Why does my kiln have a 1/2" hole in the center of the lid?

Ceramic pieces release gases and water vapor during firing. Venting allows these gases to escape from the firing chamber. The hole in the lid is designed to improve venting. For most ceramic materials, leave the lid closed and peephole plugs out during the entire firing. For additional venting of lusters and overglazes, vent the lid with the lid prop, which is near the front of the kiln.