The most challenging firing that ceramic artists undertake is the bisque firing. It is also, more often than not, the one firing that we pay the least amount of attention to—until it’s too late! Here are a few tips for better, faster firings.
Defining the Terms
Black/Carbon Coring: The result of incomplete oxidation of the organic impurities present within the clay body, resulting in a discoloration of the interior bisque-ware body ranging from light gray to charcoal gray.
Bump Crack: A mostly invisible crack that occurs from handling bone-dry ware. This structural defect rarely manifests until after the glaze firing. Bump cracks are not straight like a thermal shock crack but rather slightly jagged and almost always emanate from the outside edge or rim of the piece.
Mechanical Water/Water of Plasticity: The water used to mix a clay body. It is driven out of the clay by drying techniques used from ambient temperatures to 212°F (100°C).
Thermal Shock: A structural failure of the clay body that can occur during the heating or cooling cycle as the ceramic enters or exits quartz inversion at approximately 1050°F (566°C). These are clean hairline cracks. If the shard’s edge is sharp like a broken piece of glass, then it occurred during cooling. If the shard’s edge is rounded or dull, then the crack developed while heating up.
Venturi Burner: A natural-draft burner that incorporates the physics of a trumpeted tube to propel fuel out of the burner tube and into the kiln’s fire box.
Understanding the Bisque Firing
After an explosion in a bisque firing, not only is there a significant mess to clean up, but elements and refractories in electric kilns can also be damaged. Depending on the design of the pieces being fired, the rest of the kiln load may be damaged as well. Bump cracks as I call them, caused by mishandling or explosions of nearby pieces during the bisque firing, are difficult to see until after the glaze firing. Once the item has been glazed and fired, the crack opens up. Sometimes a damp sponge and a quick wipe over the surface of the bisque-fired piece will reveal a bump crack, but unfortunately not always. Sometimes a crack will manifest itself after glazing.
Gas Venturi burners can become occluded and then promote an intense localized reduction atmosphere inside the kiln’s chamber. Often this localized reduction causes random glaze defects of all sorts in the fired glaze kiln load. Most bisque firings require an oxidizing atmosphere in order to oxidize or burn off all the organic compounds within the clay body. In addition, an oxidizing atmosphere will evacuate all gasses present in any relatively unstable oxide compounds.
In order to prevent explosions and make sure all organic materials are burned out, as well as maintain the proper oxidizing environment, many potters choose to use slow bisque firing schedules. It is possible, however, to fire some pots successfully with a less conservative firing schedule once variables like organics and water content are understood.
Bisque Firing Techniques
Two basic approaches to bisque firing are utilized around the world. The first is bisque firing to a temperature that is lower than the maturity temperature of the clay body for better glaze adhesion and safer handling of ware while glazing. The second is bisque firing to a temperature that is higher than the maturity temperature of the glazes to vitrify forms and ensure complete burnout of organics. For mid-range clay bodies, I suggest bisque firing from cone 08 to 06 and glaze firing to cone 5 or 6. If you are using tongs when dipping pieces into glaze, and the clay body was bisque fired lower than cone 08, the ware will be too soft to grasp with the tongs without gouging its surface.
Chemical and Mechanical Water
There are two kinds of water present in every clay body formula. First, there is chemical water. Chemical water is the water that is physically bound to the clay at the molecular level. I have never seen an explosion from chemical water in my career and it is not as critical to eliminate as mechanical water.
Mechanical water is the water that is added in the formulation of your clay for throwing, handbuilding, press molding, or slip casting. Mechanical water is often referred to as water of plasticity; without water, clay bodies are not plastic. It is this added water that must be dried from pieces in order to yield successful firings.
Drying and 0% Water
Successful bisque firing depends on proper drying techniques. These include drying your greenware to a bone-dry state and using heat to drive off any remaining mechanical water. Allowing glazed bisque ware to dry before firing it to a maturing temperature, and drying ceramic decals prior to firing are also critical. Many defects in the finished pottery can be traced to improper drying throughout the ceramic process.
Drying of any ceramic product revolves around two techniques, cold drying with gentle air movement and heated drying with vigorous air movement at 212°F (100°C). Evaporation promoted by a slow fan or natural draft in the cold drying stage is a far more powerful technique than drying with heat, which is primarily used to drive the ambient humidity out of clay. Both evaporation and heating are necessary to dry something to 0% moisture content.
I know many potters who use their kilns as dryers, but I never recommend this approach. Drying in a kiln causes the water vapor and steam exiting the kiln to comingle with any sulfur that might be present in the water, natural gas, or clay. Once the sulfur and water are exposed to heat, they form a mild sulfuric acid, which will attack everything metallic in your kiln and cause it to corrode. This includes the elements in an electric kiln, thermocouples, as well as any structural iron in the kiln’s frame.
There are as many different approaches to drying protocol as there are potters. The potters I know use a wide variety of techniques for cold and heated drying. The main goal is to dry your greenware from ambient humidity to 0% moisture prior to loading the greenware into your bisque kiln. Ambient humidity is defined as the humidity present in the atmosphere outside of the kiln. Depending on your local weather conditions, ambient humidity can run from 3% to 8% or higher by weight. This is usually enough moisture in a pot to cause it to blow up if it is heated too quickly. At 0% moisture content, the wall thickness of a piece becomes almost totally irrelevant.
