A Thermal Mass Primer
Thermal mass and insulation are the two primary characteristics that describe an oven's ability to absorb and hold heat, and make it useful for cooking. An oven's thermal mass describes the part of the oven that absorbs and reflects heat from a fire, making it available for cooking, while insulation describes the oven's ability to stop heat from escaping. or leaking, out of the oven—where it is lost.
The thermal mass of a wood fire oven can vary widely, from a thin, natural clay shell to a massive 12" thick brick bread oven. When considering which oven is right for your application -- whether it is in your home or in a restaurant, there are a number of factors to take into account.
Too much thermal mass is very bad. Heat, like most things in nature, likes equilibrium. If one side of a thermal mass, such as a block of concrete, is hot and the other is cool, nature will try to balance that heat by moving it from the hot spot to the cool spot -- eventually reaching an equilibrium where everything is just warm.
In a wood fired oven, this means that it is strongly desirable that the entire thermal mass of the dome and cooking floor fully heated when it is time to cook. If they are not, where the inner face of the oven is hot, but the outer areas of the dome and floor are still cool, then heat on the inside faces of the oven will "wick" away from the oven chamber, in an attempt to create equilibrium in the whole thermal mass.
For example, if it requires six hours to fully heat up an oven's thermal mass, that oven will not cook correctly the entire six hours it is heating up. Even though you are adding more and more fuel, heat is continually moving away from the oven chamber as fast as you can replenish it. With this design it is virtually impossible to maintain the high heat required for the perfect Pizza Napoletana.
At the same time, too little thermal mass can also be problematic. While a thin clay oven shell might heat up quickly, it does not posses the heat holding ability to cook larger volumes of food, larger numbers of pizza, or bread. A thinner oven will quickly start giving up heat as soon as its fire has stopped, creating a range of problems for the chef.
The composition and density of an oven's thermal material is also important. Alumina and silica are two materials that have both high heat conductivity and high heat holding capability, and when used with a calcium aluminate binder (not Portland cement), they provide excellent cooking and longevity characteristics. An oven rich in these materials will heat up more quickly, hold heat longer and last much longer than an oven made from natural clay.
An oven built using 2-3" of engineered refractory is perfect for a vast majority of home and garden applications.