Deciding whether to bisque fire or not is something many of us question. Can the work handle being once fired? Will eliminating the process save money? Learn the pros and cons to help you make an informed decision.
Defining the Terms
Black Coring: Classified as a chemical defect where the interior of the clay body is black or dark gray because organic materials are not being completely removed in an oxidation kiln atmosphere. Once the clay is vitrified, oxygen can no longer penetrate the clay, trapping carbon. Black coring can lead to bloating if the carbon has not been removed from the clay.
Bloating: Classified as a physical defect where the clay body develops bubbles when gases expand and are trapped as the clay becomes vitrified.
Bone Dry: A ceramic object that can still contain mechanical and chemical water but is dry to the touch and brittle.
Carbon Trapping: Carbon is formed from decomposing organic matter in the clay body when there is not enough oxygen present during the firing, resulting in black coring.
Oxidation Atmosphere: In hydrocarbon-fueled kilns, more air than fuel is present during combustion, as opposed to electric kilns where no fuel is ignited.
Reduction: A kiln firing using more fuel than air in the combustion process, resulting in a carbon monoxide atmosphere. The subsequent reaction takes oxygen away from metallic oxides.
Sintering: Occurs in the early stages of firing where the clay particles bond together, partially melting. During this stage, porosity decreases, strength increases, and water absorption decreases.
Thermocouple: A device to measure temperature inside the kiln, which provides information to the kiln controller. After repeated firings, thermocouples will eventually wear out and need to be replaced.
Contemporary Bisque Firing
Bisque firing is the preliminary process of transforming the bone-dry clay into an easier-to-handle, durable, porous ware ready for subsequent glaze firing. It creates the first stages of sintering where the clay platelets start to fuse together. While the temperature is increasing in the kiln, the clay goes through several chemical and physical changes.
The majority of potters fire a bisque kiln between cone 010 (1657°F (903°C)) and cone 04 (1945°F (1063°C)), depending on the particular clay body they are using. The general rule is that the lower the bisque firing temperature, the greater the porosity of the ware. A low-temperature bisque firing is weaker, and the actual water weight in the glaze can crack the ware.
The pyrometric cone parameters and firing cycles used in bisque firing are fairly broad compared to a glaze firing. Most stoneware and low-fire clay bodies can be bisque fired at cone 06, with porcelain clay bodies fired slightly higher to cone 04. The range of glaze absorption depends in part on the temperature reached and the individual clay-body formula.
Kilns used for bisque-firing can be hydrocarbon-fueled kilns (natural gas, propane, oil, or wood) and fired in oxidation, neutral, or reduction atmospheres, the last two of which can cause problems in bisque ware; or in electric kilns, which fire in an oxidation atmosphere and release detrimental carbonaceous materials and sulfur from the clay.
Bisque Firing Process
• Preheating, Mechanical Water Removal to 180°F (82°C): Preliminary heating of the kiln for several hours, depending on the thickness and size of the ware, allows moisture to be drawn off slowly and safely.
• Water Smoking 212–392°F (100–200°C): The early stages of the firing remove free water in the pores between clay crystals through evaporation. This stage is referred to as releasing free or mechanical water in the ware. Steam can be observed exiting from the cone viewing holes in the kiln.
• Removal of Organic Materials 572–1292°F (300–700°C): Removal of organic materials, lignite, and carbonaceous matter in an excess oxidation atmosphere with sufficient time will ensure clean clay for eventual higher temperature firing.
• Chemical Water Removal 842–1112°F (450–600°C):The next stage is known as dehydroxylation, which drives off chemical water in the clay. At this point, the clay cannot be reconstituted into a plastic state. This stage removes the chemical water part in the clay formula: Al203 2Si02 2H20.
• Quartz Inversion 1063°F (573°C):Crystalline quartz inversion occurs in the clay body, with silica quartz expanding in volume suddenly at the same time that shrinkage occurs. A reverse of this process happens upon cooling.
• Sulfur and Organic Materials Released 1292–1652°F (700–900°C): Sulfur, lignite, and other organic materials are released which, depending on the individual clay deposit, can have minor or significant levels of various materials due to the mining process. In many instances, due to inadequate venting in electric kilns, sulfur or organic burning odors can be detected. If not removed during this temperature range, they can result in the clay body black coring or bloating at higher temperatures.
