Appendix 4. Thermal Mass Primer
Overview
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 is heated and
provides heat to the oven chamber, while insulation
describes the oven's ability to stop heat from escaping, or
leaking, out of the oven -- where it is lost.
Thermal Mass
The thermal mass of a wood-fired oven can vary widely,
from a simple 1 1/2" thick clay shell to a massive 12" thick
brick bread oven. The barrel vault oven described in books
and on the Internet has a 9” thick dome and hearth, which is
much too massive for backyard cooking or a commercial
pizzeria. When considering which oven is right for your
application -- whether it is in your home or in a restaurant,
you should take a number of factors 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 equilibrium
where everything is just warm.
In a wood fired oven, this means that it is strongly desirable
for the thermal layers in an oven's dome and cooking
surface to be completely hot when it is time to cook. If they
are not, the heat in the oven will continue to "wick" away
from the oven chamber, in an attempt to create equilibrium
in the whole thermal mass.
For example, if it requires 6 hours to fully heat up an oven's
thermal mass, that oven will not cook well for the entire six
hours it is heating up. Even though you are adding more
and more fuel, the heat is 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 3-minute pizza.
At the same time, too little thermal mass can also be
problematic. While an inexpensive 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 begin start giving up heat as soon
as the fire has stopped, creating a range of problems for the
chef. As a side note, the Forno Bravo ovens all have
enough thermal mass for typical home baking bread or
roasting a turkey.
The composition up of an oven's thermal material is critical.
Alumina and silica are two materials that have both high
heat conductivity and high heat holding capability. An oven
rich in these materials will heat up more quickly, and hold
heat longer, than an oven made from clay, brick, or even
standard firebricks.
By tailoring the thermal mass of the oven to the intended
us, either residential or commercial, bread or pizza, it is
possible to produce the optimal oven for a range of
applications.
An oven built using 2"-3” of engineered refractory is perfect
for a vast majority of home and garden applications.
Insulation
While too much thermal mass is bad, there is no such thing
as too much insulation. Luckily, through modern
manufacturing advances it is possible to readily and cost-
effectively insulate a wood-fired oven. In fact, it is possible
to make insulation cost and space trade-offs for a particular
oven installation.
Which is a good thing. As the optimal oven design demands
a thermal layer that can be efficiently heated throughout, it
also demands that the oven's insulating layers be able to
stop the heat in that layer, in order to hold it inside the oven.
There is a range of insulating products that you can put to
use:
Pompeii Oven Instructions
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