Although published or factory-set firing schedules are valuable tools, they are only meant to be a starting point. Learn the reasons for customizing your firing schedule to fit your kiln size and shape, as well as your work.
Defining the Terms
Manual Kiln: An electric kiln equipped with switches and usually a kiln sitter. The kiln sitter automatically shuts off the kiln when a pyrometric cone bends to maturity. Most kiln sitters have a clock motor that acts as a backup shut-off. The kiln shuts off when the timer runs out of time.
Digital Kiln: An electric kiln equipped with a digital temperature controller. The controller receives a millivolt signal from the thermocouple (temperature sensor) and converts that signal into a temperature. Digital kilns now dominate the electric-kiln market.
Ramp-Hold Firing Mode: A digital firing schedule is divided into segments. Each segment has a rate of temperature change, a target temperature, and a hold at the target temperature. A program in ramp-hold firing mode is written as a chart with columns for segment number, rate, temperature, and hold time.
Cone-Fire Firing Mode: The Orton Ceramic Foundation created a ramp-hold firing schedule for each cone number; the schedules are called cone-fire mode. The last segment is programmed at a rate of 108°F (60°C) temperature rise per hour, to conform to the most commonly used column in the Orton pyrometric cone chart. To program a schedule in cone-fire mode, enter 1) the cone number, 2) a rate of slow, medium, or fast, and if desired, 3) a candling period, 4) a hold, and 5) slow cooling.
Thermocouple Offset: The accuracy of the temperature sensor, or thermocouple, is affected by age, electromagnetic interference from nearby relay wires, and oxidation of wire connections. You can compensate for these factors by adjusting Thermocouple Offset, which is found in the Options of temperature controllers.
Thermal Mass: The ability of a material to absorb, store, and release heat. An object that is heated inside a kiln has thermal mass. The heavier the object, the more energy needed to heat it. Kiln shelves and posts are heavy thermal masses. To improve heat distribution, place them where the kiln is the hottest.
The factory preset firing settings (called cone-fire firing mode) that come with a digital controller eliminate most basic errors. If your firing needs are met with the cone-fire firing mode, and you are happy with the glazes that result from that firing, you will probably never need ramp-hold mode. But, for those of you looking to customize your firing to make adjustments for drying clay or for special projects such as crystalline glazes, large tiles, thick sculptures, etc., then ramp-hold firing mode will help you do that.
Digital kilns have two firing modes: cone-fire and ramp-hold (also called vary-fire). Many potters use only the cone-fire mode. When I work on ceramic kilns in the field, the test program used in the kiln factory is almost always still saved in the controller, which indicates that ramp-hold had not been used since the kiln left the factory. While the cone-fire mode works great for many scenarios, using the ramp-hold mode to customize the firing conditions so that they’re optimal for your work and studio situation can be beneficial and enrich your kiln-firing experience.
Manual Kilns vs. Digital Kilns
I often hear artists say that their glazes turn out better in their manually fired kiln than in a newer digital kiln. I suspect this is because they have fired their manual kiln for years and intuitively know how to tweak it for best results. They haven’t yet learned to do that with their digital kiln.
If you think the results from your manual kiln are better than those from you digital kiln in side-by-side comparisons of glaze tests, then the first step to determining the reason for this is to fill your manual kiln as you normally would for a glaze firing and plan to be present to monitor the entire firing so you can chart time and temperature rise, as well as some of the cool-down phase.
To determine how your manual kiln (the type with switches and a kiln sitter) is firing, fire it in the usual way. Insert a thermocouple into the kiln through a peephole plug, and monitor the temperature with a pyrometer. Write down the temperature every 15–30 minutes or so. Create a graph that uses angled lines for temperature rises and horizontal lines for steady temperature holds. Use the vertical or Y axis to represent temperature rise, and the horizontal or X axis to represent elapsed time.
