Ash Glaze (Haiyu): The Origin of High-Fired Japanese Glazes
Haiyu (ash glaze) is a glaze made by blending the ash of burnt wood or straw with feldspar and clay. Brushed or poured onto a pot and fired at high temperature, the minerals in the ash melt into a glassy skin.
A closely related term is natural ash glaze. In both, ash turns to glass. The difference is that natural ash glaze falls by chance from burning wood inside the kiln, while ash glaze is mixed by a person and applied to the pot. Waiting for the ash, or brushing the ash on. That single choice changes how the glaze lands, how repeatable it is, and which kilns can fire it.
Ash glaze sits at the very start of glazed pottery in Japan. It is still one of the most widely used glazes in studio work, and the same name produces completely different colours in different hands.
Glazes as a whole are classified in Japanese pottery glazes. This article digs into only those built on ash.
Ash glaze and natural ash glaze
Let us set the two most easily confused terms side by side first.
| What to look at | Ash glaze | Natural ash glaze |
|---|---|---|
| Where the ash comes from | Ash gathered and refined by a person | Ash from wood burning in the kiln |
| How it is applied | Dipped, poured or brushed on | Settles naturally during firing |
| How much it covers | The whole of the chosen surface | Only the side the ash falls on. Often just one face |
| Kilns that can fire it | Wood, gas and electric | Wood only |
| Repeatability | Blending and firing get you close | Never twice the same |
Natural ash glaze is what the kiln lays onto unglazed fired pottery, and it takes days of wood firing in an anagama or a noborigama to appear. Ash glaze is that phenomenon rebuilt so a person can repeat it. So it fires in an electric kiln, and it can coat the inside of a pot evenly.
From ash that fell, to ash that was applied
Ash-glazed pottery was first made in earnest in Japan at the Sanage kilns, in what is now Aichi Prefecture.
| Period | What happened |
|---|---|
| Around the 8th century | Pots are placed near the flame so ash lands on them. Natural ash glaze is now being aimed for |
| Early 9th century | Brushing an ash glaze on before firing appears, and ash-glazed ware goes into real production |
| Late 9th century | Ash-glazed ware from the Sanage kilns reaches its peak |
| Kamakura period onward | Ash and iron glaze techniques pass to Seto |
At first, pots were simply set where falling ash would catch them. In time, someone realised the ash could be painted on beforehand. The shift from natural ash glaze to ash glaze is the process of turning chance into technique.
That line carries into Seto ware, and in the Momoyama period it produces ki-seto, an ash glaze fired yellow in an oxidising flame, and oribe, an ash glaze turned green with copper. Shino, known for its white glaze, is built mainly on feldspar and belongs to a different line.
The work of turning ash into glaze
Ash is not a glaze as it comes. Makers generally prepare it in this order.
- Burn Burn firewood, straw or prunings and collect the ash. Remove any unburnt charcoal
- Leach Soak the ash in water, stir, pour off the top. Repeat many times to draw out the alkali
- Sieve Pass it through a fine sieve to even out the particles
- Blend Combine the dried ash with feldspar and clay to set how it melts and what colour it becomes
Leaching can take weeks, and some makers start by producing their own ash. Which tree, burnt where. That stays in the final colour, which is why plenty of makers insist on preparing ash themselves rather than buying it.
Different ash, different colour
Ash is mostly calcium, and that is what melts the glaze. The small amounts of iron and silica alongside it vary with the species of tree and the soil it grew in. This is why the same word, ash glaze, can mean an entirely different glaze once the ash changes.
| Ash | Character |
|---|---|
| Mixed wood ash | Ash from burning assorted woods. The most basic material for ash glaze |
| Pine ash | Ash from pine firewood. Makers often name it explicitly and use it deliberately |
| Straw ash | High in silica, so it does not fully melt and clouds white. The basis of white glazes |
| Fruit tree ash | Ash from prunings such as mandarin. Sometimes from trees local to the region |
One more thing decides the colour: the flame. The trace iron in the ash turns yellow or amber in an oxygen-rich flame, and blue or green when the oxygen is held back. Fire an iron-heavy glaze in reduction and push the blue-green as far as it goes, and you arrive at celadon.
