Ash glaze (hai-yu or kai-yu) is a traditional ceramic technique that creates natural glass-like glazes from wood ash, featuring humble yet profound colors and flowing patterns that captivate with their beauty. Used since the Nara period in ancient times, this glaze has supported Japanese ceramic culture, offering the charm of variations created by materials and firing processes, allowing us to enjoy the beauty of “kiln transformation” (youhen) where no two pieces are alike.
This article provides an easy-to-understand explanation of ash glaze history, characteristics, wood ash formulation methods, and firing techniques for beginners. Why not explore the world of ceramics more deeply while experiencing the gentle beauty woven by natural materials?
Table of Contents
What is Ash Glaze (Hai-yu/Kai-yu)?
Ash glaze lidded jar Sue ware KaiyuFutaTsubo Sueki
Ash glaze (hai-yu/kai-yu) is one of the oldest glazes used in ceramics, referring to the natural glass-like coating created by firing natural ash, primarily wood ash, at high temperatures. Its characteristic lies in the calcium and alkali components contained in the ash reacting with clay to form a transparent, greenish glass layer.
Though simple, the subtle color variations and flowing patterns that change according to firing conditions, raw material composition, and the reduction or oxidation atmosphere within the kiln are valued as “natural beauty” that cannot be achieved by industrial products. Contemporary ceramic artists continue to create works that utilize the individuality of materials like clay, ash, and flame by pursuing “natural glaze landscapes,” making ash glaze one of the techniques that symbolizes Japan’s pottery culture.
Natural “Glass” Glaze Created When Mixed Wood Ash, Straw Ash, Feldspar, and Clay Melt Together at High Temperatures
The appeal of ash glaze lies in utilizing familiar natural materials directly to create what could be called “natural glass” – a glass-like coating produced through high-temperature firing. The main glaze ingredients are ash components from burned mixed wood, straw ash, bamboo ash, and other combustion residues, which are rich in calcium, potassium, magnesium, and other elements.
These ashes are fired at high temperatures together with feldspar and clay, causing them to melt and form a glass-like layer on the clay surface. In oxidation firing particularly, the ash components take on a transparent to yellow-green luster, while in reduction firing they change to deep blue-green or olive colors.
The flowing patterns, gradations, and spots created by materials and firing conditions represent accidental beauty, and since the same recipe produces different expressions with each firing, ceramic artists cherish this as “flame landscape” or “kiln transformation” (youhen). Even today, the trend continues to value such natural material-derived glazes while making intentional adjustments based on chemical analysis.
Origins in Chinese Six Dynasties Celadon, Japanese Roots in Sue Ware Natural Ash Glaze
Tracing the origins of ash glaze leads back to ash-glazed pottery that appeared during the Yin-Zhou period (15th century BCE). This involved applying high-fire glazes mixed from plant ash and clay, and by the Later Han to Six Dynasties period, the technology was refined at kilns like Yuezhou in Jiangnan, completing the transparent feldspar-wood ash glazes known as celadon.
In Japan, Sue ware production began around the mid-5th century, but natural ash glaze from naturally falling ash melting on vessel surfaces can be clearly confirmed from the late 6th century onward. This phenomenon became the foundation for later ash glaze culture.
From the late Nara to early Heian period, artificial ash glaze mixing ash was developed at Sanage kilns, and by the 9th century, bowls, dishes, and jars were being systematically glazed. From the late Heian through Kamakura periods, ash-glazed pottery was mass-produced in Seto, Mino, and other regions, forming Japan’s unique aesthetic of “enjoying landscape.” Against this historical background, ash glaze continues to be inherited as a representative glaze that produces naturally rich expressions in contemporary ceramics.
Natural Ash Glaze vs. Formulated Ash Glaze – Differences in Formation and Intent
Ash glaze can be broadly divided into two types: “natural ash glaze” and “formulated ash glaze.”
**Natural ash glaze** occurs when ash from wood kilns naturally falls onto vessels during firing, melting at high temperature to spontaneously form a glass coating. This technique, seen in Sue ware and medieval Shigaraki pottery, is appealing for its flowing patterns and color changes created by chance and ash components.
