It’s frustrating when potters spend many hours in forming and glazing their work only to have a less-than-successful firing. Take a deep dive into some of the possible clay and glaze defects caused when firing a kiln.
Defining the Terms
BTUs: A measurement of heat in terms of energy often stated in burner specifications.
Carbon Coring: Unless carbonaceous material is removed from a clay body, residual deposits can form gases at higher temperatures resulting in the clay body bloating.
Chemical Water: The water bound up with many ceramic materials such as clay, 1 alumina, 2 silica, 2 water: 1Al203∙2Si02∙2H20.
Cristobalite: A crystalline form of silica which has a different molecular structure, after it is heated above 3133°F (1723°C).
Mechanical Water: The water attached to clay particle surfaces, which is driven off after 212°F (100°C).
Quartz Inversion: A phase of silica that goes through a sudden expansion at 1063ÆF (573ÆC), approximately dull red heat in the kiln.
Shivering: As a glaze cools in the kiln it is under extreme compression, often described as coming off of the fired clay body similar to paint chipping.
There are many common defects that occur during firing, but I am sure potters have encountered others. A good place to start troubleshooting is to calculate out of every 100 pots, how many are defective. If the number is higher than 10 there is a problem that has to be solved either in the forming, drying, bisque firing, glaze application, or glaze firing stages. While this is a rough gauge, it allows for an examination of the production cycle and finding out where most of the defects originate.
Temperature Ranges to Consider Before Firing the Kiln1
Bisque Firing
The first stages of drying are accomplished when mechanical water is released from the clay particle surfaces at 212°F (100°C). Water can average 40% of the clay’s moist weight.2 While the pot may feel dry, mechanical and chemical water is still contained within the clay. Pots can crack or explode in the bisque firing due to the rapid release of water turning to steam in the ware. It is not unusual to have nearby pots crack or explode from bisque pieces flying around the kiln (1).
Chemical water is driven off in the 842°–1112°F (450°–600°C) range at approximately visible dull red heat in the kiln. It constitutes approximately 14% of the clay.3 At this stage, glazes and clays have minor shrinkage rates and large increases in porosity. The clay can never be turned back into a plastic mass again.
Quartz Inversion Fast Cooling
The quartz inversion zone (1063°F (573°C)) is where quartz (silica) in the clay body goes from a low to high-temperature form resulting in a sudden expansion. Firing and cooling the kiln during this range requires slow temperature increases and decreases to prevent the pots from cracking (2). Fast cooling can also result in glaze crazing (a fine network of lines in the fired glaze) as it is subjected to extreme tension upon cooling. If when unloading the kiln, you hear “ping” sounds coming from the pots, shut the door and wait.
Carbon Coring
The kiln should be fired in complete oxidation from 573°F to 1291°F (300°C to 700°C). Carbonaceous materials, coal, and other combustible materials must be driven off completely. A reducing atmosphere (more fuel than oxygen in combustion or a neutral atmosphere, equal amounts of fuel and air) can cause the development or trapping of existing carbon in the clay (3). At higher temperatures, if carbon is still present, it forms gases, which can result in black coring and bloating in the clay body.
Under-Firing
If the clay body does not reach its maturing temperature, it can be less durable in use. Immature clay can also cause glaze crazing or leaking, as a glaze can never be considered a sealant. Immature glaze surfaces can be easily abraded in daily use, and glaze colors can appear muted (4). Under-fired ware can be subject to acid attack causing a bleaching effect in glaze colors. Similar faults are present in pottery fired too fast to the correct maturing temperature as the clay and glaze have less time to vitrify.
Over-Firing
Firing the clay body past its maturity range can cause excessive shrinkage, bloating, and warping (5). Due to excessive vitrification (glass buildup) in the clay, which can be very brittle and subject to cracking in use. In over-firing conditions, pottery can fuse to the kiln shelf. Glazes fired past their maturity can run on vertical surfaces or pool in horizontal areas. The glaze surface texture can become overly glossy, and crazing can develop (a fine network of lines in the glaze) due to clay body–glaze non-fit.
