A green run on a pot may come from ash deposited in a wood kiln or from a glaze mixed and applied before firing. Hai-yū (灰釉) covers ash glazes with different recipes and processes. Knowing which process was used tells you more than colour alone.
Image note: Unless a caption identifies a ZenKiln catalogue photograph, the kiln, historical-object, glaze-preparation, and comparison scenes are editorial illustrations—not documentary photographs, verified museum objects, or records of a particular firing or workshop. The linked museum records support the historical examples discussed in the text.
The cover is an editorial visualization of a vessel beside a kiln, not a documentary photograph or a record of how a particular pot was fired.
What "hai-yū" means
Plant ash supplies fluxes and other minerals that help form a glaze. Naturally deposited ash and a prepared ash glaze can differ in composition, thickness and firing conditions.
Sue ware and deliberately applied ash glaze
High-temperature kiln technology from the Korean peninsula helped establish Sue ware in Japan during the fifth century. Wood ash could settle and melt on the pots. These high-fired ceramics followed much earlier Japanese earthenware production.
The Met’s ninth-century Sue jar shows natural ash glaze. The Suntory Museum’s Sanage bottle record describes deliberately applied ash glaze by the early ninth century. The objects document different processes without proving that every early glaze began as an accident.
How wood ash turns into glass
Calcium, potassium and other constituents of ash can help a glaze melt. The ash source, clay, added ingredients and firing schedule affect the result.
Iron can contribute green, amber, or brown tones, influenced by glaze composition and firing atmosphere. Ash glazes can also be pale, cloudy, or matte. Colour alone cannot reveal a complete recipe or firing history.
Potters may work with uneven deposits or seek a consistent applied finish. Thickness and conditions within the kiln contribute to the variations.
Wood ash, Ki-Seto and related glazes
Dobai refers to wood ash used as a material, not automatically to every applied ash glaze. Seto and Mino developed important ash-glaze traditions. Ki-Seto’s yellow surfaces are one related subject, introduced in the Encyclopædia Britannica; individual recipes and textures vary.
Celadon and some ash glazes share iron colouring and aspects of glaze chemistry, but celadon has a separate East Asian history. Our seiji study explains that tradition.
The language of ash: shizen-yū, goma, bidoro, yōhen
Natural ash effects are associated with wood-fired yakishime, ware fired without an applied glaze, from regions including Shigaraki, Bizen, Iga and Tokoname. The Robert Yellin Yakimono Gallery glossary explains several terms:
- Shizen-yū (自然釉). Natural ash glaze formed from deposits during firing.
- Goma (胡麻), “sesame”. A scatter of ash-glaze spots, particularly associated with Bizen.
- Bidoro. Glassy pools of glaze, associated especially with Iga; the term comes from Portuguese vidro, glass.
- Yōhen (窯変). The broader term for changes in colour and surface arising in the kiln.
Tea practitioners have valued many such firing surfaces. Look at the maker’s choices and the object’s condition; an uneven mark can be intentional, incidental or later damage.
Comparing contemporary surfaces
The following pieces offer flowing glaze and contrasting colours. Use the maker’s description to establish whether ash was deposited in firing, included in a prepared glaze or absent from the recipe:
- Shigaraki Hechimon bud vase — compare the green glaze run and the surrounding surface texture.
- Shigaraki Hechimon Meiyō egg vase — a rounded form with green glaze over a speckled body.
- Shigaraki-yaki Banju Natsume vase (24 cm) — a larger dark form with a contrasting gold-coloured accent; check the listed finish.
- Kōzan Kutani leaping-rabbit matcha bowl — white rabbit motifs over a speckled grey ground. Grey colour alone does not identify an ash glaze.
Browse Shigaraki Ware and Ikebana Vases & Decor by form and finish. Within Color of Utsuwa, compare seiji celadon green and kuro-yū, the black glazes of Japanese ceramics as related glaze subjects with their own histories.
FAQ
Is ash-glazed pottery safe to use for food and drink?
Use pieces intended for food or drink and follow their care instructions. A glassy appearance does not establish suitability. Decorative vases have a different use, and wear or repairs need separate assessment.
Sources: the Met’s Sue jar, Suntory Museum’s Sanage record and the glaze glossary linked above. The contemporary products illustrate visible surfaces, not a verified glaze formula.
