Crazing (Kannyu): Cracks in Glaze That Aren't Damage

The first time you pick up a Hagi ware yunomi and see the fine lines running densely across its surface, more than a few people think: "is this cracked?" And in general, picking up a piece of pottery and finding some kind of fissure in it, many must think it is broken and about to fall apart. Countless lines running from the inside of a bowl to the outside like the contour lines of a map, looking somehow cracked. That pattern is called kannyu, crazing.

The writer was also unsettled at the first encounter — "surely this is a defect?" But a maker smiled and said, "this is what grows," and it took years to understand what those words meant. Crazing is not a flaw in a piece. Rather it is one of the core aesthetic ideas of Japanese ceramic culture: it changes as the piece is used, marking a history that belongs to that piece alone.

This article explains carefully, from how crazing comes about scientifically, through the regions whose crazing is most appealing, to how to nurture and care for it. By the time you finish reading, those fine lines should look rather lovable.

What Crazing Is

The Science: A Difference in Shrinkage Between Glaze and Body

To understand crazing, you first need to know what happens when pottery is fired in a kiln and then cooled.

Pottery consists broadly of two things: the body, and the glaze. The body is the piece itself, clay fired hard; the glaze is a thin glassy layer applied over it. As this glaze melts and sets, a smooth gloss is born on the piece.

The point is that body and glaze differ in how much they shrink with heat. After firing at over 1,000°C in the kiln, a piece is cooled gradually. Body and glaze then each try to shrink at their own pace. When the glaze shrinks more than the body, the thin glassy layer is left in a stretched state, and eventually, unable to bear the internal pressure, it runs with fissures. This is crazing.

It is the same principle by which glass cracks when cooled abruptly. Shrinkage varies with the chemical composition of the glaze. Makers manipulate that composition deliberately, controlling whether crazing appears at all, and whether it appears fine or coarse.

The depth of crazing varies too. Some stops at the glaze surface; some reaches the body. In the latter case moisture and pigments from tea or coffee soak in readily, which connects directly to the process of "growing."

Crazing Is Not a Failure but a Character

In modern industrial tableware manufacture, crazing is treated as a defect of quality. In mass-produced ware, where a uniform look and durability are required, the shrinkage of body and glaze is matched strictly so that crazing does not arise.

In the context of traditional Japanese ceramics, however, crazing carries an entirely different meaning. In the world of tea especially it is bound deeply to the aesthetics of wabi and sabi, and has long been prized as a beauty of chance that cannot be made artificially.

No pattern of crazing is ever born exactly twice, even from the same maker, the same glaze, the same kiln. The place in the kiln, a slight difference in cooling rate, a subtle variation in the thickness of the body — countless variables overlap, and the crazing of that piece alone is born. It is also proof of a one-of-a-kind nature that cannot be copied.

There is the further aspect of "growing." A crazed piece takes liquid and residue into itself as it goes on being used, and its colour changes over time. The new state is not the finished state; it changes form within its relationship with the person using it. Exactly this dynamic beauty is why so many lovers of pottery are drawn to crazing.

The appeal of Japanese ware lies in the fact that each single piece has a different expression, its keshiki. On the appeal of meeting Japanese ware in the spirit of Ichigo Ichie, the articles below cover it too.

• Japanese Pottery and Ichigo-Ichie: The Beauty of One-of-a-Kind Handmade Pieces
• Japanese Pottery Keshiki: How to Read Glaze Drips, Flame Marks, and Kiln Effects

Wares and Regions With Beautiful Crazing

Wares characterised by crazing exist all across Japan, each with a different character.

Hagi Ware, Yamaguchi: Crazing That Changes Seven Times

Hagi ware, made mainly in the city of Hagi in Yamaguchi Prefecture, is known as the ware that "grows" most of all in Japan. There is even a phrase, "the seven changes of Hagi," saying that the colour and expression of a piece change as many as seven times as it is used (a figure of speech, of course; in practice gradual changes accumulate).

The Hagi body blends feldspar, pottery stone and sand, with coarser particles than other regions and very high absorbency. The combination of this porous structure with the fissures of the crazing is what makes liquid soak in so readily.

As you go on drinking tea, tea stain soaks into the fine grooves of the crazing, and a body that was whitish shifts gradually to a warm amber and brown. A Hagi tea bowl used for ten or twenty years takes on a depth like that of ancient pottery.

