|Monthly Tech-Tip |
In ceramic kilns the firing schedule is typically managed automatically by an electronic controller. But that may not mean that ware gets automatically fired to the correct temperature and atmosphere.
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An electronic device attached to a kiln (usually an electric kiln). These controllers are usually capable of firing a kiln to a specific schedule and can shut it off at the right time, soak it for a specified period, and cool it down at a controlled rate. All industrial kilns, electric or gas, have controllers to assure repeatable firing conditions. All modern electric hobby kilns are equipped with controllers. Hobby and pottery gas kilns are also increasingly employing devices to control the schedule as well as the atmosphere.
Controllers for electric kilns work on a duty cycle, switching the power to the elements on for a few seconds, then off for a few seconds. The controller adjusts the on/off durations based on how the temperature is following the program. Technically, the controller switches relays (either mechanical or solid-state) on and off, and they turn the power on and off to the elements. During a firing, relays trigger thousands of times, and you guessed it, they are the most failure-prone part of the system.
Controllers have built-in programs and can also be programmed manually by entering the rate, temperature and hold time for each step. Hobbyists are more prone to rely on the built-in pre-programmed schedules, using these to fire off-the-shelf glazes to low and medium temperatures. However, via manual programming, these controllers have revolutionized our ability to create special-purpose glazes (e.g. crystalline, silky mattes). By creating drop-and-soak schedules (e.g. PLC6DS), they can reduce glaze defects and improve surface quality and brilliance. Slow cool schedules (e.g. C6DHSC) are important to enhancing visual effects that depend on crystal growth (e.g. in rutile glazes) and for mattes.
Even hobbyists have a key motivation for learning to program their controller manually: To be able to fine-tune or even correct the final firing temperature. This is because using the built-in cone-firing programs on many kilns fires well above the temperature a cone would confirm correct. For example, suppose a cone 6 bends to four o'clock at around 2200F on your pyrometer. But the cone fire program fires it to 2232! Obviously, this is going well beyond a cone 6 firing. This is difficult to convey to a pottery community inclined to totally trust controllers on their easiest working mode, it has almost gone to the point that "cone 6", for example, is no longer defined as what an Orton cone indicates, it is defined as "a program option in a controller".
Older kilns can often be retrofitted with controllers. Some integrate tightly into the control of individual elements, these can potentially work well if they come with relays that can withstand the thousands of on/off cycles of a typical firing. Another option is for the controller to have one large super relay, the main kiln power cable plugs into it and it plugs into the wall. These obviously cost more but require no modification to the kiln. Even if the device is guaranteed to work in your kiln a further caution is also in order: Some of the replacements may employ older programmable devices. For example, the Bartlett LED/Keypad V6-CF or 3K controllers are now old and have been superseded by their touch-screen Genesis device.
Many DIY controllers have been made, they are based on microcontrollers or Raspberry Pi computer boards. These are not CSA or UL approved of course, so fire insurance coverage will be implicated. A warning found on one site may affect your decision to try one of them: Most kiln manufacturers state "Do Not Fire Unattended". Commercial controllers deal with many fault modes that this controller does not address. Intermittent thermocouple connections, stuck relay, shorted element and weak elements, these faults have all happened to me and the control just keeps saying "kiln thinks kiln can, kiln thinks kiln can". I had to intervene. Stuck relay and bad thermocouple readings are runaway conditions that could become meltdown/fire/death. "DO NOT FIRE UNATTENDED!"
Another option is to use a kiln monitor, these are independent of kiln control, they just watch using their own thermocouple. Monitors record firing data and enable comparing what happened with fired ware compared with was supposed to happen. Or comparing actual firing curves with intended ones, either in past tense or real time.
Every potter should have one of these. This one has a Bartlett Genesis electronic controller, you will never go back after having one. Start with a kiln like this and then graduate to having a large, second kiln. We have done 950 firings on this one in the past few of years, it is still like new. Ongoing testing is the key to the constant development of your products and their quality.
