Making pottery involves many steps and variables in forming, drying, glazing, and firing, which can change slowly over time or occur in the next kiln firing. It is often said, the only thing consistent about ceramics is its inconsistency.
Defining the Terms
Clay Body: A mixture of clay(s), fluxes, and non-plastic materials to achieve a specific firing range, color, and forming procedure.
Coefficient of Expansion: The expansion and contraction of materials during heating and cooling.
Crazing: A series of fine lines in the fired glaze, which was under tension when cooling.
Glaze Formula: A combination of fluxes, clays, alkaline earths formulated for a specific temperature range, surface texture, opacity, color, and kiln firing atmosphere to produce a glaze.
Immature Clay Body: One with low strength, poor durability, high porosity, and exhibits problems with glaze fit.
Orange Peel: Sodium vapor in salt or soda kilns reacts with alumina and silica in the clay body to produce a pebble-like surface.
Oxidation Atmosphere: Fuel is burnt completely due to an excess of oxygen.
Physical Packing: Physical sizes of materials in a clay body formula—think of a jar filled with peas, marbles, and golf balls. Their various sizes leave little empty space in the jar. Ball clays (small platelets), stoneware clays (medium-size platelets), and fireclays (large platelets) fill this clay body formula.
Reduction Atmosphere: An atmosphere deprived of oxygen, producing incomplete combustion which reduces oxides in clay and glaze.
Shivering: As the glaze cools it is under extreme compression. This defect often looks like paint chips flaking off the pot.
Vitrification: During the firing process, materials in the clay body melt into a glass phase, which reduces porosity.
Often the most troublesome factors in glazes changing are intermittent or random events, as it can be difficult to find a specific cause of the failure. How many times have we copied a glaze formula from books or magazine articles only to find it produced a much different result than expected? Occasionally, we see another potter’s work with the same glaze formula we copied down, but the results are different when fired on our own pots. What are the factors that influence glazes?
Listed in this article are several things to consider before mixing and firing any glaze copied from a book, magazine, website, or a fellow potter. As a starting point, consider if any of these items might be the cause of not being able to duplicate a glaze. For each category, there are recommended questions to ask yourself before committing time and labor to a glaze that is less than preferred.
Particle Size
Do you know all the mesh sizes in the glaze formula?
The particle sizes of ceramic raw materials are critical factors in their ability to melt. A smaller particle size denotes increased surface area, which melts faster and more efficiently than a larger particle size, which may not fully dissolve in glazes. Finer glaze particles or mesh sizes stay in suspension longer compared with large particle materials that can sink to the bottom of the glaze bucket. In clay body formulas, finer materials increase physical packing in conjunction with medium and larger size materials. For example:
Silica (a glass-forming oxide and major component in any glaze) can be purchased in 60-, 100-, 200-, 325-, and 400-mesh sizes, along with finer sizes available by special order (the larger mesh numbers indicate smaller size particles). Frequently, a glaze formula will not specify a mesh size for silica. In such instances, use 325-mesh silica.
Nepheline syenite (a feldspathoid sodium-based feldspar used as a flux in high-temperature glazes) is processed in mesh sizes 16, 30, 40, 60, 80, 100, 200, 270, and 325, any one of which can alter the melting point and fired opacity of a glaze. If not stated, use 200 or 270 mesh in a glaze formula. Be specific about the mesh size of any material when re-ordering. Test a glaze whenever changing particle sizes.
Whiting (calcium carbonate) is another common glaze material that comes in different mesh sizes, from larger 200 mesh to fine mesh 325, all of which look like white powder. Larger particles of whiting, being denser, can cause the wet glaze to sink in the bucket. Additionally, the larger particle size does not easily dissolve in the liquid glaze, causing suspended carbonate bubbles to be released in the forming glaze (1). Finer mesh whiting goes into melt more easily as it has greater surface area.
