Stick with basic principles to fire successfully. (That applies to all types of firings.)
The Problems with Firing Ceramics Too Fast
Recent Q&A: Monitoring a kiln at night
Announcements: A historical glass DVD; an advanced kiln maintenance seminar
THE PROBLEMS WITH FIRING CERAMICS TOO FAST
One time a ceramist visited the factory and told me, “I always leave the kiln switches turned to high. I load the kiln. When I’m ready to fire, I press the Kiln Sitter button. When the firing is done, the Kiln Sitter shuts off the kiln. I leave the switches on high and haven’t touched them in years.”
Firing the kiln at full speed is risky. Ceramists want to fire their kilns faster to save energy. But fast firing causes more mistakes than any other firing practice. In this article I will describe the problems that can arise at each firing stage when the kiln is fired too fast.
Stage One: Water Smoking (Up to 800 – 900°F)
All clays contain water. The first stage of firing burns off the water in ceramic greenware. As the water heats, it expands and is pushed out of the clay. This is called water smoking.
Fast firing during this stage can lead to disaster. If the kiln heats past the boiling point of water (212°F), the water in the clay will expand too quickly. When the ware breaks during firing, it is usually at this first critical stage. Water boiling inside the clay can explode, damaging not only the ware but also the walls of the kiln. When the ware sounds like popcorn inside the kiln, you know you have fired too fast.
During water smoking, keep the kiln switches turned to low. If you are using a pyrometer or digital controller, maintain temperature at 200°F or lower. Another important point: vent the kiln so that the water can escape. If the water is trapped inside the kiln, it will absorb into the firebricks, slowing the firing and wasting electricity. To vent the kiln, raise the lid an inch or more and leave the peephole plugs out. Do not lower the lid and insert the plugs until all signs of water vapor have disappeared from the firing chamber. To check for water vapor, hold a small mirror near the vented lid. Water vapor will fog the mirror. (Hold the mirror near the lid for only a few seconds. If the mirror becomes too warm, the moisture will no longer fog the mirror.)
Dry the greenware thoroughly before firing. A dehumidifier is helpful especially if you live in a humid area. If you load wet greenware into the kiln, keep the kiln at 200°F or lower until the moisture has disappeared. Though you can fire the ware safely by drying it in the kiln at low temperature, it is less expensive to dry it before firing. Drying greenware in the kiln not only raises the electrical cost of the firing, but it also rusts the kiln.
Stage Two: Dehydration and Quartz Inversion (900° - 1100°F)
After the initial water smoking stage, the clay continues to lose water at a molecular level. This continues to about 1100°F. At this stage, the clay changes chemically into fired ware. At around 1060°F, the clay goes through quart inversion, which is the point at which the silica in the clay expands.
During stage two, the molecular water must exit the clay slowly, especially if the pieces are large. At quartz inversion, the clay will break if heated too quickly.
Stage Three: Oxidation (500° - 2000°F)
Impurities burn out of the clay from the very beginning of the firing, but especially during the Oxidation stage. The kiln atmosphere must have oxygen to burn the impurities. This is another reason venting is so important at the beginning of firing. The ware must be heated slowly to give impurities, such as carbon, enough time to become gases and combine with oxygen. The thicker the ware and the heavier the load, the slower the ware must be fired.
As the clay reaches higher temperatures, the components of the clay fuse together, sealing the clay surface. At this point, remaining carbon and other impurities will become trapped inside the ware. As the clay heats further, the trapped gases expand beneath the surface, bloating and cracking the ware.
Glaze pinholes, bubbles, crazing, and peeling are often caused by gases that had not burned out fully in the greenware firing. These gases are pushed to the surface during the glaze firing. Lead-free glazes are especially sensitive to trapped gases. These glazes lack the flux that smooths out glaze defects.
Trapped impurities turn white bodies gray and weaken the finished ware. They cause mildew in porcelain.
