Let me walk you through a typical bake in my backyard oven. As we light the fire, heat the masonry up, make the pizza, bake the bread, and roast the rest of our meal, you'll learn how these ovens work and why they do such a good job at making pizza and bread.
First Step: Light the fire
A small fire at the front of the oven gets things started. My backyard oven has four inches of brick surrounded by six inches of insulation. Today, I am trying to bake for as long as possible, so my goal is to build an efficient fire that will thoroughly heat those four inches of brick. The insulation will trap the heat in the brick, not letting it slip out into the cool winter air.
Step Two: How hot is the oven? Is it ready yet mom??
I've got a few tricks to test how hot the oven. I notice that the soot has burned off the dome, meaning the surface of the brick is above 650 degrees Fahrenheit. I notice that the inner dome glows a little, meaning it is quite a bit hotter that 650 degrees. The ash has turned white. Finally, if I take my infrared thermometer, I see that the surfaces of the brick dome pushes above 1200 degrees because my thermometer can register a temperature. I'll let the fire go another half hour or so, just to make sure that that intense heat migrates into those four inches of brick and doesn't just stay in the oven chamber or on the surface of the brick.
Step Three: Pizza! Why the obsession with hearth pizza?
The guys who taught me to make pizza, Peter and Edward Fath, bake Neapolitan style pizza that requires a super hot oven and a hot hearth to bake on. We try and bake our pizza when the hearth is at 750 degrees Farenheit and the dome bricks are around 850 degrees. We'll pull out most of the coals and bank the rest against on side of the hearth. A small fire will keep the oven at pizza temperature for as long as the party requires. A 750 degree hearth gives the pizza the fast char, the little bit of stiffness, and the perfectly singed toppings that makes Neapolitan pizza famous. Although Peter and Edward will make their pizza inside as well, they have to adjust their recipe and bake on a pizza stone because the pizza bakes much slower at the 550 degrees of a home oven.
Step Four: Pull the fire out. How does the oven stay hot?
Once the pizza is done, we pull out the rest of the coals. We are not longer actively heating the oven. As the oven chamber cools down, the heat that we packed into the four inches of brick will slowly move out of the brick and into that baking chamber. If we fired the oven long enough and got those bricks hot enough, the oven will stay within bread baking temperature for many hours.
Step Five: Bake the bread. Why does the oven bake bread so well?
The answer has two parts: heat and steam. Clay and masonry ovens have three kinds of heat. They conduct heat (transfer through contact) from the hearth floor into the bottom of the loaf. They convect heat (transfer with moving hot air) as hot air cycles from the dome to the hearth, across the surface of the bread. The ovens also radiate heat (heat transferred across space like the sun gives the earth heat) from the clay or masonry walls into the loaves. This unique combination of three kinds of heat transfer is essential to the baking of high-rising loaves of bread that have a gradient of color (and flavor) across its thick crust. Finally, clay and masonry ovens trap moisture instead of venting it out. An oven's worth of bread will release 10-20% of their weight in water as it bakes, effectively steaming themselves. Moist air makes the bread crust more supple as it bakes and allows the loaf to spring higher than it would in a dry environment. It also helps enzymes on the surface of the bread turn starch into sugar, and that sugar will caramelize to make a yummy, shiny, red-brown colored crust. Professional bakers and home bakers will try and find ways to add steam to their home ovens. In a masonry or clay oven, you just need to fill the oven up and the bread will steam itself!
Step Six, Seven, Eight....Bake until the heat runs out, or you get tired
Over time, the bricks will release their heat into the oven chamber and the oven temperature will continue to drop. Vegetables roast beautifully at 400 degrees, chicken does well at 350 degrees, casseroles and baked beans will slow bake at 300 degrees. I'm working on baking an old-school pumpernickel at 250 degrees (see "What makes pumpernickel dark?" if you're curious). With the last bit of heat, you can dry out wood for the next bake or dry out jerky or fruit for munching. Even in our small masonry oven, we can roast into the next day.