The potter loaded her kiln by feel — decades of burned fingertips had taught her exactly how the heat moved. It rose from the element at the base, climbing one shelf at a time until the whole chamber glowed. She had just closed the peepholes when she heard the crack: the door seal had split on the left side, letting in a tongue of cold air. The leftmost column of shelves cooled immediately — she could see the color drain from the clay. The right three columns, shielded by the door's intact half, held their heat. She patched the seal and brought the whole kiln back to temperature by morning.