Are fireclays different than other clays? This is a common question asked by potters, usually after a bad result with this type of clay. Learn more about the material, including its pros and cons.
Defining the Terms
Air-floated: Clay that is milled and separated by its density and size by a rising stream of air.
Fireclays: A group of clays low in fluxing oxides and high in alumina and silica, able to withstand high temperatures without deforming. When formed into refractory bricks, they are primarily used in glass-melting furnaces and high-temperature industrial applications. Fireclays are not refined or marketed for use by potters.
PCE (Pyrometric Cone Equivalent): Pyrometric cone equivalent is the number assigned to a pyrometric cone that bends when heated to a specific temperature.
Pyroplastic Deformation: Permanent deformation of a ceramic material when subjected to increasingly high temperatures.
Fireclays are one of several groups of clays, such as earthenwares, stonewares, kaolins, ball clays, and bentonites; with coarse-mesh fireclays being the least plastic in forming operations. Furthermore, each group of clays has individual characteristics, specifically fired color, strength, organic content, platelet size, chemical content, and deformation point or PCE (pyrometric cone equivalent), which corresponds to the bending of a pyrometric cone in conjunction with heat work due to time, temperature, and kiln atmosphere.
Within each group there are many individual clays. For example, the ball clay group will contain Tennessee #1, Tennessee #10, Thomas ball clay, Kentucky OM 4, and Mississippi M&D, all having similar characteristics as listed above. Or the fireclay group, which contains Hawthorn Bond, Lincoln, Greenstripe, IMCO 400, and IMCO 800, the last three of which are air-floated. Large particle size can play a significant role in fireclay defects. Smaller particles can be incorporated into the clay body formula successfully, but can change the clay body’s handling characteristics.
When specific groups of clays are combined such as stoneware, ball clays, and kaolins, along with feldspar and silica, the combination can form a clay-body formula. It is important to note that fireclays do not function alone in a clay-body formula. However, statistically, fireclays are the most problematic group for potters as the other materials and clays are refined for a larger market use. For example, kaolins are used in paint, paper, and drug industries where consistency and quality control are required. Potters using kaolins can take advantage of their quality-control standards and use them in clay and glaze formulas.
As the name implies, fireclays are resistant to heat due to their high-alumina (Al203) and low-flux (alkalis/alkaline earths) content. As a group of clays, they can be described as hydrated alumina silicates with trace amounts of other minerals such as iron, sulfur, free silica, calcium, sodium, potassium, titanium, lime, and magnesia.
Fireclays have a PCE of 27 (2961°F (1627°C)) to 32 (3123°F (1717°C)). Since most pottery kilns do not fire above cone 12 (2419°F (1326°C)) this type of clay plays an important part in reducing pyroplastic deformation, which is an irreversible deformation of clay when subjected to higher temperatures, especially (but not exclusively) encountered in stoneware and porcelain clay-body formulas. Fireclays are primarily used in the construction of pottery kilns and furnaces where their high temperature, strength, and durability are critical. There are five standard classifications of fireclay bricks, according to the American Society of Testing Materials (ASTM): (I) super duty, (II) high duty, (III) medium duty, (IV) low duty, and (V) semi silica, corresponding to their temperature ranges.
Origin of Fireclays
The geologic formation of fireclays depends on rocks from the earth’s crust weathering by wind, rain, heat, abrasion, freezing and changes in temperature. These raw materials were transported to acidic waters containing roots and vegetation, resulting in finely divided minerals containing silicates of alumina along with other materials. After the Glacial Period (800 to 600 million years ago), the waters withdrew, leaving sediment and silt ground up by the receding glaciers. Streams moved the remaining fine particle material to other locations, resulting in different types of fireclay, depending on the local geology. Almost all fireclays are formed by their movement along streams, eventually forming beds. Clays of this type contain impurities gathered on their travels, which can contribute to their plastic qualities, fired color, particle size, organic content and refractory nature. Fireclays are deep mined and associated with seams of coal and lignite.
