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Zone control in electric ceramic kilns has been around for a number of years. However, it has only started to become more popular among pottery producers within the past few years, as automation and improved software have made it more adjustable, accurate and user-friendly. As potters look for ways to spend less time watching their kilns and more time producing high-quality pottery, they are increasingly finding that modern automatic zone control technology can help them meet their objectives.
Conventional ControlTypical ceramic top-loading electric kilns are based on designs that are now almost a half-century old and have always had a problem with temperature uniformity, especially from top to bottom. Typically, the center or top of the kiln fires hotter, while the bottom fires colder. This differential can be up to a whole cone or more.
A number of techniques are used to counteract this effect. One of the oldest methods, which has been used in batch kilns from time immemorial, is to learn the firing characteristics of the kiln and load the kiln accordingly. For example, certain pieces that won't be as affected by the temperature differences might be loaded in the cold spots, while heavier pieces might be loaded in the center to absorb more heat. This has the obvious disadvantage of constraining the productive output of the kiln. For instance, it would be difficult to uniformly fire 100 coffee mugs with the same glaze in a conventional polygonal electric kiln.
Another technique used in many electric kilns is to grade the power output of the elements so that more power goes toward the top and bottom of the kiln. This can produce a very even firing temperature if it is done correctly. The problem with this method, however, is that the power output of the elements changes with age, so the heating characteristics of the kiln may become less uniform over time. Additionally, if one element becomes damaged (by glaze, for instance) and has to be changed, it is likely that all the elements in the kiln will have to be replaced; otherwise, the new element will create a "hot spot" that can affect the quality of the ware. Kilns with graded elements also offer less flexibility in changing the loading patterns.
Another method that has not been well explored commercially is the use of different insulation thicknesses in different parts of the kiln. For instance, more insulation could be used on the top and bottom, where the heat losses are greater, than on the sides. However, calibrating the required difference would be difficult and, unlike the resistance values of graded elements, which can be adjusted easily, insulation offers finite room for adjustment. Additionally, as with graded elements, the adjustment is static and limits the flexibility of the kiln.
Another method of evening out temperatures in an electric top-loading kiln is to use a downdraft vent. This system pulls a small amount of air down from the top of the kiln and out the bottom, counteracting the natural tendency of heat to rise. Tests at L&L Kiln Mfg. have shown about a 1/2 cone uniformity improvement using this technology.
Sophisticated AlternativesA more recent way of dealing with uniformity issues is through the use of zone control. This technology has gained widespread use over the last several decades in high-volume industrial manufacturing to achieve critical results for important processes such as firing million-dollar loads of ceramic-based superconducting wire. In the '70s, manual zone control technologies began to be implemented in some top-loading electric pottery kilns. The use of manually controlled, infinitely adjustable switches allows control of manual kilns zone by zone. When used in conjunction with a multi-thermocouple pyrometer system, this system allows potters to see which zone is getting hotter and make adjustments while firing. While such control can significantly improve the quality of the ware, the amount of time required to "babysit" the kiln and the technical knowledge required to make the necessary adjustments are significant drawbacks.
In the past few years, newer electronic controls designed specifically for electric kilns have begun to incorporate true automatic, multi-zone operation. These controls typically use three separate thermocouple inputs and three relay outputs for power control. Each zone of the kiln is independently controlled on a separate control loop-sophistication that was previously only available in very expensive controllers. With the rapid advances in computer technology, the cost of such systems has dropped dramatically.
The main advantage of automatic zone control technology is that it allows dynamic adjustment of the kiln to accommodate changing elements and different loads. Typical uniformity is within 1/2 a cone or better (see Figure 1).
The downside of automatic zone control is that it can slow down the kiln because it works by turning off power to hotter zones. In the past, this sometimes resulted in an error code shutting off the kiln because it was firing too slowly. However, this technology has matured over the past few years. Many of today's controls include the ability to turn zone control off during all but the last segment of the program to speed up firing. The degree of allowable difference can also be changed from one thermocouple to the next. The typical default setting is 25°F, but this can be raised or lowered. For instance, a very tight temperature tolerance of 10°F might be required when firing glass. Most ceramic bisque work, on the other hand, is much less critical, and would do fine with a "lag" setting of 50°F. Another algorithm in the control automatically changes the ramp rates to allow the whole kiln to match the speed of a slow zone.
With these sophisticated software features, much of the initial trouble of electronic zone control has been eliminated. The key to good functioning now is to ensure that there is enough power in the kiln to compensate for the effects of the zone control. (Check with your kiln supplier or a certified electrician to determine the appropriate wattage.)