What Makes A Glaze A Shino?
Introducing Shino Glazes
Shino glazes refer to a style of pottery glazes first developed in the Mino province of Japan in the 16th century. The term “Shino” comes from the Japanese word for Mino. Shino glazes are characterized by their milky white color and the network of fine cracks that form in the glaze during the firing process.
Shino glazes were originally developed by local Japanese potters experimenting with locally available materials like feldspar. The potters found that by combining feldspar with clays high in alumina and silica, they could create glazes that formed delicate cracking patterns when fired in wood-burning kilns. The volatile sodium from the wood ash reacted with the glaze to produce carbon dioxide bubbles that left behind the crackled texture.
Over the centuries, Shino glazes became associated with traditional Japanese aesthetics like wabi-sabi that find beauty in imperfection. The subtle cracking patterns were prized for their quiet, rustic elegance. Shino glazes remain popular today among ceramic artists who appreciate their subdued visual textures and ties to Japanese culture and history.
Sources:
A Brief History of Shino Glazes, Their Use, and Contribution to Contemporary Potters
Clay Body Composition
Shino glazes were originally developed in Japan and work best on high-iron, stoneware clay bodies that fire between cone 9 and cone 11. Some common clay types used for Shino glazes include:
- Stoneware clays – These include clays like Shigaraki, B-Mix, and speckled buff stoneware. Stoneware clays tend to be dense, with moderate to high iron contents that interact well with Shino glazes during firing. According to Digitalfire, Shino glazes were originally tested on tokoname stoneware. (https://digitalfire.com/glossary/shino+glazes)
- Red/brown clays – High iron red and brown firing clays are also excellent choices. The iron content produces nice activity and breaking with Shino glazes. Redart, a commercially available red stoneware, is commonly used.
- Porcelain – Some porcelain clays can also be used, but need higher iron contents. Adding 3-5% red iron oxide can help produce better effects with Shino glazes on porcelain bodies. (https://community.ceramicartsdaily.org/topic/15371-clay-for-shino-glazes-suggestions/)
In general, mid-range stoneware and iron-rich red/brown clays produce the best results for Shino glazes. The higher iron contents allow for more dynamic effects and interest.
Feldspar Content
Feldspars are key ingredients in traditional Shino glaze recipes. They provide critical fluxes that promote melting, gloss development, and the formation of an glassy surface at high temperatures. According to Digitalfire, originally Shinos were a two-part mix of about 70-80% high-alumina, high-sodium feldspar and 20-30% clay.
Typical Shino recipes call for significant amounts of potassic feldspars like potassium feldspar or nepheline syenite. Sodium feldspars may also be used. Feldspar contents of 50-70% are common in Shino glaze recipes fired to cone 10-11. The high feldspar content helps the glaze melt smoothly and develop the trademark milky, glossy surface of Shino wares. However, too much feldspar can lead to excessive melting and runny glazes.
In summary, high percentages of alkaline feldspars between 50-80% are a defining ingredient of Shino glazes. They enable the development of Shino’s classic surface textures and appearance.
Sources:
https://digitalfire.com/glossary/shino+glazes
Flux Materials
Shino glazes contain high amounts of fluxes, materials that promote melting and flow in a glaze. Besides feldspar, some other common flux materials used in Shino glazes include nepheline syenite, lithium carbonate, dolomite, and calcium carbonate. Nepheline syenite is a popular flux in Shino glazes because it provides a high alumina content to promote the formation of a silica-rich breakable glaze surface. Lithium carbonate acts as a powerful flux when combined with potash and soda feldspars. Dolomite and calcium carbonate also aid melting and flow. The high flux content is key to creating the characteristic Shino textures of a satiny smooth base glaze with a cracked and peeled top layer.
Alumina and Silica Content
Shino glazes typically have a high silica, low alumina composition. The ratio of silica (SiO2) to alumina (Al2O3) is a key factor that gives Shino glazes their distinctive matte, textured aesthetic.
A high silica content in the clay body and glaze promotes the growth of crystals, while the alumina acts as a flux to facilitate melting at lower temperatures. According to sources, Shino recipes often have a silica:alumina ratio between 4:1 to 8:1[1]. This produces a glaze that readily forms micro-crystalline structures yet has enough flux to melt and achieve an even coating.
Increasing silica while reducing alumina accentuates the matteness and textural effects in the fired glaze. On the other hand, too much alumina produces a more glossy, smooth glaze with less pronounced crystalline formation[2]. Therefore, balancing the silica and alumina is key for achieving the unique aesthetic that defines a Shino glaze.