Hagi glazes are often feldspathic glazes called hagi-yu, with variations by maker running from milky white to a translucent pale green. Crazing running across those soft tones looks like a map drawn on porcelain skin, and matches deeply the aesthetic eye of Japanese wabi tea.

Raku Ware, Kyoto: The Keshiki of Wabi

Raku ware is said to have been founded in the sixteenth century by the potter Chojiro under the direction of Sen no Rikyu — ware born for the tea ceremony. It has its own technique: shaped by Tebineri alone without a wheel, and fired at low temperature, around 800–1,000°C.

Raku crazing differs a little from that of other wares. Low firing leaves the glaze incompletely vitrified, the surface often has a matt quality, and the crazing is sometimes not especially fine. But the appeal of raku lies less in the visual pattern of the crazing than in the whole keshiki.

Keshiki is a word used in the world of tea, a collective term for patterns born of chance on the surface of a piece — Yōhen (kiln transformation), the flow of glaze, crazing, unevenness of colour. With raku tea bowls, black raku and red raku differ in clay, glaze and firing, and the keshiki that appears differs entirely. Fine crazing running through the deep lacquer black of black raku can look like a nebula in space.

Kohiki: Delicate Fissures in a White Skin

Kohiki (white slip ware) is a technique of applying a white slip over an iron-rich red body, then a transparent glaze over that, and firing. Its name is said to come from "a whiteness as though powder had been drawn over it."

With kohiki, the differences in shrinkage within a three-layer structure — body, slip, glaze — produce the crazing. Crazing running over the white slip is extremely delicate, giving the impression of thin pencil lines drawn on a white skin.

Use a kohiki piece over a long time and the pigment of the red body beneath the slip seeps through, while tea, coffee and the colour of food soak into the slip, shifting the whole toward a warmer tone. The slip is also more absorbent than the body, so depending on use the colour can enter in patches. There is a culture of enjoying this as "the keshiki of kohiki."

Today kohiki is made in Mashiko, Kasama, Mino and elsewhere, with a wide range from everyday rice bowls and mugs to tea bowls.

• Mashiko Pottery Guide: Japan’s Folk Craft Heartland
• Kasama Ware: Japan's Most Free-Spirited Pottery Town
• Mino ware: Functional beauty born from one of Japan's largest production centers

Korai Tea Bowls: A Beauty of Crazing That Came From Korea

Indispensable in speaking of the beauty of crazing are the Korai tea bowls. These are bowls made in the Joseon period around the sixteenth century, prized by Japanese tea masters as tea utensils; after reaching Japan they were carefully handed down among daimyo houses and tea masters as meibutsu, named pieces.

Among Korai bowls, the ido bowl is characterised by a large, powerful form and a subdued yellow glaze close to the colour of loquat. During high firing the glaze melts and flows abundantly, and on cooling a crawled glaze pattern called kairagi is born. The expression of kairagi combined with crazing matched the aesthetic eye of wabi tea perfectly, prized to the point of the saying "first Ido, second Raku, third Karatsu."

Today Japanese potters too have learned much from the techniques of Korai bowls, making work that deliberately uses crazing. The influence of Korai bowls can be called one foundation of the beauty of crazing in contemporary Japanese ceramics.

How to Nurture Crazing: The Colour Changes as You Use It

The Mechanism by Which Character Emerges

The expression "nurturing" is often used of crazed pieces. What does that nurturing mean specifically?

The grooves of crazing look like fine lines to the naked eye, but at a microscopic level they are fine fissures continuing into the body. Liquid is drawn into these fissures by capillary action. In ware you drink tea from, tea stain; with coffee, the pigment of coffee; with sake, the components of the sake — little by little they accumulate and soak in.

Grooves that were transparent at first gradually take on colour as use continues. This change is not a matter of months but a slow process over years and decades. Drinking tea once a day — the accumulation of that marks the memory of time into a piece.

The pace of change differs greatly with the absorbency of the body. With a highly absorbent body like Hagi ware, change appears comparatively early. With a refined body close to porcelain, change is very gradual.

On this culture of nurturing ware, particular to Japanese ceramics, the article below covers it as well.

• Japanese Ceramics That Grow: How Handmade Pottery Changes and Deepens with Every Use

What Soaks In and Colours It

What colours crazing is mainly the following.

