Two common firing issues occur when a glaze is under tension while cooling, revealing a series of lines in the fired glaze surface (crazing). The opposite fit defect occurs when the glaze is under extreme compression. This is a glaze-clay body non-fit issue producing chips of glaze falling off the fired pot (shivering).
After firing, the most stable glazes are under slight compression as opposed to tension loading or extreme compression. As a general rule, the ceramic materials used in glazes are weaker in tension resulting in more crazing defects as opposed to the less encountered shivering where the glaze is under excessive compression loads.
However, less attention is being paid to the area and processes taking place between the clay body and the glaze layer during a kiln firing. Such zones are not readily visible until a glazed pot is broken, revealing an intermediate layer between the clay body and glaze.
During a firing, a thin layer of glaze forms over the clay body causing it to melt during the firing. This layer is both aesthetic and functional, providing a smooth surface for future cleaning. The glaze can never be considered a sealant due to imperfections such as blistering, pinholes, immaturity, crawling, or an array of defects that allow moisture to penetrate beneath its surface. On a technical level, the molten glaze must attack and bond chemically with the underlying clay body resulting in a fusion of both the clay body and glaze. The area between the clay body and glaze is called the interface or interfacial region. This zone has a structure and chemistry of neither the glaze nor the clay body. Every glazed ceramic object has this hidden zone. Simply stated, the interface area resides between where the clay body ends and the covering glaze begins. During the firing when the glaze has vitrified it combines and dissolves the underlying clay body to some extent.1 The exact composition of the forming interface depends in part on the clay body formula, whether it has been once fired or bisque fired, the glaze formula, the kiln firing cycle, the kiln atmosphere, and the end-point temperature reached.
Interestingly, another ceramic composition is forming in the interface region, which has elements of both clay and glaze. However, this area is never a sharp dividing line as the clay body and glaze bond to each other in varying degrees. This reaction depends on how the fluxes interact with silicates present in both the clay and glaze.2 In high-temperature clay bodies the interface shows greater development due to the increased fluxing action of both the clay and glaze. How much bonding takes place depends on several factors such as the clay body porosity or its non-porous nature, the degree of glass (vitrification) formation in the glaze, the composition of the glaze, and the thickness of the glaze layer.3 One or several of these combined factors can influence the extent of interface development. As a result, there is a diffusion of elements in both the clay body and glaze during interface development.4
High-Fire Clay-Glaze Interface
Many of us do not consider this potential bonding area when choosing a clay body and glaze combination as it is unseen once the pot or sculpture is fired. In most situations, this lack of knowledge does not lead to a clay body or glaze fault. However, when breaking open a high-fired stoneware glazed pot the interface is well developed and almost seamless. In stoneware-range firings above cone 6 (2232°F (1222°C)), most clay body and glaze combinations successfully bond or fuse together in the interface area. The fired glaze looks and feels as if it is seamlessly integrated into the clay body (1).
The development of the interface or lack of it in part determines the fired strength of the pottery and its potential for a successful glaze fit. In high-temperature pottery there is a greater integration of the clay body and glaze resulting in wider tolerances of thermal expansion with less chance of several types of glaze defects. The most frequent of which are crazing or shivering. Higher temperatures produce increased strength in clay body/glaze bonding, resulting in the potential for better glaze fitting.
At high temperatures above cone 6, the dissolving action in the glaze can flux and melt iron or manganese particles in the clay body resulting in large visible brown (iron) or black (manganese) spots in the fired glaze. Often these spots have an irregular or bleeding edge due to the fluxing of the metallic particle contained in the clay body (2). In reduction kiln atmospheres, iron and manganese specking is more noticeable due to the fluxing action in the kiln atmosphere on any metallic oxides.
Conversely, at low temperatures and/or within oxidation kiln atmospheres there is less interface fusion and less vitrification of the clay body and glaze. Either one or both conditions result in iron and manganese particles in the clay body fluxing less and not as noticeably in the fired ware.
The porosity or non-porosity of the interface layer can alter the vitreous qualities of a covering glaze.5 When the glaze is vitrified it can leach and dissolve the surface areas of the underlying clay body. Many different clay bodies work with countless diverse glazes; however, when they do not, it is important to recognize and correct the interface defect. Some clay bodies will draw part of the flux content from the covering glaze during the firing process. This condition can occur when the clay body is not fully vitreous, wicking or defusing glaze oxides causing opacity in light transmission or dry surface textures in the covering glaze. Often using a different clay body with the glaze will solve this problem. Here, in part, we are relying on statistics as another clay body has a higher probability of being suited to the covering glaze. A longer or higher firing cycle could be considered, which would cause greater vitrification in the clay body but might also result in over-fired glazes.