Recently, I bisque fired 13½×13½×¾-inch clay tiles with 0% water content to cone 5 in 2 hours in my industrial Noritake roller hearth kiln. The ramp between ambient temperature and 1000°F (538°C) occurred within 15 minutes. Out of 24 tiles, we did not lose a single one. Prior to this successful test, I tried the same test with only 2% water by weight in the tile and we lost every one of the 24 test tiles. This extreme test demonstrates that once the mechanical water has been driven out of the clay, catastrophic defects from steam are no longer a concern.
Measuring 0% Moisture
The technique for measuring 0% moisture in your pottery is to run your items through a cold and heated drying cycle and then weigh the item. Put it back into the heated dryer, leave it for another hour, and weigh it again. If it is the same weight, then it is dry. If it is lighter, then dry it until it no longer loses weight.
Of course, it is nearly impossible to emulate the rapid climb of a roller hearth kiln with a studio potter’s kiln. However, once you have dried the greenware to 0% moisture, it will be nearly impossible to lose pottery to steam explosions. With no mechanical water in the clay or dried glaze coating on bisque ware, many glaze defects (for example, pinholes in matte or satin glaze finishes) can also be mitigated, if water is the culprit.
Suggested Firing Ramps
When I fire a small top loading kiln to bisque temperature, I program the controller to ramp up as rapidly as possible and yet still allow the entire chamber to heat evenly. The density of the kiln load, relative organic content of your clay, target temperature, along with the size of the kiln will dictate how fast you can fire. If you are using an electric top-loading kiln and you have a convection fan, then use the fan to help even out the temperature more quickly.
There are three methods of heat transfer: conduction, convection, and radiation. Electric kilns rely almost entirely on radiation, with some conduction. If you can introduce a draft, then convection will occur and your kiln’s chamber will heat up faster and more uniformly.
Due to these variables, I do not have a specific ramp that I can recommend other than that you can program your ramp much faster than 300°F (149°C) per hour. If you are bisque firing to cone 08, and your work is at 0% moisture, a four-hour ramp to the final firing temperature is not an unreasonable expectation.
If you manage to get your kiln to climb at this rate, then a hold or soak at the end of the ramp is highly recommended. Holding the kiln at temperature allows the heat to evenly spread throughout the firing chamber. You can judge the duration of the hold by looking into the kiln through a spyhole while wearing welding glasses. If the core of the kiln’s load is much darker than the edges, then keep the soak/hold going until the color of the kiln is more uniform. Pyrometric cones placed throughout the load are another good indicator of uniform temperatures when you unload your kiln. Sometimes I use an infrared thermometer to check temperature up to 1000°F (538°C), through the spyhole. If your kiln is very uneven during the ramp up to 1000°F (538°C) then it is advisable to slow it down.
Some clay bodies might have high concentrations of organic impurities. This is especially true for most terra cottas. In this case, it is advisable to fire your ware slowly until you exceed 500°F (260°C). I also recommend that you get as much oxygen into the kiln’s chamber as possible to facilitate a more complete oxidation of the organics in the clay.
Oxidizing Atmosphere in Bisque Firing
Oxidation of bisque ware during the entire firing schedule is an essential technique in order to produce glazes relatively free of blemishes. Pinholes, surface divots, and white spots in hand-painted decorations, as well as a host of minor blemishes are often caused by bisque kiln atmospheres that are neutral to slightly reducing. In an electric kiln (neutral atmosphere) with a convection fan, simply open the fan slightly to introduce an oxidation atmosphere. If you have a damper on the kiln, then pull the damper open a bit to allow oxygen to enter the chamber.
In a gas kiln, the proper burner orifice size coupled with the proper damper setting and the correct gas pressure will all contribute to an oxidizing atmosphere. Once a gas kiln’s chamber shows color (starting at approximately 1200°F (649°C)) if there is a visible flame shooting out of the top damper, that indicates a reduction atmosphere. If your kiln is outside and you are firing during the day, then it might be impossible to see the reduction flame. Test fire at night in order to properly set the dampers and gas pressure for bisque firings. Another way to test the kiln atmosphere is to roll up a paper torch and place it in front of the open bottom spyhole. If the flame from the torch jumps away from the spyhole, there is back pressure, indicating a reduction atmosphere. If the flame from the torch seems unaffected by the spyhole, the kiln is burning a neutral flame. If the flame from the torch is sucked into the kiln, then the kiln is burning an oxidizing flame. It is the oxidizing flame that is desired.
Fast Firing Goals
By practicing proper drying and speeding up your bisque firing schedules, you will enjoy less expense for bisque firings, less wear and tear on your kiln, and less defects in your fired ware. If you experience explosions in your bisque firings, I can assure you that it has very little to do with your firing curve and everything to do with the moisture content of your greenware.
the author Larry Camm has 50 years experience as a studio potter and industrial ceramic factory consultant, and was a professor at the state university and local college level for 11 years. He holds a master’s degree in ceramic art from California State University at Long Beach.