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?
Answer: Convection and radiation.
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.