A fire brick is a block of ceramic material that can withstand high temperatures and is used to line furnaces, kilns, fireboxes, and fireplaces. For example, they use it to surround pipes, conduits, etc. in walls as fire breaks. There are two types of fire bricks: hard fire bricks and soft fire bricks.
Hard fire bricks are very dense, hard and durable but not great for insulating. While insulating firebricks can be easily cut with any kind of saw, hard fire bricks require special equipment, like a tile cutting saw, to cut. Design your kiln to take full advantage of available shapes, and when possible to minimize the amount of hard bricks you have to cut.
They are used for their structural qualities. Hard fire bricks are used on the inside of things like kilns because they absorb and conduct heat easily, which allows for the high temperatures needed in cooking and firing. They can be often used as the main building component of large kilns, chimneys, fireboxes and burner ports—anywhere around direct flame.
Soft fire bricks are soft and light in weight. They are made from a refractory clay body containing combustible materials. They can be easily cut by handheld hack saw or any other hand tool like chisel or even drill bit. This kind of firebrick has high porosity and excellent insulating properties. When fired, the materials burn out leaving a sponge like matrix of air pockets, which serve to provide insulating qualities to the brick. Soft firebricks absorb about half the energy as hard firebricks during a firing.
Soft fire bricks range from 2000°F to 3300°F and are used as the brick of choice for constructing electric kilns or as insulating liners in reduction kilns. Soft fire bricks are used on the outside of kilns because their very low thermal conductivity allows them to insulate and keep the heat inside the kiln. Soft fire bricks can also be used as a base for small scale soldering and precious metal clay firing with hand-held soldering irons and blowtorches. In this case, the soft fire brick protects the hard surface beneath it from damage.