The next step is to see if you can reproduce these conditions using ramp-hold mode in your digital kiln. Using your graph as a reference, program that firing in ramp-hold mode in the digital kiln. Add a segment every time the firing speed changes (where the temperature rose more quickly or at a higher rate in a given 15–30 minute interval). Add holds at the end of segments where needed. If your manual kiln cools more slowly due to thicker walls than the digital kiln, add a slow cooling to the digital program. Load the kiln the same way you loaded the manual kiln for a glaze firing. The tightness or looseness of the stack and of the placement of pots on each shelf and the size of the pieces all have an effect on a firing, so keep these variables as close to the setup in the first firing as possible. After the firing, compare the results. If the firing results in the digital kiln are still different, it may be because one kiln has a downdraft vent; because of a difference in heat distribution between the kilns; or because of a difference in heat work. Measure heat work in both kilns with witness cones. Even in a digital kiln, witness cones are still the best measurement of heat work.
Large Kilns vs. Smaller Test Kilns
Have you ever tested glazes in a small kiln or a test kiln, and then fired the same glazes on larger pieces in a larger kiln, and noticed that the glazes turned out differently in the larger kiln, even when you programed both kilns to the same cone number at the same rate?
There are several reasons why this could happen. Even if the small test kiln and large kiln were programmed the same, the kilns may differ in natural heating and cooling rates. A 120-volt test kiln may fire slower than a large 240-volt kiln. Or the large kiln may fire slower due to a greater thermal mass and wall thickness. Additionally, the larger kiln likely cools much slower.
To fix this, monitor the kilns to determine the actual rates. If your controllers don’t record the firing profiles, monitor the firings and write down the temperatures every 15 minutes. Alter a cone-fire schedule in ramp-hold, so that both kilns fire at the same heating and cooling rates. Changing the temperature with thermocouple-offset settings (look these up in your controller manual to learn more) in one of the kilns may also improve glaze uniformity between them.
Customize the Slow-Cooling Schedule
I am often asked to explain the slow-cooling feature in the cone-fire firing mode and how to customize it. Because the cooling rate of a kiln can alter the appearance of certain glazes, manufacturers of kiln controllers added a slow-cooling schedule to the cone-fire mode. The slow-cooling rate and end temperature depend on the brand of controller. In a ramp-hold firing mode, you can use a sequence of several cooling rates and change the slow-cooling end temperature.
Using Schedules for Crystalline Glazes
One way to use firing schedules to your advantage is when firing crystalline glazes. Specialty glazes such as crystallines usually need a specific ramp-hold program or schedule to achieve the exact effect desired. Typically, crystalline glazes are heated to a particular temperature, and then held at progressively lower temperatures while cooling. To create a crystalline schedule that works in your kiln, experiment by changing each segment and take detailed notes. As a starting point, fire the glaze to maturity, then let the kiln cool to 2010°F (1098°C) and hold that temperature. Let the kiln cool to 1990°F (1087°C), and cool slowly to 1840°F (1004°C). Some of the most beautiful glazes have complex firing schedules that go way beyond cone-fire mode.
Troubleshooting Error Messages
Error messages such as E1 or FTL mean the kiln can’t reach the programmed cone. There is a way to slow the firing rate to avoid getting an error message. As long as all the elements are heating, you may be able to reach the programmed cone by slowing the rate—even if the kiln has been shutting off with an E1 or FTL error message.
The last segment of a typical cone-fire schedule has a rate of 108°F (60°C) temperature rise per hour. Recently, a customer’s kiln slowed down to 102°F (56°C) at 2150°F (1176°C). The kiln couldn’t keep up with the cone-fire schedule and had been shutting off with an error code. By slowing the rate within a customized firing schedule, the kiln could reach cone 6. Also, if you slow the rate of the last segment, the cone will bend at a lower temperature. (Please see the Orton cone charts (www.ortonceramic.com/pyrometric-cones-resources), which show how firing speed affects cone temperatures.)
To determine the kiln’s actual firing rate, divide the temperature rise during 5 minutes by 5 and multiply that number by 60. That will give you the temperature rise per hour. Note: Some controllers also give the actual temperature rise when you select program review at any given point during a firing.
the author Arnold Howard spent most of his career at Paragon Kilns. He now maintains kilns as an independent technician, working between San Antonio and Dallas, Texas. He also works remotely as a consultant. You can reach him at arnoldhoward@gmail.com.