Kiln Size
Do you know the size of the kiln which produced the glaze?
The size of the kiln can play an important part in the development of surface texture, light transmission, color, and hardness of the glaze and clay body. Larger kilns can have greater thermal mass, which is composed of kiln bricks, posts, shelves, and pots, all items that radiate heat during heating and cooling cycles. They can also cause a different glaze reaction compared to smaller kilns with less thermal mass, which dissipate heat at a faster rate. The longer clay and glaze stay in the maturing range, the greater the vitrification of both (2).
Small test kilns can yield different results in clay body and glaze maturity due to less thermal mass. Their heating and cooling curve is compressed even when firing to the same cone as a larger kiln (3).
Kiln Firing Cycle
Do you know the firing cycle of the kiln that produced the glaze?
A fast kiln firing cycle will produce an immature clay body and glaze, causing a weak, porous clay body and glazes that can have a dull color or dry surface texture. Too long of a firing cycle can cause the glaze to become excessively glossy or run off vertical surfaces and possibly warp the clay. Slow cooling, depending in part on the glaze formula, can also cause devitrification or crystal growth in the glaze. When crystals are small and dense, the glaze can become opaque or have a bleached dull color (4).
Cone Reading
Do you know the position of the firing cone when the kiln was shut off?
A few degrees on a bending cone can represent a significant difference in the glaze result. Additionally, potters can read the melting position of pyrometric cones at different positions. Many consider the cone reaching its correct temperature when it bends to the 3 o’clock or 9 o’clock positions. Some potters interpret the cone going over completely as a reference point. Decide on a cone reading position, note your glaze results; if acceptable, use that position every time. Large pyrometric cones are designed to be set on an 8-degree tilt from vertical for accurate results.
Glaze Fuming
Do you know what glazes were fired next to your glaze?
Under certain conditions of temperature, kiln atmosphere, proximity of glazes, and the combination of glaze materials, one glaze can vaporize off, which could alter the color of another glaze. A fuming reaction is most noticeable when a glaze formula with chrome oxide is placed next to a glaze containing tin oxide, resulting in a pink blush. Testing two different glazes placed next to each other is the best recommendation, as in many instances fuming is difficult to predict.
Materials No Longer in Production
Do you know which supplier the glaze materials were bought from? Or where they were mined and processed?
At some point you will discover a glaze (or clay body) material is no longer available. Economics play a large part in keeping any raw material available. Surprisingly, materials are geologically present but financially not worth mining or processing. Large industrial users may switch or discontinue a material used in making their product, and the supplier drops it.
Or in some instances it is not profitable just to sell relatively small quantities to the pottery market. Any substitution can cause a different glaze outcome than expected. Many glaze formulas were first developed using feldspars, clays, or other raw materials, which are no longer in production. Potters can make the mistake of using the same raw material in their studio, thinking it is still being sold in the ceramics market. It is always best to call ceramics suppliers to assure the material is still available.
Incorrect Substitution
Did you need to substitute materials in your batch that were different than the original materials?
A common mistake occurs when using potassium (potash) feldspar in place of a sodium (soda) based feldspar, which is required in the glaze formula. Feldspars should be substituted within their group—sodium, potassium, or lithium—for accurate results. Be aware, over the years some glaze formulas have been altered due to material substitutions, the sum total of which can alter a glaze. Always test a glaze that has a known substitution.
Metallic Coloring Oxides
Do you know the supplier and grade of metallic coloring oxide used in the original glaze formula?
Metallic coloring oxides can differ in metal concentration, particle size, and percentages of trace elements. As with other ceramic raw materials, there are many processors of metallic coloring oxides. Each grade can affect the intensity of color generated in a glaze. When possible, stay with the same supplier of a metallic coloring oxide and periodically inquire if they have changed sources.
Clay Body to Glaze Interaction
Do you know what clay body was used under the original glaze?
