When we form clay, on a fundamental level, we are transmuting thought into matter. It is the closest thing to magic, where you have nothing, and then something appears. Having a reliable clay body formula allows for making objects, using your head, heart, and hands, as the English potter Bernard Leach once proclaimed.
Defining the Terms
Air-Floated: A rising stream of air of controlled velocity separates solid clay particles by density and size.
Calcined Clay: A clay heated until its chemical water is removed and it loses its plastic properties.
Clay Body: A combination of clay(s), fluxes, and fillers to achieve a specific firing temperature, fired color, forming method, and function in oxidation or reduction kiln atmospheres.
Clay Platelet: Hexagonal plates with an approximate ratio of thickness to length of 10 to 1. Plates can range in size from 100u to 0.1u particle diameter.7
Coefficient of Expansion: The change in the length, area, or volume of the clay body based on temperature.
End Point: The highest temperature reached to achieve a vitreous, mature clay body and glaze.
Eutectic: A combination of two or more materials that lowers the melting point of each.
Oxidization Atmosphere: In the kiln-firing process, a greater ratio of oxygen than fuel is present throughout the firing cycle.
Pyroplastic Deformation: A clay body permanently bending or warping subject to a high-temperature firing.
Reduction Atmosphere: An atmosphere produced with a higher fuel-to-air ratio, creating carbon monoxide, which draws oxygen away from metallic oxides.
Vitreous or Vitrification: A heated clay body develops a glassy phase in its composition without deformation and is characterized by low porosity.
The fundamental element when making pottery is clay, a group of mineral aggregates consisting of hydrous aluminum silicates of layered plates becoming plastic when wet, rigid when dry, and vitrified when heated. Long before any rudimentary glaze was developed, there was always clay, formed into functional and symbolic objects.
A clay body is composed of one or more materials: clays for plasticity, fillers such as grog and silica to facilitate thermal coefficient of expansion,1 and silica to reduce warping and facilitate glaze fit. The last material is a flux, which brings together the combination of clay, silica, and fillers into a glass-like state. Ideally, all four elements act in coordination to produce a durable, plastic clay body at a given temperature range and firing atmosphere.
The Choice of a Cone-6 Clay Body
While porcelain formulas contain kaolin, feldspar, and silica, and earthenware clay body formulas can have as few as one or two clays, a high-temperature stoneware clay body offers the widest range of the various materials used in such formulas. There is no perfect clay body formula. However, some will work better than others. The Z Clay Body has a proven track record over the years and offers a starting point for an explanation of the material choices that make up the formula.*
Traditionally, the cone-9 (2300°F (1260°C)) temperature range has been the choice of many potters since the 1950s and earlier. In part, its history dates back to the firing temperatures used in Chinese, Korean, and Japanese pottery. In the US, early American stoneware pottery was produced in 1722.2
In the past few years, a lower-temperature stoneware range has yielded faster kiln-firing cycles, less wear on the kiln, and reduced fuel costs while still producing dense, vitreous, durable pottery. Correctly formulated, existing cone-9 clay body and glaze colors and surfaces are reproducible at cone 6 (2232°F (1222°C)).
A high-temperature stoneware clay body formulated for cone 6 under the right conditions of kiln firing will produce dense vitreous clay with absorption rates under 2%.3 For functional pottery, an unglazed, fired cup should hold liquids without leaking. There is always the possibility of liquid penetrating the glaze layer into the porous underlying clay body. In some instances, mold can grow on semi-absorbent, exposed clay surfaces during use. Many clay-body and glaze defects occur due to an absorbent clay body.
Glaze only provides a smooth surface for cleaning and offers aesthetic options. It is never intended to be a sealant. Functional stoneware clay-body formulas should be vitreous enough to diminish the potential of glaze crazing and staining.
The minimum and maximum percentages of each material depend on several factors such as the firing temperature, kiln atmosphere, method of forming, fired color, glaze compatibility, and the individual pottery or sculpture requirements. However, by staying within the Cone 6 Throwing/Handbuilding Limits—Minimum –Maximum of each raw material, a dense, vitreous clay body is possible.
