POTTERY PRODUCTION PRACTICES: How to Choose an Electric Kiln
March 1, 2008
The available varieties of kilns have grown so steadily over the years that the selection process can be daunting. Following are several selection criteria that will help you sort through the confusion of choosing from so many different possibilities.
TemperatureThe kiln you choose must be rated hot enough for the ware you intend to fire: 2350°F for porcelain and stoneware; 2000-2300°F for low-fire ceramics; and 1400-1700°F for china painting, glass fusing, glass slumping, enameling and bead annealing.
It is a good idea to buy a kiln that will fire hotter than you need it to. If you are firing glass to 1500°F, buy a kiln rated to 1700°F. If you fire ceramics to cone 6, buy a cone 10 kiln. As heating elements age, they draw less and less power. Generally, the higher the kiln’s maximum temperature rating, the longer the elements will last. This is because even after they begin to wear, elements still draw enough amperage to fire the ware.
Another advantage to having a higher temperature capacity is that during periods of low voltage, your kiln will still likely reach the temperature you need.
SizeIn general, the larger the kiln, the lower the cost per cubic foot of interior space. Divide a kiln’s price by its cubic feet, and you’ll see what I mean. Choose a kiln that will fire the largest ware that you produce, and decide how often you want to fire the kiln. Do you want to fire many small loads or a few large ones? Some people prefer to fire frequent small loads to see how special effects turn out before spending time on other projects. Others would rather fire fewer large loads.
Determine how long it will take you to make enough ware to fill a kiln of a given size. Do you think your needs will expand later? Kiln owners will typically tell you to buy more capacity than you currently need, because you’ll probably outgrow your kiln later.
Before purchasing a 10- or 12-sided top-loading kiln, visit a studio that has one. Reach down inside the kiln to be sure you are tall enough to load it. This is an important consideration that is often overlooked. I know people who cannot touch the floor of their kiln, so they need to leave a shelf supported by posts in the bottom. If you have difficulty loading a studio kiln, consider a short and wide 12-sided, 22-in.-deep kiln.
If you fire ware of a particular size, such as tiles or bowls, plan the kiln load on paper. Draw diagrams of different-sized kiln shelves and determine how many pieces of ware will fit onto each shelf. You may find that the ware fires most efficiently in a particular size kiln. For instance, the 10- and 12-sided kilns can both fire 10 in. bowls. Since both kilns fire four bowls per shelf, however, the bowls fire more efficiently in the 10-sided kiln than in the 12-sided.
The 10-sided kiln is also a good choice for those who need short firing cycles. Since 10-sided kilns are smaller than 12-sided, they can heat and cool faster. In addition, kiln shelves for 10-sided kilns are lighter than those for 12-sided kilns and are easier to lift.
ElectricalWill you need a new circuit installed for your kiln? (Only a licensed electrician should install a new circuit, and copper wiring should be used, not aluminum.) Homes in the U.S. and Canada usually receive 120/240 volts. If your studio is in a business district, strip mall or school, it is likely that your voltage is 208, not 240. It is important that you know your voltage before ordering the kiln. 208-volt and 240-volt circuits use the same wall outlets, so you can’t visually tell them apart.
Call your power company or electrician if you are not sure about your voltage or phase. If you fire a 240-volt kiln on a 208-volt circuit, the kiln will fire slowly and may never reach maximum temperature. This is an expensive mistake, because you will need to order new elements that are the correct voltage, and the switch box may have to be rewired as well.
Contrary to logic, 240-volt kilns do not necessarily fire hotter or faster than 120-volt kilns; some 120-volt kilns can reach 1000°F in five minutes.
Round or SquareOn a per-cubic-foot basis, “round” kilns (6-, 7-, 8-, 10- and 12-sided) are less expensive than square because they are easier to build. Ceramists usually buy the round models, while schools and potters sometimes buy the large square kilns because they are especially durable and slow cooling.
Top- or Front-LoadingFront-loading kilns are preferred for enameling, where pieces are removed from the kiln at 1450°F. Using a top-loading kiln for enameling would be difficult, since the heat from the kiln rises when you open the lid. Ceramists typically use small front-loaders for glaze testing and small pieces. Large front-loading studio kilns are easier to load than top-loading models because you don’t have to bend down into the kiln.
Firebrick or Ceramic FiberThough ceramic fiber heats and cools faster, insulated firebrick (used in most kilns) outlasts ceramic fiber. In addition, heating elements are easy to replace in a firebrick kiln because they are exposed in firebrick grooves. Most ceramic fiber kilns use elements embedded into the ceramic fiber, so the elements cannot be replaced. Instead, the ceramic fiber firing chamber and elements have to be replaced as a single unit.