What to look for in ash glaze
Ash glaze melts well and runs well at high temperature. That behaviour is exactly what you see on the finished pot.
- Running Molten glaze pulls downward and draws streaks. Thin at the rim, thick at the belly
- Pooling Glaze collects in the well of a dish or in carved grooves and clears like glass
- Crazing Fine cracks form in the glaze as it cools. The lines surface as the piece is used
- Clay showing through Where the glaze is thin, the colour of the clay beneath comes through. The choice of clay changes everything
On the cups at the top of this page, pale blue glaze has run in patches across the clay and the blue is deepest where it gathered. Where the glaze is thin, the grain of the clay shows through. Seeing thick and thin at once, on a single pot, is what ash glaze looks like.
Cracks in the glaze are covered in crazing.
Ash-glazed pottery at Nokaze
The ash-glazed pieces we carry fall into two lines, different in both clay and colour.
Karatsu clay and natural ash: pine ash glaze
Working in Yokosuka, Kanagawa, Naomi Fujii is committed to glazes made from Karatsu clay and natural ash, and values the expressions that arise on their own. Her pine-ash-glazed mugs and sake cups are finished so that pale blue glaze runs across a surface where the grain of the clay is visible. They are stoneware, so colour works into the crazing with use and the surface develops over time.
Her work is gathered in the FUJII NAOMI collection.
Ash glaze on porcelain: hai-ao and hai-kohaku
In Kyoto, Asami Maeda applies glazes she names with the character for ash, hai-ao (ash blue) and hai-kohaku (ash amber), over a porcelain body. The glaze gathers in the valleys of patterns raised by relief carving and slip trailing, so the depth of the carving becomes the depth of the colour. Over a white body it reads lighter than ash glaze on clay.
Her technique is covered in the artisan article. The pieces are gathered in the Maeda Asami collection.
We also carry ash-glazed work from a range of other makers, each with their own take on the glaze.
Living with ash-glazed pottery
How you care for it depends less on the ash glaze than on whether the body is stoneware or porcelain.
- Ash glaze on stoneware The clay absorbs water and stains can enter through the crazing. Sealing before first use is worth doing
- Ash glaze on porcelain It absorbs no water, so no sealing is needed. Simply wash and use
On stoneware, tea and coffee work into the crazing little by little and the piece changes. Enjoying that as part of the object is keinen bika. Do not leave it soaking, and dry it thoroughly before putting it away. Everyday handling is covered in stoneware care.
Related reading
Glazes
- Shizen-yu: Natural Ash Glaze Explained
- Seihakuji: White Porcelain Where Blue Pools in the Carving
- Celadon (Seiji): The Blue-Green Glaze of Iron and Fire
- Ki-Seto: The Yellow Glaze With Green Bleeding Through
- Oribe Ware: The Green Glaze That Broke the Rules
- Japanese Pottery Glazes: The Main Types Explained
Regions and history
- Seto Ware: The Kiln Town That Named Japanese Pottery
- Japan's Six Ancient Kilns: All 6 Regions Compared
- A Short History of Japanese Ceramics, Jomon to Now
Makers and care
- Artisan Maeda Asami | Japanese Porcelain Patterned in Relief, Not Paint
- Crazing (Kannyu): Cracks in Glaze That Aren't Damage
- Medome: How to Seal Unglazed Pottery Before First Use
- Japanese Stoneware Care: What Porcelain Never Needs
See ash-glazed pottery
Which tree's ash, and what kind of flame. The colour of an ash glaze carries the maker's choices straight through. Even in photographs, the running and the pooling differ from piece to piece. Karatsu clay under a pine ash glaze, sent to you directly from the maker.
See Naomi Fujii's potteryFAQ
How does ash glaze differ from natural ash glaze?
Do ash-glazed pieces need seasoning before first use?
Why does the same ash glaze look different from piece to piece?
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New to Japanese pottery? Start here — its history, how pottery differs from porcelain, the clays and glazes behind each style, and how to care for a piece. → Japanese Pottery: The Complete Guide