Fire bricks is different from that of red bricks in the chemical composition, color, shape and properties.
Fire bricks are made of clay containing alumina and silica. They are naturally white and heavy with a low porosity. The denser properties also give the brick greater resistance to damage from abrasion.
Fire bricks can be classified into three grade: low, medium and high. High-grade fire brick contains higher percentages of alumina and gets hotter than the lower grades. It is more conductive and transfer more heat to the bottom. Additionally, high-grade fire brick has less resistance to thermal shock and may likely to crack, spall and fail. Medium-grade fire brick is the most heat resistant and ideal for a brick oven, reaching top temperatures of around 900 degrees F. Fire bricks are strong and won't crack under the pressure of constant heating and cooling, making them perfect for use in ovens.
Red brick is made by pulverizing clay or shale. The maximum heat resistance for red bricks and common firebricks is about the same. Red bricks has a greater rate of expansion and are more likely to spall or flake when equally heated. This can occur when a hot fire is built and the surface of the brick is heated too quickly or when a jet of water is sprayed on the bricks.
Red bricks can also be used in a brick oven. In ovens, red bricks will heat up, retain heat, cook, bake, roast, re-fire, absorb conduct store and hold the heat from wood fire and perform the same way as fire bricks do. But it is not as hard and durable as fire bricks. It may flake or crack. Additionally, it does not conduct heat as well as fire brick and takes longer to reach adequate cooking temperatures.
One advantage of red brick over fire brick is its low price. It can also be recycled from other old projects. If you have a tight budget, red bricks can be used as a good substitute for fire bricks.