Colorants
Typical colorants used to tint Shino glazes include:
- Iron oxide, which produces reddish and brownish hues
- Copper carbonate, which creates greenish tones
- Cobalt oxide, which lends blue colors
- Rutile, which can create yellowish and reddish tints
- Nickel oxide, for gray and brown colors
- Manganese dioxide, which gives purplish-brown shades
These metal oxides serve as ceramic stains when added in small amounts to the glaze. The oxides undergo chemical reactions during firing to produce the final colors. Interesting effects can be achieved by layering and combining multiple Shino glazes tinted with different metal oxides.
Firing Temperature
Shino glazes are typically fired between cone 8 and cone 11 to achieve the characteristic breaks, texture, and color (Shino Glazes). The optimal firing range is around cone 9-10 (2300°F-2380°F or 1260°C-1300°C). Firing to these mid-range stoneware temperatures allows the feldspar glass to mature fully while still retaining enough viscosity to encourage carbon trapping. If fired too low, the glaze will lack the glassiness and breaks associated with Shino. If fired too high, it can become too fluid and lose the distinctive carbon-trapped texture (Carbon Trapping Explained, Plus a Great Shino Glaze Recipe).
Atmosphere Effects
The atmosphere in which Shino glazes are fired significantly impacts the finished results. Shino glazes react strongly to both oxidation and reduction atmospheres in the kiln.
In an oxidation atmosphere, Shino glazes can exhibit a range of colors from white to pale blue, green, pink or yellow. The oxides in the glaze oxidize fully, producing lighter and brighter colors. Minimal carbon is trapped.
In a reduction atmosphere, Shino glazes trap carbon from the atmosphere which produces grayish spots and streaks on the surface. This effect is referred to as “carbon trapping” and is a signature characteristic of Shino glazes fired in reduction. The trapped carbon can range from faint gray spotting to heavy black spotting depending on the length and temperature of the reduction atmosphere. According to the article “Shino glazes are distinctive in their ability to trap carbon and contribute blushing” from Fireborn Pottery (https://www.fireborn.com/wp-content/uploads/2015/10/Shino.pdf), the sodium in Shino glazes melts at a lower temperature which allows it to readily trap excess carbon from the kiln atmosphere.
The differences between oxidation and reduction atmospheres have a significant visual impact on Shino glazes. Potters take advantage of these reactions to achieve a wide variety of aesthetic effects from subtle carbon trapping to dramatic breaking.
Texture and Breaks
A hallmark of Shino glazes are the distinctive textures and breaks in the glaze surface. The high alumina and silica content in Shino glazes makes them prone to crawling and breaking as the glaze melts and flows over the clay body during firing.
As the glaze begins to melt, it contracts and pulls apart, leaving behind areas of exposed clay body surrounded by glaze pools. This gives Shino glazes their characteristic “cracked earth” look, with a complex web of fine cracks covering the surface.
The glaze also exhibits a high viscosity as it melts, making it resistant to running and pooling smoothly. So glaze thickens and piles up in certain areas, while thinning out in others. This variability in thickness leads to intriguing color variations across the surface.
In some areas, the glaze may even bubble up and burst from gasses escaping through the glaze melt, creating volcano-like craters and spots on the surface. Subtle greenish zinc crystals may also emerge along crack lines as the zinc flux interacts with alumina and silica.
All of these crawling, breaking, bubbling, and crystallizing effects come together to produce the earthy, rugged, and varied textures that Shino glazes are so well-known for (NicoleAquillanoCeramics). The textures make each Shino glazed piece completely unique.
Examples of Shino Glazes
Shino glazes produce surfaces with distinctive textures and breaks that set them apart. When fired, the glaze moves and gathers in certain areas, creating variegated textures and effects. The photos below showcase the range of patterns and colors possible with Shino glazes:
This photo gallery from Bracker’s Good Earth Clays shows examples of the striking crackle effects and earthy colors of Shino glazes on ceramic pieces: https://www.facebook.com/brackersgoodearthclays/photos/. The layered glazes gather and pool in certain areas, leaving other sections with a matte, exposed clay surface. Shades range from reddish browns to grayish blues and greens.
The random, organic breaks in the glaze result in one-of-a-kind works where no two pieces have the exact same patterning. The crackled spots provide striking contrast with the glossy pooled glaze. Skilled ceramic artists can guide the glaze effects using techniques like wax resist or specific brushwork. However, Shino glazes always contain an element of chance in the final fired surface.
