The Stull chart is an important tool for predicting a glaze outcome—and should be acknowledged for that—but it also has limitations. Unfortunately, these limitations are often overlooked. This article gives an example of a glaze family that doesn’t work for the Stull chart.
Defining the Terms
Gloss: When a surface is glassy, such that specular reflectance is possible and reflects via classic physics.
Matte: When the surface of a glaze is textured, such that it doesn’t show specular reflectance and instead diffuses reflectance. This is most often the case of surface crystallization (calcium, magnesium, etc.).
Semi-Matte: An intermediate between matte and gloss, such that some texture is present to break up specular reflectance, but it does not fully possess diffuse reflectance properties.
Stull Chart: A commonly referenced chart that plots silica:alumina ratios of a glaze to attempt to predict its outcome and physical properties.
Unity Molecular Formula or UMF: A ratio of similarly-behaving elements in a glaze, in which elements in a column can replace/substitute others that are expressed as R2O/RO fluxes, R2O3 stabilizers (this classifies borates as stabilizers, when they’re a flux/special case), and RO2 as glass-formers.
Vitrification: A measure of how cured or sealed/water-tight a clay or glaze is by its total melted nature after being fired.
Defining Ceramic Glazes
If you do a broad internet search exploring what makes up a ceramic glaze or what the defining factors of ceramic glaze are, you will almost always be met by the same results: silica, flux (mostly alkali and alkali earth metals), and alumina. The ceramics world determined (by convenience and tradition?) that if one of these three components is not present in a mix, a functional glaze will not be formed. You will likely also come across some discussions about improper ratios of silica, alumina, and flux in a glaze contributing to predictions of crazed, matte, or underfired properties. Each of these components plays a specific role in the development of a glaze.
Silica, for example, is an important ingredient to create the glossy and glass-forming component of a ceramic glaze.1
The flux is often present in a feldspar (but borates show up heavily in mid-range and low-fire ceramics) that functions as a melting agent. Fluxes interact with other materials typically of higher melting temperatures and will lower their melting points allowing ceramic recipes to be fired in a kiln with far lower temperatures and energy consumption.2
Alumina plays a stabilizing role in the glaze mix composition. When a glaze is fired and melting occurs, many oxides are released to produce the glass structure of a glaze and alumina serves to reinforce and stabilize the glass structures.3 With thousands of years of mixing glazes, it seems to many that these are the absolute conditions for developing a glaze. As such, the plotting of alumina:silica ratios is important in developing a glaze, but is often over-emphasized and treated somewhat as the religion of pottery, which is more wrong than it is right.
A Stull chart is a tool developed to understand what the result of a fired glaze recipe is likely to appear as with some physical properties to boot. It was developed 112 years ago (Stull, 1912), which is considered to be completely obsolete in terms of science.4 As a scientist, I would also argue that Stull’s Silica:Alumina chart wasn’t intended to be used as the absolute tool that it is used as today. It was just used to communicate some behavior patterns in glazes to a very limited degree. It takes SiO2 amounts (x-axis) and the Al2O3 amounts (y-axis) based on the glaze’s Unity Molecular Formulas (UMF) to plot a point of the glaze on the graph.5
The region on the graph that the glaze is plotted in is then used to predict the state of the glaze when it is fired: matte, crazed, underfired/unfused, semi-matte/satin, or gloss/bright (1). If the traditional conditions of a glaze composition (silica, flux, and alumina) are missing or heavily substituted, Stull charts are no longer applicable and cannot predict the outcome of a fired glaze. And as the world transitions away from cone 10–11 pottery, Stull charts become less and less accurate but it seems as though no one is discussing this.
Will McComb, the clay and glaze technician at Kentucky Mudworks, states, “I think the Stull chart gets misunderstood. People have a tendency to treat it with an almost religious fervor. It’s a graph. It charts glaze behavior at certain quantities and proportions of alumina and silica. And it does a wonderful job predicting glaze behavior within that framework. But you have to have more context to fully interpret it. The ‘standard’ Stull chart predicts glaze behavior when fluxes are set to 0.3 R2O to 0.7 RO at cone 11, and the RO is calcium. This is a very specific context. As soon as you start messing with that ratio, or using different alkaline earths, or loading the glaze up with colorants (which, oops, are also fluxes), or adding boron (which only ‘maps’ to the Stull chart at specific proportions at specific temperatures) your chart will change dramatically. This is especially true for predictions about crazing, which don’t take into account material differences in CTE or the fact that expansion fit is determined by two variables, not one. Your clay body is an absolutely essential variable in predicting expansion-fit behavior. Not only that, but also with the classic ratios, many of the lines on Stull that are straight lines implying hard regions are actually more like curves. They also shift a bit with alkaline earth selection and they get fuzzy once you add titanium into the mix. The classic Stull map also doesn’t delineate the crystallization zone that exists in the low-alumina regions. In this zone the regions for matte and gloss still technically exist but their behaviors change radically from the middle part of the map. Again, context matters. The Stull chart is a powerful and highly useful tool, but it is only one of many tools at our disposal and for it to be useful you need to understand the context in which it exists and understand its limitations.”
