to that of a regular pizza stone. We'll see if they choose to reply or not.

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Randy

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Re:Pizza Oven Physics
« Reply #10 on: June 22, 2004, 12:33:07 PM »
Baker's pride now that is getting serious about pizza!

Randy

Arthur

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• Posts: 253
• When Brooklyn Was the World
Re:Pizza Oven Physics
« Reply #11 on: June 23, 2004, 03:34:27 PM »

I definitely wish you luck. You are going further trying to manipulate a conventional oven than I tried.  If all else fails, and you can justify spending \$490 (or \$440 for 14 inch size), I again highly recommend Bakers Pride countertop pizza oven (there are other brands, but they cost about the same).  It works.

Probably lots of people know this already, but I also just got a big 525 watt Kitchenaid mixer.  OMG, it kneaded the dough in five minutes.  I love stuff that works!

Is that Bakers Pride ok for homes? i.e., regular outlet in your house?   Also, it looks like the oven has a wire rack in the picture.  Do you put a stone (or tiles) on it?

Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Re:Pizza Oven Physics
« Reply #12 on: June 23, 2004, 03:59:17 PM »
Baker's Pride makes several countertop pizza ovens.

For the larger units which utilize a stone deck, they do not recommend the 120V model for cooking raw pizza dough... not enough wattage.

Les

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• It's Proper to use Grape Tomatoes in Wine Country
Re:Pizza Oven Physics
« Reply #13 on: June 23, 2004, 07:05:24 PM »
Is that Bakers Pride ok for homes? i.e., regular outlet in your house?   Also, it looks like the oven has a wire rack in the picture.  Do you put a stone (or tiles) on it?

Hi Authur.  It is working in my place, but let me give you a few details in case you are considering buying it.

The 16 inch size is pretty big, 24" wide, 20" deep, and 10" high.  We are fortunate to have a lot of counter space, so it fits in a corner nicely.  The oven gets really hot on the top, front, and left side; the back gets less hot but still will burn you, and the right side is vented and doesn't get hot.  We don't have children in the house, so that's not a problem, but I have burned myself several times getting used to how it works.

I am building a wood enclosure for it which will make it safer, and also allow me to store things on top and so not lose much counter space.  I found some very cool insulating stuff (pun intended) which I am going to enclose the oven in on its side and top to keep the wood enclosure from getting damaged.  Check out: http://www.koolmat.com/what.shtml.  As an alternative, I think the oven could easily be set up in a garage or laundry room etc., which we are considering doing.

The Bakers Pride PX 16 Hearthbake found here: http://www.valiantequipment.com/countertops.html  give you a choice of 120v or 220v;  we chose the 120 and using an ordinary outlet (though three pronged is required) it reaches 650 degrees without a problem (of course, we haven't seen our electric bill yet ).

As you say, it does have a wire rack, and I tried a pizza stone in it.  My worries about raising the pizza too close to the top element were unfounded.  What I didn't foresee however was that the stone got so hot it burned the bottom before the top was done.  I am going back to using pizza screens for now (which work perfectly), as the manufacturer recommends.  But since the top and bottom heating elements each have their own on/off switch, I plan to experiment later with the stone again by turning off the bottom element early.
« Last Edit: June 24, 2004, 08:33:35 PM by Les »

Arthur

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• When Brooklyn Was the World
Re:Pizza Oven Physics
« Reply #14 on: June 24, 2004, 09:05:11 AM »
Hmm...

I always thought about getting one of those, particularly this one:

http://www.selectappliance.com/exec/ce-product/bp_p18s

but as it states, not recommended for home use and not recommended for fresh dough

http://www.doughproovens.com/Homeoven.shtml

for \$3K

or maybe just waiting for Steve to go through his testing with a standard home electric oven with tiles and use that???

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cory

• Guest
Re:Pizza Oven Physics
« Reply #15 on: June 24, 2004, 09:13:04 AM »
I don't know about you guys, but my pizzas all looked better than those DOUGHPRO ones, and these are my first three ever.  I guess they probably have a good smoky/woody taste, but still...

