The Secret of "Kannyu" in Japanese Pottery: Deeper Beauty with Use

When you first picked up a Hagi-ware yunomi (teacup), many of you probably saw the fine lines covering its surface and thought, "Is this broken?" You might also often pick up a piece of pottery and see cracks, thinking it's broken and will soon fall apart. These countless lines run from the inside to the outside of the bowl, like contour lines on a map, and can look like cracks. This pattern is called "**kannyu**" or crazing.

When I first encountered it, I was also confused, thinking it might be a defective product. However, the artist smiled and told me, "This is what grows," and it took me years to understand the meaning of those words. Kannyu is not a defect in the pottery. Rather, it is one of the core aesthetics of Japanese pottery culture, where the piece changes with use, engraving its unique history.

This article will carefully explain everything from the scientific origins of kannyu, to regions where kannyu is particularly beautiful, and how to care for and "grow" it. By the time you finish reading, those fine lines will surely appear endearing to you.

What is Kannyu (Crazing)?

Scientific Mechanism: Difference in Shrinkage Rates between Glaze and Clay Body

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

Pottery is largely composed of two parts: the "clay body" and the "glaze." The clay body is the main part of the vessel, formed by firing clay, and the glaze is a thin, glassy layer applied over it. When this glaze melts and hardens, it gives the vessel a smooth, glossy finish.

The issue is that the clay body and the glaze have different thermal shrinkage rates. After being fired at temperatures exceeding 1,000 degrees Celsius in a kiln, the pottery gradually cools down. During this process, the clay body and the glaze try to shrink at their own unique rates. If the glaze's shrinkage rate is greater than that of the clay body, the thin, glassy layer is subjected to tension. Eventually, it can no longer withstand the internal pressure, and cracks appear. This is what kannyu is.

This is the same principle that causes glass to crack when cooled rapidly. The shrinkage rate varies depending on the chemical composition of the glaze. Artists intentionally manipulate this mixture to control whether kannyu "appears," "does not appear," "appears finely," or "appears coarsely."

The depth of kannyu also varies. Some cracks remain only on the surface of the glaze, while others reach the clay body. In the latter case, moisture, tea, coffee, and other pigments are more easily absorbed, which directly contributes to the "growing" process.

Kannyu is "Individuality," Not "Failure"

In modern industrial tableware manufacturing, kannyu is treated as a quality defect. For mass-produced tableware that demands a uniform appearance and durability, the shrinkage rates of the clay body and glaze are precisely matched to prevent kannyu from occurring.

However, in the context of traditional Japanese pottery, kannyu has an entirely different meaning. Especially in the world of the tea ceremony, it is deeply connected to the aesthetic sensibilities of wabi-sabi and has long been prized as an "accidental beauty" that cannot be artificially created.

No two kannyu patterns are ever exactly alike, even if the same artist uses the same glaze and fires them in the same kiln. The position in the kiln, slight differences in cooling speed, subtle variations in the thickness of the clay body—innumerable variables combine to create a unique kannyu for each piece. This also proves its irreplicable individuality.

Furthermore, there is the aspect of "growing." A piece of pottery with kannyu will absorb liquids and stains with continued use, changing color over time. The new condition is not its finished form; it transforms through its relationship with the user. This dynamic beauty is why many pottery enthusiasts are captivated by kannyu.

One of the charms of Japanese pottery is the individual expression, or "keshiki" (scenery), of each piece. For more on the appeal of these unique encounters with Japanese pottery, please see the articles below.

Pottery and Regions Where Kannyu is Beautiful

While various types of Japanese pottery feature kannyu, each possesses a distinct character.

Hagi Ware (Yamaguchi Prefecture): Kannyu that Transforms Seven Times

Hagi ware, produced mainly in Hagi City, Yamaguchi Prefecture, is known as the most "maturing" pottery in Japan. There's a saying, "Hagi no Nanamake" (Hagi's Seven Transformations), which means the color and appearance of the pottery change seven times with use (of course, this is a metaphorical expression, and in reality, it's a gradual accumulation of changes).

Hagi ware's clay body is a blend of feldspar, pottery stone, and sand, making it coarser and highly absorbent compared to other regions. This porous structure of the clay body, combined with the cracks of kannyu, makes it easy for liquids to penetrate.

As tea is continuously drunk, tea stains seep into the fine grooves of the kannyu, gradually changing the initially whitish clay body to a warm amber or brownish hue. A Hagi teacup used for 10 or 20 years develops a depth akin to ancient pottery.

