Atmospheric firing is one of the most difficult-to-control facets of the clay world. This study shows control over flashing tone or value with alumina:silica ratios, hopefully contributing to a greater understanding of atmospheric firing parameters.
Defining the Terms
Alumina (Al2O3): A common material in soda compositions to strengthen the clay body.
Flashing: A unique color or patterning—often red and orange— on a ceramic surface as a result of kiln conditions—especially soda vapors—interacting with the clay body in variable ways.
Flux: Materials (often alkali, alkali earth, or transition metal oxides) loaded into a clay body to lower the melting point of a clay or glaze.
Soda Firing: A type of atmospheric vapor firing in which soda ash is introduced to the kiln to yield unique patterns and textures on ceramic surfaces.
Vitrification: A measure of the glass-like quality of a clay or glaze that has been melted as a result of the firing process.
As a cleaner alternative to salt firing, soda firing is one of the more relatively recent innovations in ceramics that offers a unique and sometimes overlooked method of surface color and atmospheric flashing. Soda firing, as a niche offshoot of atmospheric firing, works by introducing sodium carbonate or soda ash (and/or sodium bicarbonate and whiting) into the kiln chamber near the end of the firing (1).
The soda is often introduced as a liquid slurry—though it can be added as a solid powder—and is quickly vaporized in the kiln, undergoing thermal decomposition and giving off CO2. Both the spraying and thermal decomposition of the soda allow for dispersal throughout the chamber.1 These small soda particulates undergo deposition on the surfaces of the works being fired, generating unique atmospheric patterns.
In a typical soda firing, this will principally be the silica or alumina found at the surface of the clay or slip, which form sodium silicate and sodium aluminate, respectively, when exposed to the hot/decomposed soda ash. The former is usually glossy and whitish in appearance, helping to give the finished piece a lustrous, vitrified look, whereas the latter strengthens the clay matrix against the high temperatures of the kiln.2, 3 Choice in flux material—sodium, potassium, or calcium—as well as trace metal oxides contribute to the final flashing color, which is also controlled by the oxygen content, and thus, the degree of reduction during the firing.4
Flashing and the Results
At its most basic definition, flashing refers to the unique patterning of a ceramic surface in direct response to small particulate deposition on the surface—either soda buildup, resistance to soda buildup from Shigaraki inclusions, or interactions with trace iron content in the clay (2). Among the simplest (and most studied in-depth) examples is Japanese Bizen stoneware, a type of ceramic known for bright scarlet flashing patterns caused by a high-silica, high-iron clay unique to southern Japan. The clay fires to a semi-matte gray color with no atmospheric effects. However, by wrapping rice straw onto the surface of the piece, potassium acts as a flux to induce the formation of hematite and corundum crystal structures to give Bizen stoneware its characteristic bright red flashes.5 While crystal growth on the clay surface remains a strong contributor to flashing tone, other considerations—such as oxygen levels during firing and the rate at which a piece cools—generally contribute to how easily the proper crystal lattice forms and flashing profile results.6
These chemistry interactions unfortunately reside at the center of flashing patterns even in recipes that employ more complex clay and slip compositions. To probe this phenomenon, a series of clay-body recipes were developed based on the high-alumina (low-silica) experiments of Joel Willson and Casey Beck (3). Willson’s Soda Clay 20 (High Alumina XX Saggar) recipe was adapted for tiles (4). Willson and Beck’s work show a range of high-alumina clay bodies in soda firing—mostly with different materials accessible from the US to Europe. These recipes achieve exceptional flashing surfaces, showing that almost any clay source can be used within alumina:silica:flux boundaries to accomplish a beautiful atmospheric profile. The recipe of these soda-firing clays typically uses a base of lower silica and higher alumina components relative to typical stoneware clay. A more alumina-rich substrate will hinder the reaction of soda with the clay body and contribute to greater flashing, while retaining enough silica in the formula will ensure that the piece fires to a glossier finish.7
Adding Silica
In addition to these arguments, it is important to follow particular ratios of alumina to silica in a clay body, such that the vitrifying or flashing characteristics of one material are not dominated by the other in a finished piece. Conventionally, the safe zone for this is between a 3:1 and 4:1 ratio of silica to alumina, which is best (and most easily) mapped in a Stull chart.8 At the same time, the use of a Stull chart to plot atmospheric clays is somewhat nonstandard, as these recipes tend to lie outside of its usual scale.
