PPP: Kilns: Selecting One for You

September 1, 2006
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Selecting a kiln is not as difficult or as confusing a task as it first appears. Since all manufacturers use the same brick and element wire for all but certain specialized kilns, there are not significant differences in price or performance from one brand to another. A simplified step-by-step decision process follows, focusing primarily on electric kilns

A Skutt model KM-1027 electric kiln.


Gas vs. Electric

The first decision to make in the kiln selection process is whether you want-or specifically need-a gas or electric kiln. Gas kilns fire with propane, butane or natural gas; the only difference in the kilns themselves is the size of the inexpensive orifice on each burner. Generally, firing costs for similarly sized kilns are not significantly different for gas or electric. Gas is often more expensive, because kilns do not burn the fuel efficiently and a great deal of heat is lost through venting.

The advantage of a gas kiln is that reduction firing is possible. Additionally, gas is the way to go when larger kilns are contemplated (l5 or 20 cubic feet or more), because it is often easier and less expensive to install gas service than the electrical lines necessary for such large kilns. However, there are many significant disadvantages to gas kilns, especially for the relative novice. Obvious examples are that bisque firing is best done in the relatively clean atmosphere of an electric kiln, gas kilns require significant venting (generally this involves being placed outdoors), and they require much more attention and care to properly fire.

Top vs. Front Loading

The second basic decision in kiln selection concerns top loading vs. front loading configurations. Although front loading kilns are more desirable from an ease-of-loading standpoint, they are usually much more expensive. Generally, front loading kilns are built with 41/2-in.-thick walls. Top loading kilns typically feature the brick set on edge, so the walls are only 21/2 in. thick, which reduces the number of bricks significantly. Since the bricks themselves comprise the main cost of the kiln, this keeps the price down. (Three-inch-thick bricks are also available on some models. See also Wall Thickness.)

Round vs. Square

On a per-cubic-foot basis, round (and the similarly designed oval) kilns are less expensive than square kilns because they are easier to build and use less brick (see also Top vs. Front Loading and Kiln Sizes). Depending on the size and shape of the ware to be fired, a square configuration may load more effectively, but that must be balanced against the initial cost difference.

Sectional vs. Single Piece

The advantage of a kiln made in sections is twofold. First, they are easier to move and negotiate through doorways. Second, it is easier to make repairs-particularly the replacement of bottom elements-if the kiln can be taken apart. The disadvantage of sectional kilns is a slightly greater amount of heat loss due to lack of reflecting stainless steel at the section joints. In addition, there is always the danger of damaging the electrical parts or the brick itself when disassembling, storing or reassembling, which might also lessen the seals between the sections.

Brick vs. Fiber

Used in the vast majority of kilns, insulating firebrick outlasts fiber. However, fiber heats and cools faster, which can be desirable in certain instances (though it is generally considered to be a disadvantage). Elements are easy to replace in a brick kiln, but the elements are often embedded in the fiber, so the fiber chamber itself, including the element, must be replaced as a unit. Repairs to fiber are much easier than brick repairs.

Temperature

Nearly all larger electric kilns will reach at least a cone 6, and most will reach cone 8, if not cone 10. Obviously, when equipped with the usual controls, a kiln can be fired to lower temperatures than its highest manufacturer rating. Following repeated cone 10 firings, or if there are other special firing requirements, particular attention must be paid to the wall thickness and type of element to assure satisfactory long-term performance.

Kiln Sizes

Whether firing jewelry, slip-cast items, glass, pottery or sculpture, kilns are available in sizes ranging from several inches square to the size of your house. Too small a kiln means too many firings. Too large means either wasted energy or the frustration of waiting too long for a full load. As noted previously, the larger the kiln, generally the less expensive it is per cubic foot of space to purchase and to fire. Also, it is far less expensive to fire one large kiln than two loads in a kiln half the size, due to the differences in heat loss and the time/energy required to heat up both the kiln itself and the load. The standardized sizes for top loading electric kilns are shown in Table 1.