• Beginning of Silica Melt 1652°F (900°C): Silica begins to melt, binding the clay platelets and forming metakaolin, resulting in the clay body shrinking, lowering absorption, and increasing vitrification.
• Formation of Mullite Crystals Above 1922°F (1000°C): Mullite crystals form, contributing to fired clay strength, thermal shock resistance, and reduced warping in the ware. It is often said, above dull visible red heat in the kiln (1652°F (900°C)) anything bad or good has already taken place.
Bisque Firing Temperature Ranges
Currently, bisque firing is widely practiced in most pottery studios. The parameters for successful bisque firing are fairly wide as compared to glaze firing. For example, if a bisque firing is one or two cones over or under in the range between cone 010 (1657°F (903°C)) and cone 04 (1945°F (1063°C)), it will not have a significant effect on the eventual range of glaze absorption. Keep in mind, depending on the clay body, the higher temperature range of bisque firing will decrease glaze absorption while the lower range can “wick” the glaze faster and more thickly onto the pot.
Kiln Fuels and Oxygen
What at first seems to be a fairly undemanding firing process reveals a complex transformation of raw clay. Even at relatively low bisque firing temperatures there are many changes ceramic objects go through to yield a stable state. Bisque firings can be accomplished in electric kilns or hydrocarbon-fueled kilns.
Bisque firings in electric kilns expose the ware to an oxidation atmosphere, which will remove organic carbon, chemically combined water, and sulfur from the clay. The critical factor occurs when sufficient oxygen is present in any type of kiln as it removes carbonaceous or organic materials from the clay.
Advantages of Bisque Firing
• Bisque firing increases handling strength, making moving pots and glazing easier.
• The absorbent surface of the pottery helps the glaze to adhere evenly.
• Glaze mistakes and touch-ups are easier to correct on bisque pottery as opposed to raw glazing where the clay body is structurally weaker.
• When bisque firing pots, the risk of defects decreases as the physical and chemical water is removed along with organic material and sulfur, lowering the chance of cracking or exploding ware or defects.
• Inspection of bisque ware for possible surface defects, cracks or warping, which would result in a fired defect. Running a damp sponge lightly over the ware may reveal small cracks as the moisture seeps in and reveals the defect.
Disadvantages of Bisque Firing
• Bisque firing introduces another step in the pottery production cycle.
• Using the kiln for a bisque firing increases the cost of equipment. Most electric kilns need replacement elements after 250–300 firings, depending on the temperature achieved in each firing. Additionally, thermocouple relays and brick replacement are periodic costs.
• Extra production time is spent stacking, firing, and unloading the bisque kiln.
• Running a production or hobby kiln twice doubles the cost of electricity or hydrocarbon-based fuel.
• Thermal stress during bisque firing can cause cracking, warping, or breakage if pieces are not properly dried or fired.
• Bisque firing increases the production time as the kiln has to be cooled, unloaded, pots glazed, and fired again, all of which increases the cost of production.
Bisque Firing Kiln Settings
The general rule in any bisque firing is, when in doubt, slow the firing down, as this practice will cause fewer problems than a fast firing. The insignificant added price in fuel is not worth any broken ware.
Guidelines for Bisque Firing on Auto SLOW
When placing bone-dry ware in the kiln, keep in mind mechanical and chemical water still has to be driven off during the first part of the firing. Placing wet or damp pots in the kiln requires longer periods below 180°F (82.2°C) to remove mechanical water.
For most functional pottery less than ⅜ inches (1.9 cm) thick, using the computer pre-programmed SLOW setting on the kiln takes approximately 12–14 hours to reach cone 06 (1828°F (998°C)).
For larger or thicker pottery or sculpture with walls over about 1 inch (25 mm) thick, a much slower cone-06 bisque schedule should completely remove mechanical water, chemically combined water, and carbonaceous materials before the clay begins to sinter.
Guidelines for Bisque Firing on the Ramp Setting
This schedule normally takes 18–24 hours, depending on kiln size, the mass of the sculpture, and how tightly the kiln is packed.