On the other hand, **formulated ash glaze** involves crushing natural ash, removing impurities through elutriation, and precisely mixing it with feldspar, clay, lime, and other materials before applying this glaze to vessels for firing. Formulated ash glaze offers the advantages of more stable results and reproducible intended colors, and was developed in many production areas including Seto, Mino, and Karatsu.
In contemporary ceramics, approaches combining the randomness of natural ash with the controllability of formulated ash glaze are also practiced, with ongoing attempts to pursue beauty where intent and nature compete. The diverse techniques of ash glaze continue to expand the range of ceramic expression.
History of Ash Glaze Evolution Within East Asian Ceramics
Ash glaze (hai-yu/kai-yu) is a ceramic technique that has evolved over centuries within East Asian ceramic culture, with its greatest characteristic being the glass-like coating created by melting natural ash. While following the lineage of celadon glazes developed in China, in Japan it began with natural ash glaze on Sue ware and developed through Heian period ash-glazed pottery to medieval and Muromachi period Ko-Seto.
During the Momoyama period, its “landscape” was again highly valued in tea ceramics, becoming the foundation for nurturing unique aesthetics in Shino and Oribe. The history of ash glaze in East Asia was not merely a decorative technique, but also a process that nurtured the spirit of incorporating the randomness of materials and flame to revere “natural beauty.” Below, we detail the development through each period: China, Heian to Muromachi periods, and Momoyama tea ceramics.
Blue-Green Transparent Glaze Perfected at Chinese Southern and Northern Dynasties to Song Dynasty Longquan Kilns
One technical perfection of ash glaze often cited is the celadon glaze that developed from China’s Southern and Northern Dynasties through the Song period. During the Southern and Northern Dynasties (5th-6th centuries), glazes using lime-rich ash and feldspar were developed in the lower Yangtze River region, producing yellow-green colors in oxidation firing and blue-tinged transparent glaze layers in reduction firing.
Entering the Song period (10th-13th centuries), celadon production areas in Zhejiang Province, including Longquan kilns, used carefully selected clay and high-purity feldspar and ash with reduction flames to complete clear blue-green colors like “plum green” and “powder blue.” These celadons were highly valued as imperial tribute items and export ceramics, influencing all of East Asia.
Ash glaze evolved beyond simple melting into advanced technology for adjusting transparency, thickness, and blue depth through glaze ingredient formulation and firing control, reaching the pinnacle of refinement as decorative art.
Development from Japanese Heian Ash-Glazed Pottery to Kamakura-Muromachi Seto Glaze (Ko-Seto)
The development of ash glaze in Japan began with accidental natural ash (natural ash glaze) that fell on Sue ware during production from the mid-5th century, eventually developing into intentional glazing. The ash-glazed pottery technique created at Sanage kilns in the first half of the Heian period (early 9th century) spread to Seto and Tokoname from the late 9th to late 10th centuries, creating the earliest glazed pottery called “Heian ash glaze.” These vessels are characterized by being covered entirely in transparent glaze ranging from light green to yellow-green.
In the early Kamakura period (late 12th century), Seto developed glazed pottery called “Ko-Seto glaze” by combining glazes mixed from feldspar and ash with oxidation and reduction firing. Ko-Seto initially went through an early period (early Kamakura) using only ash glaze, then from the Nanbokucho period onward, diverse decorative techniques like iron glaze, stamped patterns, and applied decorations were added, leaving a major mark on medieval Japanese ceramic industry history through its varied expressions.
During the Muromachi period, the aesthetic of “wabi-sabi” emerged under tea ceremony culture influence, and glaze flows, irregularities, and scorching began to be appreciated as landscape, further diversifying ash glaze expression.
Reevaluation in Momoyama Tea Ceramics – Ash Glaze Beauty Inherited by Shino and Oribe
Ash glaze was again highly valued in the tea ceramic world from the late Muromachi through Momoyama periods. In tea ceremony culture nurtured by Sen no Rikyu and others, the aesthetic that valued “wabi-sabi” became mainstream, respecting natural change and imperfection over artificial uniformity.