The interface is the region where the clay body ends, and the lower surface of the glaze begins. The intensity of the glaze color can be influenced by a light or dark underlying clay body color. Some clays will “wick” fluxes from the covering glaze, causing an opaque or drier glaze condition. Highly vitreous clay with absorption rates at zero or slightly higher can cause the covering glaze to slide off during the firing, exposing the underlying clay. For the most accurate results, be consistent with clay body and glaze combinations.
Changes in the Clay Body
Do you know if the glaze will fit your clay body?
White clay bodies can promote cleaner, brighter colors while darker clay bodies can mute glaze colors giving greater depth. Clay bodies containing iron can increase the breaking effects of a glaze on edges and rims of pots.
Every clay body has a coefficient of expansion—a metric of how much it expands or shrinks when heated. Ideally, this shrinkage rate should fall within the zone of the glaze’s shrinkage rate. Otherwise, if the glaze is under tension when cooling, crazing (a series of fine lines in the glaze) can develop. If a glaze is under extreme compression, shivering (sheet-like plates of fired clay peel off the clay body) can develop.
Glaze Application/Thickness
How thick or thin should the glaze be applied?
The depth of the glaze layer can play an important role in duplicating a glaze effect. Too thin a glaze layer and the color of the clay body predominates, while too thick a glaze layer can cause the glaze to run off vertical surfaces or pool excessively in horizontal areas. Unfortunately, most glaze formulas do not note glaze thickness or application techniques.
Method of Glaze Application
What is the best application method for this glaze?
Similar to glaze application thickness, spraying, dipping, or brushing can impart different layers of glazes depending on the application method used. Spraying glazes has the advantage of a uniform layer of glaze, which can easily be controlled as to thickness and blended with other glazes. The spray imparts less water to the glaze surface as opposed to brushing or dipping.
Dipping the pot can produce a consistent layer of glaze but can be difficult to control drips, while brushing can cause uneven glaze layers and leave brush marks, resulting in light to dark glaze colors.
Thin glaze application can produce weaker colors, with dry surfaces, and reveal more of the underlying clay body color. Medium thickness glaze applications can develop glaze color, true glaze surface characteristics, and possible crystal growth. Thicker glaze applications can cause dripping on vertical surfaces and pooling in horizontal areas due to a higher concentration of glaze fluxes in those areas. It is important to note that each glaze application method can change the opacity, surface quality, and glaze color.
Kiln Atmosphere
What atmosphere was the glaze fired in?
Electric kilns produce clean, consistent oxidation atmospheres. However, carbon-based fuels such as natural gas, propane, wood, coal, oil, and sawdust can produce oxidation (more oxygen than fuel in the combustion process), neutral (equal amounts of fuel and oxygen in the combustion process), and various intensities of reduction (more fuel than oxygen in the combustion process, creating carbon monoxide) atmospheres. Reduction amounts can be very difficult to reproduce, as one potter’s medium-reduction atmosphere can be another’s heavy-reduction atmosphere. Variations in the length and amount of reduction can also change clay body/ glaze colors and glaze surface textures.
Soda (sodium carbonate or sodium bicarbonate) or salt (sodium chloride) kiln atmospheres contribute an orange-peel texture on exposed clay surfaces, but can also flux or melt glaze surfaces. Matte dry glazes can appear glossy, satin, or matte. The sodium atmosphere can also create a glossier glaze surface, running glazes on vertical surfaces or glaze pooling in horizontal areas. Glaze colors can appear brighter due to an increase in vitrification.1
While electric kilns firing in oxidation atmospheres produce fairly consistent results in glazes, reduction kilns have a wide range of atmospheric conditions that can be applied during a firing, any one of which can change the color, opacity, or surface texture of the glaze. Always investigate, where possible, the amount and duration of reduction a glaze requires.
Slips/Engobes
Was a slip used under the glaze? If so, consider slip and glaze fit.
