【Japanese Pottery】Crackle (Kannyū) in Ceramics: Appreciating the Beauty of Glaze Cracks

Table of Contents

• 1. The Beauty of "Kannyū" (貫入, glaze crazing)
• 2. How Kannyū (貫入, glaze crazing) Forms
• 3. How to Prevent Kannyū (貫入) in Ceramics

1. The Beauty of "Kannyū" (貫入, glaze crazing)

In ceramics, kannyū (貫入) refers to the fine cracks that appear on the surface of a glazed piece. These fissures in the glassy glaze layer reflect light in a variety of ways, creating subtle and diverse visual effects. In both Japan and China, these crazing patterns have long been appreciated as part of the aesthetic experience.

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The cracks are given various poetic names depending on their pattern, such as Hyōretsu-mon (氷裂文, ice crack pattern), Ryūyō-mon (柳葉文, willow leaf pattern), and Gyūmō-mon (牛毛文, cow hair pattern). Some potters even enhance the visibility of the crazing by rubbing in red iron oxide (bengara) or ink immediately after firing.

A glaze that is specifically designed to produce kannyū is known as a kannyū-yū (貫入釉) or kiretsu-yū (亀裂釉, crackle glaze).

Examples of crackle glazes include seiji-yū (青磁釉, celadon glaze) and jakatsu-yū (蛇蝎釉, jakatsu glaze).

While some celadon pieces may have little to no visible kannyū, many do. One particularly admired example is the double-crazing found in the Southern Song Dynasty’s official celadon ware (Nansō kanyō 南宋官窯) in China. Modern ceramic artists such as Uichi Shimizu (清水卯一) and Mineo Okabe (岡部嶺男) have also explored layered kannyū in their celadon works. When light hits the multiple layers of crazing, it can produce a mysterious and radiant effect.

Jakatsu-yū (蛇蝎釉) is created by applying a feldspar glaze (長石釉) over a black glaze (黒釉) and firing it. Since the feldspar glaze contracts more than the black glaze during firing, it creates visible kannyū. These cracks allow glimpses of the black base layer beneath, producing a visually complex and distinctive keshiki (景色, landscape). Artists often experiment with glaze formulas to achieve this effect, which is especially seen in Satsuma-yaki (薩摩焼) and Karatsu-yaki (唐津焼).

Not only crackle-specific glazes, but many ordinary ceramic pieces also feature kannyū to some extent. Even if the crazing is initially invisible, it may gradually emerge with use. Although glaze itself is impervious to water, kannyū can allow moisture to reach the clay body (taido 胎土), causing the cracks to darken over time and become visible.

2. How Kannyū (貫入, glaze crazing) Forms

Kannyū (貫入) occurs because the clay body (sōji, 素地) and the glaze (yuyaku, 釉薬) expand and contract at different rates. All materials expand when heated and contract when cooled—this property is called thermal expansion (netsu-bōchō, 熱膨張), and the rate differs between the body and glaze.

Kannyū usually develops after firing, as the pottery cools down. When a finished piece is taken out of the kiln, it rapidly cools in contrast to the external air temperature. As both the body and glaze contract from thermal expansion, the glaze usually contracts more than the clay body. This difference results in fine cracks on the glaze surface—kannyū.

For example, if the clay body contracts by 10% while the glaze contracts by 15%, the glaze will crack to accommodate the tension. These are the visible crazing lines known as kannyū. In extreme cases, excessive glaze contraction may lead to wider cracks or even cause the glaze to flake off entirely.

On the other hand, if the clay body contracts more than the glaze, the reverse stress may occur. In this case, kannyū is less likely to form, but excessive stress on the body may result in the entire vessel cracking. The ceramic pieces we receive are those that have successfully overcome these challenges.

3. How to Prevent Kannyū (貫入) in Ceramics

While kannyū is considered an aesthetic highlight in many ceramic traditions, in certain contexts—such as Western tableware or industrial ceramics—it is seen as a defect. In such cases, manufacturers implement measures to prevent crazing.

Theoretically, the most effective way to prevent kannyū is to match the shrinkage rate of the glaze and clay body as closely as possible. Thanks to modern advancements, materials with almost identical shrinkage rates are now commonly used. Manufacturers also conduct long-term durability tests to ensure resistance to crazing over time.

This is where the “Kannyū Resistance Test” (taikannyū-sei shiken, 耐貫入性試験) comes in. By using a device called an autoclave, ceramics are subjected to high heat and pressure to simulate extended use. This allows quick assessment of whether crazing will occur over time.

In the end, the perception of kannyū varies widely between makers and users. Whether it is seen as a unique feature or as a flaw depends entirely on individual values.