Responding to this, the Mino region developed diverse tea ceramics including Shino, Oribe, and Ki-Seto. While Shino enjoys white cloudy feldspar glaze with iron painting and fire color areas as landscape, ash glaze techniques are applied in its base and glaze layers.
Oribe also employed glaze adjustments adding metal oxides to ash glaze bases for copper green glaze and black Oribe, creating designs utilizing kiln transformation like flowing patterns and glaze breaks. These tea ceramics actively incorporated the randomness of ash glaze to find beauty “between intent and nature,” conveying the Japanese ceramic spirit to today. Ash glaze matured as an important element supporting tea ceremony culture, transcending mere technique.
What are the Raw Materials and Formulation Methods of Ash Glaze?
Ash glaze (hai-yu/kai-yu) is one of the representative ceramic glazes utilizing natural materials, with its appeal lying in the simplicity of raw materials and the rich variations created through firing. The basic raw materials consist mainly of ash from burned wood and plants, with feldspar, clay, and other materials added for formulation.
The calcium, potassium, and other components contained in ash act as fluxes during high-temperature firing, covering the pottery surface with a glass-like glaze layer. Depending on formulation ratios and raw material types, transparency, color tone, and flowing patterns change dramatically, creating beauty that includes the randomness called kiln transformation (youhen).
Here, we introduce in detail the main raw materials composing ash glaze and their functions, along with formulation points, from three perspectives.
Mixed Wood Ash, Straw Ash, Bamboo Ash… CaO and K₂O in Ash Become Fluxes
The greatest characteristic of ash glaze lies in utilizing familiar natural materials. Using ash from burned plants like mixed wood ash, straw ash, and bamboo ash as main raw materials, this ash is rich in calcium oxide (CaO) and potassium oxide (K₂O). These components work as “fluxes,” melting silica and alumina contained in clay at high temperatures to form glass-like glaze films.
For example, **mixed wood ash** tends to have more CaO with a high melting point while easily producing stable transparency.
On the other hand, **straw ash** has more K₂O with a lower melting point, tending to create soft, flowing glaze tones. Bamboo ash also varies in components depending on region and burning methods, showing individual characteristics like whitish glass or greenish transparency. Such ash is collected, dried, fired, crushed, and refined for purity through elutriation by production areas and ceramic artists, being utilized as optimal glaze raw materials.
Controlling Melting Point, Transparency, and Flow Through Feldspar and Clay Ratio Adjustment
In ash glaze making, ash alone is not simply used. While ash contains abundant flux components, it often lacks stability as a glaze by itself, so raw materials like feldspar and clay are added for formulation. Feldspar contains silica and alumina, becoming the main component that reacts with ash CaO and K₂O to form glass.
Adding more feldspar raises the melting point, creating a firm, thick glaze film with transparent finish. Conversely, more ash lowers the melting point, increasing fluidity and making accidental expressions like glaze runs and flowing patterns more likely. Additionally, adding clay components controls glaze shrinkage, preventing peeling and pinholes.
These raw material ratios are delicately adjusted by each ceramic artist considering firing temperature, clay type used, and kiln reduction-oxidation atmosphere. As a result, completely different finishes can be achieved even with the same raw materials through different formulations, which is the appeal of ash glaze.
Color Development Changes to Blue, Green, Brown According to Iron, Manganese, Titanium Content
One appeal of ash glaze lies in producing diverse colors through trace elements. Raw material ash contains components from burned plants and soil, with iron (Fe), manganese (Mn), titanium (Ti), and other content greatly affecting glaze color.
With high iron content, blue-green colors are obtained in reduction firing, while yellow-brown or olive colors appear in oxidation firing. This principle has been utilized in Chinese celadon and Japanese Ko-Seto glaze. Manganese content produces purplish brown effects and increases depth, creating calm, subdued coloring.
Titanium content creates opalescent white or cream-colored opacity, adding soft texture. Since such trace elements naturally vary depending on tree species burned and ash generation methods, ash glaze shows “different faces with each kiln firing even with the same recipe.” This unpredictability is both challenge and appeal for ceramic artists, becoming the foundation for creating unique works utilizing natural materials.