Choice of Materials
Each material can influence the clay body’s handling and drying qualities. While individual materials do have distinctive characteristics, it is their combination and interaction with each other when exposed to kiln temperatures that determines the clay body’s fired strength, absorption, shrinkage, and eventual glaze fit.
As in most things ceramic, the actual scale of the pot or sculpture stresses the limits of any clay body formula. Often overlooked or not recognized are the various-sized clay platelets that contribute to a stable clay body in forming and firing operations. In a clay body, clay platelet size plays an important part in the choice of each clay in the formula. Micro-level platelet variations can alter macro-level performance in a clay body. Ideally, there should be small, medium, and large platelet structures so they can interlock when moist, leaving minimal water space between the platelets. Think of filling a glass jar with golf balls (large platelet fireclay), marbles (medium platelet stoneware clay), and peas (small platelet ball clay). Platelet size variation uses the surrounding water more efficiently to form a plastic mass. Clay body formulas with too many similar-sized platelets do not allow an efficient interlocking structure of platelets to bind together. These loose clay-water structures cause the empty spaces between platelets to fill with excessive amounts of water, which can cause warping, cracking, and excessive shrinkage in the drying and firing stages.
Explanation of Materials
Hawthorne Bond Fireclay 50 Mesh:A relatively large platelet size (average 2 microns) contributes to tooth or stand-up ability in forming operations. Hawthorne Bond Fireclay is a secondary clay moved from its site of formation. Fireclays are used, as their name implies, for their strength and durability at high temperatures. They are unyielding to pyroplastic deformation (warping when fired), which can occur with lower temperature clays. They also contribute to less shrinkage in the drying and firing stages.
Cedar Heights Goldart: A secondary clay moved from its site of formation. Goldart stoneware clay is an air-floated, semi-refractory clay frequently used in cone-6 to cone-12 clay-body formulas. Between the large particle size of the fireclay (2 microns) and fine particle size of the ball clay (0.5–0.25 microns) is the medium platelet size of stoneware clay. Due to its strength, plasticity, and forming properties, it is the foundation of the clay body formula.
Tennessee #9 Ball Clay: A secondary clay moved from its site of formation contributes a small clay platelet size (averages 0.2 microns) to the clay body. Tennessee #9 ball clay is a high-temperature, light-firing clay known for its plastic properties. High percentages of any ball clay (over 20%) can cause excessive shrinkage in the drying and firing stages along with a gummy feel in forming operations. Highly plastic clay bodies can also cause handles to crack where they join the main body of the pot in the drying and firing stages.
EPK (Edgar Plastic Kaolin): A primary or residual high-temperature, white-firing clay found on or near its site of origin. Kaolin introduces another platelet size into the clay body, increasing its working properties.
G-200 EU: A potassium-based feldspar is the primary flux, or melting agent, helping to bring the clay body into a vitreous, non-absorbent state when fired. Being non-plastic, the feldspar helps reduce shrinkage and warping in the clay body.
Cedar Heights Redart: A secondary clay moved from its site of formation. This low-fire, high-iron-content clay is used for its brown fired color and platelet size. In low percentages, it is a safer way to introduce iron into the clay body formula. Due to its iron content, when used in high percentages it can act as a flux.
Silica 200 Mesh: Reduces shrinkage and warping in the clay body and combines with other materials to form a eutectic, helping to increase vitrification.
Grog: This calcined material is stable at high temperatures and increases the moist clay’s ability to stand up in throwing and handbuilding operations. Calcining the material makes it inert and stabilizes the clay body in firing. Grog reduces warping, facilitates drying, and reduces thermal shock in the firing. Grog particle size variation is important to achieve an interlocking action with other materials and is used to cut shrinkage. Approximately a 10% addition of grog will reduce fired shrinkage by 0.75%.