Insulating Firebrick Wall ThicknessMost ceramic kiln walls are either 2½ or 3 in. thick. Kilns with 3 in. walls and lid take slightly less energy to fire due to the extra insulation. However, their main advantage is that they reach a higher temperature than their 2½ in. counterparts. They also cool more slowly, which is important when firing heavy pieces that might be prone to cracking, and for special glaze effects. To fire stoneware or porcelain, buy a kiln with walls at least 3 in. thick.
Manual or AutomaticMost manual-fire kilns operate with infinite control switches similar to those used on electric ranges. They contain a bi-metallic timer that cycles on and off. As the switch is turned clockwise, the heating elements stay on longer and longer. On High, the elements stay on continuously.
Manual-fire kilns are gradually being replaced by automatic models. If you are planning on a manual-fire because that is what you’re accustomed to, at least consider an automatic kiln. Once you understand them, automatic kilns are easier to use than the manual variety. Before making your final decision, ask your dealer to demonstrate an automatic kiln for you.
Automatic kilns are of two general types: mechanical and digital. Mechanical automatics use timers to advance the switch settings and a Dawson Kiln Sitter to turn the kiln off. Digital kilns use an electronic controller.
Some people think mechanical kilns are more reliable than digital kilns. It is true that the wiring of a digital kiln is more complicated than that of most manual-fire kilns. Digital kilns use a transformer and relays, which are often not needed in a mechanical kiln. However, digital kilns are reliable if designed properly, and ceramists have been firing them successfully for over two decades.
Digital kilns are also easier to repair than some people think. The heart of the system is a small circuit board that, in a well-designed kiln, can be removed in minutes and repaired or replaced.
The biggest mistake kiln operators make is assuming that an automatic kiln will shut off as it should every time. Every automatic kiln needs human monitoring, especially near the shut-off time. Even if the kiln you are buying is automatic, plan to be near it at the end of firing. If the kiln takes longer than expected, look through the peephole at the pyrometric cones on the shelf. The cones will warn you if the kiln has fired to maturity and should be shut off manually.
Other ConsiderationsWhen estimating the price of a ceramic kiln, include the cost of furniture (the shelves and posts stacked inside the kiln). Kiln furniture enables you to stack multiple layers of ware in the kiln. Without it, you could fire only the ware that fits on the kiln bottom.
Kiln fumes should be vented from the studio or workspace, and two types of vents are available. The overhead type works like a kitchen stove vent and pulls the fumes into a hood positioned over the kiln. A downdraft vent fits directly onto the kiln and removes a small amount of hot kiln air, mixes it with room temperature air, and vents to the outside.
Making Your SelectionConsider these criteria one by one, draw a selection chart on paper and take your time finding the best choice for you. Once you understand the differences between various types of kilns, choosing the best one for your application will be easy.
For additional information regarding kiln selection, contact Paragon Industries, L.P. at 2011 S. Town E. Blvd., Mesquite, TX 75149; (972) 288-7557; fax (972) 222-0646; e-mail email@example.com; or visit the website at www.paragonweb.com.
All photos by Arnold Howard.
SIDEBAR: Important Kiln FeaturesPeepholes
Every kiln should have at least one peephole, which is a hole drilled into the wall or door of the kiln. Even at ceramic temperatures, you can safely see inside the firing chamber of your kiln. A properly designed peephole will give you a wide view of the interior with little heat loss. Use green firing safety glasses when looking into the peephole of a hot kiln.
Why would you need to see inside the firing chamber? Even though we live in the computer age, I recommend that you include pyrometric shelf cones with every firing. Cones are a reliable means of checking the maturity of ceramic ware, and you need to be able to see how the cones on each shelf have deformed during firing.
Lid & Lid Vent
Though not so important on small kilns, the hinge on a studio kiln should be large enough to help reduce stress from the weight of an open lid. Otherwise, the weight will crack the bricks in the back of the firing chamber.
The bricks in the lid and bottom of the kiln are cemented together, and the seams are hairline-tight. A well-made lid is strong but should be handled gently. The lid is coated with a refractory coating to help reduce dust inside the kiln.
Stainless Steel Jacket
Most round kilns are wrapped with a stainless steel jacket. The jacket should be tight to strengthen the firing chamber and should be held in place with stainless steel screws or an adjustable tightening system.
Use the stand that comes with your kiln, not a substitute. A well-designed stand will evenly support the kiln without stressing the firebrick bottom. If you are buying a used kiln that has no stand, order a stand from the kiln manufacturer.