A firebrick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces.
A refractory brick is designed mainly to withstand high heat, but should also usually have a low thermal conductivity to save energy. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. Dense fire bricks have a high thermal mass which is sometimes a useful property in that they will retain heat eg in a pizza oven after the fire has been taken out. Fire bricks are made by firing a clay based composition in the kiln until it is partly vitrified, and for special purposes may also be glazed. Fire bricks usually contain 30-40% alumina and the primary raw material is usually chamotte with other materials. The maximum service temperature increases with increasing alumina content and bricks can be obtained with an alumina content of 80% and above.
In other, less harsh situations, such as a natural gas fired kiln, more porous bricks are a better choice, usually referred to as Insulation Bricks. They are weaker, but they are much lighter, easier to form, and insulate far better than dense bricks. In this case they have a low thermal mass and so cannot be used to store heat. Insulation bricks have a better thermal shock resistance than dense firebricks but the main disadvantage is their low strength. The insulation property of these bricks usually comes from perlite or vermiculite. As with the dense firebricks there is a range of grades corresponding to different maximum service temperatures. The most common is Grade 23 – 1260oC.