Willson’s Soda Clay 20 (High Alumina XX Saggar) recipe was used to test the effects of changing silica content in an atmospheric firing. Silica content was added to the clay at 2.5, 5, 7.5, 10, and 15% (with small additions of nepheline syenite as well to ensure proper vitrification and to maintain 15% feldspar). Each tile was also fired under the same cone-10 reduction soda conditions as outlined in the original recipe, with one set of tiles in effectively direct or indirect contact with the soda flow in the flame.
Reading the Results
After firing, a visible color tone gradient could be seen, in which added silica content results in a lighter flashing color profile, and color saturation is lost. Both direct and indirect soda-exposure tile sets show a gradient of flashing color saturation changes from a bright vermillion to a sandstone-yellow as silica content increased.
On tiles that were placed in a dryer part of the kiln, a range of color tones is visible, albeit with some Gohonde spotting. Despite this, lighter to darker tone control is possible! Additionally, the “high alumina clay body” content was mixed and dipped as a flashing slip over Standard 182G clay.
The Stull chart map for the 6 tiles (4) shows a slow drop in the upper right-hand corner of the chart as silica content is added. The ratios of alumina to silica used for all tiles fall between 3:1 to 4:1 of silica to alumina.
For the tiles fired in direct contact with the soda flow within the kiln, a similar saturation trend is noticeable, despite there being much more soda content and graying on the edges of the tiles. While the indirect soda tiles showed some amount of Gohonde spotting, these tiles only show the classic soda gloss/orange-peel surface and the gray edges that are common with heavy soda content.
Interestingly, direct exposure to heavier soda flow parts of the kiln does not appear to change the flashing color saturation but does result in a more pitted texture, especially at lower silica ratios. At higher levels of silica, the residue of reacted soda begins to turn from a deep, slate gray into a glassier, whiter, silicate-based material, contributing to the illusion of a greater range of tone change than with indirect soda interactions.
Other Flashing Influences
Beyond silica and alumina, there is a wide range of other materials found in a clay body that hold significant influence on the profile of a finished piece. For soda firing in particular, molochite and pyrophyllite are the conventional filler materials used to assist flashing by maintaining low raw silica in the formula.
Molochite, a calcined, powdered china clay, prevents shrinkage of the clay body prior to firing and is particularly advantageous for maintaining a low silica-to-alumina ratio, thereby strengthening the green state with minimal sacrifice to the flashing patterns of the final product.9 It is effectively a finely ground porcelainic grog.
Pyrophyllite is an aluminosilicate mineral often used in conjunction with or in place of the purer quartz silica additions, providing improved thermal shock tolerance as well as proportionally higher alumina loading of the clay composition.10 It is effectively a cheaper option to molochite when used as a filler/structural support additive in clay recipes.
If atmospheric flashing clays interest you, I highly recommend looking through www.glazy.org at Joel Willson’s and Casey Beck’s flashing clay recipes. There are a range of colors, textures, and recipe options—ranging from 20% molochite to just pyrophyllite and no molochite at all. And while the ceramics world is currently experiencing an unprecedented loss of mined materials not seen before (including Newman Red, EPK kaolin, Custer feldspar, Helmer, Gerstley borate, etc.), there are a wide range of clay recipe options still available.
Lastly, it is common knowledge in pottery that it is incredibly time consuming to mix up, let dry, and wedge up your own clay from raw ingredients. For all of my work (and this entire paper), high-alumina clays were mixed up as flashing slips and dipped over Standard 182G clay tiles. While initial clays were made and used as designed, there was a strange degree of warping and cracking in the soda firings. Upon switching to Standard 182G as the base and a high-alumina clay recipe as a flashing slip, no more work was lost to cracking/warping with the high-alumina “clay” recipes. This begets the argument: all high-alumina clay recipes are also flashing slip recipes!
the authors Scott Valentine is a third-year biochemistry student at the University of Richmond.
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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Atmospheric firing is one of the most difficult-to-control facets of the clay world. This study shows control over flashing tone or value with alumina:silica ratios, hopefully contributing to a greater understanding of atmospheric firing parameters.
Defining the Terms
Alumina (Al2O3 ): A common material in soda compositions to strengthen the clay body.
Flashing: A unique color or patterning—often red and orange— on a ceramic surface as a result of kiln conditions—especially soda vapors—interacting with the clay body in variable ways.
Flux: Materials (often alkali, alkali earth, or transition metal oxides) loaded into a clay body to lower the melting point of a clay or glaze.