The opposite condition is where the clay body is vitrified to such a high degree (absorption percentages of zero or slightly above) and translucency is present in the clay body. This condition is mostly likely—but not exclusively—to occur in porcelain clay body formulas. Due to the high amount of vitrification (glass formation) in the clay body and glaze it is difficult to tell where the interface begins or ends as both structures are seamlessly almost identical. Often, the fired clay body has a glossy surface quality, which is a result of the build-up of glass formation (3). In this state predominately gloss, fluid glazes can slide off the underlying clay body surface during the firing, exposing random areas of clay. It has been described as a sliding glass glaze on a highly vitrified clay body.
Changing the clay body to one with a lower vitrification point or firing to a lower temperature will allow the fired glaze to integrate and adhere to the clay body.
Low-Fire Clay-Glaze Interface
In low-temperature earthenware pottery, in the region of cone 010 (1657°F (903°C)) to cone 04 (1945°F (1063°C)), there can be a distinct boundary layer of glaze riding on top of the clay body. It almost looks like the glaze was painted on the clay body surface, which indicates a less well-integrated interface (4). The underdeveloped interface zone can withstand less thermal expansion resulting in potential glaze crazing and shivering defects.
The interface is not as well developed due to the high absorption rate of the clay and minimal chemical interaction between the covering glaze layers. Low-fire clay body formulas that have high absorption rates can be adjusted with the addition of a frit. However, a frit will considerably shorten the maturing range of the clay, resulting in over-fluxing or under-fluxing of the clay body formula, due to temperature variation within the same kiln firing. Firing the clay to higher temperatures and/or with longer durations can reduce its absorption rate. Additionally, in instances where a clay body does not receive a high degree of vitrification it has less fired strength and is subject to chemical or organic attack. High absorption rates in clay body formulas can, under some conditions, grow mold or stain with use.
Underglazes, Slips, and Engobes
The use of any underglaze, slip, or engobe introduces another variable in terms of glaze and clay body fit. Any intermediate area between the clay body or covering glaze can be considered another interface whether well developed or not. The underglaze, slip, or engobe has a double task as it has to fit the clay body and the covering glaze.
When applying underglazes, slips, or engobes to the clay body surface there is always the potential for glaze defects as they must fit the clay body and glaze during the drying, bisque firing, and glaze firing stages. In some instances, if the underglaze does not sufficiently bond with the underlying clay body the covering glaze will not adhere to the clay body and can flake off after the glaze firing. Interestingly, at maturing temperatures everything fits as the glaze has the viscosity of honey, bonding to all surfaces. However, during the cooling process, different shrinkage rates in the clay body, engobe, underglaze, slip, or glaze can occur resulting in failures. Faced with such situations it is often difficult to tell what part of the system is not shrinking at a compatible rate.
Underglazes, slips, and engobes to varying degrees can be considered colored clays and do not develop high levels of vitrification as opposed to glazes. As in low-fire clay body/glaze interfaces any of these intermediate interface layers can cause the same types of glaze defects such as crazing, pinholing, blistering, or shivering. For example, it is possible to have a stable clay body and glaze firing result and the same combination with the use of an engobe, slip, or underglaze might cause glaze fit issues.
While we might not always be aware of the interface area on glazed pottery, it is always present. Many aspects of working with clay, glazes, and kiln firings are initially hidden, but when revealed and understood corrections can take place. Ceramics is made up of many little bits of information. Once they are assembled whole areas of knowledge fall into place. As always, test a glaze, clay body, or underglaze combination before committing to large production runs. Additionally, test clay and glazes in the same production kiln to ensure accurate results.
1 J.R. Taylor and A.C. Bull. Ceramics Glaze Technology, Pergamon Press 1986 page 86.
2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004 page 32.
3 Cullen W. Parmelee. Ceramic Glazes, Third Edition, Cahners Books, Boston MA., page 216.
4 Richard A. Eppler/Mimi Obstler. Understanding Glazes, The American Ceramics Society, 2005 page 214.
5 J.R. Taylor and A.C. Bull. Ceramics Glaze Technology, Pergamon Press 1986 page 90.
Acknowledgment: Tony Hansen’s digitalfire.com, is a source of useful technical information for anyone making pottery.
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.