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Although published or factory-set firing schedules are valuable tools, they are only meant to be a starting point. Learn the reasons for customizing your firing schedule to fit your kiln size and shape, as well as your work.
Defining the Terms
Manual Kiln: An electric kiln equipped with switches and usually a kiln sitter. The kiln sitter automatically shuts off the kiln when a pyrometric cone bends to maturity. Most kiln sitters have a clock motor that acts as a backup shut-off. The kiln shuts off when the timer runs out of time.
Digital Kiln: An electric kiln equipped with a digital temperature controller. The controller receives a millivolt signal from the thermocouple (temperature sensor) and converts that signal into a temperature. Digital kilns now dominate the electric-kiln market.
Ramp-Hold Firing Mode: A digital firing schedule is divided into segments. Each segment has a rate of temperature change, a target temperature, and a hold at the target temperature. A program in ramp-hold firing mode is written as a chart with columns for segment number, rate, temperature, and hold time.
Cone-Fire Firing Mode: The Orton Ceramic Foundation created a ramp-hold firing schedule for each cone number; the schedules are called cone-fire mode. The last segment is programmed at a rate of 108°F (60°C) temperature rise per hour, to conform to the most commonly used column in the Orton pyrometric cone chart. To program a schedule in cone-fire mode, enter 1) the cone number, 2) a rate of slow, medium, or fast, and if desired, 3) a candling period, 4) a hold, and 5) slow cooling.
Thermocouple Offset: The accuracy of the temperature sensor, or thermocouple, is affected by age, electromagnetic interference from nearby relay wires, and oxidation of wire connections. You can compensate for these factors by adjusting Thermocouple Offset, which is found in the Options of temperature controllers.
Thermal Mass: The ability of a material to absorb, store, and release heat. An object that is heated inside a kiln has thermal mass. The heavier the object, the more energy needed to heat it. Kiln shelves and posts are heavy thermal masses. To improve heat distribution, place them where the kiln is the hottest.
The factory preset firing settings (called cone-fire firing mode) that come with a digital controller eliminate most basic errors. If your firing needs are met with the cone-fire firing mode, and you are happy with the glazes that result from that firing, you will probably never need ramp-hold mode. But, for those of you looking to customize your firing to make adjustments for drying clay or for special projects such as crystalline glazes, large tiles, thick sculptures, etc., then ramp-hold firing mode will help you do that.
Digital kilns have two firing modes: cone-fire and ramp-hold (also called vary-fire). Many potters use only the cone-fire mode. When I work on ceramic kilns in the field, the test program used in the kiln factory is almost always still saved in the controller, which indicates that ramp-hold had not been used since the kiln left the factory. While the cone-fire mode works great for many scenarios, using the ramp-hold mode to customize the firing conditions so that they’re optimal for your work and studio situation can be beneficial and enrich your kiln-firing experience.
Manual Kilns vs. Digital Kilns
I often hear artists say that their glazes turn out better in their manually fired kiln than in a newer digital kiln. I suspect this is because they have fired their manual kiln for years and intuitively know how to tweak it for best results. They haven’t yet learned to do that with their digital kiln.
If you think the results from your manual kiln are better than those from you digital kiln in side-by-side comparisons of glaze tests, then the first step to determining the reason for this is to fill your manual kiln as you normally would for a glaze firing and plan to be present to monitor the entire firing so you can chart time and temperature rise, as well as some of the cool-down phase.
To determine how your manual kiln (the type with switches and a kiln sitter) is firing, fire it in the usual way. Insert a thermocouple into the kiln through a peephole plug, and monitor the temperature with a pyrometer. Write down the temperature every 15–30 minutes or so. Create a graph that uses angled lines for temperature rises and horizontal lines for steady temperature holds. Use the vertical or Y axis to represent temperature rise, and the horizontal or X axis to represent elapsed time.