Slips or engobes (interchangeable terms) are colored clays designed to fit the clay body and covering glaze. White slips can modify the glaze, as light is reflected back through the glaze, intensifying colors. Black or dark slips can absorb light, producing darker glaze colors. Any slip has the potential for altering the glaze color. Determine if a slip was used, and if so, was it white or dark?
Aside from altering the glaze color, the use of slips can alter the thermal expansion between the clay body and glaze. For example, if the glaze contracts more than the slip layer, it can cause tension in the glaze called crazing. Conversely, if the glaze contracts less, the glaze will be under extreme compression and produce shivering. Slips are a third system at play in a glaze fit which can alter glaze results.
Change in Raw Materials
Be aware that mining locations of raw materials can change, which may alter your processed product.
It is not unusual for ceramic raw materials used in glaze formulas to change, occasionally from one bag to the next, or sometimes slowly over a period of years. Do not assume a raw material remains consistent because the name on the bag stays the same. Raw materials, even the best quality-controlled materials, can sometimes shift in chemical composition, particle size, and organic content. Luckily, potters use a few basic raw materials such as feldspars, silica, whiting, dolomite, talc, frits, kaolin, and ball clays, that are also used in large industries. Potters can take advantage as industrial users dictate the quality-control parameters for these materials.
Glazes Work Until They Don’t. Always Test.
Most glazes work as advertised in books, magazine articles, or from fellow potters. In some instances the results in your kiln are fairly close and just need a little adjustment. At that point try to determine what has to be fixed, then begin testing. The standard rule is never mix an unknown glaze formula in large quantities. Test in a small batch and place test tiles throughout the kiln on vertical tiles at least 2 or 3 inches (5 or 7.6 cm) tall. Why? Small tiles might not indicate the flow of the glaze compared to the same glaze on a larger pot. In some instances, the molten weight of the glaze over increased surface area can cause the glaze to run excessively on vertical surfaces or pool in horizontal surfaces.
Often the most troublesome changes in a glaze or clay body are intermittent failures from what is predicted. They are the most complicated to diagnose and eventually correct. These random events need accurate record keeping and research to tease out the likely issues. One way to narrow the field is to put aside the least likely causes and concentrate on the remaining faults.
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.
1 Zamek, Jeff. What Every Potter Should Know. Iola, WI: Krause Publications, 1999, page 163.
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Making pottery involves many steps and variables in forming, drying, glazing, and firing, which can change slowly over time or occur in the next kiln firing. It is often said, the only thing consistent about ceramics is its inconsistency.
Defining the Terms
Clay Body: A mixture of clay(s), fluxes, and non-plastic materials to achieve a specific firing range, color, and forming procedure.
Coefficient of Expansion: The expansion and contraction of materials during heating and cooling.
Crazing: A series of fine lines in the fired glaze, which was under tension when cooling.
Glaze Formula: A combination of fluxes, clays, alkaline earths formulated for a specific temperature range, surface texture, opacity, color, and kiln firing atmosphere to produce a glaze.
Immature Clay Body: One with low strength, poor durability, high porosity, and exhibits problems with glaze fit.
Orange Peel: Sodium vapor in salt or soda kilns reacts with alumina and silica in the clay body to produce a pebble-like surface.
Oxidation Atmosphere: Fuel is burnt completely due to an excess of oxygen.
Physical Packing: Physical sizes of materials in a clay body formula—think of a jar filled with peas, marbles, and golf balls. Their various sizes leave little empty space in the jar. Ball clays (small platelets), stoneware clays (medium-size platelets), and fireclays (large platelets) fill this clay body formula.
Reduction Atmosphere: An atmosphere deprived of oxygen, producing incomplete combustion which reduces oxides in clay and glaze.
Shivering: As the glaze cools it is under extreme compression. This defect often looks like paint chips flaking off the pot.
Vitrification: During the firing process, materials in the clay body melt into a glass phase, which reduces porosity.
