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Minnesota Clay Co. USA Galleries

Firing Large Scale Sculpture

by Nan Smith
 
The Guardian After years of monitoring the firing cycles in both gas and electric kilns it became evident that both the heating and the cooling cycles were important when firing larger scale works. In densely packed kiln loads, the objects themselves contain and radiate heat. In a kiln load where the object is a large form the heat within the chamber is contained by a smaller surface area.
 
If you use a pyrometer to monitor the cooling of a kiln, you will realize that loosely packed kilns retain less heat during the cooling cycle. In fact, the pyrometer for my kiln indicated a 400 degree drop during the first hour. Other large temperature drops, well beyond 200 degrees per hour, occurred until 700 degrees was reached. At this point the cooling slows down significantly. If one large piece spans the kiln the ceramic object is certain to sustain thermal shock resulting from a difference in temperature between the top and bottom areas of the kiln. This problem caused me to think about firing my kiln down, to gently and more evenly decrease the temperature of large scale works.
 
At first I began by turning the kiln switches to the medium setting after the maximum temperature was reached. I closely monitored the decrease in temperature seeking to create a drop of 200 degrees (Fahrenheit) per hour. I "fired down" by reversing the switch settings turning each switch top to bottom from medium to low during each successive hour. When I fire a gas kiln I keep the pilots on and brick up the burner ports to slow the cooling. I use a pyrometer to determine how quickly the temperature is declining.
Above: detail of The Guardian. c. 1997 by Nan Smith
 
Two years ago, I decided that an electronic kiln controller would be a good investment (due to the length of the "fire up" and "fire down" cycles, approximately 30 hours). Today, I use an Olympic electric kiln with a stand alone controller. The controller is often used in pre-programmed cycles; slow, medium, and fast firing speeds are options. However, controllers offer the possibility of programming a "self-designed" firing schedule. Electronic kiln controllers are produced by many kiln manufacturers and are designed to be used with their kilns. The Orton Company makes the chips for most of the electronic controllers marketed. Kiln controllers will allow you to program the firing cycle by setting the rate of climb per hour and any holding patterns desired at specific temperatures.
 
I use and teach the following information for firing large work:
 
The average safe heating climb is documented to be 200 degrees per hour in the heating phase of the firing cycle. A noted principle of firing is that the heat up and cool down of the ware should be similar or balanced. If the average heat up is safe at 200 degrees per hour one would presume that the average cool down should be the same. Practical experience has led me to advise that large or complex works be fired more slowly. Steam explosions can occur at 212 degrees, so the initial heat up must be slow. (A note: Most thicker walled ware does not fully dry in the humid Florida or similar environments. Thicker walled ware does not fully dry out until it is heated in a drying chamber, either a kiln on a pre-heat cycle or in a drying box). An overnight pre-heat cycle is a good rule of thumb for sculptural work. Sometimes this 8 to 12 hour pre-heat cycle (hold below 212 degrees fahrenheit) is not long enough. If you feel steam on your hand or see steam on a mirror or glass placed in front of the top peep hole, then continue the warming cycle until no steam is apparent. Using a pyrometer during the firing will inform you of the rate of climb. The clay object is undergoing chemical and physical changes during the firing cycle. In the bisque firing you must travel gently and slowly through these critical heating points. In the glaze firing it is best to go slowly especially during chemical quartz inversion.
 
They are:
  • 0- 212 degrees, water of plasticity burns off
  • 212 degrees, water becomes steam 650-750 degrees, chemically combined water burns off; your clay can still explode (clay is alumina, silica, and water in it's chemical composition)
  • 900-1100 degrees, chemical quartz inversion takes place (this is a structural change in the clay where the silica/flint/quartz changes from alpha to beta quartz. The quartz crystalline structure actually changes by expansion and contraction). Cracking will eventuate if this 200 degree period is traveled through too quickly. Too fast, is on the average more than 200 degrees in one hour. To avoid the hair line cracks one can post the ware up from the bottom of the kiln.
I also recommend a "fire down" on the cooling cycle (especially for large works which span the kiln vertically). The other key cooling points occur through quartz inversion between 900 and 1100 degrees Fahrenheit. It is advised not to allow the kiln to drop more than the two hundred degrees in one hour.
 
Heat up cycle:
Heating Cycle
 
Chart does not show a pre-heat cycle, the duration of one will vary on the thickness and dampness of ware
  • 0-80 degrees pre-heat cycle (until no steam is present) 80 degrees per hour until 750 degrees
  • 100 degrees per hour until 1100 degrees (pass chemical water burn off and chemical quartz inversion)
  • 150 degrees per hour to desired firing temperature for bisque or glaze
Cooling cycle:
Cooling Cycle
 
  • drop 100-150 degrees per hour to 900 degrees
  • drop 200 degrees per hour to 600 degrees
  • let cool naturally to 200 degrees and then crack the lid of the kiln
Suggested Firing Pattern for large work (using a kiln controller)
 
Heat up cycle:
  • 80 degrees per hour to 212 degrees, hold for three hours
  • 80 degrees per hour to 750 degrees, hold for two hours
  • 80 degrees per hour to 1100 degrees, no hold
  • 120 degrees per hour to desired bisque temperature, no hold
Cooling Cycle:
  • 150 degrees per hour to 750 degrees, no hold
  • 200 degrees per hour to 300 degrees, no hold, off
* Note - This schedule represents six segments in a ramp/hold cycle.
 
If you are interested in reading the most updated information on electric kiln firing a good resource is: "Firing Line" by the Orton Company, Columbus, Ohio. A hand held digital pyrometer is a great tool for reading a detailed temperature climb. The meter (which is the size of any TV remote control) can be used in multiple kilns by having a pyrometer lead hook-up in each of your kilns. The one meter can then be used to take readings in each of your shop kilns.
 
An understanding of firing principles and an interactive firing procedure will allow you to fire large scale and/or complex works successfully.