Firing and Kilns – Landscapes Created by Flame and Ash
The beauty of ash glaze lies not only in formulation skill, but in the unique “landscapes” created by flame and ash within kilns, along with management of firing temperature and atmosphere (oxidation/reduction). Glaze is not merely a coating film, but creates glass layers while reacting with clay at high temperatures, generating crystals, flowing patterns, and color changes within.
Ash glaze particularly shows completely different expressions even with the same formulation through natural ash fall in wood kilns, flame reduction power, and oxidation atmosphere control. In modern times, techniques to reproduce this in gas and electric kilns are also researched, pursuing unique beauty where “intent and chance” of natural and formulated glazes compete. Below, we detail in depth the color differences between reduction and oxidation firing, comparisons between wood and gas kilns, and landscapes created by melting and cooling in different temperature ranges.
Reduction Firing Creates Celadon Tones, Oxidation Firing Changes to Amber and Yellow-Brown
One appeal of ash glaze is that different color tones are drawn from the same raw materials depending on kiln atmosphere during firing. In oxidation firing, sufficient air is supplied, and iron content in the glaze stabilizes as ferric oxide (Fe2O3), showing warm yellow-brown, amber, and olive-system coloring.
Conversely, in reduction firing, oxygen supply is limited and incompletely burned gases steal oxygen, causing iron content to become ferrous oxide (FeO) and develop blue-green colors like celadon. This technique has been widely used throughout East Asian ceramic history from ancient Chinese Longquan celadon to Japanese Seto glaze and Momoyama tea ceramics like Oribe and Shino.
In contemporary ceramics too, how to design the harmony between celadon-tone transparency through reduction flame control and the rustic clay flavor through oxidation flame is an important element reflecting individual artist characteristics. Subtle changes also occur depending on glaze thickness, body composition, and firing time, creating unique expressions even with the same formulation.
Wood Kiln Natural Ash Glaze vs. Gas Kiln Formulated Ash Glaze Reproduction Techniques
In traditional wood kiln firing, wood ash dances in flames and naturally falls onto vessels, melting to form “natural ash glaze.” This creates unintended ash thickness, flows, and spots, with the unique landscapes brought by chance becoming appealing. In unglazed high-temperature firing like Shigaraki and Bizen pottery, glass pools from this natural ash are also revered as beauty.
Conversely, modern gas and electric kilns don’t produce natural ash fall, so “formulated ash glaze” using crushed and refined plant ash mixed intentionally is used to reproduce natural glaze. This enables stability and color control difficult to achieve in wood kilns, supporting mass production and reproduction of specific colors. However, completely reproducing the accidental expressions of natural glaze is difficult, and many ceramic artists explore landscapes approaching natural glaze even in gas kilns by utilizing oxidation-reduction switching and temperature changes during firing. Such attempts represent the fusion of traditional inheritance and modern technology.
Melting at 1,250-1,300°C, Cooling Crystals Create “Flows” and “Pools”
Ash glaze is generally fired at high temperatures of 1,250-1,300°C, where ash components CaO and K₂O react with feldspar and silica to melt into glass. In this high-temperature range, the glaze maintains viscosity while flowing, creating “flows” sliding down vessel curves and “pools” collecting in grooves and depressions.
This is the greatest appeal of natural glaze, where sculptural landscapes accidentally shaped by flame direction, ash quantity, and firing time can never be reproduced identically. Additionally, when crystallization progresses during post-firing cooling, opalescence and crystal spots appear on glaze surfaces, adding even more diverse expressions. The physicochemical reactions within kilns involving reduction or oxidation flames, iron content changes, and transparency differences from glaze thickness are complex, with craftsmen making subtle adjustments through experience and intuition.
In contemporary ceramics too, attempts are made to maximize ash glaze appeal by designing cooling speed change programs and reduction switching timing details to intentionally design “flows” and “pools.”
What are the Appeals of Ash Glaze (Hai-yu/Kai-yu)?
