Pottery kilns require very high temperatures to properly vitrify the clay and make it non-porous. Fire bricks are commonly employed as either the entire kiln structure, or for the inner layer of the kiln, where the most heat is present. Higher alumina content bricks (around 40 percent) are required for the inner surface of kilns that need to fire raku ware and stoneware, which reaches temperatures of 1,800 to 2,200 degrees Fahrenheit respectively. Soft fire bricks are often used to block up the kiln doors during firing, as they are less durable for permanent construction, and provide easily-removed insulation for the kiln door.