• Tea — green, hojicha, black　Tea, rich in tannin, leaves brown to amber stains in the crazing. When crazing in a Hagi yunomi turns a beautiful amber through years of green tea, that is the accumulation of tannin.
• Coffee and oolong tea　Rich in polyphenols, they tend to stain more darkly than tea. Used as a coffee mug, a piece can take its crazing to a deep brown or blackish brown.
• Sake and plum wine　The amino acids and sugars of the drink soak in, giving a mellow tone. Ochoko and tokkuri are representative of ware nurtured with sake.
• Soy sauce, dashi, miso soup　In ware used for soups, the pigment of soy sauce and the components of dashi soak in. If that is unwelcome, the preparation called Medome(Sealing,Seasoning) is effective (see below).

Colouring is not necessarily "dirt." Colour accumulated through proper use becomes the character of a piece and evidence of the time it has travelled. If food residue rots and soaks in, however, problems of smell and hygiene arise, so the habit of washing promptly after use matters.

Crazing and Kintsugi

In speaking of crazing, one cannot avoid its relation to kintsugi.

Kintsugi is a Japanese repair technique of joining broken or chipped ware with urushi and applying metal powder — gold, silver, tin — along the seam. Deliberately not hiding the repair but showing it beautifully as a golden line, the damage becomes a new character of the piece.

Crazing and kintsugi stand on the same aesthetic: accepting imperfection as beauty. Both are born of the idea of raising a "defect" into a "keshiki," and connect deeply with the spirit of wabi and sabi.

In practice, a long-used crazed piece can sometimes see part of its crazing enlarge into an actual crack. Moisture soaks repeatedly into crazing that has reached the body; freeze and thaw repeat, or the piece meets a sharp temperature difference, and the fissure can grow. At such a time, repair by kintsugi lets the life of the piece continue.

Apply kintsugi to a crazed piece and the golden line sometimes runs along the fine grooves of the crazing, which in turn produces a distinctive keshiki. From crazing, to breakage, to kintsugi — the sum of the time a piece has accumulated is inscribed on a single bowl.

Kintsugi has drawn attention in recent years not only within Japan but abroad. Its philosophy — repairing what is broken beautifully rather than hiding it — draws sympathy from the standpoint of sustainability as well. Using a crazed piece for a long time, and if it breaks, reviving it with kintsugi: this can be called one of the richest relationships possible with a piece of pottery.

• Kintsugi: How Japan Repairs Broken Ceramics with Gold

Caring for Crazed Ware

Medome When You First Use It

For crazed ware, especially highly absorbent Hagi, kohiki and Bizen, performing Medome(Sealing,Seasoning) when you first use it is recommended. Medome is a preparation that temporarily closes the fine pores of the body so that the colour and smell of food soak in less readily. The most popular method is simmering in rice washing water.

• 1. Prepare a pan　Add enough water to submerge the piece, plus rice washing water (or a little raw rice).
• 2. Heat from cold　Put the piece in while the water is cold and bring it slowly to the boil (avoid sudden temperature change).
• 3. Simmer gently　After 15–20 minutes over low heat, turn off the heat and let it cool as it is.
• 4. Dry it　Once cool, take it out, wipe the moisture away with a soft cloth, and dry it thoroughly.

The starch of the rice enters the fine pores of the body, making it less absorbent. This is a temporary effect, however; the starch washes out as use continues. Through repeated use the body gradually settles in.

There is no problem in use without medome, but avoiding deeply coloured food — curry, soy sauce, red wine — for the first several uses keeps the soaking-in less conspicuous.

Whether to perform medome at all is the owner's decision. If you want to enjoy "nurturing" from the very start, choosing to begin without it is also an option. Consulting the maker is another good approach.

Preventing Stains and Smells

Here are the points to watch in everyday care.

