The insulation is composed of a number of layers. The bottom is usually fire brick. The interior is lined with a layer of either firebrick, fiber board, or a combination of fiber board and fiber blanket. The sides of the kiln may be made from insulating fiber board. With all this insulation, the inner surface of the kiln may reach 2000 degrees F, while the outside remains cool enough to touch.
Most kilns used in glass studios are electric fired.
An electric kiln provides more uniform heating than a gas-fired kiln, is quieter, and requires less maintenance. A kiln can have the door on either the top or the front. Top loading kilns are easier to load and provide complete access to the inside surface of the kiln. However, burns to the face and arms are more likely when using a top-loader if you peek into the kiln to see how the firing is progressing. During peeking, particles can also fall off the lid of a top loader onto the project.