Bisque Firing in Hydrocarbon-Fueled Kilns
Inherent in the firing process of offering many and varied choices of fuel/air ratios producing diverse kiln atmospheres is the underlying benefit/fault in hydrocarbon-fueled kilns. Potters choose which damper setting, gas pressure, forced air, natural draft, and kiln firing cycle that will produce a clean bisque firing atmosphere. It is critical at all times to maintain a blue flame at the burners, which indicates an oxidation atmosphere (more air than fuel entering the kiln). Reduction or neutral atmospheres can trap carbon or sulfur in the clay and increase black coring or bloating. Once dull red heat (1000°F (537.8°C)) is achieved, the kiln atmosphere should be clear and not gray.
A pyrometer, which indicates heat increases or decreases during the firing, and pyrometric cones placed throughout a hydrocarbon-fueled kiln are the most accurate measurements of heat work (energy input during the firing normally represented in terms of temperature and time).
Load bone-dry pieces into the kiln, slowly increase the temperature by 50°–75°F (10°–24°C) per hour up to but not exceeding 200°F (93.3°C), holding 1–3 hours if pots are thick.
Increase temperature 75°–100°F (24°–37.8°C) per hour up to 600°F (315.6°C).
Increase temperature 100°–125°F (37.8°–51.7°C) per hour until you reach 1000°F (537.8°C).
After dull red heat (1000°F (537.8°C)), there should be a clear, crisp atmosphere in the kiln throughout the firing. The temperature can then be increased by 150°F (65.6°C) per hour until cone 06 is reached.
For large or thick sculptural work, the entire firing might take 18–22 hours. The key is slow early heating that avoids steam explosions and helps release carbon in the clay.
Bisque Firing Defects to Avoid
Bisque firing produces fewer defects than glaze firing. The obvious reason being there is no glaze to consider. As in any kiln firing procedure, whatever method is used, various failures can occur. Once understood, kiln firing defects can be avoided in the future. There is a direct correlation between thick pieces and defects such as cracks, carbon trap, black coring, and bloating, which are more likely to occur if the bisque firing procedures are not followed. The thicker the piece, the further organic matter and gases have to travel to exit the ware. Some defects, while not apparent in the bisque firing, will develop fully in the glaze firing.
Fast Firing: Fast firing can cause the mechanical and eventually the chemical water in the clay to release explosive steam. Pots can crack, or in some instances blow up, in the first part of the firing. This is most likely to occur near quartz inversion at 1063°F (573°C).
Carbon Trapping: Carbon is trapped in the clay during the bisque firing process. While some pots or sculptures do not indicate a gray-to-black surface discoloration, carbon can still be found in a broken cross section. A higher probability of carbon trapping occurs in thick pieces and densely stacked pieces (such as tiles), as excess oxygen cannot penetrate the clay to volatilize carbon. Carbon trapping can lead to black coring if the organic material is not completely removed from the clay. Trapped carbon can cause pinholes and blisters in glazes due to incomplete carbon-gas release through the glaze layer. Black spots or discoloration in the fired bisque can result in incomplete burnout of carbon, iron compounds, or clay contamination.
Electric kilns being fired in an oxidation atmosphere under the right conditions can efficiently remove carbon from clay, as opposed to hydrocarbon-fueled kilns where neutral or reduction atmospheres can yield carbon trapping.
Black Coring: The interior of the clay can be dark gray or black because of organic matter not being completely removed by an oxidation atmosphere during the first part of the firing. Remaining carbon in the clay creates gas that cannot exit, which in turn produces bubbles in the semi-vitreous clay. Black coring is frequently found in high-iron-bearing clay bodies as the iron is held in a reduced state due to the lack of oxygen. In some instances, interior black coring does not progress to swelling of the clay body if, at higher temperatures, carbon is oxidized out of the clay before vitrification is complete.
Bloating: If organic materials are not fully dissipated, carbon monoxide and sulfur cannot be removed from the clay, and trapped gases can expand. The interior of the clay is dark gray to black, with a deformation bubble on the surface. Bloating is not always noticeable in a bisque firing, but is often encountered at higher than bisque temperatures due to the vitreous sealing quality of the clay body. Not every black-cored pot will bloat, but bloating can occur if the organic material within the clay is not completely removed. In most instances, black coring is an internal defect compromising the clay body, whereas bloating is a visible external defect.