Ash glaze (hai-yu/kai-yu) is a representative glaze that has supported the ceramic history of Japan and East Asia, with its greatest appeal lying in the unpredictable beauty created by naturally-derived materials. Though composed of simple formulations using mixed wood ash, straw ash, bamboo ash, and other plant ashes as main components with added feldspar and clay, rich “landscapes” are created through flame, temperature, and ash melting during firing, with no two pieces having the same expression.
Additionally, color tones change dramatically through reduction and oxidation flame control, while flowing patterns, three-dimensional pools, and increased luster through use create appeal as “growing vessels” including aging changes. Below, we detail three appeals unique to ash glaze: **glass texture, patterns created by chance, and aging changes**.
Coexistence of Glass Transparency and Matte Softness
Ash glaze creates glass layers on pottery surfaces when melted ash CaO and K₂O react with feldspar and silica. These layers gain beautiful transparency through high-temperature melting and cooling, but unlike mere glass that is hard and reflects light, they also possess soft opalescent luster through dispersion of fine crystals and impurities.
They show diverse coloring – yellow-green and amber in oxidation firing, celadon blue-green in reduction firing – with glaze thickness and body color showing through to create unique depth. Additionally, techniques for creating moist matte texture by controlling glaze film shrinkage through feldspar and clay ratio adjustment are also possible.
This coexistence of contradictory textures – “transparent yet soft” – can only be realized through craftsman skill in utilizing natural materials and manipulating fire and ash chemical reactions.
Accidental Flowing Patterns and Pools – No Two Landscapes Are Alike
The essence of ash glaze lies in “landscapes” created by accidental chemical reactions and physical phenomena during firing. In wood kiln firing, ash from burning wood rises and naturally accumulates on vessels, creating partial thickness that forms “flows” and “pools.”
When this ash melts at high temperatures (approximately 1,250-1,300°C), it covers surfaces as glass glaze film, with falling traces forming unique striped patterns and gradations. Additionally, flame contact within kilns, oxidation-reduction atmosphere changes, and cooling speed also subtly change coloring and transparency, achieving irreproducible unique expressions even with the same formulation and kiln.
Such randomness is also called “kiln transformation” (youhen) and has long been valued as precious beauty in Japanese ceramics. Craftsmen calculate this randomness and intentionally pursue heterogeneous beauty, creating “living vessels” unavailable in mass-produced items.
Aging Changes and Hand Familiarity Where Luster Deepens with Use
Another major appeal of ash glaze lies in “aging changes” that deepen through use. Though ash glaze is glass-like, unlike completely non-absorbent hard glass, it contains slight irregularities and internal crystals, gradually absorbing friction, oils, and hand oils during use to increase unique luster.
New vessels have somewhat matte, subdued luster, but through daily use gradually gain moist smoothness and deep brilliance, achieving touch that seems to stick to hands. This connects to Japan’s unique aesthetic of “nurturing vessels,” where the same vessel undergoes different changes depending on the user’s lifestyle, hand shape, and contact frequency.
Additionally, small glaze cracks and color changes that develop through long use are evidence of attachment, and the process of nurturing unique landscapes over time itself could be called ash glaze’s greatest appeal.
Summary
Ash glaze (hai-yu/kai-yu) is a traditional glaze representing Japanese and East Asian ceramics, using naturally-derived materials like mixed wood ash and straw ash to create glass coatings on pottery surfaces through high-temperature firing. Its appeal lies in utilizing “natural beauty” beyond mere decoration.
Color tones changing through oxidation-reduction firing, flowing patterns and pools full of randomness from wood kilns, subtle expressions from cooling crystals – no two are alike. Additionally, the technology for freely manipulating transparency and matte texture by adjusting feldspar and clay formulations to control melting points and flow represents the crystallization of craftsman experience and intuition.
Furthermore, aging changes where luster deepens and hand familiarity increases through use further enhance ash glaze appeal. Ash glaze is an art that nurtures unique “landscapes” woven by materials, flame, ash, time, and human hands, with its value continuing to be inherited in contemporary ceramics unchanged.