Firing Conditions
Whether developing your own clay-body formula or using one from a book or fellow potter, what is often not given enough thought is the firing cycle used to produce a good result. Ceramic materials as potters use them—not industrial pottery production—need longer times to their end point temperatures to ensure maturation of the clay and glaze. Frequently, a fast firing will produce a pot that will “ring” and appear dense, but will leak in use. This is due to not enough vitrification in the still absorbent clay body. Fast firing in a glaze can cause opacity in transparent glazes, dull colors, and easily abraded glazes when used. The best clay body formula will fail if fired incorrectly.
Electric-Kiln Firing
Electric kilns are fairly straightforward in that when the electricity is turned on the kiln heats up in an oxidation atmosphere. There are many options to complete a successful firing. The question then becomes what is one firing cycle that will produce a non-absorbent clay body. For the Z Body fired to cone 6, a recommended firing cycle would be: a fully loaded kiln (as this will increase the thermal mass, producing a slow heating and cooling cycle), using the computer-controlled slow setting for the firing. The total time should take 12–15 hours to reach cone 6, depending on how densely the kiln is stacked. Open the kiln below 250°F (121°C).
Hydrocarbon-Fueled Kiln Firing
One of the advantages of firing kilns with natural gas, propane, wood, or any other hydrocarbon-based fuel is the ability to change the kiln atmosphere, which can influence clay body and glaze colors. However, this wide range of possible atmosphere conditions can cause the kiln to fire in under- or over-reduction, resulting in less successful results.
There is not just one firing cycle that works. In fact, there are many. One that has produced successful results is: from room temperature, fire in a complete oxidation atmosphere up to cone 06 (1828°F (1222°C)), taking approximately 7 hours, after which place the kiln in a slightly reducing atmosphere, proceeding with a temperature increase of 75°F (23°C) per/ hour during which the firing time should take between 8 and 9 hours to reach cone 6.
Optional Raw Material Guidelines: Cone-6 Stoneware Clay-Body Formulas
The minimum and maximum amounts of each material represent general limits for formulating a clay body. By choosing a percentage within the limits from fireclay, stoneware clay, ball clay, feldspar, and silica the total should equal 100%.
Epsom Salts (Magnesium Sulphate): A flocculant. When used at 0.3% based on the dry weight of the clay is the suggested addition to the clay mixing water as it will electrostatically charge or knit the moist clay, giving it the ability to bind together in forming operations.4 Clay bodies that contain slightly soluble materials such as nepheline syenite, a sodium-based feldspar, or some frits can be stabilized with the addition of Epsom salts.
Red Iron Oxide (Fe203): used as a clay body colorant. Over 2% can cause excessive fluxing and brittleness of the clay body, especially in reduction kiln atmospheres where it acts as a stronger flux than in oxidation kiln atmospheres. High percentages of red iron oxide can also make the clay body take on water faster and slump in forming operations.
Additive A: A blend of lignosulphonates and organic and inorganic chemicals produced by Lignotech USA. It is non-toxic. Additive A increases the plastic characteristics without the need of additional ball clays or bentonite. The increase in plasticity is most noticeable in short or non-plastic clay bodies such as raku, sculpture, jigger and ram press formulas. Additive A can also be used in soda, salt, low-fire, and tile clay bodies. Additive A substantially increases the green strength (pots that have been formed and are still not dry) and dry strength (pots that have been formed and are already dry) of clay bodies, making for less fragile ware.5
Bentonite: An extremely fine-grained clay with a small platelet size. For example, a 100-pound bag of bentonite would cover 2400 acres of surface area.6 Bentonite is derived from volcanic ash. In 1–2% increments, it increases the plasticity of the clay body.
Vee Gum T:Produced by R.T. Vanderbilt Co., and mined from selected groups of smectite ores which are refined and mixed for consistency and reliability. When added to a clay body it increases the plastic properties of the clay. Recommended use is 0.1–2% based on the dry weight of the clay body.
