1. What are the thin lines you can see inside a tea cup?
If you look closely at the inside of a tea cup used for black or green tea, you may see thin lines. Tilt the cup slightly near a bright window, and you might see a mesh-like pattern appearing in a light brown color. These lines are called *kan-nyu* (crackles). *Yūyaku* (glaze) is a thin glass-like membrane covering the surface of the vessel, and the fine cracks that occur in it are the *kan-nyu* [2]. In Hagi ware, it is said that tea stains (*cha-shibu*) soak into these lines, changing the expression of the vessel as it is used. Among tea practitioners, this change is called "Hagi no Nanabake" (Hagi's Seven Transformations) [4]. The phrase "the vessel grows" often refers to these changes. But when and how do these cracks form in the first place?
2. Do cracks in pottery form during firing or during cooling?
Surprisingly, cracks are not born during the hottest part of the firing process, but during the cooling after firing is complete. At the maximum temperature of the kiln, the glaze is melted like a liquid. As it solidifies and cools to near room temperature, if the glaze shrinks more than the *kiji* (body), it is pulled apart and cracks form [1]. This is different from "firing shrinkage," where the body shrinks while being fired [1]. You might imagine paper glued to a board shrinking as it dries and pulling against the board. However, the cause of *kan-nyu* is not the loss of moisture, but the difference in shrinkage as the temperature drops. This difference in shrinkage is explained by the difference in thermal expansion (the rate at which materials expand and contract with heat). It is said that *kan-nyu* occurs if there is a large difference in thermal expansion between the body and the glaze [3]. *Kan-nyu* is likely to occur in combinations where the glaze's contraction is greater than the body's [1]. Cracks are less of an accident and more a result of the combination of body and glaze. Hearing this, one might think all cracks are failures. How are they actually viewed in the manufacturing of tableware?
3. Why are cracks that appear later in porcelain treated as defects?
Porcelain tableware that originally had no cracks may develop them over time. This is called *keinen kan-nyu* (aging crackles) and is treated as a product defect in industrial settings [1]. This is because it not only spoils the appearance but can also cause moisture from food to seep through the cracks, leading to leaks, dirt, and mold [1]. Cracks that appear late are explained as occurring due to "hydration expansion," where the particles of the vessel's body absorb water and swell, and they are more likely to appear in glazes with high thermal expansion [1]. Both the difference in shrinkage during cooling and the body swelling after absorbing water share the same background: the difference in expansion and contraction between the body and the glaze. It is important to distinguish the types of cracks. Cracks that form during cooling and *keinen kan-nyu* that appear over time differ in when they occur and their direct causes. Viewed this way, cracks are a nuisance. However, looking back at history, there were people who intentionally created cracks.
4. How did Song Dynasty kilns in China make cracks on purpose?
Roughly a thousand years ago, during the Song Dynasty in China, celadon wares with intentionally created *kan-nyu* were produced at the imperial kilns, and the artistry of the cracks was valued [2]. Examples of using cracks for decoration also exist in Japanese pottery. These include *kikko-kan-nyu* (tortoiseshell crackles), which look like ice cracking in multiple layers, and *sumi-kan-nyu* (ink crackles), where ink is soaked into the cracks after firing for decoration [1]. The dividing line between defect and decoration seems to lie not in the presence of cracks, but in whether they were desired. So, what do cracks bring to vessels used for drinking tea?
5. How do tea stains in the cracks change the look of Hagi ware over time?
Hagi ware is a type of pottery that highlights the texture of the clay [4]. The fired surface has *kan-nyu*, and as it is used, tea stains soak into them, changing the vessel's charm. This change is called "Nanabake" (Seven Transformations) among tea practitioners [4]. This change is also called *cha-nare*, a term for how the vessel’s appearance changes as it is used for tea. The JAANUS dictionary, which explains Japanese art terms in English, states that the mellow texture of Hagi ware changes with use because the clay absorbs a small amount of liquid [5]. Thus, the change involves not just cracks, but also the absorbency of the body. The number "seven" is not a promise of exactly seven changes, but a way of saying the expression changes many times as it is used. So far, we have seen three similar stories about "cracks." Which of these is meant when people say "the vessel grows"?
6. What does it mean when people say a pottery vessel "grows"?
Let us separate and list the three phenomena: 1) Cracks formed during cooling after firing. 2) *Keinen kan-nyu* in tableware caused by the body absorbing water. 3) The "Seven Transformations" of Hagi ware, where tea stains soak in and change the color. When people say "the vessel grows," the center is likely the change in ③. The increase in the number of cracks over time and the change in appearance due to tea stains are separate phenomena. Mixing them up makes both the explanation of Hagi ware's aging and the explanation of tableware defects inaccurate. A common point is that liquid can seep in. In tableware, moisture from food can seep through cracks and cause dirt and mold [1]. In Hagi ware, tea stains seep into the cracks, while the body’s absorbency also helps change the vessel’s appearance [4]. These related processes can be a problem in one case and part of the vessel’s charm in the other. By overlaying the materials, "growing" can be read not as the vessel itself growing, but as the result of the user's time overlapping with the cracks and the absorbency of the body.
7. How can you look for cracks in the tea cups and bowls at home?
Let us try it. Look at the ceramic tea cups, bowls, and small plates in your home at an angle near a bright window. Search for thin lines on the glaze surface. If you find lines, sketch them in a notebook, noting if the lines have color or if there are areas with more or fewer lines on the same vessel. Do not touch chipped or cracked vessels, as they may cut your hands. Check with the store where you bought the vessel or with a family member before washing or caring for it. To expand your research, you can read "About Crackles in Ceramics" from the Mie Prefectural Industrial Research Institute in Japanese [1]. It lists types such as *kikko-kan-nyu* and *sumi-kan-nyu*. Celadon from the Song Dynasty imperial kilns can be explored through museum exhibition guides [2].