Fireclays, as a group, are one of the most highly contaminated clays used by potters. However, they do contribute low shrinkage and less warping in drying and firing stages. Fireclays are the weakest link in any of the clays that can compose clay body formulas. Ben-eficially, they add “tooth,” or the ability of moist clay in wheel forming operations to stabilize itself vertically. This quality alone can justify fireclays’ use in throwing and handbuilding clay body formulas. In the absence of relatively coarse particle size clays such as fireclays the clay body can exhibit thixotropic properties resulting in a “gummy” quality, deforming under pressure.
Fireclays are very refractory and used in stoneware clay body formulas above cone 6 (2232°F (1222°C)), which contributes to the fired strength in clay body. The relatively large structure of fireclay aids in particle size variation when used with smaller-sized ball clays and medium-sized stoneware clays. Imagine a glass jar filled with tennis balls, golf balls, and smaller marbles. The three sizes leave little empty space. On a mechanical level these various size clays lock in place, surrounded by a water film in the clay body. Clay body formulas with diverse particle size variations are stable in hand building and throwing operations.1
Contaminants Found in Fireclays
While many clay-body formulas might benefit from fireclays, their inconsistent quality can cause periodic defects for potters. Their unpredictable nature is due to the geological formations in which they are mined.1 There are several geologic and economic reasons for their presence and continued existence in the raw material market. There is a downside for potters when they use raw materials specified for industrial requirements. Something that industry would not consider a defect can produce flaws for the potter; thus, it’s allowed in the mine’s production batch and sold to the majority of large commercial customers. The mining and processing of fireclays does not lend itself to consistency when used by potters. A troublesome issue with any clay will occur when industry does not require it for their production. This has happened with fireclays in the past, and history dictates it will happen again. At that point, potters are left with finding a suitable substitute.
Unfortunately, as potters, with fireclays we have to take what we can get. Fireclays are often mined adjoining seams of coal and lignite (combustible sedimentary rock) and can have carbonaceous content, causing bloating (bubbles in the fired clay) and black coring (carbon trapped in the fired clay). Other impurities such as sulfur, free quartz, mica, pyrites, and calcium carbonate, can be present in random batches. One or more of these impurities can spike at any time, eventually resulting in numerous defects in clay bodies and glazes. Regrettably, many of these defects are not noticeable during forming and occur after the ware has been fired.
Economic Factors in Fireclay Production
Fireclays can also include variable particle sizes of silica, iron, and manganese. At the mine site, even with careful excavation, some of these impurities can find their way into the harvested raw clay. On a technical level all of these “bad” qualities can be refined out of the clay at a certain expense. However, on an economic level, intense processing is not required as the majority of fireclay users—steel mills, casting foundries, brick manufacturers—use the clay with minimal processing as it works well in their forming processes and products. Since potters represent less than 0.1% of the raw material market in the US, it does not make economic sense for mines or raw material processors to refine a product for such a small market share. Potters do not dictate any raw material standard to mines or raw materials processors, as we do not represent a viable market. However, other raw materials used in clay bodies and glazes are quality controlled such as feldspars, silica, frits, whiting, dolomite, kaolins, ball clays, and talc, due to larger industry requirements.
Iron, Manganese, Lime, and Copper in Fireclays
The major (but not the only) contaminants found in some types of fireclays are lime and iron. With iron, the size of the particle is critical as potters want small random brown specks (0.5 to 1 mm in diameter) in the fired clay body (2). However, large nodules of iron (4 to 6 mm in diameter) in the fired body can cause outsized brown blemishes on pottery surfaces (3). Fireclays can also contain aggregate lumps of manganese associated with the iron and copper (4), which can cause brown/black concave or convex defects in fired clay surfaces. In functional pottery, such disruptions can trap food or liquid, causing unsanitary conditions. Irregular defects in the clay body surface can occur when manganese becomes an active flux above 1979°F (1080°C) in the kiln atmosphere. This effect is intensified in reduction kiln atmospheres due to the fluxing action caused in the metal and the volatilizing of manganese.
