Used firebricks are found in the demolition of kilns, fireplaces and ovens they were originally used to construct. As a reusable material, firebricks are often harvested and re-sold via online sellers like Alibaba.com or from building material resale stores like Habitat for Humanity's ReStores.
Firebrick is composed primarily of clay like ordinary building bricks but contains higher quantities of aluminum oxide and silicon oxide. These compounds, when baked in, create a denser, more refractory material that provides lower thermal conductivity, effectively concentrating energy in high-heat applications. Firebricks are designated as low, medium or high-duty depending on the concentrations of aluminum oxide and density of the material; high-duty bricks, for example, have aluminum oxide concentrations exceeding 50 percent of the material and a density above 150 pounds per cubic foot. This type can withstand temperatures up to 2650 degrees Fahrenheit.
Different types of firebricks are used for different applications. High-duty bricks, for example, tend to be more expensive and retain heat better. They are more commonly used for furnaces and kilns in high-temperature industrial applications. Medium-duty bricks, in contrast, retain heat to a lesser degree and are used more often for brick ovens and fireplaces. Light-duty firebricks are often used to insulate equipment instead, as they have poor thermal conductivity and hold little heat.