The red controller on the right is a Skutt Kilnmaster, the blue controller to the left is an Orton Autofire. As of 2022 both of these are now ancient devices, having been replaced with newer touch panel units. The principle of operation for both is the same: They turn the power on and off in a duty cycle to control temperature rise. A 50% duty cycle, for example, sees the power on for 50% of the time. The length of individual bursts increases with kiln temperature. The controllers monitor a thermocouple in the kiln to determine the length and frequency of power bursts needed. Both of these devices are external to the kilns (but there is a big difference). The KilnMaster controller is attached to the 220V power line and the kiln power line attaches to it (there are heavy-duty electrical relays inside). The blue Autofire controller does not have internal heavy-duty relays or switches, they are in the kiln. The KilnMaster is thus more flexible since it can connect to any kiln, but it is also triple the price. In 2022 the AutoFire now has its own relays like the KilnMaster.
Notice the 220V enters lower left to a terminal block. That splits to a transformer (above) and relays (below that). The 4-07-6024 transformer supplies power to the SMT_3140 controller board (converting the 220V down to that needed by the circuit board). The controller is run by an MSP430 Texas Instruments microcontroller (lower left has closeup). That controller has inputs from the thermocouple and a current monitor (yellow donut shape around the wires going to the elements). The board has outputs that connect to the relay, that relay in turn controls the flow of current to the elements. This is a very simple appearing device, but not something to be replaced lightly. DIY controller boards documented online look tempting but they are not CSA or UL approved (so your fire insurance coverage is implicated). Commercial controllers focus on safety and liability over functionality, they handle intermittent thermocouple connections, bad thermocouple readings, stuck relays and shorted and weak elements - these are runaway conditions that could become meltdowns in a kiln controlled by a DIY device. This being said, these commercial products do have a weakness: The relays. They are mechanical devices and are the first thing to need replacement. Kiln controllers are thus particularly good at recognizing relay failure.
I document programs in my account at insight-live.com, then print them out and enter them into the controller. This controller can hold six, it calls them Users. The one I last edited is the one that runs when I press "Start". When I press the "Enter Program" button it asks which User: I key in "2" (for my cone 6 lab tests). It asks how many segments: I press Enter to accept the 3 (remember, I am editing the program). After that it asks questions about each step (rows 2, 3, 4): the Ramp "rA" (degrees F/hr), the Temperature to go to (°F) to and the Hold time in minutes (HLdx). In this program I am heating at 300F/hr to 240F and holding 60 minutes, then 400/hr to 2095 and holding zero minutes, then at 108/hr to 2195 and holding 10 minutes. The last step is to set a temperature where an alarm should start sounding (I set 9999 so it will never sound). When complete it reads "Idle". Then I press the "Start" button to begin. If I want to change it I press the "Stop" button. Those ten other buttons? Don't use them, automatic firing is not accurate. One more thing: If it is not responding to "Enter Program" press the Stop button first.
Put the pots in, select a cone, press start. It is time to rethink that approach! The Bartlett Genesis kiln controller is standard equipment on hobby and production electric kilns now. It is not meant to be run like a toaster! Good glazes are about much more than recipes, they are about firing schedules. None of the built-in "toaster schedules" have hold times on any segments, drop-and-hold sequences or controlled cools. Or even fire-to-cone accuracy. Yet such are a must for defect-free glazes, enhancing the effects of reactive glazes that must develop crystallization or variegation or firing accurately. It is easy to program: Tap the blue edit button to edit a program, tap a column of any segment to edit its value. Tap a segment number to delete or duplicate it. Search "bartlett genesis controller" on YouTube for videos on creating and editing a schedule.
Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). To actually go to cone 10 we need to manually create a program that fires higher than the built in cone-fire one. Determining how high to go is a matter repeated firings verified using a self-supporting cone (regular cones are not accurate). In our lab we keep notes in the schedule record in our account at insight-live.com. And we have a chart on the wall showing the latest temperature for each of the cones we fire to. What about cone 6? Controllers fire it to 2235, we put down a cone at 2200!
This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is overfired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay warns not to fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Calibrate the kiln sitter by using a self-supporting cone.
Cones bending badly, cones bending goodly
Cones are ceramic and bend through a narrow temperature range. They used to be actively used to determine when firings were completed but now are used to calibrate electronic devices.
Build a kiln monitoring device
Kiln Regulator 3
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, being aware of the limitations and keeping them in good repair is very important.
Kiln Controller Device List
If you are aware of devices not listed here please contact us. We want to focus on those in active development. Devices supplied on manufactured kilns are usually relabelled, they are not actually made by the kiln manufacturer.
|By Tony Hansen|
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