Stage Four: Maturity (1300° - 2300°F)
This is the stage where the heat has transformed the ware to the degree intended by the clay manufacturer. Over-glazes mature at 1000° - 1500°F; low-fire ceramics mature at 1700° - 2100°F; and porcelain and stoneware mature at 2100° - 2400°F. See your clay supplier for the recommended firing temperature of your clay. It is listed as a pyrometric cone number.
When to Speed Up the Firing
After the quartz inversion (1000° - 1100°F), clays in the bisque, or greenware firing, can be fired faster. By this stage, the water has been completely driven out of the clay. Actual speed will depend on how many impurities the clay contains. Slow the firing again during the last 200°F.
How fast you should fire depends on the thickness of ware and type of clay. Thin-walled, low-fire ceramics can be fired much faster than stoneware. Fire test pieces in a small kiln. Experiment with firing speed before risking an entire load of ware. Always test-fire clays you are unfamiliar with. If test results are satisfactory, but you want to fire faster, try further tests at increasingly faster rates.
Firing the ware too fast and with insufficient venting causes more problems than any other firing practice. So long as the clay and glaze are designed properly, you will get almost fool-proof results by firing slowly, venting thoroughly, and using witness cones. These are the basics of firing.
Some clays can be fired rapidly with no ill effect. Certain glazes even look better when fired fast. With experience, you will learn the limits of clays and glazes. You can experiment, yet still avoid the vast majority of firing mistakes, so long as you continue to follow the basics.
The last Kiln Pointer was on firing the kiln at night. Jamie Gray in Calgary, Alberta, Canada wrote, “I recently hooked up a baby monitor so that I could wake in the night and listen for the kiln relays firing without having to go to my basement studio to check the kiln. For a recent firing, I had also set the digital kiln alarm for 100 degrees above my top temperature. If things went haywire, it would beep and warn me.
“That night while I was sleeping, the alarm went off and I heard it because of the baby monitor. The relays had burned out. If I had not had the baby monitor and had not set the kiln alarm, I might have had a very unhappy situation in my studio. Bottom line: One way or another, we should never leave our kilns unattended!”
Ed and Martha Biggar are selling a DVD of glass pioneers Frances and Michael Higgins. The Higgins fused glass for over 50 years. Martha wrote, “Insights into their history, their styles and techniques, and their lives together make this DVD a fascinating view into their Riverside, Illinois studio; one that you will want to watch again and again. Perfect for artisans, collectors, or both. It is incredibly informative and can be watched several times to get more out of it each time.” For more information visit Ed and Martha’s website.
ANNOUNCEMENT: ADVANCED KILN SEMINAR
We are holding an Advanced Kiln Maintenance Seminar February 22 - 23, 2008 at the Paragon factory in Mesquite, Texas. The seminar will cover kiln design, extensive trouble shooting, three-phase power, and much more. Participants should complete the basic repair seminar or have experience repairing kilns before attending the advanced seminar. You do not need to bring tools. The seminar fee is $95.00.
Meals, Airport Pickup, Hotels
As a seminar student, you are a VIP guest at Paragon. We furnish lunches on both days and dinner the first evening. The seminar is an exciting way to meet new friends. If you arrive before 4:00 p.m. the day before the seminar, we will pick you up at Love Field or D/FW International Airport. Please call ahead with flight number, arrival time, airport, and gate number.
We will pick up students from the offices of the following hotels at 7:30 - 7:45 a.m. each seminar morning and return them at the end of the first day:
Courtyard by Marriott 972-681-3300
Hampton Inn 800-426-7866
Holiday Inn Express 972-288-9900
After the seminar, a shuttle will leave Paragon at around 12:30 – 1:00 p.m. to take students back to the airports.
For more information and to register, please call 800-876-4328 and ask for customer service. We hope you can come and look forward to visiting with you.
I hope you enjoyed your News Year’s celebration. My wife and I watched “It’s a Wonderful Life,” starring my favorite actor, James Stewart. Each time I watch that movie, I get more out of it. It is a timeless, uplifting classic with a powerful message.
With best wishes,
Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 firstname.lastname@example.org / www.paragonweb.com
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Copyright 2008, by Paragon Industries, L.P.