Some of the commonly used fire brick dimensions are 9 by 4.5 by 3 inches and 9 by 4.5 by 2.5 inches. Some fire bricks are only half as thick as traditional fire bricks; these are commonly known as "splits."
Fire bricks are made with fire clay that is heated in a kiln. Special-purpose bricks may be glazed. Fire bricks are composed of about 50 to 80 percent aluminum oxide. Bricks used with high temperatures, such as those lining steel furnaces, have a high content of silica to be able to withstand temperatures up to 3000 degrees Fahrenheit. For low temperature applications, other brick materials, such as magnesium oxide, can be used.