One way to decide on kiln size is to draw a circle (or square) the size you think you want on some newspaper. Place some of your pieces within the shape you've drawn to determine if that size will provide you with an adequate and efficient load size.

Electrical Considerations

Once your desired kiln size has been determined, the adequacy of your electrical supply needs to be checked. Most 110-volt kilns require amperage close to the 15 or 20 amps usually found in such circuits, therefore no other appliances can be operated on the same line at the same time that the kiln is in operation.

A 220-volt kiln will need a dedicated line. An electrician should check that the total capacity is available and that there is room in the main breaker box to add a line, if necessary. You can get a rough idea yourself by totaling the amps marked on each of your breakers, adding the amps of the kiln and comparing that to the total amps marked on the main breaker. At least a 10% safety factor is recommended, although this can be exceeded if the kiln will never be operated at the same time as an electric dryer, air conditioner or other high-electrical-usage appliance.

The voltage also needs to be checked. This can be done by calling your electric company or by checking your line with a simple volt meter. Breaker box labels cannot be relied on, as 208 volts is seldom identified as such. Most residences are 220/240 volts, but 208 volts, which is often found in commercial buildings and schools, is sometimes also found in homes. It is critical that the kiln matches your voltage or the performance will be unacceptable.

Residences have single-phase electrical service. Three-phase applies to 220/240 or 208 volt service, and consists of four wires instead of three. Three-phase is sometimes found in commercial and institutional buildings. When firing a kiln, there is no efficiency advantage to three-phase service, although lower amperage on each individual wire (phase) is required (thus a smaller wire capacity). An electrician can simply convert three-phase service to single-phase, or the kiln can be ordered wired for three-phase service.

It should also be noted that the three-prong configuration of the kiln plug will vary depending on the brand and amperage. Since the cord and plug on the kiln are very heavy duty, the wall receptacle should be changed, if necessary-not the kiln plug. Also note that the 30-amp plug on most electric dryers is unique to dryers, so a kiln with a 30-amp plug will not fit in a dryer outlet. By law, an adapter is not available, so it is best to change the plug end on the dryer and the receptacle in the wall.

Wall Thickness

Kiln brick comes in two thicknesses, 21/2 in. and 3 in. An analysis of the energy used for heat storage in the brick (the energy absorbed by the brick itself) vs. the energy lost through the brick shows that unless you plan a soak or a long firing at very high temperatures (where energy loss becomes more pronounced) or need to cool very slowly, as is the case with crystal glazes, the advantage of the thicker brick and walls is almost negligible.

A few kiln manufacturers also offer models with extra insulation between the brick and stainless steel cover. Again, unless you will be doing very high-temperature firings (cone 8-10), the extra cost is probably not justified. Energy savings will be quite minimal.

Elements

Elements can last from 150 to 300 or more firings, depending on their proper care and what is being fired. However, firing electric kilns to cone 10 will wear elements out much faster. After 30-50 cone 10 firings in a 21/2-in. brick-walled kiln, the kiln will no longer be capable of reaching that temperature, although the elements will not necessarily burn out or break.

All kiln manufacturers use the same wire for elements, so same-sized kilns with the same performance ratings will use the same amount of electricity. There is a newer APM element wire available and offered by several kiln manufacturers as an upgrade option, but it is quite expensive and the cost/benefit remains in question.

Controls and Controllers

Kiln controls can range from a simple on/off switch to fully programmable computers. Most kilns come minimally equipped with a variable switch (similar to a rheostat in function) and a W.P. Dawson KilnSitterĀ®, which mechanically shuts the kiln off by use of an expendable cone (a piece of clay formulated to bend at a specific temperature). A safety timer is usually added that overrides other controls and shuts the kiln off when a predetermined amount of time has elapsed. With these controls, the operator will have to return to turn the kiln up at least twice during a standard 8-hour firing.