Steve

• Posts: 2316
• Age: 53
• Location: Richmond, VA
Re:Pizza Oven Physics
« Reply #16 on: June 24, 2004, 09:13:58 AM »
I always thought about getting one of those, particularly this one:

http://www.selectappliance.com/exec/ce-product/bp_p18s

but as it states, not recommended for home use and not recommended for fresh dough

The 120-volt Bakers Pride P18S oven is not recommended for fresh dough. The 240-volt model is (you can buy this oven in 120V or 240V configurations.) The reason is that the 120V model is only 1800 watts, whereas the 240V model is 2850 watts. While both models will reach 650°F, the 120V model will take a lot longer to get there and maintain that temperature (more watts = faster heating.)

I myself am thinking about the P22S-BL (Brick Lined) oven... it's slightly more expensive, but the brick option would turn it into a real pizza oven!    I'd have to build a stand for it out in my garage and run a 240V circuit to it.

Arthur

• Registered User
• Posts: 253
• When Brooklyn Was the World
Re:Pizza Oven Physics
« Reply #17 on: June 24, 2004, 09:59:48 AM »
I would love to try one of those "small" bakers pride ovens before I bought one.  I wouldn't mind spending \$\$\$ on either an outdoor wood burning oven or the bakers pride "small" ovens if I knew they turned out better than my home oven.

Maybe I'll attend this:

http://www.newyorkpizzashow.com/index.htm

and bring some fresh dough to try out in pizza ovens!

Les

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• It's Proper to use Grape Tomatoes in Wine Country
Re:Pizza Oven Physics
« Reply #18 on: June 24, 2004, 10:46:01 AM »
I would love to try one of those "small" bakers pride ovens before I bought one.  I wouldn't mind spending \$\$\$ on either an outdoor wood burning oven or the bakers pride "small" ovens if I knew they turned out better than my home oven.

I obviously can't give you the empirical aspect of what the oven is like (unless you want to drop by California and check out mine), but I can give you a bit more info. Steve's right about the 120v taking more time to heat up of course.  Bakers Pride says to allow the oven's thermostat to cycle three times at one's selected temperature before using. The 120v takes about 40 minutes to achieve that.

I'm not sure what your oven already does, so I can't compare it and the BP.  But my regular oven barely reached 500 degrees, while the BP reaches 650 (I know because I have a digital thermometer).  Putting in a pizza doesn't cause its thermostat to kick in, so it retains heat well (which is what you'd expect for an oven made for commercial use).  On a pizza screen, at 650, it cooks a pizza in 5 and 1/2 minutes browning the cheese without burning the crust.  Compared to the results I've had with a regular oven, that is a definite improvement.

The fact that it is designed for commercial use has good and not-so-good aspects.  Not so good is how it's finished, which I think is somewhat rough.  There are sharp edges inside, and where the rack slides in and out is unfinished-looking and feeling.  The experience of using it isn't "smooth." I'm already developing little assists to make things smoother however, and I am pretty sure I will be happy living with it in the absence of a wood burning oven (which I'd love to have).

The good apsect of its commercial build is the electronics, which seem exceptionally solid.  When you use the electronic timer, for example, it feels really well made, even it's beep sounds strong (and long lasting too so you can't ignore it too long).  Temperature controls are the same way, and the individual switches for the upper and lower heating elements I really appreciate having.  When you use the oven, and it works so consistantly every time, you can tell that inspite of its rough finish, it is built to be effective.
« Last Edit: June 24, 2004, 10:50:43 AM by Les »

itsinthesauce

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• This is a sickness.
Re:Pizza Oven Physics
« Reply #19 on: June 24, 2004, 11:14:13 AM »
That Show looks like it would be well worth the effort to attend, for numerous reasons. Imagine being amongst thousands of Pizzaholics.

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