The feldspathic inclusions of Shigaraki style of ceramics not only give the ware a unique look, but also contribute to the firing process. This article discusses common approaches to Shigaraki-style pottery and different options for feldspathic inclusions in clay.
Defining the Terms
Anagama: A traditional Japanese wood-burning kiln. The direct translation is “cave kiln,” with some built over sloping hills and others dug into the ground. These often large kilns consist of a single chamber, allowing ash from the burning wood to reach the pottery inside.
Flux: Substances used to lower the melting point of a ceramic work to vitrification temperatures. Often consisting of oxides, fluxes are found in both clay bodies and glazes.
Sintering: A densification process, where tighter particle packing leads to a stronger matrix.
Vitrification: How clays melt, become glassy instead of liquid. The crystalline ceramic body is heated to its melting point, transitioning into glass and losing porosity. The threshold for vitreosity varies across industries, with maximum porosity percentages ranging from 0.5% to 5% depending on application.
The Origins of Shigaraki
Spanning back 1200 years to the Shiga Prefecture of Japan, Shigaraki ware is a distinctive pottery style used in a variety of ceramic goods, such as tiles, teapots, and cookware. Koyama Fujio, a scholar of Japanese ceramics, includes Shigaraki as one of the Six Ancient Kilns of Japan: areas rich in natural clays that have been producing ceramics for hundreds of years. Coarse deposits of feldspar mixed into the clay create bursts of white throughout a piece, often referred to as crab eyes.1 These stand out among the ceramic body’s natural rusty browns and greenish grays.
The first Shigaraki wares date back to tile production in the 8th century, when Emperor Shōmu ordered the construction of the Shigaraki Palace. Still, the style became especially popular hundreds of years later. In the 15th century, the Wabi-cha style of tea ceremony arose, emphasizing simplicity and appreciation for nature.2,3 The Shigaraki style’s earthiness fits perfectly into this philosophy, becoming very sought after. Today, Shigaraki is still appreciated for its beauty and challenges, as artists attempt to translate ancient techniques into modern methods (1).
Distinctive Clay Properties
The clay used in Shiga is harvested from the banks of Lake Biwa, Japan’s oldest and largest lake. Millions of years of sediments and organic matter depositing along the lake floor have created many types of clay, but Shigaraki ware utilizes kaolinite-based gaerome. Kaolinite’s structure is Al2Si2O5(OH)4, but it often contains metal impurities. In the case of Shigaraki ware, iron in the kaolinite reacts to the fire of the kiln, creating unique reds and oranges across the body. Gaerome means ‘frog-eyed’ clay (2), referencing the large feldspathic inclusions—these come from the naturally occurring veins of granite and rhyolite from the surrounding mountains, both of which are commonly occurring igneous rocks.4,5 Other sources of feldspar include anorthosite and nepheline syenite. Feldspar is the most common mineral in the Earth’s crust, comprising about 60%. It is classed as an aluminosilicate (having a negative Si-O-Al network), with different compositions including KAlSi3O8 or NaAlSi3O8. These alkali feldspars are the variants most commonly used in ceramics.
Feldspar’s abundance worldwide is fortunate for ceramic artists, as its ability to melt gradually makes it an efficient flux material, leading to easier clay firing. Depending on which elements are present, a feldspar can have different properties when used during firing. For example, potassium feldspar provides more protection against deformation than sodium feldspar, which lowers the melting point more.6 Clay minerals, characterized by aluminum phyllosilicate networks (like feldspar), have extremely high melting points—kaolinite melts at 3218°F (1770°C). The alkali oxides in feldspar attack this network, depolymerizing the clay and creating oxygen ions, that liquefy the clay.7 Flux can be added to both the clay body and glaze, but because traditional Shigaraki ware is left unglazed, only the naturally occurring feldspar acts as a flux.
