fire clay, have low thermal conductivity, and are designed to withstand extremely high heat. Alternatively called refractory bricks, fire clay bricks, and dense fire bricks; they are used in pizza ovens due to their ability to withstand extreme temperatures. Unlike insulating fire bricks, standard fire bricks are dense, and contain the right amount of alumina and silica to make them most suitable for constructing the hearth and dome of the oven. A sign of a good quality refractory brick is that it will keep the oven hot for over 12 hours, as it is able to hold heat without dissipating it. These bricks are required at the base of the oven, as regular building bricks may not be able to withstand the heat, and begin to crack. Fire bricks, however, due to their heavy density and low porosity, tend to last for long periods.