Fire brick

A fire brick, firebrick, fireclay brick, or refractory brick is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. Made of primarily oxide materials like silica and alumina in varying ratios, these insulating materials are able to withstand extremely high temperatures, and have a low thermal conductivity for greater energy efficiency. Refractory bricks generally range from 25-45% alumina, and ~60% silica, with additional magnesium, calcium, potassium oxides.

Usually dense fire bricks 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 and easier to form and insulate far better than dense bricks. In any case, firebricks should not spall, and their strength should hold up well during rapid temperature changes.