Not only does feldspar aid in melting the clay, but it also seeps into its pores, improving strength and heat stability while reducing water absorption and kiln shrinkage. When a flux enters the pore, it forms a layer between clay crystals that interlock as the clay cools. Depending on the clay used and the amount of flux required, feldspar can lower the melting point of a ware by 572° to 1292°F (300 to 700°C). However, it should be noted that clay does not conventionally melt but rather vitrifies, becoming a glassy, poreless solid, which is impermeable to water. Vitrification occurs when feldspar acts as the flux in a process known as “viscous flow sintering.” This is the process by which feldspar reduces its surface area, like rain droplets joining, and the resulting glassy liquid enters the pores of the clay, densifying it. In the case of kaolinite, it is dehydroxylated into meta-kaolinite (Al2Si2O5(OH)4 + heat -> Al2Si2O7 + 2H2O) before crystallization into mullite. As the melted feldspar vitrifies clay, secondary mullite crystals interlock and hold the ceramic’s structure as the glass fills gaps and makes it watertight.8
Firing Shigaraki Wares
Traditional Shigaraki ware is fired in an anagama kiln at temperatures above 1832°F (1000°C)—feldspar fully melts/ aids in vitrification at cone 10, which is 2372°F (1300°C). Wares can take several days to fire, so the anagama must be constantly replenished with new fuel. While Shigaraki artists did not glaze their work, ash from burning wood can react with the flames and clay minerals, creating the green hues that contrast with the naturally orange clay.1 A similar effect can be achieved using modern technology. In soda firing, a technique in which soda ash (sodium carbonate, Na2CO3) is added as a spray or powder to a hot kiln, color gradients form. The ash becomes sodium oxide (Na2O) on the piece as the vaporized soda reacts with silica on the exterior of the body.9
Ceramic artist Steve Davis has built his own kiln, called the Kazegama. He uses the gas-fired kiln to get the wood-fired effects (3) of a traditional anagama by adding ash in with wares to be fired:
“My family travels to Bishop, Joshua Tree, and other areas for outdoor climbing (bouldering). I don’t climb. My neck is fused, so I walk around the boulders and pick up the crumbly decomposed granite for my work. You can also gather it up in mountain road cut-outs at about 3,500 feet, where there is freezing snow and water that seeps into the granite face and lifts off the surface of the granite from the freezing water expanding in the material. There are mica sheets within the granite that will melt out into the clay wall, which you may want to remove. I bisque fire the granite in 1-inch layers across kiln shelves, which will cause the mica sheets to expand, so they can be removed before they are wedged into the clay. Otherwise, the mica will expand in the clay during the bisque firing and blow out chunks of the clay wall. Then, I screen the granite with different hardware screen sizes to separate the granite into various sizes. Small work gets the small stuff, and large pots and sculptures get the big stuff. You can get a 24-inch-wide, galvanized screen from Home Depot and build a box with it and 1 × 4-inch boards, molding to cover the bottom edge, finishing nails, and wood glue. Make two screens so the one screen fits inside the other for storage and screening other materials like wood ash, with window screen in between the two screens.”
Shigaraki Techniques Reimagined
While there are other instances of naturally occurring feldspar-containing clay deposits across the world, many ceramic artists will add the feldspar themselves to control particle size and add color. The powdered form can be mixed with frit and sintered before being vitrified to fully mix any added colorants.10 On its own, the powdered form works as a flux and gives ceramic ware a uniform body, but to get the Shigaraki effect, artists may use larger feldspar additions, such as granite chicken grit from farm supply stores ($20 for 50 pounds of crushed granite). Decomposed granite (see sidebar) is abundant in nature and can be collected for incorporation into clay. This is what chemistry professor Ryan Coppage does as well:
“In Richmond, I collect rotted granite sand from stream beds. Most of the surrounding rock is granite, such that as it weathers, it breaks apart. The stream beds then separate the feldspathic sand based on size, depending on its grain size and where it was deposited, shown by how far it was carried by water. This is then wedged into clay and fired to cone 10 in Tracy Gordon’s gas-soda kiln outside Boone, North Carolina.”
Over a dozen centuries, this ancient Japanese style has endured in popularity around the globe. Its distinct look and challenging technique have spurred ceramic artists to try it for themselves, preserving culture through modern innovation.
the authors 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. Sukie Weiner is a current Biochemistry and Molecular Biology major at the University of Richmond.
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The feldspathic inclusions of Shigaraki style of ceramics not only give the ware a unique look, but also contribute to the firing process. This article discusses common approaches to Shigaraki-style pottery and different options for feldspathic inclusions in clay.
Defining the Terms
Anagama: A traditional Japanese wood-burning kiln. The direct translation is “cave kiln,” with some built over sloping hills and others dug into the ground. These often large kilns consist of a single chamber, allowing ash from the burning wood to reach the pottery inside.
Flux: Substances used to lower the melting point of a ceramic work to vitrification temperatures. Often consisting of oxides, fluxes are found in both clay bodies and glazes.
Sintering: A densification process, where tighter particle packing leads to a stronger matrix.
Vitrification: How clays melt, become glassy instead of liquid. The crystalline ceramic body is heated to its melting point, transitioning into glass and losing porosity. The threshold for vitreosity varies across industries, with maximum porosity percentages ranging from 0.5% to 5% depending on application.