Often the most troublesome factors in glazes changing are intermittent or random events, as it can be difficult to find a specific cause of the failure. How many times have we copied a glaze formula from books or magazine articles only to find it produced a much different result than expected? Occasionally, we see another potter’s work with the same glaze formula we copied down, but the results are different when fired on our own pots. What are the factors that influence glazes?
Listed in this article are several things to consider before mixing and firing any glaze copied from a book, magazine, website, or a fellow potter. As a starting point, consider if any of these items might be the cause of not being able to duplicate a glaze. For each category, there are recommended questions to ask yourself before committing time and labor to a glaze that is less than preferred.
Particle Size
Do you know all the mesh sizes in the glaze formula?
The particle sizes of ceramic raw materials are critical factors in their ability to melt. A smaller particle size denotes increased surface area, which melts faster and more efficiently than a larger particle size, which may not fully dissolve in glazes. Finer glaze particles or mesh sizes stay in suspension longer compared with large particle materials that can sink to the bottom of the glaze bucket. In clay body formulas, finer materials increase physical packing in conjunction with medium and larger size materials. For example:
Whiting (calcium carbonate) is another common glaze material that comes in different mesh sizes, from larger 200 mesh to fine mesh 325, all of which look like white powder. Larger particles of whiting, being denser, can cause the wet glaze to sink in the bucket. Additionally, the larger particle size does not easily dissolve in the liquid glaze, causing suspended carbonate bubbles to be released in the forming glaze (1). Finer mesh whiting goes into melt more easily as it has greater surface area.
Kiln Size
Do you know the size of the kiln which produced the glaze?
The size of the kiln can play an important part in the development of surface texture, light transmission, color, and hardness of the glaze and clay body. Larger kilns can have greater thermal mass, which is composed of kiln bricks, posts, shelves, and pots, all items that radiate heat during heating and cooling cycles. They can also cause a different glaze reaction compared to smaller kilns with less thermal mass, which dissipate heat at a faster rate. The longer clay and glaze stay in the maturing range, the greater the vitrification of both (2).
Small test kilns can yield different results in clay body and glaze maturity due to less thermal mass. Their heating and cooling curve is compressed even when firing to the same cone as a larger kiln (3).
Kiln Firing Cycle
Do you know the firing cycle of the kiln that produced the glaze?
A fast kiln firing cycle will produce an immature clay body and glaze, causing a weak, porous clay body and glazes that can have a dull color or dry surface texture. Too long of a firing cycle can cause the glaze to become excessively glossy or run off vertical surfaces and possibly warp the clay. Slow cooling, depending in part on the glaze formula, can also cause devitrification or crystal growth in the glaze. When crystals are small and dense, the glaze can become opaque or have a bleached dull color (4).
Cone Reading
Do you know the position of the firing cone when the kiln was shut off?
A few degrees on a bending cone can represent a significant difference in the glaze result. Additionally, potters can read the melting position of pyrometric cones at different positions. Many consider the cone reaching its correct temperature when it bends to the 3 o’clock or 9 o’clock positions. Some potters interpret the cone going over completely as a reference point. Decide on a cone reading position, note your glaze results; if acceptable, use that position every time. Large pyrometric cones are designed to be set on an 8-degree tilt from vertical for accurate results.
Glaze Fuming
Do you know what glazes were fired next to your glaze?
Under certain conditions of temperature, kiln atmosphere, proximity of glazes, and the combination of glaze materials, one glaze can vaporize off, which could alter the color of another glaze. A fuming reaction is most noticeable when a glaze formula with chrome oxide is placed next to a glaze containing tin oxide, resulting in a pink blush. Testing two different glazes placed next to each other is the best recommendation, as in many instances fuming is difficult to predict.
Materials No Longer in Production
Do you know which supplier the glaze materials were bought from? Or where they were mined and processed?