Soda Firing: A type of atmospheric vapor firing in which soda ash is introduced to the kiln to yield unique patterns and textures on ceramic surfaces.
Vitrification: A measure of the glass-like quality of a clay or glaze that has been melted as a result of the firing process.
As a cleaner alternative to salt firing, soda firing is one of the more relatively recent innovations in ceramics that offers a unique and sometimes overlooked method of surface color and atmospheric flashing. Soda firing, as a niche offshoot of atmospheric firing, works by introducing sodium carbonate or soda ash (and/or sodium bicarbonate and whiting) into the kiln chamber near the end of the firing (1).
The soda is often introduced as a liquid slurry—though it can be added as a solid powder—and is quickly vaporized in the kiln, undergoing thermal decomposition and giving off CO2. Both the spraying and thermal decomposition of the soda allow for dispersal throughout the chamber.1 These small soda particulates undergo deposition on the surfaces of the works being fired, generating unique atmospheric patterns.
In a typical soda firing, this will principally be the silica or alumina found at the surface of the clay or slip, which form sodium silicate and sodium aluminate, respectively, when exposed to the hot/decomposed soda ash. The former is usually glossy and whitish in appearance, helping to give the finished piece a lustrous, vitrified look, whereas the latter strengthens the clay matrix against the high temperatures of the kiln.2, 3 Choice in flux material—sodium, potassium, or calcium—as well as trace metal oxides contribute to the final flashing color, which is also controlled by the oxygen content, and thus, the degree of reduction during the firing.4
Flashing and the Results
At its most basic definition, flashing refers to the unique patterning of a ceramic surface in direct response to small particulate deposition on the surface—either soda buildup, resistance to soda buildup from Shigaraki inclusions, or interactions with trace iron content in the clay (2). Among the simplest (and most studied in-depth) examples is Japanese Bizen stoneware, a type of ceramic known for bright scarlet flashing patterns caused by a high-silica, high-iron clay unique to southern Japan. The clay fires to a semi-matte gray color with no atmospheric effects. However, by wrapping rice straw onto the surface of the piece, potassium acts as a flux to induce the formation of hematite and corundum crystal structures to give Bizen stoneware its characteristic bright red flashes.5 While crystal growth on the clay surface remains a strong contributor to flashing tone, other considerations—such as oxygen levels during firing and the rate at which a piece cools—generally contribute to how easily the proper crystal lattice forms and flashing profile results.6
These chemistry interactions unfortunately reside at the center of flashing patterns even in recipes that employ more complex clay and slip compositions. To probe this phenomenon, a series of clay-body recipes were developed based on the high-alumina (low-silica) experiments of Joel Willson and Casey Beck (3). Willson’s Soda Clay 20 (High Alumina XX Saggar) recipe was adapted for tiles (4). Willson and Beck’s work show a range of high-alumina clay bodies in soda firing—mostly with different materials accessible from the US to Europe. These recipes achieve exceptional flashing surfaces, showing that almost any clay source can be used within alumina:silica:flux boundaries to accomplish a beautiful atmospheric profile. The recipe of these soda-firing clays typically uses a base of lower silica and higher alumina components relative to typical stoneware clay. A more alumina-rich substrate will hinder the reaction of soda with the clay body and contribute to greater flashing, while retaining enough silica in the formula will ensure that the piece fires to a glossier finish.7
Adding Silica
In addition to these arguments, it is important to follow particular ratios of alumina to silica in a clay body, such that the vitrifying or flashing characteristics of one material are not dominated by the other in a finished piece. Conventionally, the safe zone for this is between a 3:1 and 4:1 ratio of silica to alumina, which is best (and most easily) mapped in a Stull chart.8 At the same time, the use of a Stull chart to plot atmospheric clays is somewhat nonstandard, as these recipes tend to lie outside of its usual scale.
Willson’s Soda Clay 20 (High Alumina XX Saggar) recipe was used to test the effects of changing silica content in an atmospheric firing. Silica content was added to the clay at 2.5, 5, 7.5, 10, and 15% (with small additions of nepheline syenite as well to ensure proper vitrification and to maintain 15% feldspar). Each tile was also fired under the same cone-10 reduction soda conditions as outlined in the original recipe, with one set of tiles in effectively direct or indirect contact with the soda flow in the flame.