Direct Flame Impingement: Pots or sculptures placed too close to a heat source, whether electric coils or hydrocarbon-fueled burners, can receive direct heat in a localized area. This can produce a crack in the bisque ware due to an unequal thermal gradient where one area of the pot is subjected to more heat than surrounding areas. Additionally, the heat-impinged area will be less absorbent and can deform due to increased vitrification or glass formation.
In electric kilns, pots should be placed no closer than ¼ inch (6.5 mm) to the elements. In hydrocarbon-fueled kilns, the safe distance to prevent direct flame impingement depends in part on the burner type, fuel pressure, and bag wall placement within the kiln.
Low Bisque Firing: Higher absorption rates occur with lower bisque firing temperatures. Glaze will “wick” in faster and apply more thickly due to the overly porous nature of the ware. Low bisque firing can also decrease the strength of the ware, and in some instances the unbalanced glaze application can cause cracking due to the water weight contained in the glaze.
High Bisque Firing: Higher bisque firing temperatures will decrease clay absorbency and increase its shrinkage. Glazes will apply more thinly and take longer to dry. Extremely high bisque firing temperatures can cause warping, and vitrification can trap carbon in the clay, leading to bloating, black coring, or pinholes and blisters in the glaze firing. Black spots and surface discolorations can result from iron or contaminants starting to melt. Extremely high bisque firing temperatures can cause pots to stick to the kiln shelves.
High bisque firing showing discoloration of the clay. Not being present when the kiln is turned off is a low-probability, high-consequence event. While automatic kiln shut-off devices are extremely effective, just one failure can lead to the “Chernobyl” effect where the shelves, posts, pots, and bricks melt.
the author Jeff Zamek started his career 50 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
References:
Hammer, Frank and Janet. The Potter’s Dictionary of Materials and Techniques, 6th Edition, Bloomsbury Publishing.
Article in Pottery Invention and Invocation in East Asia and the Near East by Kevin Gibbs, Cambridge Archaeological Journal.
Leach, Bernard. The Potters Book, Publisher Faber Finds, 2011.
Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of
Pennsylvania Press, 2004, page 26.
Bloomfield, Linda. Science for Potters, American Ceramics Society, 2017.
Lawrence, W.G. Ceramic Science for the Potter. Chilton Book Company, 1972, page 39.
Renfew, Colin, and Bahn, Paul. Archaeology: Theories, Methods, and Practice, a widely used archaeology textbook on the origins of pottery.
Bloomfield, Linda. Science for Potters, American Ceramics Society, 2017 page 116.
Acknowledgements: My editor, Jim Fineman My professors at Alfred University College of Ceramics, NY.: Val Cushing, Ted Randall, Robert Turner, Daniel Rhodes and W.G. Lawrence Richard Buncamper
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Deciding whether to bisque fire or not is something many of us question. Can the work handle being once fired? Will eliminating the process save money? Learn the pros and cons to help you make an informed decision.
Defining the Terms
Black Coring: Classified as a chemical defect where the interior of the clay body is black or dark gray because organic materials are not being completely removed in an oxidation kiln atmosphere. Once the clay is vitrified, oxygen can no longer penetrate the clay, trapping carbon. Black coring can lead to bloating if the carbon has not been removed from the clay.
Bloating: Classified as a physical defect where the clay body develops bubbles when gases expand and are trapped as the clay becomes vitrified.
Bone Dry: A ceramic object that can still contain mechanical and chemical water but is dry to the touch and brittle.
Carbon Trapping: Carbon is formed from decomposing organic matter in the clay body when there is not enough oxygen present during the firing, resulting in black coring.
Oxidation Atmosphere: In hydrocarbon-fueled kilns, more air than fuel is present during combustion, as opposed to electric kilns where no fuel is ignited.
Reduction: A kiln firing using more fuel than air in the combustion process, resulting in a carbon monoxide atmosphere. The subsequent reaction takes oxygen away from metallic oxides.
Sintering: Occurs in the early stages of firing where the clay particles bond together, partially melting. During this stage, porosity decreases, strength increases, and water absorption decreases.