At some point you will discover a glaze (or clay body) material is no longer available. Economics play a large part in keeping any raw material available. Surprisingly, materials are geologically present but financially not worth mining or processing. Large industrial users may switch or discontinue a material used in making their product, and the supplier drops it.
Or in some instances it is not profitable just to sell relatively small quantities to the pottery market. Any substitution can cause a different glaze outcome than expected. Many glaze formulas were first developed using feldspars, clays, or other raw materials, which are no longer in production. Potters can make the mistake of using the same raw material in their studio, thinking it is still being sold in the ceramics market. It is always best to call ceramics suppliers to assure the material is still available.
Incorrect Substitution
Did you need to substitute materials in your batch that were different than the original materials?
A common mistake occurs when using potassium (potash) feldspar in place of a sodium (soda) based feldspar, which is required in the glaze formula. Feldspars should be substituted within their group—sodium, potassium, or lithium—for accurate results. Be aware, over the years some glaze formulas have been altered due to material substitutions, the sum total of which can alter a glaze. Always test a glaze that has a known substitution.
Metallic Coloring Oxides
Do you know the supplier and grade of metallic coloring oxide used in the original glaze formula?
Metallic coloring oxides can differ in metal concentration, particle size, and percentages of trace elements. As with other ceramic raw materials, there are many processors of metallic coloring oxides. Each grade can affect the intensity of color generated in a glaze. When possible, stay with the same supplier of a metallic coloring oxide and periodically inquire if they have changed sources.
Clay Body to Glaze Interaction
Do you know what clay body was used under the original glaze?
The interface is the region where the clay body ends, and the lower surface of the glaze begins. The intensity of the glaze color can be influenced by a light or dark underlying clay body color. Some clays will “wick” fluxes from the covering glaze, causing an opaque or drier glaze condition. Highly vitreous clay with absorption rates at zero or slightly higher can cause the covering glaze to slide off during the firing, exposing the underlying clay. For the most accurate results, be consistent with clay body and glaze combinations.
Changes in the Clay Body
Do you know if the glaze will fit your clay body?
White clay bodies can promote cleaner, brighter colors while darker clay bodies can mute glaze colors giving greater depth. Clay bodies containing iron can increase the breaking effects of a glaze on edges and rims of pots.
Every clay body has a coefficient of expansion—a metric of how much it expands or shrinks when heated. Ideally, this shrinkage rate should fall within the zone of the glaze’s shrinkage rate. Otherwise, if the glaze is under tension when cooling, crazing (a series of fine lines in the glaze) can develop. If a glaze is under extreme compression, shivering (sheet-like plates of fired clay peel off the clay body) can develop.
Glaze Application/Thickness
How thick or thin should the glaze be applied?
The depth of the glaze layer can play an important role in duplicating a glaze effect. Too thin a glaze layer and the color of the clay body predominates, while too thick a glaze layer can cause the glaze to run off vertical surfaces or pool excessively in horizontal areas. Unfortunately, most glaze formulas do not note glaze thickness or application techniques.
Method of Glaze Application
What is the best application method for this glaze?
Similar to glaze application thickness, spraying, dipping, or brushing can impart different layers of glazes depending on the application method used. Spraying glazes has the advantage of a uniform layer of glaze, which can easily be controlled as to thickness and blended with other glazes. The spray imparts less water to the glaze surface as opposed to brushing or dipping.
Dipping the pot can produce a consistent layer of glaze but can be difficult to control drips, while brushing can cause uneven glaze layers and leave brush marks, resulting in light to dark glaze colors.
Thin glaze application can produce weaker colors, with dry surfaces, and reveal more of the underlying clay body color. Medium thickness glaze applications can develop glaze color, true glaze surface characteristics, and possible crystal growth. Thicker glaze applications can cause dripping on vertical surfaces and pooling in horizontal areas due to a higher concentration of glaze fluxes in those areas. It is important to note that each glaze application method can change the opacity, surface quality, and glaze color.
