Continuous vs. Periodic Kilns
Before delving into specifics, it’s important to recognize that there are two basic types of kilns: continuous (tunnel) kilns and periodic (intermittent) kilns. Continuous kilns are always firing, so they never cool. The ware to be fired is loaded into cars and slowly goes through the kiln on a track or rail. After cooling, the still-moving cars are unloaded and reloaded to begin the circuit again. These kilns are used for industrial manufacturing. Periodic kilns are the ones most familiar to potters. They are fired on an intermittent schedule and is not firing all the time. The kilns are loaded, brought to temperature, cooled, then unloaded. Below are the most common types of periodic kilns used by potters. The firebox is where the flame enters. The damper is at the top of the kiln and controls the exhaust (and, by association, the kiln’s atmosphere). The stack area is where the pots are set and it’s between the firebox and damper. Although an updraft kiln tends to be less fuel efficient than a downdraft kiln, most commercially built fuel-burning kilns are updrafts. This is mainly due to their simplicity to build, pack, and ship. Downdraft kilns consist of four main components: the firebox, the stack area, the damper, and the chimney. The addition of the chimney helps create draw or air flow. Sprung arch downdraft kilns are probably the most commonly potter-built kiln type in the United States. Sprung arches do, however, displace weight outward. Due to this outward tension, sprung arch kilns are framed with angle iron to support the arch and reduce any movement of the firebricks used to build the kiln. Sprung arch kilns may be designed as either updraft or downdraft kilns. Catenary arch kilns are often quite beautiful because it is a graceful form. However, the interior does not lend itself to efficient use of the space in the stack area. Like sprung arch kilns, a catenary arch kiln may be either an updraft or downdraft kiln, depending on how the air and flame flow through the kiln. Multi-chambered hill-climbing kilns are often downdraft in style. Hill-climbing kilns that are one long chamber are often closer to an updraft air-flow pattern. Examples of hill-climbing kilns include the ancient kilns built in Britain during the Roman occupation. It’s also seen in anagama kilns, which are experiencing a revival in Japan and elsewhere. These kilns are shaped in a circular cone, dome, or bottle form. They are often built on or out from a vertical wall, where the firebox is located. They are updraft kilns and during the Industrial Age, they were typically fired with coal. Most bottle kilns still being fired today are used in industry, but even they seem to be disappearing as continuous kilns become more common. This arrangement allows the car to be pulled out from the main body of the kiln. Once loaded and pushed back into place, it forms the floor and one side of the kiln as a whole. Unlike the cars in a continuous kiln, the car of a periodic kiln is an integral part of the kiln itself. The top of the kiln may be lifted by handles, but only if it is small and light. Otherwise, the top may be raised by a pulley system or a lever and counter-weights. Because of the full access a top-hat kiln gives to the ware, they can make very good kilns for raku. Consideration must be given, however, to the fact that the top will be incandescently hot. It requires a place to safely rest after being removed from the base.