Thermocouple: A device to measure temperature inside the kiln, which provides information to the kiln controller. After repeated firings, thermocouples will eventually wear out and need to be replaced.
Contemporary Bisque Firing
Bisque firing is the preliminary process of transforming the bone-dry clay into an easier-to-handle, durable, porous ware ready for subsequent glaze firing. It creates the first stages of sintering where the clay platelets start to fuse together. While the temperature is increasing in the kiln, the clay goes through several chemical and physical changes.
The majority of potters fire a bisque kiln between cone 010 (1657°F (903°C)) and cone 04 (1945°F (1063°C)), depending on the particular clay body they are using. The general rule is that the lower the bisque firing temperature, the greater the porosity of the ware. A low-temperature bisque firing is weaker, and the actual water weight in the glaze can crack the ware.
The pyrometric cone parameters and firing cycles used in bisque firing are fairly broad compared to a glaze firing. Most stoneware and low-fire clay bodies can be bisque fired at cone 06, with porcelain clay bodies fired slightly higher to cone 04. The range of glaze absorption depends in part on the temperature reached and the individual clay-body formula.
Kilns used for bisque-firing can be hydrocarbon-fueled kilns (natural gas, propane, oil, or wood) and fired in oxidation, neutral, or reduction atmospheres, the last two of which can cause problems in bisque ware; or in electric kilns, which fire in an oxidation atmosphere and release detrimental carbonaceous materials and sulfur from the clay.
Bisque Firing Process
• Preheating, Mechanical Water Removal to 180°F (82°C): Preliminary heating of the kiln for several hours, depending on the thickness and size of the ware, allows moisture to be drawn off slowly and safely.
• Water Smoking 212–392°F (100–200°C): The early stages of the firing remove free water in the pores between clay crystals through evaporation. This stage is referred to as releasing free or mechanical water in the ware. Steam can be observed exiting from the cone viewing holes in the kiln.
• Removal of Organic Materials 572–1292°F (300–700°C): Removal of organic materials, lignite, and carbonaceous matter in an excess oxidation atmosphere with sufficient time will ensure clean clay for eventual higher temperature firing.
• Chemical Water Removal 842–1112°F (450–600°C): The next stage is known as dehydroxylation, which drives off chemical water in the clay. At this point, the clay cannot be reconstituted into a plastic state. This stage removes the chemical water part in the clay formula: Al203 2Si02 2H20.
• Quartz Inversion 1063°F (573°C): Crystalline quartz inversion occurs in the clay body, with silica quartz expanding in volume suddenly at the same time that shrinkage occurs. A reverse of this process happens upon cooling.
• Sulfur and Organic Materials Released 1292–1652°F (700–900°C): Sulfur, lignite, and other organic materials are released which, depending on the individual clay deposit, can have minor or significant levels of various materials due to the mining process. In many instances, due to inadequate venting in electric kilns, sulfur or organic burning odors can be detected. If not removed during this temperature range, they can result in the clay body black coring or bloating at higher temperatures.
• Beginning of Silica Melt 1652°F (900°C): Silica begins to melt, binding the clay platelets and forming metakaolin, resulting in the clay body shrinking, lowering absorption, and increasing vitrification.
• Formation of Mullite Crystals Above 1922°F (1000°C): Mullite crystals form, contributing to fired clay strength, thermal shock resistance, and reduced warping in the ware. It is often said, above dull visible red heat in the kiln (1652°F (900°C)) anything bad or good has already taken place.
Bisque Firing Temperature Ranges
Currently, bisque firing is widely practiced in most pottery studios. The parameters for successful bisque firing are fairly wide as compared to glaze firing. For example, if a bisque firing is one or two cones over or under in the range between cone 010 (1657°F (903°C)) and cone 04 (1945°F (1063°C)), it will not have a significant effect on the eventual range of glaze absorption. Keep in mind, depending on the clay body, the higher temperature range of bisque firing will decrease glaze absorption while the lower range can “wick” the glaze faster and more thickly onto the pot.
Kiln Fuels and Oxygen
What at first seems to be a fairly undemanding firing process reveals a complex transformation of raw clay. Even at relatively low bisque firing temperatures there are many changes ceramic objects go through to yield a stable state. Bisque firings can be accomplished in electric kilns or hydrocarbon-fueled kilns.
