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Glaze maturity and firing rates are specific to each potter and depend on the variables imposed during the firing process-i.e., heating rate, soak times and atmosphere. Most potters use pyrometric cones to determine when their firing is complete. Following defined firing rates for cones, such as those available from the Orton Ceramic Foundation, will allow a potter to accurately repeat a past firing cycle as long as the temperature is increased at the same rate as the past firing cycle. But if you're only using pyrometric cones, how can you identify consistent increases in temperature at Orton's specified heating rates?
Cone LimitationsPyrometric cones are a good indicator that a target temperature has been approximated and that the glaze has matured to specific requirements as long as the atmosphere and heating rates have been repeated. However, any changes in these variables, such as using an oxidation or reduction atmosphere, can dramatically affect the deformation of a cone near its intended peak indication point.
General notes taken from a standard Orton cone chart indicate that the temperature equivalents for each cone number apply only to Orton standard pyrometric cones when heated at the tested calibration rates of 108°F per hour or 270°F per hour. Heating rates should be maintained uniformly during the last several hundred degrees of temperature rise. Additionally, temperature equivalents are not necessarily those at which cones will deform if the firing conditions are different from those under which calibration determinations were made. (All determinations for the Orton cone chart in this example were made in an air atmosphere). Finally, for reproducible results, care should be taken to ensure that cones are set in a plaque with the bending face at the correct angle of 82¼ from horizontal. (Self-supporting cones at 82¼ are available.)
For these reasons, potters looking to achieve reproducible, consistent firing results should consider using thermocouples along with cones to identify heating rates and final glaze development for all firing cycles.
Avoiding DriftThermocouples are classified by the base metal used in their wires and are identified by letters, such as Type K (Chromel® [nickel-10% chromium (+)] vs. Alumel® [nickel-5% aluminum and silicon (-)], Type R (platinum-13% rhodium [+] vs. platinum [-]) and Type S (platinum-10% rhodium [+] vs. platinum [-]). The upper operating temperature limit, durability and cost are the main criteria used in selecting a given thermocouple type.
Cost is typically a big concern for most potters. For this reason, many potters use a Type K 8-gauge element, which is a relatively low-cost option suitable for use in oxidizing or inert atmospheres at temperatures up to 2300°F. However, any money saved up front can easily be lost over the next several years through inaccurate temperature readings and the need for thermocouple replacement, as these thermocouples tend to oxidize and drift over time. A test performed by NASA on Type K thermocouples indicated a loss of approximately 5 degrees for every 600 hours of firing (see Table 1). After just 200 hours at a peak temperature of 2300°F, the thermocouples exhibited a 25-degree drift. While this is still more accurate than pyrometric cones, which provide an indication of temperature within ±45 degrees, it is not an ideal situation.
A better option is a Type R or S platinum thermocouple, which can be used in air or inert atmospheres to 2700°F. While these thermocouples are more expensive, they are also much more durable, making them well worth the extra initial cost. A NASA-conducted test on a platinum thermocouple (-48 gauge AWG, which is a much smaller gauge than normally used) run at 2446°F for 10,000 hours indicated only a 5.22-degree drift over the entire cycle. For most potters, this indicates a lifetime of firing without the need to calibrate or replace their thermocouples.
An Accurate IndicationOf course, an accurate thermocouple isn't very useful without an accurate temperature indicator. With an analog indicator, the kiln temperature might change as much as 25 to 50 degrees before you can see the indicator needle move. A digital indicator, on the other hand, provides a temperature reading to within a tenth of a degree.
Many potters view thermocouples and temperature indicators as costly and unnecessary additions to the firing process. However, the expense can be quickly offset through reductions in fuel/energy consumption and, most importantly, the ability to fire the kiln in a timely manner. With accurate thermocouples and a digital indicator, you can instantly see how any adjustments made during firing (gas flow, exhaust, etc.) affect the kiln temperature (or the zone temperature, if multiple thermocouples in different zones are used). If you were only using pyrometric cones, you wouldn't be able to see these effects for hours; by then, an entire load of pottery might be wasted.
I use three Type S platinum thermocouples with digital indicators to monitor the top, middle and bottom levels of my kiln. These thermocouples were purchased in 1985 and have not yet needed replacement. My firings are consistent and repeatable, and I use a minimum amount of fuel.
Continue using cones-you never know when you will experience a power outage, and cones still provide valuable information. (I use cones in every firing.) But I highly recommend the use of platinum thermocouples and digital indicators combined with cones for any firing cycle and, as always, accurate documentation of the firing cycle for future troubleshooting.
For Further ReadingPark, Richard M., "Thermocouple Fundamentals," online at http://www.marlin-mfg.com/tcfundamentals.pdf.
Jerry Wagner began his wholesale pottery operation, Wagner Pottery, in September 2001. He currently divides his time between producing pottery and building kilns for potters and industrial ceramic manufacturers as a kiln engineer at SBL Kiln Services Inc., Bridgeville, Pa. If there is a specific tip or technique that you would like to see covered in a future column, contact Jerry at (724) 438-6957 or firstname.lastname@example.org.