Over-Reduction
When more fuel than air is introduced into the glaze firing, carbon can be trapped in the clay body. At higher temperatures it can cause bloating and black coring (6). Over-reduction can yield a brittle clay body subject to breaking in daily use. An over-reduced clay body can cause glaze shivering due to extreme glaze compression upon cooling. Cracking can also occur when pots are used in relatively low-temperature kitchen ovens. Glazes can appear dark gray or black due to their heavily-reduced metallic oxide content.
Under-Reduction
When not enough reduction is introduced during the glaze firing the clay body color can become pale and bleached (7). Glazes can appear lighter in color, and in under-reduction atmospheric conditions, copper reds can be muted or totally absent.
Flame Impingement
Pottery subject to direct flame or placed too close to heating coils in an electric kiln can cause glaze blisters or glaze drips on one side of the pot (8). Satin-matte or matte glazes in the flame impingement areas can become glossy due to over vitrification of the clay or glaze.
Slow Firing: Too Much or Too Little
Significantly, ceramic materials in clay and glazes react to absolute temperature and the time it takes to reach that temperature. Excessively longer firing times to glaze and clay-body maturity cause increased melting in both. Slow firing can result in increased amounts of glass formation in the clay body resulting in excessive shrinkage and warping. Pottery can also adhere to the kiln shelf due to excess glass formation in the clay. The longer time to glaze maturity can also result in satin matte and matte glazes appearing glossy. Gloss glazes can run on vertical surfaces or pool on horizontal surfaces due to a longer firing time.
Slow firing can also increase the amount of cristobalite development in the clay body, which builds up after cone 8 (2280°F (1249°C)). At this temperature range, quartz changes to cristobalite with the help of fluxes other than feldspar. Upon cooling in the 392°F (200°C) range, cristobalite can result in pots cracking. In some instances, oven-related ware can crack from cristobalite inversion in kitchen oven use. While not a significant problem in electric kilns and some hydrocarbon-fueled kilns due to their relatively fast firing cycles, cristobalite inversion is most often encountered in wood-fired kilns that stall out after cone 8 and take an extremely long time to reach end-point temperatures.4
Bisque Firing Recommendations
For most functional pottery forms (cups, bowls, pitchers, covered jars, bottles, etc.) that are dry to the touch when placed in the bisque kiln, a 12-hour total firing time is recommended. For example, a 7–10-cubic foot (0.2–0.2-m3) electric kiln firing on the automatic slow setting should take 9 to 12 hours, depending on how densely packed the kiln is.
Slower bisque firing times are required if the pots are thicker than ½ inch (1.3 cm) or taller than 14 inches (35.6 cm). Longer bisque firing times are essential if plates, tiles, wide-based forms, or thicker forms are fired. A common firing mistake takes place when a potter falls into the habit of firing their functional pottery with a “safe” bisque-firing cycle and then fires large sculptural pieces with the same cycle. Larger and/or thicker pieces need slower temperature increases to safely release their mechanical and chemical water. Fast bisque firing can also trap organic material in the clay even if the clay is fired to the appropriate pyrometric cone. Kiln venting during the 572°F to 1292°F (300°C to 700°C) (approximately visible dull red heat in the kiln) range allows organic matter in the clay to be released in an oxidation atmosphere.5
Glaze Firing Recommendations
There are many ways to fire electric and gas kilns. A good result requires understanding the kiln’s firing characteristics and being able to duplicate every firing with minimal kiln adjustments.
Electric Kilns
Kilns smaller than 7 cubic feet (0.2 m3) require more thoughtful firing cycles due to their decreased thermal mass, which can heat and cool faster than larger kilns.
The slow setting in a 7–10-cubic foot (0.2–0.2-m3) electric kiln should take 10 to 12 hours depending on the kiln load. A fully loaded kiln with pots, posts, and shelves will increase thermal mass, slowing down the firing and cooling cycle. If many glazes look under fired, a longer firing time is required and can be accessed through the computer-controlled program on newer kilns.
