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Pyrometric Cones
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Pyrometric cones are used worldwide to monitor ceramic firings in industrial kilns, pottery kilns, and small hobby kilns where the consistent temperature is important to the quality of the final product being fired. Pyrometric cones measure heat-work, the effect of time, and temperature. Over time, kilns sometimes drift from their setpoint even though "Nothing has changed". Pyrometric cones will be the first to let you know your ceramic firing is under control or that subtle changes are taking place within your kiln, allowing you to take corrective action before they impact your ware.
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All you really need to know about cones
Although Orton pyrometric cones were developed in the late 1800s to provide the ceramist with a tool to measure the firing of ceramic products, cones have stood the test of time and continue to provide verification of the combined effect of time and temperature (heat-work) on ceramic products around the globe. The development of electronic controllers paired with thermocouples has certainly proved invaluable in the control of the firing cycle of ceramic kilns, but the controllers cannot measure heat-work.
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The starting premise of cone development was that a blend of ceramic materials compounded to exacting proportions should behave similarly to the ceramic ware being fired in the kiln. A great deal of work later the starting premise was confirmed and from this work grew a series of blends that would deform at an exceptionally predictable temperature and be highly reproducible. Further investigation determined that the form of the cone that worked best was a slender, triangular pyramid and identified the variables that impacted the accuracy of the cone, and procedures were developed to allow the ceramist to compensate for processing variables.
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The practical use of cones is for verification of the thermal uniformity and reproducibility of the heat work delivered by a specific heating profile. The final degree of cone bending is dependent on the heating rate during the last 100°C (180°F) of the firing profile and the final temperature. It is important to remember that the cone bends over a fairly small temperature range and even though the cone's degree of bending may appear significant it may actually only represent a few degrees of temperature. Most ceramic products are no more sensitive than the cone to temperature since they share many of the same ingredients. The value of the cone is to provide verification that the kiln is delivering the expected amount of heat work each time a specific heating profile is applied and verify that the heat work is being delivered uniformly within the kiln ware load when multiple cones are placed within the load setting.
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Selecting the proper cones
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Utilize the Orton Cone Temperature Equivalents Chart to select the correct cone numbers you will need. Selecting Orton's self-supporting cones will solve the issue of correct mounting height and angle for you. For further assistance selecting the proper cone, see "Selecting Cone Types" at Pyrometric Cones - Resources.
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Example: You are firing to 1,147°C and the heating rate for the final 100°C is 75°C/hr. Select the heating rate column closest to your rate, which is 60°C, you scan down to 1,147°C, which is greater than the temperature equivalent for cone 2 and less than the temperature equivalent for cone 3, therefore your target cone should be cone 3. If your firing profile includes a soak at the maximum temperature then you should increase the target cone by one to two cone numbers for a one-hour soak and two to three cone numbers for a soak of two hours.
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Once you have defined the cone number you need for your kiln's firing profile initially you should select one cone number above and one cone number below your target cone. Fire a test of the three cones in your kiln to determine the exact fit to your firing profile. Measure the target cone's deformation with the Orton cone measurement template. If the target cone's final position after firing is between 25 and 75 degrees then you have selected the correct cone for your kiln's profile. Templates are available from Orton.
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Now it is time to put the cones to work for you. To evaluate heat-work uniformity with your kiln, place the three-cone series at the equivalent of top, middle and bottom; right, center and left. Then evaluate the cones' degree of bending at each location. This process should help you identify any hot or cold spots within your kiln. The second use of the cone is for quality assurance of the firing process. On a routine basis run the three cones at predetermined locations; top middle, and bottom in the center of the ware. A brief look at these cones on a routine basis will provide verification of consistent heat-work delivery to the ware and also provide an early warning if changes in the firing profile are occurring. Keeping a written log or digital photo of each firing's cone positions will provide a reference for future kiln performance and help you recognize changes in the firing profile before they become major problems.
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