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The Hidden Zone
Two common firing issues occur when a glaze is under tension while cooling, revealing a series of lines in the fired glaze surface (crazing). The opposite fit defect occurs when the glaze is under extreme compression. This is a glaze-clay body non-fit issue producing chips of glaze falling off the fired pot (shivering).
After firing, the most stable glazes are under slight compression as opposed to tension loading or extreme compression. As a general rule, the ceramic materials used in glazes are weaker in tension resulting in more crazing defects as opposed to the less encountered shivering where the glaze is under excessive compression loads.
However, less attention is being paid to the area and processes taking place between the clay body and the glaze layer during a kiln firing. Such zones are not readily visible until a glazed pot is broken, revealing an intermediate layer between the clay body and glaze.
During a firing, a thin layer of glaze forms over the clay body causing it to melt during the firing. This layer is both aesthetic and functional, providing a smooth surface for future cleaning. The glaze can never be considered a sealant due to imperfections such as blistering, pinholes, immaturity, crawling, or an array of defects that allow moisture to penetrate beneath its surface. On a technical level, the molten glaze must attack and bond chemically with the underlying clay body resulting in a fusion of both the clay body and glaze. The area between the clay body and glaze is called the interface or interfacial region. This zone has a structure and chemistry of neither the glaze nor the clay body. Every glazed ceramic object has this hidden zone. Simply stated, the interface area resides between where the clay body ends and the covering glaze begins. During the firing when the glaze has vitrified it combines and dissolves the underlying clay body to some extent.1 The exact composition of the forming interface depends in part on the clay body formula, whether it has been once fired or bisque fired, the glaze formula, the kiln firing cycle, the kiln atmosphere, and the end-point temperature reached.
Interestingly, another ceramic composition is forming in the interface region, which has elements of both clay and glaze. However, this area is never a sharp dividing line as the clay body and glaze bond to each other in varying degrees. This reaction depends on how the fluxes interact with silicates present in both the clay and glaze.2 In high-temperature clay bodies the interface shows greater development due to the increased fluxing action of both the clay and glaze. How much bonding takes place depends on several factors such as the clay body porosity or its non-porous nature, the degree of glass (vitrification) formation in the glaze, the composition of the glaze, and the thickness of the glaze layer.3 One or several of these combined factors can influence the extent of interface development. As a result, there is a diffusion of elements in both the clay body and glaze during interface development.4
High-Fire Clay-Glaze Interface
Many of us do not consider this potential bonding area when choosing a clay body and glaze combination as it is unseen once the pot or sculpture is fired. In most situations, this lack of knowledge does not lead to a clay body or glaze fault. However, when breaking open a high-fired stoneware glazed pot the interface is well developed and almost seamless. In stoneware-range firings above cone 6 (2232°F (1222°C)), most clay body and glaze combinations successfully bond or fuse together in the interface area. The fired glaze looks and feels as if it is seamlessly integrated into the clay body (1).
The development of the interface or lack of it in part determines the fired strength of the pottery and its potential for a successful glaze fit. In high-temperature pottery there is a greater integration of the clay body and glaze resulting in wider tolerances of thermal expansion with less chance of several types of glaze defects. The most frequent of which are crazing or shivering. Higher temperatures produce increased strength in clay body/glaze bonding, resulting in the potential for better glaze fitting.
At high temperatures above cone 6, the dissolving action in the glaze can flux and melt iron or manganese particles in the clay body resulting in large visible brown (iron) or black (manganese) spots in the fired glaze. Often these spots have an irregular or bleeding edge due to the fluxing of the metallic particle contained in the clay body (2). In reduction kiln atmospheres, iron and manganese specking is more noticeable due to the fluxing action in the kiln atmosphere on any metallic oxides.
Conversely, at low temperatures and/or within oxidation kiln atmospheres there is less interface fusion and less vitrification of the clay body and glaze. Either one or both conditions result in iron and manganese particles in the clay body fluxing less and not as noticeably in the fired ware.
The porosity or non-porosity of the interface layer can alter the vitreous qualities of a covering glaze.5 When the glaze is vitrified it can leach and dissolve the surface areas of the underlying clay body. Many different clay bodies work with countless diverse glazes; however, when they do not, it is important to recognize and correct the interface defect. Some clay bodies will draw part of the flux content from the covering glaze during the firing process. This condition can occur when the clay body is not fully vitreous, wicking or defusing glaze oxides causing opacity in light transmission or dry surface textures in the covering glaze. Often using a different clay body with the glaze will solve this problem. Here, in part, we are relying on statistics as another clay body has a higher probability of being suited to the covering glaze. A longer or higher firing cycle could be considered, which would cause greater vitrification in the clay body but might also result in over-fired glazes.
