I received an interesting note from a customer: "I know of someone who complained that there was a gap between her lid and body of the kiln and that she could see the inside of her kiln through that gap. She had been told by Paragon's customer service that it's supposed to be that way and she isn't having any heat loss from the gap. What?? That's the most ridiculous statement I've ever heard!"
The complaint of the lid is a normal misunderstanding. The kilns are manufactured using a special insulated firebrick that expands at high temperatures. The lid is installed so the lid by the hinge does not touch the top of the firebrick sidewall. This is to allow the sidewall brick to expand vertically and not put pressure on the lid. Furthermore, the slot in the metal through which the hinge rod goes is oval in shape not round. This is again to allow the lid to move and "float" on top of the kiln as the kiln grows vertically during the firing. All kiln manufacturers understand this issue and design kilns with "float" in the lid. There is a colored glow visible between the top of the sidewalls and the lid. The kiln lid of any of our ceramic, pottery, or top loading glass kilns uses the same design concept.
A significant portion of the lid touches the top of the kiln and it is normally the front of the kiln. Furthermore, the lid at temperature bows with the edges bowing up and the middle bowing down toward the firing chamber. This will be exaggerated in larger diameter kilns and almost negligent in smaller diameter kilns. This is how the brick material behaves at temperature. The amount of heat "escaping" is very small and has almost no impact on the kiln's performance. Allow me to explain.
People do have a perception that if they see any light around the lid, or the lid is not touching the top of the kiln sides all around, massive amounts of heat are escaping or there is a hazard. This is not correct.
First, almost every kiln sold by Paragon and the other kiln manufacturers are certified to Underwriter Laboratories Section 499 that regulates kilns. If the design itself were hazardous they would not allow it to be certified.
Second, the assumption that heat "escapes" assumes the impact of convection on the firing process. Above 1100 F the heat transference in kilns is through radiant heat, not convection. Physics dictates that the particle density of air diminishes rapidly with temperature. Convection relies on particles physically colliding to transfer heat energy. At higher temperatures, the density of the molecules diminishes to the point that convection is not an effective method of heat transference. Radiant energy transfers heat at these temperatures.
This is the same as the sun on your face on a cloudy day. When the sun is out, you feel warm on your face, and when clouds appear, the temperature you feel drops. This is why the glass kilns use top elements to uniformly heat flat glass and why you should not use shelves stacked in glass kilns. The glass that does not receive direct radiant heat will not heat properly. If convection were the only method of heat transference, this would not be the case.