The glaze used for Hagi ware is often a feldspar glaze called "Hagi-glaze," with various variations from milky white to translucent pale green, depending on the artist. The kannyu running through its soft colors is like a map drawn on the porcelain's surface, deeply aligning with the aesthetic sense of Japanese wabi-cha.

Raku Ware (Kyoto): Scenery of Wabi

Raku ware is pottery created for the tea ceremony, said to have been founded in the 16th century by the potter Chojiro under the guidance of Sen no Rikyu. It features a unique technique of shaping solely by hand-building without a potter's wheel and firing at low temperatures (around 800-1,000 degrees Celsius).

The kannyu in Raku ware is slightly different from other pottery. Due to low-temperature firing, the vitrification of the glaze is incomplete, and the surface of the vessel often has a matte texture, so the kannyu may not be as fine. However, the appeal of Raku ware lies more in its overall "keshiki" (scenery) than in the visual pattern of the kannyu.

"Keshiki" is a term used in the world of the tea ceremony, referring to the collective name for patterns created by serendipity on the vessel's surface, such as kiln changes (kamahen), glaze flow, kannyu, and uneven coloring. In the case of Raku tea bowls, the clay, glaze, and firing methods differ for black Raku and red Raku, resulting in entirely different keshiki. The subtle kannyu running through the deep black of black Raku can resemble a cosmic nebula.

Kohiki: Delicate Cracks on a White Surface

Kohiki is a technique where a white slip (shirodoro) is applied over a red clay body rich in iron, and then a transparent glaze is applied before firing. Its name is said to come from its "powdered white" appearance.

In Kohiki, the difference in shrinkage rates among the three layers—clay body, slip, and glaze—creates kannyu. The kannyu running across the white slip is extremely delicate, giving the impression of fine pencil lines drawn on a white surface.

As Kohiki pottery is used over time, the pigments from the red clay body beneath the slip may seep through, or tea stains, coffee, and food colors may absorb into the slip, gradually changing the overall tone to a warmer hue. Furthermore, the slip is more absorbent than the clay body, and depending on how it's used, colors may stain unevenly. There's a culture of appreciating this as the "Kohiki keshiki" (Kohiki scenery).

Today, Kohiki pottery is produced in various regions such as Mashiko, Kasama, and Mino, offering a wide range of items from everyday rice bowls and mugs to tea bowls.

Goryeo Tea Bowls: The Beauty of Kannyu from Korea

When discussing the beauty of kannyu, Goryeo tea bowls (Korai Chawan) are indispensable. These are tea bowls made in the Korean Joseon Dynasty around the 16th century, treasured by Japanese tea masters as tea utensils. After arriving in Japan, they were cherished as "meibutsu" (famous articles) and passed down among prominent daimyo families and tea masters.

Among Goryeo tea bowls, Ido Chawan (well-shaped tea bowls) are characterized by their large, powerful form and a dull yellow glaze close to persimmon color. During high-temperature firing, the glaze melts and flows abundantly, creating a wrinkled pattern called "kairagi" (plum blossom skin) upon cooling. The combination of this kairagi and kannyu perfectly aligned with the aesthetic sense of wabi-cha, leading to its high esteem, being ranked as "first Ido, second Raku, third Karatsu."

Today, Japanese potters also learn much from the techniques of Goryeo tea bowls, creating works that intentionally incorporate kannyu. The influence of Goryeo tea bowls can be said to be one of the foundations of kannyu beauty in contemporary Japanese pottery.

How to "Grow" Kannyu: How It Changes Color with Use

The Mechanism Behind How It Gets Better with Use

The phrase "to grow" pottery with kannyu is often used. What exactly does "to grow" mean?

The grooves of kannyu, which appear as fine lines to the naked eye, are, at a microscopic level, thin cracks that extend deep into the clay body. Liquids are drawn into these cracks by capillary action. If it's a teacup, tea stains; if it's for coffee, coffee pigments; if it's for sake, components of sake gradually accumulate and permeate.

The kannyu grooves, initially transparent, gradually become colored with continued use. This change is not on a scale of months, but a slow process over years or even decades. Drinking tea once a day—this accumulation engraves the memory of time into the pottery.

The pace of change varies greatly depending on the absorbency of the pottery's clay body. For highly absorbent clay bodies like Hagi ware, changes appear relatively quickly. On the other hand, for refined clay bodies closer to porcelain, changes are very gradual.

For more about the unique Japanese culture of "growing" pottery, please refer to the article below.