CMC (Sodium Carboxymethylcellulose): derived from pure cellulose. There are over 100 different grades of CMC. In clay body formulas, CMC increases plasticity from 0.1– 2% based on the dry weight of the clay body.
Beer, Apple Cider Vinegar, and Yeast:In the past, these additives were used in a water mixture to start mold growth in the moist clay, with the result of increasing its plastic properties. The mold acts as a binding agent in the water space between the clay platelets. Moist clay can sometimes spontaneously grow mold depending on the conditions in the studio. However, some types of black mold can cause voids in the moist clay, which can impede forming and firing operations.
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 Potters Health & Safety Questionnaire are available from Jeff Zamek/ Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
* “The Perfect Clay Body,” Zamek, Jeff. What Every Potter Should Know, First Edition, Krause Publication, 1999, page 76. ** The amount and duration of a reduction kiln atmosphere can influence the fired color of the clay body and its absorption percentage. *** Additions of grog up to 20% are recommended for sculpture bodies.
1. McColm, Ian J. Dictionary of Ceramic Science and Engineering, Second Edition, Plenum Press, 1994, page 71. 2. Webster, Donald, Decorated Stoneware Pottery of North America, First Edition, Charles E. Tuttle Company, Inc., 1971, page 28. 3. Clay body absorption rates can vary depending on the clay body formula, end point temperature reached in the kiln, time to the end point temperature, time held in the maturing zone, and kiln atmosphere. 4. Epsom salts (magnesium sulphate) is a flocculant and electrolyte added to the clay mixing water causing clay platelets to aggregate together. 5. Zamek, Jeff. What Every Potter Should Know, First Edition, Krause Publication, 1999, page 93. 6. Lawrence, W.G. Ceramic Science for the Potter. Chilton Book Company, 1972, page 39. 7. Ibid., 38, 39.
Acknowledgements:
“Why we make things and why it matters” by Peter Korn, from the quote, transmuting thought into matter. Tony Hanson, Digitalfire, supplied information on Epsom salts, digitalfire.com/material/epsom+salt.
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When we form clay, on a fundamental level, we are transmuting thought into matter. It is the closest thing to magic, where you have nothing, and then something appears. Having a reliable clay body formula allows for making objects, using your head, heart, and hands, as the English potter Bernard Leach once proclaimed.
Defining the Terms
Air-Floated: A rising stream of air of controlled velocity separates solid clay particles by density and size.
Calcined Clay: A clay heated until its chemical water is removed and it loses its plastic properties.
Clay Body: A combination of clay(s), fluxes, and fillers to achieve a specific firing temperature, fired color, forming method, and function in oxidation or reduction kiln atmospheres.
Clay Platelet: Hexagonal plates with an approximate ratio of thickness to length of 10 to 1. Plates can range in size from 100u to 0.1u particle diameter.7
Coefficient of Expansion: The change in the length, area, or volume of the clay body based on temperature.
End Point: The highest temperature reached to achieve a vitreous, mature clay body and glaze.
Eutectic: A combination of two or more materials that lowers the melting point of each.
Oxidization Atmosphere: In the kiln-firing process, a greater ratio of oxygen than fuel is present throughout the firing cycle.
Pyroplastic Deformation: A clay body permanently bending or warping subject to a high-temperature firing.
Reduction Atmosphere: An atmosphere produced with a higher fuel-to-air ratio, creating carbon monoxide, which draws oxygen away from metallic oxides.
Vitreous or Vitrification: A heated clay body develops a glassy phase in its composition without deformation and is characterized by low porosity.
The fundamental element when making pottery is clay, a group of mineral aggregates consisting of hydrous aluminum silicates of layered plates becoming plastic when wet, rigid when dry, and vitrified when heated. Long before any rudimentary glaze was developed, there was always clay, formed into functional and symbolic objects.
A clay body is composed of one or more materials: clays for plasticity, fillers such as grog and silica to facilitate thermal coefficient of expansion,1 and silica to reduce warping and facilitate glaze fit. The last material is a flux, which brings together the combination of clay, silica, and fillers into a glass-like state. Ideally, all four elements act in coordination to produce a durable, plastic clay body at a given temperature range and firing atmosphere.