The next step is to see if you can reproduce these conditions using ramp-hold mode in your digital kiln. Using your graph as a reference, program that firing in ramp-hold mode in the digital kiln. Add a segment every time the firing speed changes (where the temperature rose more quickly or at a higher rate in a given 15–30 minute interval). Add holds at the end of segments where needed. If your manual kiln cools more slowly due to thicker walls than the digital kiln, add a slow cooling to the digital program. Load the kiln the same way you loaded the manual kiln for a glaze firing. The tightness or looseness of the stack and of the placement of pots on each shelf and the size of the pieces all have an effect on a firing, so keep these variables as close to the setup in the first firing as possible. After the firing, compare the results. If the firing results in the digital kiln are still different, it may be because one kiln has a downdraft vent; because of a difference in heat distribution between the kilns; or because of a difference in heat work. Measure heat work in both kilns with witness cones. Even in a digital kiln, witness cones are still the best measurement of heat work.
Large Kilns vs. Smaller Test Kilns
Have you ever tested glazes in a small kiln or a test kiln, and then fired the same glazes on larger pieces in a larger kiln, and noticed that the glazes turned out differently in the larger kiln, even when you programed both kilns to the same cone number at the same rate?
There are several reasons why this could happen. Even if the small test kiln and large kiln were programmed the same, the kilns may differ in natural heating and cooling rates. A 120-volt test kiln may fire slower than a large 240-volt kiln. Or the large kiln may fire slower due to a greater thermal mass and wall thickness. Additionally, the larger kiln likely cools much slower.
To fix this, monitor the kilns to determine the actual rates. If your controllers don’t record the firing profiles, monitor the firings and write down the temperatures every 15 minutes. Alter a cone-fire schedule in ramp-hold, so that both kilns fire at the same heating and cooling rates. Changing the temperature with thermocouple-offset settings (look these up in your controller manual to learn more) in one of the kilns may also improve glaze uniformity between them.
Customize the Slow-Cooling Schedule
I am often asked to explain the slow-cooling feature in the cone-fire firing mode and how to customize it. Because the cooling rate of a kiln can alter the appearance of certain glazes, manufacturers of kiln controllers added a slow-cooling schedule to the cone-fire mode. The slow-cooling rate and end temperature depend on the brand of controller. In a ramp-hold firing mode, you can use a sequence of several cooling rates and change the slow-cooling end temperature.
Using Schedules for Crystalline Glazes
One way to use firing schedules to your advantage is when firing crystalline glazes. Specialty glazes such as crystallines usually need a specific ramp-hold program or schedule to achieve the exact effect desired. Typically, crystalline glazes are heated to a particular temperature, and then held at progressively lower temperatures while cooling. To create a crystalline schedule that works in your kiln, experiment by changing each segment and take detailed notes. As a starting point, fire the glaze to maturity, then let the kiln cool to 2010°F (1098°C) and hold that temperature. Let the kiln cool to 1990°F (1087°C), and cool slowly to 1840°F (1004°C). Some of the most beautiful glazes have complex firing schedules that go way beyond cone-fire mode.
Troubleshooting Error Messages
Error messages such as E1 or FTL mean the kiln can’t reach the programmed cone. There is a way to slow the firing rate to avoid getting an error message. As long as all the elements are heating, you may be able to reach the programmed cone by slowing the rate—even if the kiln has been shutting off with an E1 or FTL error message.
The last segment of a typical cone-fire schedule has a rate of 108°F (60°C) temperature rise per hour. Recently, a customer’s kiln slowed down to 102°F (56°C) at 2150°F (1176°C). The kiln couldn’t keep up with the cone-fire schedule and had been shutting off with an error code. By slowing the rate within a customized firing schedule, the kiln could reach cone 6. Also, if you slow the rate of the last segment, the cone will bend at a lower temperature. (Please see the Orton cone charts (www.ortonceramic.com/pyrometric-cones-resources), which show how firing speed affects cone temperatures.)
To determine the kiln’s actual firing rate, divide the temperature rise during 5 minutes by 5 and multiply that number by 60. That will give you the temperature rise per hour. Note: Some controllers also give the actual temperature rise when you select program review at any given point during a firing.
the author Arnold Howard spent most of his career at Paragon Kilns. He now maintains kilns as an independent technician, working between San Antonio and Dallas, Texas. He also works remotely as a consultant. You can reach him at arnoldhoward@gmail.com.
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