Pottery Grows, Expressions Change: The Japanese Pottery Culture Where Attachment Deepens with Use

What Stains and Colors It

The main substances that color kannyu are as follows:

Tea (green tea, hojicha, black tea): Teas rich in tannins leave brownish-amber stains in the kannyu. When green tea is continuously drunk from a Hagi yunomi, the kannyu takes on a beautiful amber color due to this accumulation of tannins.

Coffee/Oolong Tea: Due to their high polyphenol content, they tend to stain darker than tea. If used continuously as a coffee mug, the kannyu can become a deep reddish-brown to blackish-brown.

Sake/Umeshu (plum wine) and other alcoholic beverages: The amino acids and sugars in sake permeate, resulting in a mellow hue. Sake cups (ochoko) and tokkuri (sake bottles) are prime examples of vessels that "grow" with sake.

Soy Sauce/Dashi/Miso Soup: For bowls used for liquid dishes, soy sauce pigments and dashi components permeate. If this is undesirable, a pretreatment called "medome" (sealing the pores) is effective (explained later).

Discoloration is not necessarily "dirt." The colors accumulated through proper use become part of the pottery's individuality and a testament to the time it has lived. However, if food residues ferment and seep in, it can cause odor and hygiene issues, so it's important to wash the pottery promptly after use.

The Relationship Between Kannyu and Kintsugi

When discussing kannyu, it's impossible not to mention its relationship with kintsugi.

Kintsugi is a Japanese repair technique where broken or chipped pottery is joined with lacquer, and then metal powders like gold, silver, or tin are applied to the seams. Instead of hiding the repair, it is beautifully showcased as golden lines, transforming the damage into a new individuality for the vessel.

Kannyu and kintsugi both stand on the same aesthetic principle of "accepting imperfection as beauty." Both stem from the idea of elevating "defects" into "scenery" and are deeply connected to the spirit of wabi-sabi.

In reality, parts of the kannyu in a piece of pottery that has been used for a long time can sometimes expand and develop into actual cracks. When kannyu that has reached the clay body repeatedly absorbs moisture, goes through cycles of freezing and thawing, or is exposed to sudden temperature changes, the cracks can enlarge. In such cases, repairing it with kintsugi extends the life of the pottery.

When kintsugi is applied to pottery with kannyu, the golden lines sometimes run along the fine grooves of the kannyu, creating a unique "keshiki" (scenery) in itself. Starting with kannyu, then cracking, then kintsugi—the culmination of the time the pottery has accumulated is engraved on a single tea bowl.

In recent years, kintsugi has gained attention not only in Japan but also overseas. The philosophy of "beautifully repairing broken items without hiding them" resonates with the principles of sustainability. Using pottery with kannyu for a long time, and then restoring it with kintsugi if it breaks—this can be said to be one of the richest relationships with pottery.

For more about the culture of kintsugi, please also see below.

Kintsugi: How Japan Repairs Broken Ceramics with Gold

Care Instructions for Pottery with Kannyu

Pre-treatment (Medome) for New Pottery

For pottery with kannyu, especially highly absorbent Hagi ware, Kohiki, and Bizen ware, it is recommended to perform "medome" (sealing the pores) before first use. Medome is a pre-treatment that temporarily seals the pores of the clay body, making it less likely for food colors and odors to seep in.

The most popular method is "boiling in rice water":

Place the pottery in a pot with enough water to submerge it, and add rice water (or a small amount of raw rice).
Put the pottery into the cold water and slowly bring it to a boil (avoid sudden temperature changes).
Simmer on low heat for about 15-20 minutes, then turn off the heat and let it cool naturally.
Once cooled, remove it, wipe off any moisture with a soft cloth, and dry it thoroughly.

The starch from the rice penetrates the pores of the clay body, making it less absorbent. However, this effect is temporary, and the starch will wash out with continued use. The clay body will gradually become seasoned with repeated use.

Even if you don't do medome, there's no problem with using the pottery. However, avoiding intensely colored foods (like curry, soy sauce, or red wine) for the first few uses can make any staining less noticeable.

Please note that whether to perform medome is up to the owner's discretion. If you want to enjoy the "growing" process from the beginning, you might choose to start using it without medome. Consulting with the artist might also be a good idea.

Preventing Stains and Odors

Here are some key points for daily care:

Wash promptly after use: Leaving food residues for a long time makes it easier for pigments and odors to seep in deeply. Wash it promptly after use or soak it in water.

