We regularly encounter a myth in kiln firing: heat rises so the top of the kiln is always hotter than the bottom. This is something we all learned years ago and see in everyday life. However, in the kiln world, this is both true and false. The answer depends upon which temperature the statement regards. At temperatures below 1100°F this is generally true, but above 1100°F it is generally false. Let me explain.
Convection, the normal action of hotter molecules rising and cooler ones falling, occurs at temperatures below 1100°F. This normal action of molecules allows hotter molecules to physically collide with cooler molecules and transfer the heat energy. As temperatures approach 1100°F, the amount of convection diminishes quickly as the primary method of heat transference, because there are so few molecules remaining in the fire chamber. The density of air diminishes dramatically with temperature. As the density of air diminishes, the number of air molecules decreases, and the ability to transfer heat by collision decreases accordingly.
The primary method of heat transfer at temperatures above 1100°F is radiation. This is transferring heat from the elements to the ware by line of sight high energy, high frequency "light." Think of sunlight on a cool, clear day. You can feel the radiant heat from the sun on your face, but when the cloud passes over, it feels much cooler. This is line of sight radiant heat transfer. The molecules between the earth and the sun are not transferring the heat by bouncing into each other and heating the earth. Space is cold. Except for annealing, the temperature at which most of the critical heat work occurs on art glass is above 1100°F. Therefore, radiant heat is the primary method by which heat is transferred to glass, and convection has little to no impact on the glass.
Now that we understand how heat energy is transferred, here are a few points of how this principle is applied in the design of a kiln. First, most glass kilns, other than the smallest models, have elements in the top. Smaller models with 8” x 8” or smaller interior chamber sizes do not require top elements as their uniformity of radiant heat distribution will be sufficient with side elements only. The glass is no more than about 3 or 4 inches from an element. Most kilns with an interior diameter above 10 inches have elements in the top. The top elements are normally laid out in a pattern in the lid to radiate heat evenly onto a flat piece of glass. This reduces the temperature variances on the glass and prevents cracking. Moreover, this is why stacking shelves is discouraged. They act as a barrier and prevent the radiant heat generated by the elements in the lid from reaching the glass on the lower shelf. If shelves are used with top element kilns, the bottom shelf will severely under-fire almost every time.
Second, the kiln’s shelf or art glass should not be placed in the firing chamber in such a way that they act as a barrier between the elements and the thermocouple. The thermocouple should reflect the temperature that is experienced by the glass.
Third, if firing projects with drape molds, drop molds, etc., where a large percentage of the glass faces the side wall of the kiln and not the top elements, the use of kilns with side elements is encouraged for this application. Most kilns that are above 7 to 9” deep either come with top and side elements as standard in the design or have side elements as an option at the time of purchase. The idea is the same. As parts of the glass bend and no longer face the top elements, the side elements provide the radiant heat to the glass that now faces the inside walls. There are no drawbacks to firing flat glass in a kiln with top and side elements.
Many times people ask if they can fire glass in old ceramic kilns, which have only side elements. The answer is yes, it can be fired, but it usually takes much testing to adjust the programs to have successful firing. Better firing results are obtained by slowing down the firing profiles and doing smaller sized projects. The smaller glass pieces have diminished capacity for temperature variances across the piece. Large flat pieces are very difficult to fire successfully in side element only kilns.
Remember these points when firing to improve your results and end another kiln myth.