The Choice of a Cone-6 Clay Body
While porcelain formulas contain kaolin, feldspar, and silica, and earthenware clay body formulas can have as few as one or two clays, a high-temperature stoneware clay body offers the widest range of the various materials used in such formulas. There is no perfect clay body formula. However, some will work better than others. The Z Clay Body has a proven track record over the years and offers a starting point for an explanation of the material choices that make up the formula.*
Traditionally, the cone-9 (2300°F (1260°C)) temperature range has been the choice of many potters since the 1950s and earlier. In part, its history dates back to the firing temperatures used in Chinese, Korean, and Japanese pottery. In the US, early American stoneware pottery was produced in 1722.2
In the past few years, a lower-temperature stoneware range has yielded faster kiln-firing cycles, less wear on the kiln, and reduced fuel costs while still producing dense, vitreous, durable pottery. Correctly formulated, existing cone-9 clay body and glaze colors and surfaces are reproducible at cone 6 (2232°F (1222°C)).
A high-temperature stoneware clay body formulated for cone 6 under the right conditions of kiln firing will produce dense vitreous clay with absorption rates under 2%.3 For functional pottery, an unglazed, fired cup should hold liquids without leaking. There is always the possibility of liquid penetrating the glaze layer into the porous underlying clay body. In some instances, mold can grow on semi-absorbent, exposed clay surfaces during use. Many clay-body and glaze defects occur due to an absorbent clay body.
Glaze only provides a smooth surface for cleaning and offers aesthetic options. It is never intended to be a sealant. Functional stoneware clay-body formulas should be vitreous enough to diminish the potential of glaze crazing and staining.
The minimum and maximum percentages of each material depend on several factors such as the firing temperature, kiln atmosphere, method of forming, fired color, glaze compatibility, and the individual pottery or sculpture requirements. However, by staying within the Cone 6 Throwing/Handbuilding Limits—Minimum –Maximum of each raw material, a dense, vitreous clay body is possible.
Choice of Materials
Each material can influence the clay body’s handling and drying qualities. While individual materials do have distinctive characteristics, it is their combination and interaction with each other when exposed to kiln temperatures that determines the clay body’s fired strength, absorption, shrinkage, and eventual glaze fit.
As in most things ceramic, the actual scale of the pot or sculpture stresses the limits of any clay body formula. Often overlooked or not recognized are the various-sized clay platelets that contribute to a stable clay body in forming and firing operations. In a clay body, clay platelet size plays an important part in the choice of each clay in the formula. Micro-level platelet variations can alter macro-level performance in a clay body. Ideally, there should be small, medium, and large platelet structures so they can interlock when moist, leaving minimal water space between the platelets. Think of filling a glass jar with golf balls (large platelet fireclay), marbles (medium platelet stoneware clay), and peas (small platelet ball clay). Platelet size variation uses the surrounding water more efficiently to form a plastic mass. Clay body formulas with too many similar-sized platelets do not allow an efficient interlocking structure of platelets to bind together. These loose clay-water structures cause the empty spaces between platelets to fill with excessive amounts of water, which can cause warping, cracking, and excessive shrinkage in the drying and firing stages.
Explanation of Materials
Hawthorne Bond Fireclay 50 Mesh: A relatively large platelet size (average 2 microns) contributes to tooth or stand-up ability in forming operations. Hawthorne Bond Fireclay is a secondary clay moved from its site of formation. Fireclays are used, as their name implies, for their strength and durability at high temperatures. They are unyielding to pyroplastic deformation (warping when fired), which can occur with lower temperature clays. They also contribute to less shrinkage in the drying and firing stages.