Deposits of lime (calcium hydroxide) randomly found in fireclay can cause clay body defects depending on the particle size. Many potters have experienced a semi-elliptical ⅛–½-inch (0.3–1.3-cm) crack in either their bisque- or glaze-fired ware. Nodules of lime expand as they take on atmospheric moisture, causing pressure on the surrounding clay. The resulting half-moon-shaped crack, with a conical hole and a white speck at the bottom, is called a lime pop, which disrupts the clay body surface. The same lime in powder form does not exert the considerable internal forces of expansion in the clay body when in contact with moisture.
Fireclays are primarily used as a component in mid- to high-temperature clay-body formulas. They are not found in porcelain formulas as their iron and manganese content would negate a white fired color. Fireclays are less frequently used in low-temperature white or red clay-body formulas due to their dark fired color, minimum plasticity, and high-temperature range. In trying to investigate clay-body defects, a general rule comes into play. The first place to look is the fireclay component. Statistically, this is the first material to explore, and once discounted, then move on to other possibilities for the failure.
Mitigation of Fireclay Defects
There are several steps that can moderate fireclays’ inherent defects, but in some instances not eliminate them completely.
Sieving fireclays through a 30-mesh screen will remove the larger contaminant particles. For example, Hawthorne Bond Fireclay 50 mesh, when sent through a 30-mesh sieve will leave approximately 1–3% contaminant on the screen. Nevertheless, if this small percentage found its way into a clay body formula, visible concave or convex defects might occur on the fired clay-body surface. Fireclay defects are most prevalent in reduction-fired ware as the kiln atmosphere over-fluxes or melts iron and manganese in the fireclay component of the clay-body formula. Some ceramics suppliers will use finer-mesh fireclays in clay-body formulas to the detriment of the clay’s handling properties. Often the clay is gelatinous and deforms easily in wheel and handbuilding operations.
Sheffield Pottery Supply in Sheffield, Massachusetts, has been offering clay screening services as part of its quality control program. Their raw materials refining system is unique as it removes damaging particles from fireclays and other types of clay. Additionally, their use of an iron filtration magnet removes all or some of the iron contaminants found in fireclays (5). While screening will be an extra expense, a bad load of clay will significantly cost more in terms of lost ware and time to produce the pottery. Often smaller, powder-sized fireclay contaminants are incorporated into the fusion of the clay without resulting in a defect. As with other contaminants, size does matter as larger particles do not blend into the clay mix and can cause concern.
Some fireclays, such as Green Stripe, IMCO 400, and IMCO 800, are air-floated, greatly reducing their particle size and the possibility of defects. However, as with any fine-particle clay, it can decrease the workability and handling qualities of the clay which would require the use of coarser-particle fireclays in the clay body formula. And here we have the circular problem with fireclays: their larger particle size aids in workably but contributes to defects in the fired ware.
If you are mixing your own high-temperature formulas, start with 25% or less fireclay, with other materials contributing to a well-balanced clay body. As with any clay-body formula, special considerations might apply depending on the individual potter’s requirements.
Again, why use fireclay at all? The right mesh size and correct percentage in a clay body formula will contribute to the handling and high-temperature firing characteristics while reducing warping in drying and firing. Some of the most stable clay-body formulas are composed in part of small (ball clay), medium (stoneware clay), and large (fireclay) platelet-sized clays which mechanically lock into each other when moist.
1 Zamek, Jeff. What Every Potter Should Know. Iola, WI Krause Publication, 1999. Page 76.
Acknowledgments John Cowen, president of Sheffield Pottery, and Patsy Cowen, director of purchasing, supplied information on the raw-material screening system he developed for use with various clays.
the author Jeff Zamek started his career 48 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.