The Origins of Shigaraki
Spanning back 1200 years to the Shiga Prefecture of Japan, Shigaraki ware is a distinctive pottery style used in a variety of ceramic goods, such as tiles, teapots, and cookware. Koyama Fujio, a scholar of Japanese ceramics, includes Shigaraki as one of the Six Ancient Kilns of Japan: areas rich in natural clays that have been producing ceramics for hundreds of years. Coarse deposits of feldspar mixed into the clay create bursts of white throughout a piece, often referred to as crab eyes.1 These stand out among the ceramic body’s natural rusty browns and greenish grays.
The first Shigaraki wares date back to tile production in the 8th century, when Emperor Shōmu ordered the construction of the Shigaraki Palace. Still, the style became especially popular hundreds of years later. In the 15th century, the Wabi-cha style of tea ceremony arose, emphasizing simplicity and appreciation for nature.2,3 The Shigaraki style’s earthiness fits perfectly into this philosophy, becoming very sought after. Today, Shigaraki is still appreciated for its beauty and challenges, as artists attempt to translate ancient techniques into modern methods (1).
Distinctive Clay Properties
The clay used in Shiga is harvested from the banks of Lake Biwa, Japan’s oldest and largest lake. Millions of years of sediments and organic matter depositing along the lake floor have created many types of clay, but Shigaraki ware utilizes kaolinite-based gaerome. Kaolinite’s structure is Al2Si2O5(OH)4, but it often contains metal impurities. In the case of Shigaraki ware, iron in the kaolinite reacts to the fire of the kiln, creating unique reds and oranges across the body. Gaerome means ‘frog-eyed’ clay (2), referencing the large feldspathic inclusions—these come from the naturally occurring veins of granite and rhyolite from the surrounding mountains, both of which are commonly occurring igneous rocks.4,5 Other sources of feldspar include anorthosite and nepheline syenite. Feldspar is the most common mineral in the Earth’s crust, comprising about 60%. It is classed as an aluminosilicate (having a negative Si-O-Al network), with different compositions including KAlSi3O8 or NaAlSi3O8. These alkali feldspars are the variants most commonly used in ceramics.
Feldspar’s abundance worldwide is fortunate for ceramic artists, as its ability to melt gradually makes it an efficient flux material, leading to easier clay firing. Depending on which elements are present, a feldspar can have different properties when used during firing. For example, potassium feldspar provides more protection against deformation than sodium feldspar, which lowers the melting point more.6 Clay minerals, characterized by aluminum phyllosilicate networks (like feldspar), have extremely high melting points—kaolinite melts at 3218°F (1770°C). The alkali oxides in feldspar attack this network, depolymerizing the clay and creating oxygen ions, that liquefy the clay.7 Flux can be added to both the clay body and glaze, but because traditional Shigaraki ware is left unglazed, only the naturally occurring feldspar acts as a flux.
Not only does feldspar aid in melting the clay, but it also seeps into its pores, improving strength and heat stability while reducing water absorption and kiln shrinkage. When a flux enters the pore, it forms a layer between clay crystals that interlock as the clay cools. Depending on the clay used and the amount of flux required, feldspar can lower the melting point of a ware by 572° to 1292°F (300 to 700°C). However, it should be noted that clay does not conventionally melt but rather vitrifies, becoming a glassy, poreless solid, which is impermeable to water. Vitrification occurs when feldspar acts as the flux in a process known as “viscous flow sintering.” This is the process by which feldspar reduces its surface area, like rain droplets joining, and the resulting glassy liquid enters the pores of the clay, densifying it. In the case of kaolinite, it is dehydroxylated into meta-kaolinite (Al2Si2O5(OH)4 + heat -> Al2Si2O7 + 2H2O) before crystallization into mullite. As the melted feldspar vitrifies clay, secondary mullite crystals interlock and hold the ceramic’s structure as the glass fills gaps and makes it watertight.8
Firing Shigaraki Wares
Traditional Shigaraki ware is fired in an anagama kiln at temperatures above 1832°F (1000°C)—feldspar fully melts/ aids in vitrification at cone 10, which is 2372°F (1300°C). Wares can take several days to fire, so the anagama must be constantly replenished with new fuel. While Shigaraki artists did not glaze their work, ash from burning wood can react with the flames and clay minerals, creating the green hues that contrast with the naturally orange clay.1 A similar effect can be achieved using modern technology. In soda firing, a technique in which soda ash (sodium carbonate, Na2CO3) is added as a spray or powder to a hot kiln, color gradients form. The ash becomes sodium oxide (Na2O) on the piece as the vaporized soda reacts with silica on the exterior of the body.9
Ceramic artist Steve Davis has built his own kiln, called the Kazegama. He uses the gas-fired kiln to get the wood-fired effects (3) of a traditional anagama by adding ash in with wares to be fired:
“My family travels to Bishop, Joshua Tree, and other areas for outdoor climbing (bouldering). I don’t climb. My neck is fused, so I walk around the boulders and pick up the crumbly decomposed granite for my work. You can also gather it up in mountain road cut-outs at about 3,500 feet, where there is freezing snow and water that seeps into the granite face and lifts off the surface of the granite from the freezing water expanding in the material. There are mica sheets within the granite that will melt out into the clay wall, which you may want to remove. I bisque fire the granite in 1-inch layers across kiln shelves, which will cause the mica sheets to expand, so they can be removed before they are wedged into the clay. Otherwise, the mica will expand in the clay during the bisque firing and blow out chunks of the clay wall. Then, I screen the granite with different hardware screen sizes to separate the granite into various sizes. Small work gets the small stuff, and large pots and sculptures get the big stuff. You can get a 24-inch-wide, galvanized screen from Home Depot and build a box with it and 1 × 4-inch boards, molding to cover the bottom edge, finishing nails, and wood glue. Make two screens so the one screen fits inside the other for storage and screening other materials like wood ash, with window screen in between the two screens.”
