A
fire brick,
firebrick, or
refractory brick is a block of
refractory ceramic material used in lining
furnaces,
kilns,
fireboxes, and
fireplaces. A refractory
brick is built primarily to withstand high temperature, but will also usually have a low
thermal conductivity for greater energy efficiency. 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. In other, less harsh situations, such as in an electric or
natural gas fired
kiln, more porous bricks, commonly known as "kiln bricks" are a better choice. They are weaker, but they are much lighter, easier to form, and insulate far better than dense bricks. In any case, firebricks should not
spall and their strength should hold up well under rapid temperature change.