Techno File: Clay Bodies for Soda Casey Beck

Appears in the September 2023 issue of Ceramics Monthly.

Soda firing is infinitely expansive in the results that can be achieved. Casey Beck’s current work, resulting from seven years of research, involves using high-alumina clay bodies fired in a soda kiln without applying glaze or slip to the exterior. 

Defining the Terms 

Soda Firing: Near the peak temperature of a firing, soda ash is introduced into the gas- or wood-fueled kiln, volatilizing and combining with the surface of the clay, and the silica within it, to form a glaze on the work. The soda firing referenced in this article utilizes the cone-10 temperature range in a gas kiln. 

Clay Body: A combination of plastic clays, fluxes, fillers, etc. 

Flux: A melting agent, which aids in vitrifying the clay body. 

Filler: Non-plastic materials added to clay to promote strength and reduce shrinkage. 

Plastic Clay: The component that introduces the qualities that we look for in a clay body: plasticity, or the ability to stretch and hold its shape. 

Silica to Alumina Ratio (SiO2:Al2O3): Ratio of silicon dioxide to aluminum oxide in a clay body. In this article, the silica will always be in relation to the alumina, which is set at one. 

Silica-to-Alumina Ratio in Clay Bodies 

Silica-to-alumina ratios in common ceramic formulas can determine the matteness and quality of not only a glaze, but also the soda glaze that forms on the surface of a clay body during a soda firing. Typical porcelain and stoneware recipes tend to have a silica-to-alumina ratio between 4:1 and 5:1. In a soda firing, these clay bodies develop a desirable high-gloss soda glaze on the surface. According to my research and definition, high-alumina clay bodies have a ratio ranging between 3:1 and 4:1. Gail Nichols writes in her book, Soda, Clay and Fire, that “good fire color and glaze qualities occur when the clay’s silica/alumina ratio is close to 3.0[:1].” When the ratio begins to dip below 3:1, the clay has a harder time forming the soda glaze, but produces stronger flashing. When the ratio is higher than 4:1, the clay begins to lose its color response, while the soda glaze becomes glossier, losing its sense of depth. Note: The silica and alumina added to a clay body do not have to come from pure materials, as both are found in many materials that are already used in clay-body formation, such as various clays and feldspars. 

Setting Material Limits to Achieve Proper Vitrification 

Due to the high-alumina content in my clay bodies, they tend to be more refractory than standard clay resulting in a higher maturing point, thus they need a more powerful flux in order to vitrify properly to cone 10. To promote effective vitrification, a flux mixture of approximately 25% is usually needed. About 50% of the clay body should be comprised of plastic clays and the remaining 25% fillers such as pyrophyllite, silica, and molochite. 

Note: A stoneware clay body may need less flux than a porcelain body, as fireclays often contain secondary fluxes. Stonewares also tend to be higher in iron, which will produce a greener soda-glaze surface as opposed to lower iron clays, which produce a bluer glaze. 

A common misconception is that adding alumina hydrate or a similar material to the clay body is an effective way to lower the silica-to-alumina ratio. Although this may sometimes work, there are other materials that will lower the silica-to-alumina ratio and offer other properties that we look for in a workable clay body such as a different clay, pyrophyllite, or the choice of flux. These material limits are mere guidelines. As you begin to test new clay bodies, you can push beyond these guidelines to further understand the materials and their effects when fired together, in order to achieve desirable results. 

Clay Body Recipe Formulation Using Glazy.org 

Glazy.org is a free online tool in which you can input ceramic recipes of any kind, adjust them as needed, and share your research with a broader community. For developing clay bodies, we can find the silica-to-alumina ratio using Glazy.org and the analyses it provides. 

For a body that already exists, you can input the recipe into Glazy and use the calculator feature to change proportions and substitute materials. One way to achieve a high-alumina clay body is to change the recipe’s proportions by decreasing the silica content in favor of increasing the clay. This can also be done by substituting additional materials, for example, XX Sagger ball clay has less silica and more alumina than OM 4 ball clay. If you are using a stoneware clay, reducing the fireclay content, and adding kaolin can also lower the silica. You can also reduce the silica and add pyrophyllite and molochite to the clay body. While you make these various changes, use the analysis portion of the calculator to maintain or lower the silica-to-alumina ratio. 

Once you understand the basics of clay bodies and hot to alter them to contain higher alumina levels, it is easy to begin developing them from scratch. For example, I may begin with 30–40% kaolin and/or fireclay, then add 10–20% ball clay for the plastic clay component. I will then add around 25% feldspars and secondary fluxes. Nepheline syenite and soda feldspars tend to offer a stronger fluxing power than potash feldspars, while also offering sodium, which promotes a stronger color response in the flashing of the clay. From here, the silica-to-alumina ratio can be adjusted with the fillers. Start by including 5–15% silica and/or pyrophyllite, then molochite, kyanite, mullite, or another non-plastic grog. Once you reach a clay body totaling 100, you can go back and fine tune each material slightly to adjust the silica to alumina ratio. 

