Careful control of the drying and ﬁring stages is necessary for successful completion of large-scale sculpture and tilework. Although climatic conditions vary from studio to studio, it is possible for everyone to effectively manage the moisture level within clay; that is, the plastic working period for clay can be held and extended, and the drying balanced so that cracking and warping can be virtually eliminated. Similarly, careful control of the ﬁring cycle (both heating and cooling) is key to producing work that pushes limits of design and scale.
To keep their clay pliable, many ceramists resort to spraying with water before wrapping it in plastic. Often the misted water runs into the form’s recessions, puddling and creating wet areas. The result can be disastrous. Moisture cracks may develop where puddling has occurred. As a ceramics educator, I have even seen pieces collapse after this type of wet wrapping.
The controlled addition of water can be accomplished by evenly spraying a light mist over the entire surface of the form. Sufﬁcient time should then be permitted for the misted water to be absorbed by the clay—the shine disappears and the satin surface reappears. This very important step is accomplished in just a few moments.
Moist paper towels placed on the wareboard or kiln shelf at the base of the sculpture prior to wrapping can help maintain pliability between work sessions. The moisture is locked into the enclosed area by covering with multiple layers of lightweight plastic (dry cleaners’ bags work well). On average, I use at least eight layers to carefully wrap a large sculpture. One thing that should be taken into account is that moisture will condense on the inside surface of the plastic.
When the form is securely wrapped, the water content will even out; however, the result will vary, depending on the ambient humidity, as well as the level of ﬁllers (grog, sand) within the clay recipe. I always keep a complex form under wrap for a week to even the moisture content, before beginning to dry it out.
To begin a controlled drying phase, I wrap the sculpture or tile with towels (cotton bath towels provide perfect absorption), then cover with eight layers of lightweight plastic. I use more than one towel on most forms, and make sure to place them evenly against the clay. I am also careful not to create a downward pull or extra weight on any part of the form. The cloth towels blot the water from the clay’s surface. Moist towels are removed and replaced.
After the clay begins to change color (at the end of the leather-hard phase), the plastic is removed; however, the towels are kept on the surface to promote even drying. In effect, the cloth blocks uneven air ﬂow while drawing water from the surface of the clay.
When is the clay really dry? Many artists simply say that it should not feel moist or cold to the touch; however, under these circumstances, the core may not be fully dry. This is especially true when wall thicknesses are K inch or more. A false “sense of dryness” has led many to ﬁre too quickly, causing explosions when water turns to steam.
Preheating for Core Drying
Core drying can be effectively accomplished during a preheat cycle in electric and gas kilns. Steam explosions can occur at 212°F, so the initial heat-up period must be very slow. Use of a pyrometer or kiln controller will allow you to monitor the heat climb. An average safe heat climb is purported to be 200°F per hour; however, practical experience has led me to ﬁre large or complex works more slowly.
Using an electronic controller for my electric kiln, I allow a rise in temperature of 80°F per hour during the preheat cycle, holding the kiln at 212°F to ensure thorough drying. The peepholes are left unplugged and the kiln vent is turned on at the beginning of the preheat cycle.
I often rotate elements, turning the bottom on, then off, the middle on, then off, etc. You can hold your electric kiln at 212°F by experimenting with the lowest switch settings (and the number switches that are on the lowest setting). At any point below or at 212°F if you feel or see steam (on a mirror or glass placed in front of the peephole), continue to hold at that temperature until no steam is apparent.
Preheating in gas kilns varies, depending upon the burner systems. I use a pyrometer to gauge the heat increase and manipulate the door, damper, pilots and blowers to be certain that the climb is gradual and preferably not more than 100°F per hour and a half.
Firing Principles for Large Scale
The clay object undergoes chemical and physical changes during the ﬁring cycle. When bisque ﬁring large-scale works, heat rise must be gentle and slow through critical points: 0–212°F, when water of plasticity becomes steam; 650°–750°F, when chemically combined water escapes; 900–1100°F, when quartz inversion takes place (this is a structural change in the clay where the silica/ﬂint changes from alpha to beta quartz). The quartz crystalline structure actually changes by expansion and contraction. Cracking will eventuate if this 200° range is passed through too quickly—more than 200°F in one hour. It is best to go slowly through quartz inversion in the glaze ﬁring as well.
If the average heat up is safe at 200°F per hour, presumably, the average cool down should be the same. For large-scale works, I do not allow the kiln to drop more than the 200°F in one hour.
After years of monitoring the ﬁring cycles in both gas and electric kilns, I have found that both the heating and the cooling cycles are important when ﬁring large-scale sculpture and tiles. 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, it is not unusual for the pyrometer for my kiln to indicate a 400°F drop during the ﬁrst hour.
Other large temperature drops, well beyond 200°F per hour, occur until 700°F is reached. At this point, the cooling slows down signiﬁcantly. If I were ﬁring only one large piece, there would be a danger of thermal shock resulting from a difference in temperature between the top and bottom area of the kiln. Realizing this led me to ﬁring my kiln down to decrease the temperature gently and more evenly.
Initially, 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 not more than 200°F per hour. To continue “ﬁring down,” I turned 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.
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 of approximately 30 hours). Now, I use an electric kiln with a stand-alone controller. An electronic controller often has preprogrammed cycles for slow, medium and fast ﬁring speeds; however, it is also possible to program a custom ﬁring schedule. I’ve programmed my controller with a ramp/hold cycle to ﬁre up and down.
Suggested Firing Schedules
To successfully ﬁre (bisque and glaze) using a pyrometer, I recommend the following schedule:
• Turn on kiln vent.
• 0–80°F preheat cycle (until no steam is present).
• 80°F per hour until 750°F.
• 100°F per hour until 1100°F (past chemical water escape and chemical quartz inversion).
• 150°F to desired ﬁring temperature for bisque or glaze.
• Turn off kiln vent.
• Drop 100°–150°F per hour to 900°F.
• Drop 200°F per hour to 600°F.
• Let cool naturally to 200°F, then crack the lid of the kiln.
To ﬁre (bisque and glaze) using a kiln controller, try the following:
• Turn on kiln vent.
• 80°F per hour to 212°F, hold for three hours.
• 80°F per hour to 750°F, hold for two hours.
• 80°F per hour to 1100°F.
• 120°F per hour to desired bisque temperature.
• Turn off kiln vent.
• Drop 150°F per hour to 750°F.
• Drop 200°F per hour to 300°F.
Stacked Kilns with Dual Controllers
When ﬁring tall pieces (signiﬁcantly taller than 27 inches in height), I stack my two kilns one on top of the other. If a sculpture is within 4 inches of the kiln height, I add a blank extension ring. If not, I use a minimum of two rings from my second kiln and plug the two kilns into separate receptacles (my electrical supply is able to handle two 208-volt kilns on separate breakers).
When I purchased my electronic controller, I quickly realized that unless two controllers could be worked simultaneously, or unless a controller was built to run six rings, I would once again need to resort to monitoring my ﬁrings with a pyrometer for up to 30 hours. I found that no single controller was made to do this job, but that it is possible to program two controllers with the same schedule to ﬁre the two stacked kilns with only a minimum temperature (an average of 20°F) difference between the top and bottom kilns.
Just remember that, when producing large-scale sculpture and tilework, as much creative thought must be given to the drying and ﬁring as any other aspect of making the work.