Hai-yū: The Ash Glaze That Started Japanese Pottery
10 min readIf the last chapter of this series left porcelain behind for the iron black of stoneware, this one goes all the way back to the beginning. Ash glaze — hai-yū (灰釉) in Japanese — is the oldest high-fired glaze in East Asia, and it was not so much invented as discovered, in the ash of the wood fire itself. Before cobalt, before enamel, before gold, a Japanese pot's first color came from a dusting of ash that melted in the kiln into a thin skin of glass. This seventh chapter of Color of Utsuwa follows hai-yū from an accident on the shoulder of an ancient jar to the deliberate green-gold surfaces of Shigaraki and Bizen still made today.
What "hai-yū" means
Hai-yū joins hai (灰, ash) with yū (釉, glaze): literally, "ash glaze." It names the whole family of glazes whose essential ingredient is the ash of burned wood or straw. That ash, rich in lime and other minerals, is the flux that lets a hard, glassy surface form on a pot at kiln temperature. Ash glaze arrives in two ways, and the difference decides how a piece looks. In natural ash glaze, flying ash settles on the pot inside the kiln and melts on its own. In applied ash glaze, the potter collects the ash, mixes it into a slurry, and coats the pot before firing. The chemistry is identical; only the hand of the potter differs.
Atomic fact — ash glaze is the oldest high-fired glaze in East Asia. The earliest glazes in China and Japan were not applied by hand at all. They formed by accident when wood ash, carried on the flames of a hot kiln, landed on the shoulders of pots and fused with the silica in the clay into a thin, uneven glass. Potters then spent centuries learning to control on purpose what the fire had first done by chance.
The accident on an ancient jar
Japan's first meeting with glaze came with a new kind of kiln. In the early fifth century, high-temperature kiln technology reached Japan from the Korean peninsula, and with it came sue ware (須恵器) — a hard, grey, high-fired stoneware unlike the soft earthenware made before. Sue ware was mostly unglazed, but in the long, fierce heat of the sloping anagama (穴窯, "hole kiln"), wood ash sometimes drifted onto the pots and melted into patches of greenish, glassy glaze. Nobody had planned it. It was the fire's own signature, left here and there where the ash had gathered thickest.
You can still see that first glaze in museum collections. The Metropolitan Museum of Art holds a ninth-century Heian-period jar described simply as "stoneware with natural ash glaze (Sue ware)"; its Japanese title, 須恵器 灰釉壺, names the ash glaze outright. On a piece like this the glaze is not a coat but an event — a run of green-brown gloss down one side, the rest of the pot left bare. This is hai-yū in its wild state, and for centuries it was the only glaze Japanese potters knew.
How wood ash turns into glass
The magic of ash glaze is really just chemistry, and understanding it explains every color the glaze can take. Wood ash is full of lime (calcium oxide) along with potassium and other minerals. Those act as fluxes — substances that lower the temperature at which silica, the glass-former in clay and rock, will melt. Spread ash over a silica-rich clay body, heat it past roughly 1,200°C, and the ash and the surface of the pot fuse into a genuine glass. The pot, in effect, grows its own glaze.
Atomic fact — the green comes from iron and fire. Wood ash always carries a little iron, and that iron is the colorant in a classic ash glaze. How the kiln is run decides the tone. In a reduction firing, where oxygen is starved, the iron shifts toward its ferrous form and the glaze reads a cool green — the same chemistry that colors celadon. In an oxidation firing, with air to spare, the iron stays ferric and the glaze warms toward amber, olive, and yellow-brown.
This is why an ash-glazed surface is never quite uniform. The thickness of the ash, the path of the flame, the iron in that particular tree's wood, the atmosphere in that corner of the kiln — all of it registers in the finished glaze. Two pots fired side by side can leave the same kiln wearing different colors.
From accident to intention: dobai, Ki-Seto, and celadon
Once potters understood that ash made glaze, they began to gather it, wash it, and apply it on purpose. Applied wood-ash glaze — dobai (土灰) in the workshop — became the backbone of glazed pottery in central Japan. In the Seto and Mino districts of present-day Aichi and Gifu, whole traditions grew up around ash and iron-ash glazes. One of the most loved is Ki-Seto (黄瀬戸), "yellow Seto": an iron-ash glaze that, according to the Encyclopædia Britannica, was made in the Mino area from the late Muromachi period (1338–1573) onward and deepened to its warm signature yellow in the Momoyama era. Ki-Seto is ash glaze grown up — the same lime-and-iron recipe, now controlled closely enough to hold a steady color across a whole bowl.
Follow that control one step further and you arrive at celadon. A celadon glaze (seiji) is a refined lime glaze fired in reduction so that a small, carefully judged amount of iron turns a clear, even green. In other words, the jade-green porcelain we met earlier in this series is a direct, purified descendant of the accidental green on that Sue ware jar. If you have read the chapter on seiji, the jade-green world of Japanese celadon, you have already met ash glaze's most polished child.
