Firebricks, also called fire bricks, fireclay bricks or refractory bricks, are blocks of ceramic material made to withstand extremely high temperatures. They are used to line furnaces, kilns, fireboxes, fireplaces and pizza or bread ovens. They are made mainly of oxides such as silica and alumina, typically about 25 to 45 percent alumina and around 60 percent silica, and their low thermal conductivity helps hold heat in.
Dense firebricks are used where heat, abrasion, ash or slag are harshest, such as wood-fired kilns and metal furnaces, while lighter, more porous insulating firebricks suit electric or gas kilns. A good firebrick should not crack or spall when temperatures change quickly. The two standard sizes are 9 by 4.5 by 3 inches and 9 by 4.5 by 2.5 inches.
Source: Wikipedia: Fire brick
Image: Torpedo car refractory bricks by Morgan Riley, CC BY 3.0, via Wikimedia Commons.