Reading the Results
After firing, a visible color tone gradient could be seen, in which added silica content results in a lighter flashing color profile, and color saturation is lost. Both direct and indirect soda-exposure tile sets show a gradient of flashing color saturation changes from a bright vermillion to a sandstone-yellow as silica content increased.
On tiles that were placed in a dryer part of the kiln, a range of color tones is visible, albeit with some Gohonde spotting. Despite this, lighter to darker tone control is possible! Additionally, the “high alumina clay body” content was mixed and dipped as a flashing slip over Standard 182G clay.
The Stull chart map for the 6 tiles (4) shows a slow drop in the upper right-hand corner of the chart as silica content is added. The ratios of alumina to silica used for all tiles fall between 3:1 to 4:1 of silica to alumina.
For the tiles fired in direct contact with the soda flow within the kiln, a similar saturation trend is noticeable, despite there being much more soda content and graying on the edges of the tiles. While the indirect soda tiles showed some amount of Gohonde spotting, these tiles only show the classic soda gloss/orange-peel surface and the gray edges that are common with heavy soda content.
Interestingly, direct exposure to heavier soda flow parts of the kiln does not appear to change the flashing color saturation but does result in a more pitted texture, especially at lower silica ratios. At higher levels of silica, the residue of reacted soda begins to turn from a deep, slate gray into a glassier, whiter, silicate-based material, contributing to the illusion of a greater range of tone change than with indirect soda interactions.
Other Flashing Influences
Beyond silica and alumina, there is a wide range of other materials found in a clay body that hold significant influence on the profile of a finished piece. For soda firing in particular, molochite and pyrophyllite are the conventional filler materials used to assist flashing by maintaining low raw silica in the formula.
If atmospheric flashing clays interest you, I highly recommend looking through www.glazy.org at Joel Willson’s and Casey Beck’s flashing clay recipes. There are a range of colors, textures, and recipe options—ranging from 20% molochite to just pyrophyllite and no molochite at all. And while the ceramics world is currently experiencing an unprecedented loss of mined materials not seen before (including Newman Red, EPK kaolin, Custer feldspar, Helmer, Gerstley borate, etc.), there are a wide range of clay recipe options still available.
Lastly, it is common knowledge in pottery that it is incredibly time consuming to mix up, let dry, and wedge up your own clay from raw ingredients. For all of my work (and this entire paper), high-alumina clays were mixed up as flashing slips and dipped over Standard 182G clay tiles. While initial clays were made and used as designed, there was a strange degree of warping and cracking in the soda firings. Upon switching to Standard 182G as the base and a high-alumina clay recipe as a flashing slip, no more work was lost to cracking/warping with the high-alumina “clay” recipes. This begets the argument: all high-alumina clay recipes are also flashing slip recipes!
the authors Scott Valentine is a third-year biochemistry student at the University of Richmond.
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 Nichols, G. (2006). Soda, Clay and Fire, American Ceramic Society, cited in Soda Firing Techniques, Tips and Recipes, Ceramic Arts Daily (2009), 2-5 https://internationalschoolofceramicart.wordpress.com.
2 Beck, C. (2020). Soda Firing. Beck Pots, https://www.beckpots.com.
3 Li, Y., Song, L., Chen, G. et al. (2025). A Study of the Influence and Mechanism of Alumina Ceramic Powder on the High-Temperature Strength of NaCl–Na2CO3 Cores in Die-Casting Production. Inter Metalcast 19, 1516–1531. https://doi.org/10.1007/s40962-024-01400-x.
4 Zamek, J. (2019). Q&A: Soda Firing. Ceramics, Art and Perception, (113), 136-137. https://newman. richmond.edu/login?qurl=https%3A%2F%2Fwww.proquest.com%2Ftrade-journals%2Fq-amp-soda-firing%2Fdocview%2F2296080761%2Fse-2%3Faccountid%3D14731.
5 Kusano, Y., et al. (2009). Science in the Art of the Master Bizen Potter. Accounts of Chemical Research, 43(6), 906-915. https://pubs.acs.org/doi/pdf/10.1021/ar9001872?ref=article_openPDF.
6 Ibid.
7 Beck, C. (September 2023). Clay Bodies for Soda. Ceramics Monthly (744), 54-56. https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/techno-file-clay-bodies-for-soda.
8 Ibid.
9. Nichols, G. (2001). Technical and Aesthetic Investigations in Soda Glaze Ceramics. [Unpublished doctoral dissertation]. Monash University.
10. Ibid.
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