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Are fireclays different than other clays? This is a common question asked by potters, usually after a bad result with this type of clay. Learn more about the material, including its pros and cons.
Defining the Terms
Air-floated: Clay that is milled and separated by its density and size by a rising stream of air.
Fireclays: A group of clays low in fluxing oxides and high in alumina and silica, able to withstand high temperatures without deforming. When formed into refractory bricks, they are primarily used in glass-melting furnaces and high-temperature industrial applications. Fireclays are not refined or marketed for use by potters.
PCE (Pyrometric Cone Equivalent): Pyrometric cone equivalent is the number assigned to a pyrometric cone that bends when heated to a specific temperature.
Pyroplastic Deformation: Permanent deformation of a ceramic material when subjected to increasingly high temperatures.
Fireclays are one of several groups of clays, such as earthenwares, stonewares, kaolins, ball clays, and bentonites; with coarse-mesh fireclays being the least plastic in forming operations. Furthermore, each group of clays has individual characteristics, specifically fired color, strength, organic content, platelet size, chemical content, and deformation point or PCE (pyrometric cone equivalent), which corresponds to the bending of a pyrometric cone in conjunction with heat work due to time, temperature, and kiln atmosphere.
Within each group there are many individual clays. For example, the ball clay group will contain Tennessee #1, Tennessee #10, Thomas ball clay, Kentucky OM 4, and Mississippi M&D, all having similar characteristics as listed above. Or the fireclay group, which contains Hawthorn Bond, Lincoln, Greenstripe, IMCO 400, and IMCO 800, the last three of which are air-floated. Large particle size can play a significant role in fireclay defects. Smaller particles can be incorporated into the clay body formula successfully, but can change the clay body’s handling characteristics.
When specific groups of clays are combined such as stoneware, ball clays, and kaolins, along with feldspar and silica, the combination can form a clay-body formula. It is important to note that fireclays do not function alone in a clay-body formula. However, statistically, fireclays are the most problematic group for potters as the other materials and clays are refined for a larger market use. For example, kaolins are used in paint, paper, and drug industries where consistency and quality control are required. Potters using kaolins can take advantage of their quality-control standards and use them in clay and glaze formulas.
As the name implies, fireclays are resistant to heat due to their high-alumina (Al203) and low-flux (alkalis/alkaline earths) content. As a group of clays, they can be described as hydrated alumina silicates with trace amounts of other minerals such as iron, sulfur, free silica, calcium, sodium, potassium, titanium, lime, and magnesia.
Fireclays have a PCE of 27 (2961°F (1627°C)) to 32 (3123°F (1717°C)). Since most pottery kilns do not fire above cone 12 (2419°F (1326°C)) this type of clay plays an important part in reducing pyroplastic deformation, which is an irreversible deformation of clay when subjected to higher temperatures, especially (but not exclusively) encountered in stoneware and porcelain clay-body formulas. Fireclays are primarily used in the construction of pottery kilns and furnaces where their high temperature, strength, and durability are critical. There are five standard classifications of fireclay bricks, according to the American Society of Testing Materials (ASTM): (I) super duty, (II) high duty, (III) medium duty, (IV) low duty, and (V) semi silica, corresponding to their temperature ranges.
Origin of Fireclays
The geologic formation of fireclays depends on rocks from the earth’s crust weathering by wind, rain, heat, abrasion, freezing and changes in temperature. These raw materials were transported to acidic waters containing roots and vegetation, resulting in finely divided minerals containing silicates of alumina along with other materials. After the Glacial Period (800 to 600 million years ago), the waters withdrew, leaving sediment and silt ground up by the receding glaciers. Streams moved the remaining fine particle material to other locations, resulting in different types of fireclay, depending on the local geology. Almost all fireclays are formed by their movement along streams, eventually forming beds. Clays of this type contain impurities gathered on their travels, which can contribute to their plastic qualities, fired color, particle size, organic content and refractory nature. Fireclays are deep mined and associated with seams of coal and lignite.
