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.
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.
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.