Washing method: Wash gently with a soft sponge and neutral detergent. Avoid abrasive sponges or strong detergents, as they can damage the glaze. Dishwashers are generally not recommended due to sudden temperature changes and strong water jets that can expand kannyu; handwashing is preferred.

Drying: After washing, wipe off moisture and dry thoroughly before storing. Storing while damp can hinder drying and lead to odors or mold.

If stains bother you: Soaking briefly in diluted vinegar water (about 9 parts water to 1 part vinegar) can help alleviate tannins and other pigments. However, avoid prolonged soaking to prevent any adverse effects on the pottery. Also, stains that don't come off even with this method should be accepted as "grown colors"—this is a good way to appreciate pottery with kannyu.

Microwave/Oven: Using highly absorbent pottery in the microwave can cause the absorbed moisture in the clay body to heat up and potentially cause breakage. Especially for pottery with kannyu, check for dryness before use and follow the manufacturer's or artist's recommendations.

Long-term storage: If not used for a long time, wrap it in soft paper (like washi) and store it in a dry place. Storing it in a paulownia wood box is ideal, and this method is common, especially for tea bowls used as tea ceremony utensils.

For more detailed care instructions for Japanese ceramics, please refer to the article below.

Caring for Japanese Pottery: How to Cherish Your Pieces for a Lifetime

Frequently Asked Questions About Kannyu (Crazing)

Q1. Are the fine cracks (kannyu) on the surface of pottery different from defects or breaks?

They are not defects or flaws (cracks). Kannyu refers to the fine lines that run only through the glassy layer (glaze) covering the surface, not the pottery body (clay) itself being broken. In traditional Japanese pottery, it has long been cherished as an "accidental beauty" or individuality that cannot be artificially created.

Q2. Why does "kannyu" appear on the surface of pottery?

Kannyu is caused by the "difference in thermal shrinkage rates" between the pottery body (clay) and the glassy layer (glaze) on its surface. After being fired at high temperatures in a kiln, during the cooling process, the glaze tends to shrink more than the clay body. When the glassy layer can no longer withstand this pulling force, fine lines appear, creating kannyu.

Q3. What does it mean to "grow" pottery with kannyu?

It means that by regularly using the pottery to drink tea, coffee, sake, etc., pigments gradually seep into the tiny gaps of the kannyu, and the texture of the pottery changes to a richer, more profound state. Hagi-ware's "seven transformations" and Kohiki are particularly famous for this. It's a unique and beautiful Japanese culture where the pottery accumulates depth and history over years and decades that it didn't possess when new.

Q4. What are the care instructions for first-time use and daily precautions for pottery with kannyu?

If you want to prevent food colors and odors from rapidly transferring when you first start using it, it's recommended to perform "medome" (sealing the pores) by boiling it in rice water. For daily care, the most important points to prevent mold and odors are "washing promptly after use" and "drying thoroughly before storing." Since it is sensitive to sudden temperature changes, it is generally recommended to avoid dishwashers and microwaves, and to handwash instead.

Q5. What should I do if kannyu develops into a real crack after many years of use?

If, by any chance, kannyu repeatedly absorbs moisture or is subjected to temperature stress and develops into an actual crack or break, it can be revived using the traditional Japanese repair technique called "kintsugi." By deliberately not hiding the break and beautifully decorating it with lacquer and gold powder, the pottery gains even more unique value as the culmination of its history, further enriching its character.

Cracks are the Pottery's Resume: The Charm of Japanese Ceramics That Enjoy "Growing"

Kannyu is a scientifically inevitable phenomenon arising from the difference in shrinkage rates between glaze and clay body. However, Japanese pottery culture has cherished this accidental creation as a unique "individuality."

The "seven transformations" of Hagi ware, the wabi aesthetics of Raku ware, the delicate white cracks of Kohiki, the historical beauty of Goryeo tea bowls—each piece of pottery expresses its own unique character through kannyu.

And holding that pottery, pouring tea into it daily, allowing colors to seep in little by little—that's not just using the pottery; it's "growing" it. In 10 or 20 years, that piece of pottery will have a unique landscape engraved with your shared time.

Kannyu is truly the pottery's resume. Who used it, what it drank, what it ate, what kind of time it spent—the record of all these experiences accumulates in every single fine groove.

The greatest charm of pottery with kannyu lies in its paradoxical beauty: the more you use it, the more it approaches completion—a process never found in industrial products.

The next time you visit a pottery market or a specialty store, if you find a piece of pottery with fine lines on its surface, be sure to pick it up. It is not a flaw or a defect; it is a blank page for the story that is about to begin with you.

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