The language of ash: shizen-yū, goma, bidoro, yōhen
While Seto and Mino refined ash into an even coat, another set of kilns did the opposite and made the fire's own ash glaze the entire point. In the wood-fired stoneware of Shigaraki, Bizen, Iga, and Tokoname — yakishime (焼締), high-fired unglazed ware — natural ash glaze became a prized landscape rather than a flaw, and potters built a rich vocabulary for its effects. The Robert Yellin Yakimono Gallery's glossary of Japanese glazes catalogs many of them:
- Shizen-yū (自然釉) — "natural ash glaze," the general name for glaze formed by ash in the kiln with nothing applied by hand.
- Goma (胡麻) — "sesame," the speckled scatter of melted pine ash across a surface, a signature of Bizen ware.
- Bidoro — from the Portuguese vidro, "glass": pools of glossy green where the ash has run thick and vitrified, especially admired on Iga pieces.
- Yōhen (窯変) — "kiln change," the broad word for the unpredictable shifts of color and surface that the fire leaves behind.
These are the surfaces the tea aesthetic learned to love: irregular, unrepeatable, marked by the fire. The wabi-sabi (侘寂, the beauty of the imperfect and impermanent) sensibility found in a run of natural ash glaze exactly what it valued — a beauty no potter could sign, because the kiln always had the last word.
Hai-yū in the ZenKiln collection
The most direct place to see hai-yū in living work is Shigaraki, one of Japan's Six Ancient Kilns, where the Hechimon art line of Marui Seitō still builds vases whose glaze owes as much to the fire as to the glaze bucket. Look for the green pooling, the running drips, and the ash "hood" gathered across a shoulder:
- Shigaraki Hechimon bud vase with an ash-glaze drip — a hand-built stoneware vase whose glassy green bidoro is natural ash glaze caught mid-run, the effect most prized on wood-fired ware.
- Shigaraki Hechimon Meiyō egg vase — a speckled tan Shigaraki body wearing a green ash-glaze hood across its top, a clean, sculptural study in hai-yū.
- Shigaraki-yaki Banju Natsume vase (24 cm) — a larger Hechimon vase carrying hai-kaburi, the "ash-blanket" surface, where drifted ash and iron meet a single stroke of gold.
- Kōzan Kutani leaping-rabbit matcha bowl — ash glaze on a tea bowl rather than a vase: three raised-white rabbits hop across a speckled grey ash-glaze ground.
To browse more, see the full Shigaraki Ware collection and our Ikebana Vases & Decor. And if you are reading Color of Utsuwa in sequence, set this chapter beside its refined descendant, seiji celadon green, and its iron-dark cousin, kuro-yū, the black glazes of Japanese ceramics.
Frequently asked questions about Japanese ash glaze (hai-yū)
What is hai-yū?
Hai-yū (灰釉) means "ash glaze" in Japanese. It is a ceramic glaze whose key ingredient is wood or straw ash, which melts at high kiln temperature to form a glassy surface. It is the oldest high-fired glaze in East Asia, and it appears both as natural ash glaze, formed by ash settling on a pot inside the kiln, and as applied ash glaze, coated on before firing.
What is the difference between natural and applied ash glaze?
Natural ash glaze (shizen-yū) forms inside the kiln when flying wood ash lands on a pot and melts on its own, leaving irregular runs and pools. Applied ash glaze uses ash that the potter has collected, washed, and mixed into a liquid to coat the pot deliberately before firing. The chemistry is the same; the difference is whether the potter or the fire placed the glaze.
Why is ash glaze often green?
The green comes from iron. Wood ash always contains a little iron, and when a piece is fired in a reduction, or low-oxygen, atmosphere, that iron produces a cool green — the same effect that gives celadon its color. Fired in oxidation, with more air, the same glaze turns amber, olive, or yellow-brown instead.
Is ash glaze the same as celadon?
Not quite, but they are closely related. Celadon (seiji) is a refined, carefully controlled lime glaze fired in reduction to give an even jade green. It evolved directly from early wood-ash glazes, so celadon can be thought of as ash glaze purified and standardized into a consistent color.
Which Japanese wares are known for ash glaze?
Natural ash glaze is a hallmark of wood-fired stoneware such as Shigaraki, Bizen, Iga, and Tokoname, where it is admired as shizen-yū, goma, and bidoro. Applied ash and iron-ash glazes are central to Seto and Mino wares, including the yellow iron-ash glaze known as Ki-Seto.
Is ash-glazed pottery safe to use for food and drink?
A fully fired, vitrified ash glaze is a stable glass surface like any other high-fired glaze. That said, many ash-glazed pieces are wood-fired stoneware or decorative vases rather than tableware, and some are best hand-washed. Always follow the care card included with each ZenKiln piece, and treat vases as decorative objects rather than food vessels.
Editor's note. ZenKiln works directly with the Shigaraki kilns behind the pieces linked above and hand-packs every order in Japan. Where firing methods, historical periods, and named traditions are cited here, they reflect the broadly documented history of Japanese ceramics, drawn from sources including the Metropolitan Museum of Art and the Robert Yellin Yakimono Gallery. We describe eras and traditions rather than attaching specific works to makers we cannot verify. — ZenKiln Editorial
