The main ingredient in these Hy-C Medium-Duty Fire Bricks is fireclay, which contains mostly alumina and silica, elements capable of withstanding high temperatures. These hard bricks are available in several grades, depending on their composition and properties, which determine the application of the product. Proper selection of the appropriate grade is critical. The fire bricks are rated medium-duty and have a temperature rating to 2700 degrees F maximum and a high thermal shock resistance. These medium-duty fire bricks are standard in most high efficiency wood burning appliances.