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 a natural gas fired kiln, more porous 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.
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In the making of firebrick, fireclay is baked 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 × 4½ × 3 in. (230 mm × 115 mm × 75 mm) and the other is 9" × 4½" × 2½". 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" × 4½" × 1¼". 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.[1] Here is a description of the plant circa 1904:
| “ | The grounds of the plant cover 133 acres. The company has three mines in the city and one mine at Glencoe, Mo., 30 miles away, where about 61 acres are owned. All the mining is done below ground by pick and blast. About 50 men are constantly employed disembowelling 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. -- The Clayworking Plants of St. Louis (1904 book)[1] | ” |
Fire bricks usually contain 30-40% aluminium oxide or alumina and 50% silicon dioxide or silica. They can also be made of chamotte and other materials. For bricks of extreme refractory character, the aluminium oxide content can be as high as 50-80% (with correspondingly less silica),[2] and silicon carbide may also be present.
The silica firebricks that line steel-making furnaces are used at temperatures up to 1650°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, is used to make the insulating tiles of the space shuttle.
A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces. Common red clay brick are used for chimneys and wood-fired ovens.