Bisque firings in electric kilns expose the ware to an oxidation atmosphere, which will remove organic carbon, chemically combined water, and sulfur from the clay. The critical factor occurs when sufficient oxygen is present in any type of kiln as it removes carbonaceous or organic materials from the clay.
Advantages of Bisque Firing
• Bisque firing increases handling strength, making moving pots and glazing easier.
• The absorbent surface of the pottery helps the glaze to adhere evenly.
• Glaze mistakes and touch-ups are easier to correct on bisque pottery as opposed to raw glazing where the clay body is structurally weaker.
• When bisque firing pots, the risk of defects decreases as the physical and chemical water is removed along with organic material and sulfur, lowering the chance of cracking or exploding ware or defects.
• Inspection of bisque ware for possible surface defects, cracks or warping, which would result in a fired defect. Running a damp sponge lightly over the ware may reveal small cracks as the moisture seeps in and reveals the defect.
Disadvantages of Bisque Firing
• Bisque firing introduces another step in the pottery production cycle.
• Using the kiln for a bisque firing increases the cost of equipment. Most electric kilns need replacement elements after 250–300 firings, depending on the temperature achieved in each firing. Additionally, thermocouple relays and brick replacement are periodic costs.
• Extra production time is spent stacking, firing, and unloading the bisque kiln.
• Running a production or hobby kiln twice doubles the cost of electricity or hydrocarbon-based fuel.
• Thermal stress during bisque firing can cause cracking, warping, or breakage if pieces are not properly dried or fired.
• Bisque firing increases the production time as the kiln has to be cooled, unloaded, pots glazed, and fired again, all of which increases the cost of production.
Bisque Firing Kiln Settings
The general rule in any bisque firing is, when in doubt, slow the firing down, as this practice will cause fewer problems than a fast firing. The insignificant added price in fuel is not worth any broken ware.
Guidelines for Bisque Firing on Auto SLOW
When placing bone-dry ware in the kiln, keep in mind mechanical and chemical water still has to be driven off during the first part of the firing. Placing wet or damp pots in the kiln requires longer periods below 180°F (82.2°C) to remove mechanical water.
For most functional pottery less than ⅜ inches (1.9 cm) thick, using the computer pre-programmed SLOW setting on the kiln takes approximately 12–14 hours to reach cone 06 (1828°F (998°C)).
For larger or thicker pottery or sculpture with walls over about 1 inch (25 mm) thick, a much slower cone-06 bisque schedule should completely remove mechanical water, chemically combined water, and carbonaceous materials before the clay begins to sinter.
Guidelines for Bisque Firing on the Ramp Setting
This schedule normally takes 18–24 hours, depending on kiln size, the mass of the sculpture, and how tightly the kiln is packed.
Bisque Firing in Hydrocarbon-Fueled Kilns
Inherent in the firing process of offering many and varied choices of fuel/air ratios producing diverse kiln atmospheres is the underlying benefit/fault in hydrocarbon-fueled kilns. Potters choose which damper setting, gas pressure, forced air, natural draft, and kiln firing cycle that will produce a clean bisque firing atmosphere. It is critical at all times to maintain a blue flame at the burners, which indicates an oxidation atmosphere (more air than fuel entering the kiln). Reduction or neutral atmospheres can trap carbon or sulfur in the clay and increase black coring or bloating. Once dull red heat (1000°F (537.8°C)) is achieved, the kiln atmosphere should be clear and not gray.
A pyrometer, which indicates heat increases or decreases during the firing, and pyrometric cones placed throughout a hydrocarbon-fueled kiln are the most accurate measurements of heat work (energy input during the firing normally represented in terms of temperature and time).
For large or thick sculptural work, the entire firing might take 18–22 hours. The key is slow early heating that avoids steam explosions and helps release carbon in the clay.
Bisque Firing Defects to Avoid
Bisque firing produces fewer defects than glaze firing. The obvious reason being there is no glaze to consider. As in any kiln firing procedure, whatever method is used, various failures can occur. Once understood, kiln firing defects can be avoided in the future. There is a direct correlation between thick pieces and defects such as cracks, carbon trap, black coring, and bloating, which are more likely to occur if the bisque firing procedures are not followed. The thicker the piece, the further organic matter and gases have to travel to exit the ware. Some defects, while not apparent in the bisque firing, will develop fully in the glaze firing.