On older model kilns, leaving the medium switches on longer will increase the firing time. If only one glaze is under fired, a glaze adjustment might be required. Ceramic materials like longer times to temperatures as opposed to holding at temperature. Holding the kiln at temperature is not recommended (except in special situations such as growing crystals) as it can “boil off” lower-melting oxides in the glaze causing drips or blisters.
Gas Kilns—Cone 9
There are many methods for firing hydrocarbon-based kilns just like there are many ways to have failures due to optional firing cycles. Listed is one of several methods that have yielded good results.
When firing a 20–120-cubic foot (0.6–3.4-m3) kiln the following recommendations apply. A complete oxidation firing (more air than fuel in the burners) from room temperature to cone 06 (1828°F (997°C)) and then a slightly reducing atmosphere, which can be achieved by moving the damper in until there are 3 inches (7.6 cm) of back pressure coming out of the bottom cone viewing port. Additionally, decrease the amount of air from the burners to achieve a yellow flame. A slightly muddy kiln atmosphere should be created until the end of the firing. The correct balance of air, fuel, and damper setting will yield an increase of 75°F (23°C) to 85°F (29°C) per hour from cone 06 (1828°F (997°C)) until the end of the firing. This corresponds to approximately an 8-hour period from cone 06 (1828°F (997°C)) to cone 9 (2300°F (1260°C)). You are on the right track if during the firing, not many adjustments are required to the damper, gas pressure, and air settings.
Gas Kilns—Cone 6
The same firing procedures are enacted when firing to cone 6 (2232ÆF (1222ÆC)); however, the time from cone 06 to cone 6 will be decreased by approximately 3 hours as compared to a cone 9 firing.
the author Jeff Zamek started his career 57 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 Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
1 Lawrence, W.G. Ceramic Science for the Potter. Chilton Book Company, 1972, pages 115, 116. 2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 380. 3 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 380. 4 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 117. 5 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 116.
We understand your email address is private. You will receive emails and newsletters from Ceramic Arts Network. We will never share your information except as outlined in our privacy policy. You can unsubscribe at any time.
You have read of of your complimentary articles for the month.
For unlimited access to Ceramics Monthly premium content, subscribe right now for as low as $4.85/month.
We understand your email address is private. You will receive emails and newsletters from Ceramic Arts Network. We will never share your information except as outlined in our privacy policy. You can unsubscribe at any time.
Subscribe to Ceramics Monthly
It’s frustrating when potters spend many hours in forming and glazing their work only to have a less-than-successful firing. Take a deep dive into some of the possible clay and glaze defects caused when firing a kiln.
Defining the Terms
BTUs: A measurement of heat in terms of energy often stated in burner specifications.
Carbon Coring: Unless carbonaceous material is removed from a clay body, residual deposits can form gases at higher temperatures resulting in the clay body bloating.
Chemical Water: The water bound up with many ceramic materials such as clay, 1 alumina, 2 silica, 2 water: 1Al203∙2Si02∙2H20.
Cristobalite: A crystalline form of silica which has a different molecular structure, after it is heated above 3133°F (1723°C).
Mechanical Water: The water attached to clay particle surfaces, which is driven off after 212°F (100°C).
Quartz Inversion: A phase of silica that goes through a sudden expansion at 1063ÆF (573ÆC), approximately dull red heat in the kiln.
Shivering: As a glaze cools in the kiln it is under extreme compression, often described as coming off of the fired clay body similar to paint chipping.
There are many common defects that occur during firing, but I am sure potters have encountered others. A good place to start troubleshooting is to calculate out of every 100 pots, how many are defective. If the number is higher than 10 there is a problem that has to be solved either in the forming, drying, bisque firing, glaze application, or glaze firing stages. While this is a rough gauge, it allows for an examination of the production cycle and finding out where most of the defects originate.
Temperature Ranges to Consider Before Firing the Kiln1
Bisque Firing
The first stages of drying are accomplished when mechanical water is released from the clay particle surfaces at 212°F (100°C). Water can average 40% of the clay’s moist weight.2 While the pot may feel dry, mechanical and chemical water is still contained within the clay. Pots can crack or explode in the bisque firing due to the rapid release of water turning to steam in the ware. It is not unusual to have nearby pots crack or explode from bisque pieces flying around the kiln (1).