Cedar Heights Goldart: A secondary clay moved from its site of formation. Goldart stoneware clay is an air-floated, semi-refractory clay frequently used in cone-6 to cone-12 clay-body formulas. Between the large particle size of the fireclay (2 microns) and fine particle size of the ball clay (0.5–0.25 microns) is the medium platelet size of stoneware clay. Due to its strength, plasticity, and forming properties, it is the foundation of the clay body formula.
Tennessee #9 Ball Clay: A secondary clay moved from its site of formation contributes a small clay platelet size (averages 0.2 microns) to the clay body. Tennessee #9 ball clay is a high-temperature, light-firing clay known for its plastic properties. High percentages of any ball clay (over 20%) can cause excessive shrinkage in the drying and firing stages along with a gummy feel in forming operations. Highly plastic clay bodies can also cause handles to crack where they join the main body of the pot in the drying and firing stages.
EPK (Edgar Plastic Kaolin): A primary or residual high-temperature, white-firing clay found on or near its site of origin. Kaolin introduces another platelet size into the clay body, increasing its working properties.
G-200 EU: A potassium-based feldspar is the primary flux, or melting agent, helping to bring the clay body into a vitreous, non-absorbent state when fired. Being non-plastic, the feldspar helps reduce shrinkage and warping in the clay body.
Cedar Heights Redart: A secondary clay moved from its site of formation. This low-fire, high-iron-content clay is used for its brown fired color and platelet size. In low percentages, it is a safer way to introduce iron into the clay body formula. Due to its iron content, when used in high percentages it can act as a flux.
Silica 200 Mesh: Reduces shrinkage and warping in the clay body and combines with other materials to form a eutectic, helping to increase vitrification.
Grog: This calcined material is stable at high temperatures and increases the moist clay’s ability to stand up in throwing and handbuilding operations. Calcining the material makes it inert and stabilizes the clay body in firing. Grog reduces warping, facilitates drying, and reduces thermal shock in the firing. Grog particle size variation is important to achieve an interlocking action with other materials and is used to cut shrinkage. Approximately a 10% addition of grog will reduce fired shrinkage by 0.75%.
Firing Conditions
Whether developing your own clay-body formula or using one from a book or fellow potter, what is often not given enough thought is the firing cycle used to produce a good result. Ceramic materials as potters use them—not industrial pottery production—need longer times to their end point temperatures to ensure maturation of the clay and glaze. Frequently, a fast firing will produce a pot that will “ring” and appear dense, but will leak in use. This is due to not enough vitrification in the still absorbent clay body. Fast firing in a glaze can cause opacity in transparent glazes, dull colors, and easily abraded glazes when used. The best clay body formula will fail if fired incorrectly.
Electric-Kiln Firing
Electric kilns are fairly straightforward in that when the electricity is turned on the kiln heats up in an oxidation atmosphere. There are many options to complete a successful firing. The question then becomes what is one firing cycle that will produce a non-absorbent clay body. For the Z Body fired to cone 6, a recommended firing cycle would be: a fully loaded kiln (as this will increase the thermal mass, producing a slow heating and cooling cycle), using the computer-controlled slow setting for the firing. The total time should take 12–15 hours to reach cone 6, depending on how densely the kiln is stacked. Open the kiln below 250°F (121°C).
Hydrocarbon-Fueled Kiln Firing
One of the advantages of firing kilns with natural gas, propane, wood, or any other hydrocarbon-based fuel is the ability to change the kiln atmosphere, which can influence clay body and glaze colors. However, this wide range of possible atmosphere conditions can cause the kiln to fire in under- or over-reduction, resulting in less successful results.
There is not just one firing cycle that works. In fact, there are many. One that has produced successful results is: from room temperature, fire in a complete oxidation atmosphere up to cone 06 (1828°F (1222°C)), taking approximately 7 hours, after which place the kiln in a slightly reducing atmosphere, proceeding with a temperature increase of 75°F (23°C) per/ hour during which the firing time should take between 8 and 9 hours to reach cone 6.