Changing the Parameters
Based on the parameters of the Stull chart, we wanted to see if we could make a mid-range glaze with no feldspar or silica, which does sound a little crazy if you talk to most potters who mix their own glazes. This would remove one (or two?) entire standard component(s) from the Stull chart and your silica content would only be coming from clay in the glaze, which would mostly create a static alumina:silica ratio. I wanted to flux this with something that didn’t plot well on the Stull chart, so I chose borates (I have an old supply of Gerstley borate, but Gillespie borate works mostly the same) and used an almost obscene amount of red and black iron oxide to pigment the borate-fluxed EPK mix—and also contribute some fluxing nature as well. After some 30 tests, I ended up with the following recipe and resulting surface from a drop-and-soak firing schedule at cone 6 (2).
Interestingly, the gunmetal blooms only show up on a slow cool and are only directed toward the inside of the kiln, where heat is present for longer, radiating off of other pots or kiln furniture. This glaze started as equal amounts of EPK:borates:BIO:RIO and equivalences of borates (Gillespie or Gerstley) were added until it fluxed aggressively, and even started to run down the test tile (3). Not surprisingly, and based on the limitations of the Stull chart, this plots horribly (4). It plots as crazed, matte, and underfired, of which it is actually none of those things. It is a completely vitrified, non-crazed, satin glaze that does have the drawback of microwaving a little “hot” because it is almost 30% iron oxide by weight. And we all know what happens when you put a fork or spoon in the microwave.
The next logical step is to formulate glazes that should plot on the Stull chart, as being satin, gloss, etc. This includes the additions of silica to just move the location and expected properties of the glaze formulation on the plot. For example, adding a small amount of silica, such that the recipe should be translucent and clear but still crazed, via the plot below, creates a matte/satin glaze (5).
As you can see, this is not a clear, bright glaze. You’d have to take our word for it that it is not crazed as well, but you can also tell that it is matte, despite not charting as matte.
When increasing the silica content and dropping the Gillespie borate content to make an “ideal” glaze, you should expect the glaze to be a nice clear, glossy amber color if going solely on the Stull chart (6). Instead, it is an opaque, satin, not-fully-cured glaze that does not at all have the properties that the Stull chart suggests it should.
The Stull chart doesn’t work for borate system glazes, nor for anticipating flux contributions by metal oxide pigments.
It should be noted that these glazes don’t map to Stull even if you use the Extended UMF on Glazy.org. The Extended UMF is actually a pretty great predictive tool but even it falls apart when you have these really extreme chemistries, “extreme” as in low alumina, very low-alkali metals, iron subbing in as the primary alkaline earth over calcium and magnesium, and substantially more boron than needed to melt the glaze under normal conditions. But on that note, just to play devil’s advocate, Glaze 3 does in fact chart well to the left and down from the “ideal gloss” region if you use the Extended UMF. Still, its surface isn’t explained by its position on the map. Using some interpretive skill, we could predict that this glaze may well be a microcrystalline glaze, and potentially a matte based on its chemistry. But the map won’t tell us this. We would need a solid base of experience and some creativity in interpreting the UMF.
Conclusion
While Stull charts are valuable tools for anticipating glaze properties, there are limitations to them, which are becoming more substantial as the world moves away from cone-10 firings and standard silica-clay-feldspar glaze formulations. By all means, continue to use Stull charts when looking at glazes, but also be open to working outside of that shrinking box as the world shifts toward more energy-efficient firing options, pottery finished at lower cones, and glazes developed with more alternative flux content like borates and talc, etc.
the authors Anthony Marzano is a current student studying biology at the University of Richmond. Will McComb is the clay and glaze technician at Kentucky Mudworks. He is committed to increasing clay and glaze literacy for all ceramic artists. Ryan Coppage is currently chemistry teaching faculty at the University of Richmond. He fiddles with various glaze projects and makes a reasonable number of pots. To see more, visit www.RyanCoppage.com.
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The Stull chart is an important tool for predicting a glaze outcome—and should be acknowledged for that—but it also has limitations. Unfortunately, these limitations are often overlooked. This article gives an example of a glaze family that doesn’t work for the Stull chart.
Defining the Terms
Gloss: When a surface is glassy, such that specular reflectance is possible and reflects via classic physics.