Several kiln manufacturers also offer a feature whereby the kiln will ramp up automatically. Computer controllers that permit almost unlimited hands-off control over the entire process are the ultimate example of this. These controllers are very simple to use and generally have several preset programs for normal firing needs. They also permit very slow ramping up or down between different temperatures, as well as the ability to soak or hold a temperature, which can be very useful in overcoming certain glaze defects. Saving one kiln load in this manner can easily cover the added cost of the controller, to say nothing of saving you time and mental anguish.

Zone Controls

Most kilns on the market have all of the elements controlled as a single zone. Differences in temperature uniformity due to greater heat loss at the top and bottom are compensated for by using different rated elements top to bottom. This is normally effective, except when loads are uneven or when an element needs to be replaced.

Elements degrade over time, and installation of an entire set of new elements to regain uniformity for temperamental glaze firings is expensive. Thus, several kiln manufacturers offer zone controls, either manual or automatic.

Kiln Placement

A 50-amp or less kiln with a plug usually does not require a permit, although local codes may vary. The kiln should be placed on a non-flammable surface (not wood or linoleum) and be kept 2 ft from a 1-hour firewall, which is what is usually found on garage walls that abut living quarters. Obviously, flammables should not be stored nearby.

Venting

Venting of the kiln is usually not necessary from a health point of view under most firing conditions. Although some hazardous gases, such as sulfur and carbon monoxide, are emitted, studies have shown that they dissipate to well below dangerous threshold levels if the kiln is in an open area and there is air circulation (such as in a garage). However, there may well be detectable odors that you will want to exhaust to the outside.

There are several types of vents. A few kiln manufacturers offer (at extra cost) built-in venting, whereby the fumes are removed from the side at various heights. There are also fan-powered hood vents and vents that sit below the kiln and draw the fumes through the kiln and out the bottom. Venting systems that draw air through the kiln can improve the vibrancy of colors by introducing more oxygen to the kiln atmosphere. They also eliminate the need to prop open the top or deal with peephole plugs during the initial firing stage.

Other Costs

When estimating the cost of a kiln, be sure to include the cost of kiln furniture (shelves and posts to stack ware on different levels within the kiln), which will amount to about an additional 5-10% of the kiln's cost. Generally, shelves are available to fit standard kilns in full or half sizes (5/8 to 1 in. in thickness), and posts are available in 1-in. increments. Thicker kiln shelves are recommended when firing to higher temperatures to minimize potential warping of the shelves. After determining the size of the ware to be fired, keep in mind the following when deciding kiln furniture needs:
  • A shelf on the brick bottom is recommended to protect the brick.
  • The largest pieces can be placed on the top shelf to avoid tall, and thus less stable, posting.
  • At least 1 in. of space should be left between the ware and the shelf above to allow for air circulation.
  • Large shelves can be quite heavy and unwieldy.
  • Large-bottomed ware, such as platters, should not span two half-shelves.
  • Smaller posts can be stacked (carefully) on larger ones for more height.

It's also a good idea to estimate your firing costs. In the formula shown, A = the amperage rating from the plate usually found on the side of the control box, B = the voltage rating from the plate usually found on the side of the control box (if 220 or 240, use 230), C = your firing time in hours, multiplied by .6 (since the kiln is only on full power at the end of the firing), and D = the cost per kilowatt from your electric bill:

Note that while a longer firing will increase the energy cost, the increase will not be large because the kiln will not be on full power for the additional time, and the .6 factor in C of the formula would be lower. The formula assumes a normal, even-increase, 8-hour firing. Greater accuracy can be achieved by following the same procedure for multiple, smaller time segments. A 3-hour drying on the lowest setting might use a factor of .1 in calculating C, for instance, and then .6 or .7 for the remaining time.

Editor's note: Reprinted with permission from Clay Planet. For more information, visit http://www.clay-planet.com .

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