Fireclays, as a group, are one of the most highly contaminated clays used by potters. However, they do contribute low shrinkage and less warping in drying and firing stages. Fireclays are the weakest link in any of the clays that can compose clay body formulas. Ben-eficially, they add “tooth,” or the ability of moist clay in wheel forming operations to stabilize itself vertically. This quality alone can justify fireclays’ use in throwing and handbuilding clay body formulas. In the absence of relatively coarse particle size clays such as fireclays the clay body can exhibit thixotropic properties resulting in a “gummy” quality, deforming under pressure.
Fireclays are very refractory and used in stoneware clay body formulas above cone 6 (2232°F (1222°C)), which contributes to the fired strength in clay body. The relatively large structure of fireclay aids in particle size variation when used with smaller-sized ball clays and medium-sized stoneware clays. Imagine a glass jar filled with tennis balls, golf balls, and smaller marbles. The three sizes leave little empty space. On a mechanical level these various size clays lock in place, surrounded by a water film in the clay body. Clay body formulas with diverse particle size variations are stable in hand building and throwing operations.1
Contaminants Found in Fireclays
While many clay-body formulas might benefit from fireclays, their inconsistent quality can cause periodic defects for potters. Their unpredictable nature is due to the geological formations in which they are mined.1 There are several geologic and economic reasons for their presence and continued existence in the raw material market. There is a downside for potters when they use raw materials specified for industrial requirements. Something that industry would not consider a defect can produce flaws for the potter; thus, it’s allowed in the mine’s production batch and sold to the majority of large commercial customers. The mining and processing of fireclays does not lend itself to consistency when used by potters. A troublesome issue with any clay will occur when industry does not require it for their production. This has happened with fireclays in the past, and history dictates it will happen again. At that point, potters are left with finding a suitable substitute.
Unfortunately, as potters, with fireclays we have to take what we can get. Fireclays are often mined adjoining seams of coal and lignite (combustible sedimentary rock) and can have carbonaceous content, causing bloating (bubbles in the fired clay) and black coring (carbon trapped in the fired clay). Other impurities such as sulfur, free quartz, mica, pyrites, and calcium carbonate, can be present in random batches. One or more of these impurities can spike at any time, eventually resulting in numerous defects in clay bodies and glazes. Regrettably, many of these defects are not noticeable during forming and occur after the ware has been fired.
Economic Factors in Fireclay Production
Fireclays can also include variable particle sizes of silica, iron, and manganese. At the mine site, even with careful excavation, some of these impurities can find their way into the harvested raw clay. On a technical level all of these “bad” qualities can be refined out of the clay at a certain expense. However, on an economic level, intense processing is not required as the majority of fireclay users—steel mills, casting foundries, brick manufacturers—use the clay with minimal processing as it works well in their forming processes and products. Since potters represent less than 0.1% of the raw material market in the US, it does not make economic sense for mines or raw material processors to refine a product for such a small market share. Potters do not dictate any raw material standard to mines or raw materials processors, as we do not represent a viable market. However, other raw materials used in clay bodies and glazes are quality controlled such as feldspars, silica, frits, whiting, dolomite, kaolins, ball clays, and talc, due to larger industry requirements.
Iron, Manganese, Lime, and Copper in Fireclays
The major (but not the only) contaminants found in some types of fireclays are lime and iron. With iron, the size of the particle is critical as potters want small random brown specks (0.5 to 1 mm in diameter) in the fired clay body (2). However, large nodules of iron (4 to 6 mm in diameter) in the fired body can cause outsized brown blemishes on pottery surfaces (3). Fireclays can also contain aggregate lumps of manganese associated with the iron and copper (4), which can cause brown/black concave or convex defects in fired clay surfaces. In functional pottery, such disruptions can trap food or liquid, causing unsanitary conditions. Irregular defects in the clay body surface can occur when manganese becomes an active flux above 1979°F (1080°C) in the kiln atmosphere. This effect is intensified in reduction kiln atmospheres due to the fluxing action caused in the metal and the volatilizing of manganese.