Matte: When the surface of a glaze is textured, such that it doesn’t show specular reflectance and instead diffuses reflectance. This is most often the case of surface crystallization (calcium, magnesium, etc.).
Semi-Matte: An intermediate between matte and gloss, such that some texture is present to break up specular reflectance, but it does not fully possess diffuse reflectance properties.
Stull Chart: A commonly referenced chart that plots silica:alumina ratios of a glaze to attempt to predict its outcome and physical properties.
Unity Molecular Formula or UMF: A ratio of similarly-behaving elements in a glaze, in which elements in a column can replace/substitute others that are expressed as R2O/RO fluxes, R2O3 stabilizers (this classifies borates as stabilizers, when they’re a flux/special case), and RO2 as glass-formers.
Vitrification: A measure of how cured or sealed/water-tight a clay or glaze is by its total melted nature after being fired.
Defining Ceramic Glazes
If you do a broad internet search exploring what makes up a ceramic glaze or what the defining factors of ceramic glaze are, you will almost always be met by the same results: silica, flux (mostly alkali and alkali earth metals), and alumina. The ceramics world determined (by convenience and tradition?) that if one of these three components is not present in a mix, a functional glaze will not be formed. You will likely also come across some discussions about improper ratios of silica, alumina, and flux in a glaze contributing to predictions of crazed, matte, or underfired properties. Each of these components plays a specific role in the development of a glaze.
Silica, for example, is an important ingredient to create the glossy and glass-forming component of a ceramic glaze.1
The flux is often present in a feldspar (but borates show up heavily in mid-range and low-fire ceramics) that functions as a melting agent. Fluxes interact with other materials typically of higher melting temperatures and will lower their melting points allowing ceramic recipes to be fired in a kiln with far lower temperatures and energy consumption.2
Alumina plays a stabilizing role in the glaze mix composition. When a glaze is fired and melting occurs, many oxides are released to produce the glass structure of a glaze and alumina serves to reinforce and stabilize the glass structures.3 With thousands of years of mixing glazes, it seems to many that these are the absolute conditions for developing a glaze. As such, the plotting of alumina:silica ratios is important in developing a glaze, but is often over-emphasized and treated somewhat as the religion of pottery, which is more wrong than it is right.
A Stull chart is a tool developed to understand what the result of a fired glaze recipe is likely to appear as with some physical properties to boot. It was developed 112 years ago (Stull, 1912), which is considered to be completely obsolete in terms of science.4 As a scientist, I would also argue that Stull’s Silica:Alumina chart wasn’t intended to be used as the absolute tool that it is used as today. It was just used to communicate some behavior patterns in glazes to a very limited degree. It takes SiO2 amounts (x-axis) and the Al2O3 amounts (y-axis) based on the glaze’s Unity Molecular Formulas (UMF) to plot a point of the glaze on the graph.5
The region on the graph that the glaze is plotted in is then used to predict the state of the glaze when it is fired: matte, crazed, underfired/unfused, semi-matte/satin, or gloss/bright (1). If the traditional conditions of a glaze composition (silica, flux, and alumina) are missing or heavily substituted, Stull charts are no longer applicable and cannot predict the outcome of a fired glaze. And as the world transitions away from cone 10–11 pottery, Stull charts become less and less accurate but it seems as though no one is discussing this.
Will McComb, the clay and glaze technician at Kentucky Mudworks, states, “I think the Stull chart gets misunderstood. People have a tendency to treat it with an almost religious fervor. It’s a graph. It charts glaze behavior at certain quantities and proportions of alumina and silica. And it does a wonderful job predicting glaze behavior within that framework. But you have to have more context to fully interpret it. The ‘standard’ Stull chart predicts glaze behavior when fluxes are set to 0.3 R2O to 0.7 RO at cone 11, and the RO is calcium. This is a very specific context. As soon as you start messing with that ratio, or using different alkaline earths, or loading the glaze up with colorants (which, oops, are also fluxes), or adding boron (which only ‘maps’ to the Stull chart at specific proportions at specific temperatures) your chart will change dramatically. This is especially true for predictions about crazing, which don’t take into account material differences in CTE or the fact that expansion fit is determined by two variables, not one. Your clay body is an absolutely essential variable in predicting expansion-fit behavior. Not only that, but also with the classic ratios, many of the lines on Stull that are straight lines implying hard regions are actually more like curves. They also shift a bit with alkaline earth selection and they get fuzzy once you add titanium into the mix. The classic Stull map also doesn’t delineate the crystallization zone that exists in the low-alumina regions. In this zone the regions for matte and gloss still technically exist but their behaviors change radically from the middle part of the map. Again, context matters. The Stull chart is a powerful and highly useful tool, but it is only one of many tools at our disposal and for it to be useful you need to understand the context in which it exists and understand its limitations.”