Optional Raw Material Guidelines: Cone-6 Stoneware Clay-Body Formulas
The minimum and maximum amounts of each material represent general limits for formulating a clay body. By choosing a percentage within the limits from fireclay, stoneware clay, ball clay, feldspar, and silica the total should equal 100%.
Epsom Salts (Magnesium Sulphate): A flocculant. When used at 0.3% based on the dry weight of the clay is the suggested addition to the clay mixing water as it will electrostatically charge or knit the moist clay, giving it the ability to bind together in forming operations.4 Clay bodies that contain slightly soluble materials such as nepheline syenite, a sodium-based feldspar, or some frits can be stabilized with the addition of Epsom salts.
Red Iron Oxide (Fe203): used as a clay body colorant. Over 2% can cause excessive fluxing and brittleness of the clay body, especially in reduction kiln atmospheres where it acts as a stronger flux than in oxidation kiln atmospheres. High percentages of red iron oxide can also make the clay body take on water faster and slump in forming operations.
Additive A: A blend of lignosulphonates and organic and inorganic chemicals produced by Lignotech USA. It is non-toxic. Additive A increases the plastic characteristics without the need of additional ball clays or bentonite. The increase in plasticity is most noticeable in short or non-plastic clay bodies such as raku, sculpture, jigger and ram press formulas. Additive A can also be used in soda, salt, low-fire, and tile clay bodies. Additive A substantially increases the green strength (pots that have been formed and are still not dry) and dry strength (pots that have been formed and are already dry) of clay bodies, making for less fragile ware.5
Bentonite: An extremely fine-grained clay with a small platelet size. For example, a 100-pound bag of bentonite would cover 2400 acres of surface area.6 Bentonite is derived from volcanic ash. In 1–2% increments, it increases the plasticity of the clay body.
Vee Gum T: Produced by R.T. Vanderbilt Co., and mined from selected groups of smectite ores which are refined and mixed for consistency and reliability. When added to a clay body it increases the plastic properties of the clay. Recommended use is 0.1–2% based on the dry weight of the clay body.
CMC (Sodium Carboxymethylcellulose): derived from pure cellulose. There are over 100 different grades of CMC. In clay body formulas, CMC increases plasticity from 0.1– 2% based on the dry weight of the clay body.
Beer, Apple Cider Vinegar, and Yeast: In the past, these additives were used in a water mixture to start mold growth in the moist clay, with the result of increasing its plastic properties. The mold acts as a binding agent in the water space between the clay platelets. Moist clay can sometimes spontaneously grow mold depending on the conditions in the studio. However, some types of black mold can cause voids in the moist clay, which can impede forming and firing operations.
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 Potters Health & Safety Questionnaire are available from Jeff Zamek/ Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
* “The Perfect Clay Body,” Zamek, Jeff. What Every Potter Should Know, First Edition, Krause Publication, 1999, page 76.
** The amount and duration of a reduction kiln atmosphere can influence the fired color of the clay body and its absorption percentage.
*** Additions of grog up to 20% are recommended for sculpture bodies.
1. McColm, Ian J. Dictionary of Ceramic Science and Engineering, Second Edition, Plenum Press, 1994, page 71.
2. Webster, Donald, Decorated Stoneware Pottery of North America, First Edition, Charles E. Tuttle Company, Inc., 1971, page 28.
3. Clay body absorption rates can vary depending on the clay body formula, end point temperature reached in the kiln, time to the end point temperature, time held in the maturing zone, and kiln atmosphere.
4. Epsom salts (magnesium sulphate) is a flocculant and electrolyte added to the clay mixing water causing clay platelets to aggregate together.
5. Zamek, Jeff. What Every Potter Should Know, First Edition, Krause Publication, 1999, page 93.
6. Lawrence, W.G. Ceramic Science for the Potter. Chilton Book Company, 1972, page 39.
7. Ibid., 38, 39.
Acknowledgements:
“Why we make things and why it matters” by Peter Korn, from the quote, transmuting thought into matter.
Tony Hanson, Digitalfire, supplied information on Epsom salts, digitalfire.com/material/epsom+salt.
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