This page explains the different types of fire brick refractory and what the trade-offs are between them. It should help you decide what types of fire brick you are going to use, and it can help you find the right fire brick refractory at your local masonry supply store. It might be useful to take this page with you when you go shopping.
Here are some basic fire brick refractory types:
Medium duty fire brick. This is the fire brick refractory that we recommend for both the cooking floor and dome of the Pompeii oven. medium duty fire bricks are comprised of roughly 35% alumina and 50% silica, heat up quickly, easily withstand the 900F heat your oven will reach, and are designed for the rapid heat-up and cool down (thermal cycling) that your oven will experience. fire brick will also reach the heat required for pizza more quickly than clay refractory brick, as they are more efficient at conducting heat.
Further, because fire brick is designed to withstand thermal cycling, your oven will last longer, though for most home ovens this is not an important issue, and your oven will probably outlast you whichever brick you choose.
When choosing your fire brick, look for a refractory brick with straight edges for your cooking floor. It is important that the bricks in the floor fit against each other, and a curved edge will result in a gap between the fire bricks and in your cooking floor.
A typical fire brick measures 9″x4.5″x2.5″, weighs about 8 pounds and is yellow.
Red fire brick. This is the traditional red fire brick that you find at Home Depot and at masonry supply stores. Fire bricks are made from clay, and fired in a kiln. They are typically made from local clay, as shipping is too expensive, and fired to between 2000F – 3000F (high enough to fuse the minerals). You can use clay brick in the oven dome, but we would not recommend using them in the oven floor. There are trade-offs to consider.
There are two shortcoming to using a fire brick in your oven dome. First, thermal cycling will cause fire brick to spall, where little pieces of the fire brick flake off, and could cause individual fire bricks to crack. It has happened to us. Second, fire brick is not as good a conductor as fire brick and as a result will take longer to heat up.
Our view is that unless cost is a prohibiting factor, we would recommend fire brick. For example, a 42″ oven some has roughly 180 bricks in the dome, so the difference in brick cost should be around $100. In the context of the overall cost of the oven, and large amount of human capital you will be investing in your oven, we think the extra cost of worth it.
If your choice is to build your oven with fire brick or not at all, we would strongly recommend building your oven with fire brick.
Red fire bricks are typically used for building the decorative arch and optional sides around the oven vent and vent landing, and can be used for any decorative feature.
There are three types of fire brick that you should avoid.
Concrete fire brick. They are made from standard Portland cement-based concrete and are air dried, not kiln fired. They will not withstand the heat inside your oven.
High duty fire brick. These brick have very high alumina content, get very hot (1500F and up) and are designed for continual high-heat applications, such as furnaces. They are expensive, and will get too hot for some of your oven uses, such as baking bread and roasting. In general, pizza wants heat between 750F and 900F, while bread and roasts cooks best between 500F and 600F. (Note that brick ovens are able to cook at higher temperatures without burning because of the moist heat inside the oven and shorter cooking times.)
Insulating fire brick. These light-weight refractory bricks are designed to stop heat, and as such have low conduction and low heating holding capacity. They are often used to insulate industrial equipment. A typical insulating fire brick weighs about 2 lbs, compared with an 8 lb standard duty fire brick
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