Deposits of lime (calcium hydroxide) randomly found in fireclay can cause clay body defects depending on the particle size. Many potters have experienced a semi-elliptical ⅛–½-inch (0.3–1.3-cm) crack in either their bisque- or glaze-fired ware. Nodules of lime expand as they take on atmospheric moisture, causing pressure on the surrounding clay. The resulting half-moon-shaped crack, with a conical hole and a white speck at the bottom, is called a lime pop, which disrupts the clay body surface. The same lime in powder form does not exert the considerable internal forces of expansion in the clay body when in contact with moisture.
Fireclays are primarily used as a component in mid- to high-temperature clay-body formulas. They are not found in porcelain formulas as their iron and manganese content would negate a white fired color. Fireclays are less frequently used in low-temperature white or red clay-body formulas due to their dark fired color, minimum plasticity, and high-temperature range. In trying to investigate clay-body defects, a general rule comes into play. The first place to look is the fireclay component. Statistically, this is the first material to explore, and once discounted, then move on to other possibilities for the failure.
Mitigation of Fireclay Defects
There are several steps that can moderate fireclays’ inherent defects, but in some instances not eliminate them completely.
Sieving fireclays through a 30-mesh screen will remove the larger contaminant particles. For example, Hawthorne Bond Fireclay 50 mesh, when sent through a 30-mesh sieve will leave approximately 1–3% contaminant on the screen. Nevertheless, if this small percentage found its way into a clay body formula, visible concave or convex defects might occur on the fired clay-body surface. Fireclay defects are most prevalent in reduction-fired ware as the kiln atmosphere over-fluxes or melts iron and manganese in the fireclay component of the clay-body formula. Some ceramics suppliers will use finer-mesh fireclays in clay-body formulas to the detriment of the clay’s handling properties. Often the clay is gelatinous and deforms easily in wheel and handbuilding operations.
Sheffield Pottery Supply in Sheffield, Massachusetts, has been offering clay screening services as part of its quality control program. Their raw materials refining system is unique as it removes damaging particles from fireclays and other types of clay. Additionally, their use of an iron filtration magnet removes all or some of the iron contaminants found in fireclays (5). While screening will be an extra expense, a bad load of clay will significantly cost more in terms of lost ware and time to produce the pottery. Often smaller, powder-sized fireclay contaminants are incorporated into the fusion of the clay without resulting in a defect. As with other contaminants, size does matter as larger particles do not blend into the clay mix and can cause concern.
Some fireclays, such as Green Stripe, IMCO 400, and IMCO 800, are air-floated, greatly reducing their particle size and the possibility of defects. However, as with any fine-particle clay, it can decrease the workability and handling qualities of the clay which would require the use of coarser-particle fireclays in the clay body formula. And here we have the circular problem with fireclays: their larger particle size aids in workably but contributes to defects in the fired ware.
If you are mixing your own high-temperature formulas, start with 25% or less fireclay, with other materials contributing to a well-balanced clay body. As with any clay-body formula, special considerations might apply depending on the individual potter’s requirements.
Again, why use fireclay at all? The right mesh size and correct percentage in a clay body formula will contribute to the handling and high-temperature firing characteristics while reducing warping in drying and firing. Some of the most stable clay-body formulas are composed in part of small (ball clay), medium (stoneware clay), and large (fireclay) platelet-sized clays which mechanically lock into each other when moist.
1 Zamek, Jeff. What Every Potter Should Know. Iola, WI Krause Publication, 1999. Page 76.
Acknowledgments John Cowen, president of Sheffield Pottery, and Patsy Cowen, director of purchasing, supplied information on the raw-material screening system he developed for use with various clays.
the author Jeff Zamek started his career 48 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.
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