Kilns are rather big pieces of equipment and they get really, really hot. These two factors alone can make choosing the right kiln for your shop a bit intimidating. Once you’re armed with some know-how, however, making a good buying decision becomes so much easier, and you can be far more confident in your decision. Much of the information in this post is excerpted, with permission of the author, from a previously published article written by John S. Hohenshelt, president of Paragon Industries, a well-known manufacturer of kilns. Thanks, John!
The marketplace offers a great number of different kilns, each with a wide array of features, sizes, shapes, designs and control systems. In this post, we take a look at a number of factors that should be considered when you’re thinking about choosing a new kiln. So, let’s organize these variables into seven main criteria and go through each of them.
It is a good idea to buy a kiln that will fire hotter than you need it to. This is because, as heating elements age, they draw less and less power and, generally speaking, the higher the kiln’s maximum temperature rating, the longer the elements last. Once the elements begin to wear, a slightly higher-rated kiln still draws enough amperage to fire your pieces. So, if you are firing glass to 1500°F, buy a kiln rated to 1700°F. If you fire ceramics to cone* 6, buy a kiln rated for a cone 10.
Another advantage to higher temperature capacity is that, during periods of low voltage, your kiln will still likely reach the temperature you need. In general, the maximum temperatures required for various materials are:
- 1800°F: COPPRclay™
- 1650°F: PMC3™, PMC+™, and PMC® Flex
- 1550°F: BRONZclay™
- 1525°F: Fastfire BRONZclay™
- 1500°F: PMC® Sterling
Glass & Enameling
- 1400°−1700°F: glass fusing, glass slumping, enameling, bead annealing, glazed china
Porcelain, Stoneware & Ceramics
- 2350°F: porcelain and stoneware
- 2300°F: low-fire ceramics
*Cone number refers to a pyrometric cone. These cones, manufactured from various oxide mixtures, are used to provide a visible indication of how the kiln is heating. (See more information on cones in our white paper, “Choosing An Electric Kiln.”)
Whatever size kiln you choose, your pieces need to fit easily into the kiln with plenty of space around and between them to enable the heat to circulate efficiently. Consider:
- What size are the pieces you create?
- Does your firing require a firing container or some type of kiln furniture?
- Do you make jewelry pieces only, or are you instead (or also) creating hollowware, sculptures or larger vessels?
- Will you be firing many small loads or a few large ones or some of both?
Some users fire frequent small loads to test design elements or to see how special effects turn out before investing time creating larger quantities of something new. Others prefer firing fewer loads with larger quantities of pieces.
Choose a kiln that will fire the largest pieces that you produce, will accommodate any container or kiln furniture you’ll be using, and evaluate how often you will need to fire the kiln. Figure how long it will take you to make enough pieces 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, to allow yourself the room to grow into in the future.
And one other consideration regarding size: The larger the kiln is, the lower the cost per cubic foot of interior space is likely to be. Divide the price of several kilns by their cubic feet, and you’ll see how this bears out.
Power & Wiring Requirements
The electrical requirements of the kiln you choose are an important consideration and should factor into your choice of kilns. If you’ll need a new circuit installed for your kiln, you’ll need a licensed electrician to install a new circuit (and insist on copper wiring, not aluminum).
Homes in the U.S. and Canada are usually wired for 120/240 volts. If your studio is in a business district, strip mall, or school, though, it is likely that your voltage is 208, not 240. It is important that you know your voltage before choosing a kiln. Outlets wired for 208-volt circuits look the same as those wired for 240-volt circuits—you can’t tell just by looking; call your power company or electrician if you’re not sure about your voltage or phase. (If you run a 240-volt kiln on a 208 volt circuit, the kiln will fire slowly and may never reach maximum temperature. This can be an expensive mistake, because you will need to order new elements of the correct voltage and possibly have the switch box rewired to resolve the incompatibility.)