Throughout this process, I recommend formulating several similar clay bodies to be mixed and tested at the same time. Beginning with a line blend may help to understand the impacts of slight material changes in a clay body. Some tests that you can run are substituting one clay or feldspar for another, or substituting pyrophyllite for the silica. Make these substitutions in 5% or 10% increments and place them around the kiln to see the full range of effects that occur. From these tests, you can directly see the impact that changing these materials and amounts has on the surface quality. 

Silica Sand 

Silica sand heavily influences a clay body’s surface development during the firing, as it creates small pits where the sand breaches the surface of the clay. The pits created by 30–60-mesh grains of sand can sometimes become covered in soda glaze or create small orange-peel-like pitting, whereas sand larger than 30 mesh can begin to visually break up the surface as the soda flows around it. Since silica alone has a high melting point, these sand particles seem to be resilient to the soda and as the soda glaze forms during the firing, rivulets flow around the sand. As little as 1% 10-mesh silica sand can make for a visually stimulating surface, while 10% can create a very busy surface. 

Mixing Clay 

When working with new clay, it is helpful to test it in small 1000– 5000-gram batches before committing to a full 150-pound batch. This small batch will provide enough clay to make a shrinkage bar, a small tile for absorption testing, and a few small pieces. Mix the test batch in a 5-gallon bucket as a thick slip and then allow the clay to firm up on a plaster slab before wedging and using it. 

If you establish a clay body that you like and works well, moving onto a larger batch in a clay mixer is a good idea, however, it may be less plastic due to the clay particles not being sufficiently hydrated. Adding 1–2% bentonite (first blunged in water) to the mixer beforehand can help reduce the shortness. 

Firing 

The method of firing the work is just as important as the clay bodies themselves, as it can accentuate the qualities of these specifically formulated clay bodies. High-alumina clay bodies may require extra soda added to the kiln than you would with regular silica-rich clay bodies to achieve a desirable soda glaze. My preference requires ¹⁄3 to ½ pound of soda ash per cubic foot of stacking space in the kiln. For example, for the 5-cubic-foot kiln that I fire, I use about 2½ pounds of soda ash, and about 9 pounds in my 22-cubic-foot kiln. 

During the introduction of soda, the atmosphere further affects the surfaces. Reducing the kiln can produce carbon-trap gray tones, while oxidizing the atmosphere can create a white soda glaze. 

Here is my typical firing schedule: 

• Fire kiln to cone 9 overnight in a neutral atmosphere. 
• 8–10am: Introduce soda as the kiln slowly continues to heat, beginning to bend cone 10. 
• 10–11am: Soak in neutral atmosphere to drop cone 10. 
• 11–11:30am: Soak in medium to light reduction. 
• 11:30–Noon: Soak in medium to heavy reduction. 
• Noon–5pm: Down fire in varying atmospheres to 1600°F (871°C). If I want darker surfaces, I will lean toward reduction during this time. If I want lighter surfaces, I will lean toward oxidation. Sometimes a combination of reduction and oxidation is used to create surfaces that reflect both atmospheres. 

Finding Absorption Rates 

The final part of testing involves finding the absorption rates of your clay body, which is imperative for the longevity of the work, especially pottery that is meant to be used. To find the absorption rate of a clay body, fire a small 2×2-inch tile in the soda kiln along with your other work. Once fired, record the initial weight of the tile, then boil it in water for an hour, turn off the heat, and let it sit in the water overnight. The following day, pat the tile dry and weigh it again to measure the final weight. Use the following equation to find the absorption of the clay: 

• (wet weight-dry weight)/(dry weight) × 100 = % Absorption A 0% absorption rate is ideal. 

Final Thoughts and a Call to Artists 

Although this article gets to the basics of formulating high-alumina clay bodies for the soda kiln, it does not fully cover a few other key factors in the outcome of a soda firing. The method of soda introduction, the atmosphere, and the duration of the soak, and cooling of the kiln play a pivotal role in both surface and color development with bare clays. Your current firing method may work well for high alumina bodies; however, you may wish to experiment with each variable to determine the best-suited firing method for your work. 

As the world of high-alumina clay usage expands, I would like to invite anyone working with this type of clay to tag their posts on social media with #HighAluminaClay. This can provide a resource for artists with similar research interests to find each other and create a community where we can collectively expand our knowledge of soda firing. 

Author note: My research has stemmed from Gail Nichols’ research and book, Soda, Clay, and Fire, and began with generous support from the Jerome Project Grant awarded through the Northern Clay Center (Minneapolis, Minnesota) in 2020. 

the author Casey Beck is currently a third-year MFA candidate at the University of Nebraska-Lincoln where he is studying ceramics. Beck has been a resident artist at the Cub Creek Foundation in Virginia and Faenza Art Ceramic Center in Italy, was awarded the 2020 Jerome Ceramic Artist Project Grant through Northern Clay Center, and is a 2023 Ceramics Monthly Emerging Artist. 

*digitalfire.com