• Wash soon after use　Left for long with food still on it, pigment and smell soak in deeply. Once you have finished, either leave it in water or wash it soon.
• How to wash　Wash gently with a soft sponge and neutral detergent. Avoid abrasive sponges and strong detergents, which can damage the glaze. A dishwasher's sudden temperature change and strong water flow can enlarge crazing, so hand washing is recommended as a rule.
• Drying　After washing, wipe the moisture away and dry thoroughly before storing. Stacked and stored while damp, pieces dry poorly and this can cause smell and mould.
• If a stain concerns you　Soaking briefly in diluted vinegar water (about 9 parts water to 1 of vinegar) can ease pigments such as tannin. Considering the effect on the piece, though, avoid long soaking. And accepting a stain that will not come out even so, as "grown colour," is the skilful way to live with crazed ware.
• Microwave and oven　Using highly absorbent ware in a microwave can break it, as moisture taken into the body heats up. With crazed ware especially, check that it is dry before use and follow the maker's recommendation.
• Long storage　When not using a piece for a long period, wrap it in soft paper such as washi and store it somewhere with little humidity. A paulownia box is ideal, and this is the usual method for tea bowls used as tea utensils.

For more detail on caring for Japanese ceramics, see the article below as well.

• Japanese Ceramics Care Guide

Frequently Asked Questions About Crazing

Q1. Does crazed ware leak?

Crazing in the glaze layer is not a crack through the body, so it does not normally leak. With highly absorbent bodies like Hagi and kohiki, though, leaving water in for a long time can make the foot faintly damp. If you have the habit of drying a piece after use, there is no problem.

Q2. Does crazing increase with use?

It does. It is normal for new crazing to come in little by little as temperature changes accumulate through use, rather than all appearing straight from the kiln. You may even hear a small "ping." That is not the sound of a piece breaking but of the glaze moving.

Q3. Is there a trick to colouring crazing faster?

What works best is using it every day. Drinking tannin-rich green tea or hojicha, or polyphenol-rich coffee, from the same piece brings change out faster. Soaking it in strong tea for hours to hurry things along is not recommended — it causes patchy staining.

Q4. Can crazed ware go in the dishwasher?

As a rule we recommend hand washing. A dishwasher's sudden temperature change and strong water flow can widen crazing, or leave it insufficiently dried and cause smell. With a refined body close to porcelain it can be fine, so check the maker's guidance.

The Fissures Are the Piece's Record: The Appeal of Ceramics You Nurture

Crazing is a scientifically inevitable phenomenon, born of a difference in shrinkage between glaze and body. Yet Japanese ceramic culture has cherished that product of chance as character.

The seven changes of Hagi ware, the wabi keshiki of raku, the delicate white fissures of kohiki, the historical beauty of Korai bowls — each piece holds its own expression through crazing.

And taking that piece in hand, pouring tea into it every day, letting colour soak in little by little. That is not simply using a piece; it is nurturing one. In ten years, in twenty, that piece will hold a keshiki nothing else has, inscribed with the time it spent with you.

Crazing is precisely a piece's record. Who used it, what was drunk from it, what was eaten from it, what time was spent with it — that record accumulates in every one of those fine grooves.

Time that belongs to that piece alone, never born in an industrial product. The greatest appeal of crazed pottery is a paradoxical beauty: the more it is used, the closer it comes to being finished.

The next time you visit a pottery market or a specialist shop and find a piece with fine lines running across its surface, do take it in hand. That is neither damage nor defect, but a blank page of the story about to begin with you.

Related Articles and Guides

More on the Culture and History of Japanese Ceramics

• The History of Japanese Ceramics
• The Aesthetic of Wabi-Sabi Hidden in Japanese Ceramics — Nurturing a Japanese ceramic
• Kintsugi: How Japan Repairs Broken Ceramics with Gold
• What Wabi-Sabi Ceramics Bring Into our Life
• Japanese Ceramics That Grow: How Handmade Pottery Changes and Deepens with Every Use
• Japanese Pottery and Ichigo-Ichie: The Beauty of One-of-a-Kind Handmade Pieces
• Japanese Pottery Keshiki: How to Read Glaze Drips, Flame Marks, and Kiln Effects

More on Japan's Kiln Regions

• A Style Guide to Japanese Ceramics by Kiln Region
• Hagi Ware: Four Centuries of Living Japanese Ceramics Loved by Tea Masters
• Hagi Ware and Bizen Pottery: Two Philosophies of Wabi-Sabi in Japanese Ceramics
• Raku Ware: The Tea Bowl Born from the Philosophy of Japan

How to Choose Japanese Ceramics

• Why Japanese Pottery Is Worth Owning
• Japanese Stoneware and Porcelain Explained
• Japanese Pottery: Earthenware, Stoneware and Porcelain Explained

How to Buy Japanese Ceramics

• About Us｜Japanese pottery & culture
• Buy Japanese Pottery Online
• New Arrivals