Fast Firing: Fast firing can cause the mechanical and eventually the chemical water in the clay to release explosive steam. Pots can crack, or in some instances blow up, in the first part of the firing. This is most likely to occur near quartz inversion at 1063°F (573°C).
Carbon Trapping: Carbon is trapped in the clay during the bisque firing process. While some pots or sculptures do not indicate a gray-to-black surface discoloration, carbon can still be found in a broken cross section. A higher probability of carbon trapping occurs in thick pieces and densely stacked pieces (such as tiles), as excess oxygen cannot penetrate the clay to volatilize carbon. Carbon trapping can lead to black coring if the organic material is not completely removed from the clay. Trapped carbon can cause pinholes and blisters in glazes due to incomplete carbon-gas release through the glaze layer. Black spots or discoloration in the fired bisque can result in incomplete burnout of carbon, iron compounds, or clay contamination.
Electric kilns being fired in an oxidation atmosphere under the right conditions can efficiently remove carbon from clay, as opposed to hydrocarbon-fueled kilns where neutral or reduction atmospheres can yield carbon trapping.
Black Coring: The interior of the clay can be dark gray or black because of organic matter not being completely removed by an oxidation atmosphere during the first part of the firing. Remaining carbon in the clay creates gas that cannot exit, which in turn produces bubbles in the semi-vitreous clay. Black coring is frequently found in high-iron-bearing clay bodies as the iron is held in a reduced state due to the lack of oxygen. In some instances, interior black coring does not progress to swelling of the clay body if, at higher temperatures, carbon is oxidized out of the clay before vitrification is complete.
Bloating: If organic materials are not fully dissipated, carbon monoxide and sulfur cannot be removed from the clay, and trapped gases can expand. The interior of the clay is dark gray to black, with a deformation bubble on the surface. Bloating is not always noticeable in a bisque firing, but is often encountered at higher than bisque temperatures due to the vitreous sealing quality of the clay body. Not every black-cored pot will bloat, but bloating can occur if the organic material within the clay is not completely removed. In most instances, black coring is an internal defect compromising the clay body, whereas bloating is a visible external defect.
Direct Flame Impingement: Pots or sculptures placed too close to a heat source, whether electric coils or hydrocarbon-fueled burners, can receive direct heat in a localized area. This can produce a crack in the bisque ware due to an unequal thermal gradient where one area of the pot is subjected to more heat than surrounding areas. Additionally, the heat-impinged area will be less absorbent and can deform due to increased vitrification or glass formation.
In electric kilns, pots should be placed no closer than ¼ inch (6.5 mm) to the elements. In hydrocarbon-fueled kilns, the safe distance to prevent direct flame impingement depends in part on the burner type, fuel pressure, and bag wall placement within the kiln.
Low Bisque Firing: Higher absorption rates occur with lower bisque firing temperatures. Glaze will “wick” in faster and apply more thickly due to the overly porous nature of the ware. Low bisque firing can also decrease the strength of the ware, and in some instances the unbalanced glaze application can cause cracking due to the water weight contained in the glaze.
High Bisque Firing: Higher bisque firing temperatures will decrease clay absorbency and increase its shrinkage. Glazes will apply more thinly and take longer to dry. Extremely high bisque firing temperatures can cause warping, and vitrification can trap carbon in the clay, leading to bloating, black coring, or pinholes and blisters in the glaze firing. Black spots and surface discolorations can result from iron or contaminants starting to melt. Extremely high bisque firing temperatures can cause pots to stick to the kiln shelves.
High bisque firing showing discoloration of the clay. Not being present when the kiln is turned off is a low-probability, high-consequence event. While automatic kiln shut-off devices are extremely effective, just one failure can lead to the “Chernobyl” effect where the shelves, posts, pots, and bricks melt.
the author Jeff Zamek started his career 50 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
References:
Acknowledgements:
My editor, Jim Fineman
My professors at Alfred University College of Ceramics, NY.: Val Cushing, Ted Randall, Robert Turner, Daniel Rhodes and W.G. Lawrence
Richard Buncamper
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