Many famous archaeological sites have been subjected to destructive fires, whether hostile or accidental, including Near Eastern cities constructed largely of mud-brick. But how long did it take to burn down a city? The mud-bricks themselves provide a valuable record. By heating experimental bricks of different sizes, shapes and compositions to high temperatures, the minimum duration of an ancient conflagration can be calculated. The resulting equations were applied to bricks from the destruction of Tel Megiddo at the end of the Iron Age I, and indicate that the burning lasted a minimum of two to three hours: a much shorter period than expected.