Shigaraki Techniques Reimagined
While there are other instances of naturally occurring feldspar-containing clay deposits across the world, many ceramic artists will add the feldspar themselves to control particle size and add color. The powdered form can be mixed with frit and sintered before being vitrified to fully mix any added colorants.10 On its own, the powdered form works as a flux and gives ceramic ware a uniform body, but to get the Shigaraki effect, artists may use larger feldspar additions, such as granite chicken grit from farm supply stores ($20 for 50 pounds of crushed granite). Decomposed granite (see sidebar) is abundant in nature and can be collected for incorporation into clay. This is what chemistry professor Ryan Coppage does as well:
“In Richmond, I collect rotted granite sand from stream beds. Most of the surrounding rock is granite, such that as it weathers, it breaks apart. The stream beds then separate the feldspathic sand based on size, depending on its grain size and where it was deposited, shown by how far it was carried by water. This is then wedged into clay and fired to cone 10 in Tracy Gordon’s gas-soda kiln outside Boone, North Carolina.”
Over a dozen centuries, this ancient Japanese style has endured in popularity around the globe. Its distinct look and challenging technique have spurred ceramic artists to try it for themselves, preserving culture through modern innovation.
the authors 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. Sukie Weiner is a current Biochemistry and Molecular Biology major at the University of Richmond.
1 Kanazawa, A. The Flavor of the Earth: The Rustic Ceramics of Shigaraki. Entoten. https://www.entoten.com/2015/02/25/the-flavor-of-the-earth-the-rustic-ceramics-of-shigaraki/ (accessed 2025-10-26).
2 Gutierrez, E. Shigaraki--Garden of Earthy Delights. Japan Quarterly 1999, 46 (2), 66–77.
3 Shigaraki Ware and Tea Ceremony Culture / Shigaraki Ware and Tea Ceremony Culture Special Website. https://tanukimura.com/shigaraki/chanoyu_en.html (accessed 2025-10-26).
4 Shigaraki [Outline and history]. Journey. One thousand years. The Six Ancient Kilns. https://en.sixancientkilns.jp/shigaraki/ (accessed 2025-10-26).
5 Craft Journey #2 /Shigaraki - Clay/. newa. https://newa-cat.com/en-us/blogs/story/craft-journey-2 (accessed 2025-10-26).
6 Feldspar - an overview | ScienceDirect Topics. https://www.sciencedirect.com/topics/materials-science/ feldspar (accessed 2025-10-26).
7 Goltsman, B. M.; Yatsenko, E. A. Modern Fluxing Materials and Analysis of Their Impact on Silicate Structures: A Review. Open Ceramics 2024, 17, 100540. https://doi.org/10.1016/j.oceram.2024.100540.
8 Wattanasiriwech, D.; Srijan, K.; Wattanasiriwech, S. Vitrification of Illitic Clay from Malaysia. Applied Clay Science 2009, 43 (1), 57–62. https://doi.org/10.1016/j.clay.2008.07.018.
9 What’s Soda Firing? - Harry Levenstein Pottery. https://web.archive.org/web/20250907022358/https:/ www.harrylevensteinpottery.com/whatissodafiring.html (accessed 2025-10-26).
10 Kayser, E. Shigaraki Surfaces. Pottery Making Illustrated 2017, 20 (2), 10–11
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