Before delving into specifics, we should first recognize that there are two basic types of kilns that all other types fall into. These two primary types are continuous (tunnel) kilns and periodic (intermittent) kilns.
Continuous kilns are always firing; 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 in industry.
Periodic kilns are the ones most familiar to potters. They are fired on an intermittent schedule; the kilns are loaded, brought to temperature, cooled, then unloaded. The kiln is not firing all the time.
Below are some types of periodic kilns used by potters.
Updraft kilns are those in which the flame is introduced into the bottom of the kiln, at or below floor level, and exhausted out the top. Updraft kilns consist of three basic components: the firebox, the damper, and the stack area.
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 is 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 are designed to force the flame and heated air to circulate through the kiln. Flame is introduced at the bottom of the kiln and naturally flows upward. The construction forces the flame back downward, to exhaust at the bottom of the kiln.
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 Arch Kilns
The term "sprung arch" refers to the roof structure of the kiln. The arch is built using a form; once completed, the form is removed and the arch will stay in place without further interior support.
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
Catenary arches, once built, are self-supporting. The arch itself itself is an upside-down replica of the curve produced when a heavy rope or chain is hung from two points on the same horizontal plane. Like sprung arches, catenary arches must be supported by a form while they are being constructed.
Catenary arch kilns are often quite beautiful; it is a graceful form. However, the interior does not lend itself to an efficient use of 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 flows through the kiln.
Many versions of hill-climbing kilns utilize catenary arches in their structure. The real hallmark, however, is that these kilns are built on a slope. The slope itself acts as a natural chimney, enhancing the kiln's draw.
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 ancient kilns built in Britain during the Roman occupation and the anagama kilns which are experiencing a revival in Japan and elsewhere.
Bottle kilns and their very close relatives, beehive kilns, are generally indistinguishable. Many people use the terms interchangeably, although the older beehive style does not have a flue at the top, whereas a bottle kiln does add a short chimney.
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 were typically fired with coal. Most bottle kilns still being fired today are used in industry, but seem to be disappearing as continuous kilns become more common.
Just as cars are used to transport ware through industrial continuous kilns, they have also been adapted to use for periodic kilns used by potters. Set on a track or on wheels, the car is the platform on which ware is stacked for firing.
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.
Generally small, top-hat kilns are built in two sections: an immobile base which includes the firebox, and a removable top section. The ware is stacked on the open base, then the top will be carefully placed over all to complete the kiln. After firing, the top will be lifted off the base so the ware can be unloaded.
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.