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 under rapid temperature change, and their strength should hold up well during rapid temperature changes.
In the making of firebrick, fireclay is fired in the kiln until it is partly vitrified, and for special purposes may also be glazed. There are two standard size of fire-brick; one is 9×4 1⁄2×3 inches (230×110×76 mm) and the other is 9×4 1⁄2×2 1⁄2 inches (230×110×64 mm). Also available are firebrick "splits" which are half the thickness and are often used to line wood stoves and fireplace inserts. The dimensions of a split are usually 9×4 1⁄2×1 1⁄4 inches (0.23×0.11×0.032 m). In the United States, fire bricks were made at the now-defunct Evens & Howard Fire Brick Company in Saint Louis, Missouri, and shipped to diverse locations around the nation as well as Canada. Here is a description of the plant circa 1904:
The grounds of the plant cover 133 acres [53.8 ha]. The company has three mines in the city and one mine at Glencoe, Mo., 30 miles [48 km] away, where about 61 acres [24.7 ha] are owned. All the mining is done below ground by pick and blast. About 50 men are constantly employed disemboweling the raw material. The company has an extensive equipment of cars and tracks, at Glencoe, for the conveying of the clay to the plant. All clay is weathered for at least six months. The clay from the mines on the plant is hauled to the dry pay by teams, being dumped into storage bins situated near the pans for which it is intended.
Fire bricks have an aluminium oxide content that can be as high as 50-80% (with correspondingly less silica). 
High temperature applications
The silica firebricks that line steel-making furnaces are used at temperatures up to 1648°C (3000°F), which would melt many other types of ceramic, and in fact part of the silica firebrick liquefies. HRSI, a material with the same composition, was used to make the insulating tiles of the space shuttle.
Non-ferrous metallurgical processes use basic refractory bricks because the slags used in these processes readily dissolve the "acidic" silica bricks. The most common basic refractory bricks using in smelting non-ferrous metal concentrates are "chrome–magnesite" or "magnesite-chrome" bricks (depending on the relative ratios of magnesite and chromite ores used in their manufacture).
Lower temperature applications
A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces. Silica bricks are the most common type of bricks used for the inner lining of furnaces and incinerators. As the inner lining is usually of sacrificial nature, fire bricks of higher alumina content may be employed to lengthen the duration between re-linings. Very often cracks can be seen in this sacrificial inner lining shortly after being put into operation. They revealed more expansion joints should have been put in the first place, but these now become expansion joints themselves and are of no concern as long as structural integrity is not affected. Silicon carbide, with high abrasive strength, is a popular material for hearths of incinerators and cremators. Common red clay brick are used for chimneys and wood-fired ovens.
- Insulating Brick, retrieved 2012-07-04
- "The Clayworking Plants of St. Louis". Brick XX (5): 232–233. May, 1904. Retrieved 2013-09-09.
- http://www.vitcas.com/refractory-bricks Refractroy bricks
- Modern Refractory Practice, Fifth Edition (Harbison–Walker Refractories: Pittsburgh, Pennsylvania, 1992), Page CR-2
- Modern Refractory Practice, Fifth Edition (Harbison–Walker Refractories: Pittsburgh, Pennsylvania, 1992), Page CR-3
- Refractory Engineering. Die Deutsche Bibliothek. 2004. ISBN 3-8027-3155-7.