Chemical water is driven off in the 842°–1112°F (450°–600°C) range at approximately visible dull red heat in the kiln. It constitutes approximately 14% of the clay.3 At this stage, glazes and clays have minor shrinkage rates and large increases in porosity. The clay can never be turned back into a plastic mass again.
Quartz Inversion Fast Cooling
The quartz inversion zone (1063°F (573°C)) is where quartz (silica) in the clay body goes from a low to high-temperature form resulting in a sudden expansion. Firing and cooling the kiln during this range requires slow temperature increases and decreases to prevent the pots from cracking (2). Fast cooling can also result in glaze crazing (a fine network of lines in the fired glaze) as it is subjected to extreme tension upon cooling. If when unloading the kiln, you hear “ping” sounds coming from the pots, shut the door and wait.
Carbon Coring
The kiln should be fired in complete oxidation from 573°F to 1291°F (300°C to 700°C). Carbonaceous materials, coal, and other combustible materials must be driven off completely. A reducing atmosphere (more fuel than oxygen in combustion or a neutral atmosphere, equal amounts of fuel and air) can cause the development or trapping of existing carbon in the clay (3). At higher temperatures, if carbon is still present, it forms gases, which can result in black coring and bloating in the clay body.
Under-Firing
If the clay body does not reach its maturing temperature, it can be less durable in use. Immature clay can also cause glaze crazing or leaking, as a glaze can never be considered a sealant. Immature glaze surfaces can be easily abraded in daily use, and glaze colors can appear muted (4). Under-fired ware can be subject to acid attack causing a bleaching effect in glaze colors. Similar faults are present in pottery fired too fast to the correct maturing temperature as the clay and glaze have less time to vitrify.
Over-Firing
Firing the clay body past its maturity range can cause excessive shrinkage, bloating, and warping (5). Due to excessive vitrification (glass buildup) in the clay, which can be very brittle and subject to cracking in use. In over-firing conditions, pottery can fuse to the kiln shelf. Glazes fired past their maturity can run on vertical surfaces or pool in horizontal areas. The glaze surface texture can become overly glossy, and crazing can develop (a fine network of lines in the glaze) due to clay body–glaze non-fit.
Over-Reduction
When more fuel than air is introduced into the glaze firing, carbon can be trapped in the clay body. At higher temperatures it can cause bloating and black coring (6). Over-reduction can yield a brittle clay body subject to breaking in daily use. An over-reduced clay body can cause glaze shivering due to extreme glaze compression upon cooling. Cracking can also occur when pots are used in relatively low-temperature kitchen ovens. Glazes can appear dark gray or black due to their heavily-reduced metallic oxide content.
Under-Reduction
When not enough reduction is introduced during the glaze firing the clay body color can become pale and bleached (7). Glazes can appear lighter in color, and in under-reduction atmospheric conditions, copper reds can be muted or totally absent.
Flame Impingement
Pottery subject to direct flame or placed too close to heating coils in an electric kiln can cause glaze blisters or glaze drips on one side of the pot (8). Satin-matte or matte glazes in the flame impingement areas can become glossy due to over vitrification of the clay or glaze.
Slow Firing: Too Much or Too Little
Significantly, ceramic materials in clay and glazes react to absolute temperature and the time it takes to reach that temperature. Excessively longer firing times to glaze and clay-body maturity cause increased melting in both. Slow firing can result in increased amounts of glass formation in the clay body resulting in excessive shrinkage and warping. Pottery can also adhere to the kiln shelf due to excess glass formation in the clay. The longer time to glaze maturity can also result in satin matte and matte glazes appearing glossy. Gloss glazes can run on vertical surfaces or pool on horizontal surfaces due to a longer firing time.