The opposite condition is where the clay body is vitrified to such a high degree (absorption percentages of zero or slightly above) and translucency is present in the clay body. This condition is mostly likely—but not exclusively—to occur in porcelain clay body formulas. Due to the high amount of vitrification (glass formation) in the clay body and glaze it is difficult to tell where the interface begins or ends as both structures are seamlessly almost identical. Often, the fired clay body has a glossy surface quality, which is a result of the build-up of glass formation (3). In this state predominately gloss, fluid glazes can slide off the underlying clay body surface during the firing, exposing random areas of clay. It has been described as a sliding glass glaze on a highly vitrified clay body.
Changing the clay body to one with a lower vitrification point or firing to a lower temperature will allow the fired glaze to integrate and adhere to the clay body.
Low-Fire Clay-Glaze Interface
In low-temperature earthenware pottery, in the region of cone 010 (1657°F (903°C)) to cone 04 (1945°F (1063°C)), there can be a distinct boundary layer of glaze riding on top of the clay body. It almost looks like the glaze was painted on the clay body surface, which indicates a less well-integrated interface (4). The underdeveloped interface zone can withstand less thermal expansion resulting in potential glaze crazing and shivering defects.
The interface is not as well developed due to the high absorption rate of the clay and minimal chemical interaction between the covering glaze layers. Low-fire clay body formulas that have high absorption rates can be adjusted with the addition of a frit. However, a frit will considerably shorten the maturing range of the clay, resulting in over-fluxing or under-fluxing of the clay body formula, due to temperature variation within the same kiln firing. Firing the clay to higher temperatures and/or with longer durations can reduce its absorption rate. Additionally, in instances where a clay body does not receive a high degree of vitrification it has less fired strength and is subject to chemical or organic attack. High absorption rates in clay body formulas can, under some conditions, grow mold or stain with use.
Underglazes, Slips, and Engobes
The use of any underglaze, slip, or engobe introduces another variable in terms of glaze and clay body fit. Any intermediate area between the clay body or covering glaze can be considered another interface whether well developed or not. The underglaze, slip, or engobe has a double task as it has to fit the clay body and the covering glaze.
When applying underglazes, slips, or engobes to the clay body surface there is always the potential for glaze defects as they must fit the clay body and glaze during the drying, bisque firing, and glaze firing stages. In some instances, if the underglaze does not sufficiently bond with the underlying clay body the covering glaze will not adhere to the clay body and can flake off after the glaze firing. Interestingly, at maturing temperatures everything fits as the glaze has the viscosity of honey, bonding to all surfaces. However, during the cooling process, different shrinkage rates in the clay body, engobe, underglaze, slip, or glaze can occur resulting in failures. Faced with such situations it is often difficult to tell what part of the system is not shrinking at a compatible rate.
Underglazes, slips, and engobes to varying degrees can be considered colored clays and do not develop high levels of vitrification as opposed to glazes. As in low-fire clay body/glaze interfaces any of these intermediate interface layers can cause the same types of glaze defects such as crazing, pinholing, blistering, or shivering. For example, it is possible to have a stable clay body and glaze firing result and the same combination with the use of an engobe, slip, or underglaze might cause glaze fit issues.
While we might not always be aware of the interface area on glazed pottery, it is always present. Many aspects of working with clay, glazes, and kiln firings are initially hidden, but when revealed and understood corrections can take place. Ceramics is made up of many little bits of information. Once they are assembled whole areas of knowledge fall into place. As always, test a glaze, clay body, or underglaze combination before committing to large production runs. Additionally, test clay and glazes in the same production kiln to ensure accurate results.
1 J.R. Taylor and A.C. Bull. Ceramics Glaze Technology, Pergamon Press 1986 page 86.
2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004 page 32.
3 Cullen W. Parmelee. Ceramic Glazes, Third Edition, Cahners Books, Boston MA., page 216.
4 Richard A. Eppler/Mimi Obstler. Understanding Glazes, The American Ceramics Society, 2005 page 214.
5 J.R. Taylor and A.C. Bull. Ceramics Glaze Technology, Pergamon Press 1986 page 90.
Acknowledgment: Tony Hansen’s digitalfire.com, is a source of useful technical information for anyone making pottery.
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.
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