Kiln Atmosphere
What atmosphere was the glaze fired in?
Electric kilns produce clean, consistent oxidation atmospheres. However, carbon-based fuels such as natural gas, propane, wood, coal, oil, and sawdust can produce oxidation (more oxygen than fuel in the combustion process), neutral (equal amounts of fuel and oxygen in the combustion process), and various intensities of reduction (more fuel than oxygen in the combustion process, creating carbon monoxide) atmospheres. Reduction amounts can be very difficult to reproduce, as one potter’s medium-reduction atmosphere can be another’s heavy-reduction atmosphere. Variations in the length and amount of reduction can also change clay body/ glaze colors and glaze surface textures.
Soda (sodium carbonate or sodium bicarbonate) or salt (sodium chloride) kiln atmospheres contribute an orange-peel texture on exposed clay surfaces, but can also flux or melt glaze surfaces. Matte dry glazes can appear glossy, satin, or matte. The sodium atmosphere can also create a glossier glaze surface, running glazes on vertical surfaces or glaze pooling in horizontal areas. Glaze colors can appear brighter due to an increase in vitrification.1
While electric kilns firing in oxidation atmospheres produce fairly consistent results in glazes, reduction kilns have a wide range of atmospheric conditions that can be applied during a firing, any one of which can change the color, opacity, or surface texture of the glaze. Always investigate, where possible, the amount and duration of reduction a glaze requires.
Slips/Engobes
Was a slip used under the glaze? If so, consider slip and glaze fit.
Slips or engobes (interchangeable terms) are colored clays designed to fit the clay body and covering glaze. White slips can modify the glaze, as light is reflected back through the glaze, intensifying colors. Black or dark slips can absorb light, producing darker glaze colors. Any slip has the potential for altering the glaze color. Determine if a slip was used, and if so, was it white or dark?
Aside from altering the glaze color, the use of slips can alter the thermal expansion between the clay body and glaze. For example, if the glaze contracts more than the slip layer, it can cause tension in the glaze called crazing. Conversely, if the glaze contracts less, the glaze will be under extreme compression and produce shivering. Slips are a third system at play in a glaze fit which can alter glaze results.
Change in Raw Materials
Be aware that mining locations of raw materials can change, which may alter your processed product.
It is not unusual for ceramic raw materials used in glaze formulas to change, occasionally from one bag to the next, or sometimes slowly over a period of years. Do not assume a raw material remains consistent because the name on the bag stays the same. Raw materials, even the best quality-controlled materials, can sometimes shift in chemical composition, particle size, and organic content. Luckily, potters use a few basic raw materials such as feldspars, silica, whiting, dolomite, talc, frits, kaolin, and ball clays, that are also used in large industries. Potters can take advantage as industrial users dictate the quality-control parameters for these materials.
Glazes Work Until They Don’t. Always Test.
Most glazes work as advertised in books, magazine articles, or from fellow potters. In some instances the results in your kiln are fairly close and just need a little adjustment. At that point try to determine what has to be fixed, then begin testing. The standard rule is never mix an unknown glaze formula in large quantities. Test in a small batch and place test tiles throughout the kiln on vertical tiles at least 2 or 3 inches (5 or 7.6 cm) tall. Why? Small tiles might not indicate the flow of the glaze compared to the same glaze on a larger pot. In some instances, the molten weight of the glaze over increased surface area can cause the glaze to run excessively on vertical surfaces or pool in horizontal surfaces.
Often the most troublesome changes in a glaze or clay body are intermittent failures from what is predicted. They are the most complicated to diagnose and eventually correct. These random events need accurate record keeping and research to tease out the likely issues. One way to narrow the field is to put aside the least likely causes and concentrate on the remaining faults.
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.
1 Zamek, Jeff. What Every Potter Should Know. Iola, WI: Krause Publications, 1999, page 163.
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