Changing the Parameters
Based on the parameters of the Stull chart, we wanted to see if we could make a mid-range glaze with no feldspar or silica, which does sound a little crazy if you talk to most potters who mix their own glazes. This would remove one (or two?) entire standard component(s) from the Stull chart and your silica content would only be coming from clay in the glaze, which would mostly create a static alumina:silica ratio. I wanted to flux this with something that didn’t plot well on the Stull chart, so I chose borates (I have an old supply of Gerstley borate, but Gillespie borate works mostly the same) and used an almost obscene amount of red and black iron oxide to pigment the borate-fluxed EPK mix—and also contribute some fluxing nature as well. After some 30 tests, I ended up with the following recipe and resulting surface from a drop-and-soak firing schedule at cone 6 (2).
Interestingly, the gunmetal blooms only show up on a slow cool and are only directed toward the inside of the kiln, where heat is present for longer, radiating off of other pots or kiln furniture. This glaze started as equal amounts of EPK:borates:BIO:RIO and equivalences of borates (Gillespie or Gerstley) were added until it fluxed aggressively, and even started to run down the test tile (3). Not surprisingly, and based on the limitations of the Stull chart, this plots horribly (4). It plots as crazed, matte, and underfired, of which it is actually none of those things. It is a completely vitrified, non-crazed, satin glaze that does have the drawback of microwaving a little “hot” because it is almost 30% iron oxide by weight. And we all know what happens when you put a fork or spoon in the microwave.
The next logical step is to formulate glazes that should plot on the Stull chart, as being satin, gloss, etc. This includes the additions of silica to just move the location and expected properties of the glaze formulation on the plot. For example, adding a small amount of silica, such that the recipe should be translucent and clear but still crazed, via the plot below, creates a matte/satin glaze (5).
As you can see, this is not a clear, bright glaze. You’d have to take our word for it that it is not crazed as well, but you can also tell that it is matte, despite not charting as matte.
When increasing the silica content and dropping the Gillespie borate content to make an “ideal” glaze, you should expect the glaze to be a nice clear, glossy amber color if going solely on the Stull chart (6). Instead, it is an opaque, satin, not-fully-cured glaze that does not at all have the properties that the Stull chart suggests it should.
The Stull chart doesn’t work for borate system glazes, nor for anticipating flux contributions by metal oxide pigments.
It should be noted that these glazes don’t map to Stull even if you use the Extended UMF on Glazy.org. The Extended UMF is actually a pretty great predictive tool but even it falls apart when you have these really extreme chemistries, “extreme” as in low alumina, very low-alkali metals, iron subbing in as the primary alkaline earth over calcium and magnesium, and substantially more boron than needed to melt the glaze under normal conditions. But on that note, just to play devil’s advocate, Glaze 3 does in fact chart well to the left and down from the “ideal gloss” region if you use the Extended UMF. Still, its surface isn’t explained by its position on the map. Using some interpretive skill, we could predict that this glaze may well be a microcrystalline glaze, and potentially a matte based on its chemistry. But the map won’t tell us this. We would need a solid base of experience and some creativity in interpreting the UMF.
Conclusion
While Stull charts are valuable tools for anticipating glaze properties, there are limitations to them, which are becoming more substantial as the world moves away from cone-10 firings and standard silica-clay-feldspar glaze formulations. By all means, continue to use Stull charts when looking at glazes, but also be open to working outside of that shrinking box as the world shifts toward more energy-efficient firing options, pottery finished at lower cones, and glazes developed with more alternative flux content like borates and talc, etc.
the authors Anthony Marzano is a current student studying biology at the University of Richmond. Will McComb is the clay and glaze technician at Kentucky Mudworks. He is committed to increasing clay and glaze literacy for all ceramic artists. Ryan Coppage is currently chemistry teaching faculty at the University of Richmond. He fiddles with various glaze projects and makes a reasonable number of pots. To see more, visit www.RyanCoppage.com.
1 Tony Hansen. SiO2 (Silicon dioxide, silica). Retrieved November 24, 2024, from https://digitalfire.com/oxide/sio2.
2 Tony Hansen. Flux. (n.d.). Retrieved November 24, 2024, from https://digitalfire.com/glossary/flux.
3 Tony Hansen. Al2O3 (Aluminum oxide, alumina). (n.d.). Retrieved November 25, 2024, from https://digitalfire.com/oxide/Al2O3.
4 Stull, R.T., Influences of variable silica and alumina on porcelain glazes. American Ceramic Society. p 62–70. 1912.
5 McLeod, Sue. Matte to glossy. Techno File. Default. Retrieved November 24, 2024, from https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/techno-file-matte-to-glossy#.
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
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