Slow firing can also increase the amount of cristobalite development in the clay body, which builds up after cone 8 (2280°F (1249°C)). At this temperature range, quartz changes to cristobalite with the help of fluxes other than feldspar. Upon cooling in the 392°F (200°C) range, cristobalite can result in pots cracking. In some instances, oven-related ware can crack from cristobalite inversion in kitchen oven use. While not a significant problem in electric kilns and some hydrocarbon-fueled kilns due to their relatively fast firing cycles, cristobalite inversion is most often encountered in wood-fired kilns that stall out after cone 8 and take an extremely long time to reach end-point temperatures.4
Bisque Firing Recommendations
For most functional pottery forms (cups, bowls, pitchers, covered jars, bottles, etc.) that are dry to the touch when placed in the bisque kiln, a 12-hour total firing time is recommended. For example, a 7–10-cubic foot (0.2–0.2-m3) electric kiln firing on the automatic slow setting should take 9 to 12 hours, depending on how densely packed the kiln is.
Slower bisque firing times are required if the pots are thicker than ½ inch (1.3 cm) or taller than 14 inches (35.6 cm). Longer bisque firing times are essential if plates, tiles, wide-based forms, or thicker forms are fired. A common firing mistake takes place when a potter falls into the habit of firing their functional pottery with a “safe” bisque-firing cycle and then fires large sculptural pieces with the same cycle. Larger and/or thicker pieces need slower temperature increases to safely release their mechanical and chemical water. Fast bisque firing can also trap organic material in the clay even if the clay is fired to the appropriate pyrometric cone. Kiln venting during the 572°F to 1292°F (300°C to 700°C) (approximately visible dull red heat in the kiln) range allows organic matter in the clay to be released in an oxidation atmosphere.5
Glaze Firing Recommendations
There are many ways to fire electric and gas kilns. A good result requires understanding the kiln’s firing characteristics and being able to duplicate every firing with minimal kiln adjustments.
Electric Kilns
Kilns smaller than 7 cubic feet (0.2 m3) require more thoughtful firing cycles due to their decreased thermal mass, which can heat and cool faster than larger kilns.
The slow setting in a 7–10-cubic foot (0.2–0.2-m3) electric kiln should take 10 to 12 hours depending on the kiln load. A fully loaded kiln with pots, posts, and shelves will increase thermal mass, slowing down the firing and cooling cycle. If many glazes look under fired, a longer firing time is required and can be accessed through the computer-controlled program on newer kilns.
On older model kilns, leaving the medium switches on longer will increase the firing time. If only one glaze is under fired, a glaze adjustment might be required. Ceramic materials like longer times to temperatures as opposed to holding at temperature. Holding the kiln at temperature is not recommended (except in special situations such as growing crystals) as it can “boil off” lower-melting oxides in the glaze causing drips or blisters.
Gas Kilns—Cone 9
There are many methods for firing hydrocarbon-based kilns just like there are many ways to have failures due to optional firing cycles. Listed is one of several methods that have yielded good results.
When firing a 20–120-cubic foot (0.6–3.4-m3) kiln the following recommendations apply. A complete oxidation firing (more air than fuel in the burners) from room temperature to cone 06 (1828°F (997°C)) and then a slightly reducing atmosphere, which can be achieved by moving the damper in until there are 3 inches (7.6 cm) of back pressure coming out of the bottom cone viewing port. Additionally, decrease the amount of air from the burners to achieve a yellow flame. A slightly muddy kiln atmosphere should be created until the end of the firing. The correct balance of air, fuel, and damper setting will yield an increase of 75°F (23°C) to 85°F (29°C) per hour from cone 06 (1828°F (997°C)) until the end of the firing. This corresponds to approximately an 8-hour period from cone 06 (1828°F (997°C)) to cone 9 (2300°F (1260°C)). You are on the right track if during the firing, not many adjustments are required to the damper, gas pressure, and air settings.
Gas Kilns—Cone 6
The same firing procedures are enacted when firing to cone 6 (2232ÆF (1222ÆC)); however, the time from cone 06 to cone 6 will be decreased by approximately 3 hours as compared to a cone 9 firing.
the author Jeff Zamek started his career 57 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 Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
1 Lawrence, W.G. Ceramic Science for the Potter. Chilton Book Company, 1972, pages 115, 116.
2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 380.
3 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 380.
4 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 117.
5 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 116.
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
Click the cover image to return to the Table of Contents