Some people worry about the line of light that appears around a kiln lid (or door) at high temperatures. They wonder if the light means heat is escaping.
The kiln expands as it heats. The larger the kiln, the greater the overall expansion of parts. Since the hot inner surface of the lid expands more than the cooler outer surface, the lid or door bows slightly toward the firing chamber. This causes a small gap, where light from the firing chamber is visible. The gap is more pronounced on the ends of an oval lid than on a round lid.
But unless the lid rises during firing (the subject of another Kiln Pointer), there is little heat loss from the gap under the lid. At high temperatures, the molecules in air are so far apart that they no longer transfer heat through convection. This is why heat does not pour out of a peephole when you remove the plug.
On some models, we add a gap between the lid (or door) and kiln at the hinge. As the kiln gets hot, the gap closes due to the expansion of the firing chamber.
