What causes firebricks to crack?

We are accustomed to using products that stay new looking for years—cars, furniture, cameras. But kilns are different. The high temperatures they reach generate tremendous stress. Since the insulating firebricks expand and contract with each firing, hairline cracks will appear in the bricks while the kiln is cold—even in a new kiln. Do not be concerned with these. They are normal. The cracks close tightly when the heated bricks expand. The cracks function as expansion joints and will not affect the firing. Though the insulating firebrick is fragile enough to carve with a fingernail, it is a miracle of physics and when properly maintained can survive for many years.