Oh, and one more thing: As contrary to logic as it seems, a 240-volt kiln does not necessarily fire hotter or faster than a 120-volt kiln. Some 120 volt kilns can reach 1000° F. in five minutes! All I’m sayin’ is: Don’t set your heart on a 240-volt model just because you think it’ll be “better” than a 120-volt model.
Kiln Shape: Round or Square?
A “round” kiln is any kiln that has more than four ‘sides.’ On 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. Artists who work with ceramics usually buy the round models while schools and potters sometimes buy the large square kilns, because they are especially durable and slow-cooling. Jewelry-makers, metal clay artists, enamelists and casters typically choose a square kiln; glass fusers tend to prefer round kilns, particularly if they are fusing large items such as plates or hollowware.
Top- or Front-Loading Configurations
Front-loading kilns are preferred for enameling, where pieces are removed from the kiln at 1450°F. This would be difficult with a top-loading kiln since the heat rises when you open the lid and you’ll have to lean into the escaping heat. In addition, front-loading studio kilns are easier to load than top-loading models because you don’t have to bend down into the kiln.
Before purchasing a 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 important. If you get a top-loading kiln into your studio and then find you can’t reach the base, you will likely have to raise the base by leaving a shelf supported on posts in the bottom permanently, wasting a good deal of firing room you may have counted on having. If you find depth is an issue, consider a shorter, but broader kiln.
Firebrick vs. Ceramic Fiber Firing Chambers
The firing chamber in any given kiln will be made with one of two materials: firebrick or ceramic fiber. Though ceramic fiber heats and cools faster—highly useful for short firing times and quick turnaround of work, insulated firebrick outlasts ceramic fiber and is easier to replace as needed. Heating elements in a firebrick kiln are exposed, set into grooves cut into the firebricks; the elements are simply removed and replaced. Most ceramic fiber kilns have elements that are embedded into the ceramic fiber and cannot be replaced; instead, the entire firing chamber (fiber and elements) is replaced as a single unit.
The thickness of the chamber walls will also be a factor in how quickly the kiln reaches temperature, how efficiently it holds at temperature and how quickly it cools. Slower cooling can be a benefit when you’re working with larger or heavier pieces or with fire-in-place gemstones where fast cooling can lead to thermal shock and cracking. If you’re going to be firing stoneware or porcelain, choose a kiln with walls at least 3″ thick.
Manual vs. Automatic & Mechanical vs. Digital
Manual-fire kilns are gradually being replaced by automatic models. Most manual-fire kilns operate with infinite-control switches, the type used on electric kitchen stoves. They contain a bi-metallic switch that cycles on and off to achieve, then maintain temperature.
Automatic kilns allow you to set the temperature you want and let the kiln get there automatically. These kilns are easier to use than manual kilns and offer more precise control over the heat, ramp speed and other variables.
Automatic kilns are offered with either mechanical or digital controls.
- Mechanical kilns automatically use timers to advance the switch settings and a separate kiln controller to turn the kiln off.
- Digital kilns use a built-in electronic controller. Many kilns now offer pre-set programs specially created for certain firing processes such as firing metal clay, glass or enamels, as well as programs that you can customize and save for your specific firing needs.
About reliability and repairs: Digital kilns use a transformer and relays (generally not needed in mechanical kilns) but, when properly designed, are highly reliable and are easier to repair than mechanical models should the need arise. Its small circuit board can be removed in minutes and repaired or replaced.
IMPORTANT! Manual or automatic, mechanical or digital, whatever you choose, all kilns require supervision, especially near the end of the firing cycle. Always arrange to be nearby at the end of firing. If you are using pyrometric cones, a quick peek will tell you if the pieces inside have been adequately fired.
When you’re ready to move forward on choosing a kiln for your shop, take some time to think through these criteria with your own work in mind. The right kiln will make a world of difference to the efficiency, convenience, and fun of your work. This information should help you narrow the many choices to a handful of kilns that are best suited to you and your shop and then help you choose the one kiln from among that handful that is the one for you. Let me know how it goes; I’d love to hear about your experience. And, if you have questions, the Rio Tech Team is always there, ready to answer your questions.