An Introduction to Japanese Ceramic Glazes

The first appearance of glaze in Japan was in the fifth century. The earliest example is found on sueki, also known as Sue stoneware, clay vessels fired using techniques transmitted from the Korean Peninsula. However, these originating glazes were not intentionally created but rather accidental products, as components naturally contained in the clay body vitrified under high heat, or ashes from the fuel used in firing reacted chemically to create the glazes by chance.

Over time, techniques emerged to recreate that glassy layer that previously had only formed naturally on vessel surfaces. Between the twelfth and sixteenth centuries, various glazes were developed and flourished across Japan, centered on regions that were and continue today to be active centers of ceramic production, such as modern-day Seto City in Aichi Prefecture.

Initially, glazed ceramics were luxury items. But they gradually became more affordable and spread among the common people starting in the Edo period (1603–1868 CE), around the first half of the seventeenth century. This shift is said to have been driven by the dramatic increase in the production of Arita ware, porcelain made around present-day Arita City in Saga Prefecture.

The main roles of glaze include:

Decoration: Glaze adds luster, color, and patterns, broadening design possibilities.

Protection: Glaze shields the surface of ceramics from scratches and dirt, enhancing durability.

Waterproofing: Glaze reduces absorption of moisture into the ceramic surface, improving hygiene.

A crucial point to understand is that the development of glaze did not originally stem from a pursuit of functionality like protection or waterproofing but from a desire for decoration—the beauty that glaze brings forth. In other words, glaze and the beauty of ceramics are deeply, fundamentally connected.

Glaze consists of three basic components: silica, alkali substances, and metallic oxides. Let’s briefly outline what each does:

Silica: Silicon oxide obtained from quartz or feldspar. It vitrifies during firing, giving hardness and luster to the ceramic’s surface.

Alkalis: Include substances like sodium and potassium. They promote the melting of silica and help maintain the appropriate temperature for vitrification.

Metallic oxides: Act like pigments, diversifying glaze color. For example, copper oxide produces green or red hues; cobalt oxide yields vivid blue; and iron oxide creates brown or yellow tones.

While glazes are often industrially manufactured and sold by specialist companies, it is not uncommon for potters to make their own in pursuit of more ideal results.

Some ceramics are deliberately fired without glaze. In such cases (just as with ancient Sue stoneware), the alkali, lime, and other components in the ashes from burning firewood may react with silicic acid in the clay body to form a natural glassy coating. This is known as shizen-yu, or “natural (ash) glaze”. With results depending on firing conditions, natural ash glaze can produce a wide range of colors such as green, yellow, and brown. The end result is what one might call the beauty of chance.

As previously mentioned, most ceramics are made by applying a glaze made up of a suspension of powdered glaze components in water to the surface of a bisque-fired clay body. The bisque firing, also known as biscuit firing or suyaki in Japanese, is conducted at around 800°C (1,472°F). After bisque firing and glazing come the main firing, or honyaki, at around 1,200°C (2,192°F).

The process of applying glaze to bisque-fired ceramics is called glazing or glaze application, and there are several methods:

The chosen method has a significant impact on the vessel’s final appearance, reflecting each potter’s concept of beauty and individual aesthetic.

Furthermore, some metallic oxides in glaze change color depending on the firing atmosphere. The two main atmospheres are oxidation firing, carried out in the presence of ample oxygen, versus reduction firing, carried out in an oxygen-deficient environment. For example, iron oxide typically produces yellow to brown or black with oxidation firing, but bluish tones with reduction firing. Copper oxide produces a green color in oxidation firing and a red color in reduction firing.

There are many types of glazes, classified in various ways, such as by component, texture, or firing temperature. Here we present one example of a classification based on color, a factor that deeply influences the beauty of ceramics:

Celadon glaze develops blue or green hues in reduction firing. These colors come from trace iron. Small amounts of iron result in a pale blue-white, such as that of blue-white porcelain, while almost no iron results in a transparent finish. A thicker glaze results in a deeper blue upon firing.

Used in Shino ware starting from the second half of the sixteenth century in southern Gifu and made solely from feldspar. Firing produces semi-translucent, milky-white surfaces with countless pinholes. The surface at the pinholes is tinged a distinctive scarlet due to iron in the clay body beneath the glaze, creating a two-tone contrast.

A mixture of plant ash and iron oxide. First used in Seto ware in the late thirteenth century, in what today is now northeastern Aichi prefecture. Iron glaze made up of 8–10 percent iron oxide yields kuro-yu, or “black glaze.” Removing black-glazed vessels from the kiln during firing and cooling them rapidly produces the glossy, jet-black finish known as Seto black.

A type of iron glaze containing 8–10 percent iron oxide. Named after Tianmu Mountain (known as Tenmoku Mountain in Japanese) in Zhejiang Province, China, as a particular area of the mountain fired ceramics in shades of black. Used for tea bowls, variations arise from differences in iron amounts and crystallization. These include the iridescent yohen tenmoku, said to exhibit seven colors of light depending on differences in appearance of iron oxide crystals that rise to the surface, and yuteki tenmoku, whose spots resemble oil floating within water. Red or brown may also appear on a vessel’s lip, another defining characteristic.

Made of plant ash mixed with 1–3 percent ferric oxide. Developed in the late 16th century when potters from the Seto region in northeastern Aichi Prefecture moved to the southern part of the Mino area and created the technique. The yellow color comes from the ferric oxide and requires oxidation firing. Strong yellow tones and textured surfaces are highly valued.

Technically a technique, not a glaze, and seen in Mino ware. It is said to be named after its finish, which looks as though white powder were sprinkled or blown over the ceramic’s surface. A red clay body is coated with white slip, then clear glaze (see note below) is applied over top. The result is a thick, soft white finish.

Note: Clear glaze: 
Made from lime mixed with feldspar or pottery stone. Lime acts as a strong flux, meaning it promotes melting and fusion; feldspar or pottery stone helps with melting, adhesion, and vitrification.

In Japan, glazes that have been altered in appearance and expressive quality under the flames of the kiln are likened to landscapes and enjoyed as if it were natural scenery. Just like natural ash glaze, this “scenery,” created by chance, is valued for its individuality and as a merit in itself. Below are a couple of examples.

Also called crazing. Refers to a pattern of surface cracks that appear due to differences in shrinkage between glaze and clay body. When the fired vessel cools rapidly, the glaze contracts more than the body, creating the cracks.

These landscapes often change subtly depending on the angle and type of light, or—for tableware—the food or beverages placed inside, revealing new charms each time.

Another delightful way to view these pieces is to consider the landscapes they resemble. Kannyu—complexly entwined wild mountain vines. Glaze drips—waterfall rivulets streaming down cliff faces. Each brings an enjoyable sense of the colorfully diverse scenery of the natural world.