|Monthly Tech-Tip |
In ceramics kilns the firing schedule of a kiln is typically managed automatically by an electronic device called a kiln controller. These are especially common on electric kilns.
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 revolutionised our ability to create special purpose glazes (e.g. crystalline, silky mattes). By creating drop-and-soak schedules (e.g. PLC6DS), they can to 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 the final firing temperature. This is because using the built-in cone-firing programs on many kilns can fire well below or above the temperature a cone would confirm correct. For example, suppose a cone bends to four oclock at around 2205 on your pyrometer but the automatic firing program shuts the kiln off at 2180: Obviously this is not achieving a cone 6 firing.
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 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.
So many glazes appear as they do because of the firing schedule (especially the cooling curve). Imagine getting an awesome result out of a kiln and not knowing (or being able to replicate) the exact firing schedule that produced it. This device reads and records the temperature once per minute. It is a Raspberry Pi computer with camera, Wifi and custom software I am developing. It costs about $100 (with the Lego case and GoPro compatible gooseneck mount). Because the device is a full-power Linux web server I can login, will be able to see the list of schedules and download any into my Insight-live.com account. And then I can link that firing it to photos of glaze test results!
Put the pots in, select a cone, press start. It is time to rethink that approach! Seriously. 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. Google "bartlett genesis controller" for videos on creating and editing a schedule.
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 375 firings on this one in the past couple of years, it is still like new. Ongoing testing is the key to constant development of your productS and their quality.
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!
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.
Red controller on the right: A Skutt Kilnmaster. Blue controller to the left of it: An Orton Autofire. These controllers both attach to a thermocouple in the kiln so they know the temperature. Both are external to the kilns (but there is a big difference). The controllers monitor the temperature change as they turn the power on in bursts, changing the length and frequency of the bursts to control temperature rise. 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 connects to the switching mechanism in the other kiln (built to receive it), thus no heavy duty relays are needed within it. The KilnMaster is more flexible since it can connect to any kiln, but it is also triple the price.
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 over fired. 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 says do not 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: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.
Devices that melt and bend in a ceramic kiln at specific temperatures when subjected to specific up ramps. Today, cones are used to calibrate controllers.
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, but if you are aware of the limitations and take precautions they are workable.