1. How Glaze Makes Plates and Tea Bowls Smooth Compared with Bare Clay
The surface of plates and tea bowls is covered by a glass-like layer called glaze [6]. This layer works to make the vessel’s surface smooth. It also helps prevent dirt from sticking, making it easier to wash [1]. What about unglazed pottery? Inside the clay, there are tiny, invisible holes scattered throughout. Before firing, glaze is a muddy state dissolved in water, so it soaks into these holes [2]. When fired, it becomes a glass layer that covers or closes the holes.
2. The Three Kinds of Ingredients That Make Up Glaze
Glaze ingredients are often explained by dividing them into three roles based on their function: things that "melt" (like lime or ash), things that "become glass" (like silica), and things that "stick to the body" (like alumina or kaolin) [4]. However, some materials like feldspar have multiple roles, so ingredients and roles do not always match one-to-one. Silica, the base of glass, has a very high melting temperature on its own. So, oxides of sodium, potassium, and calcium are added to lower the melting temperature [3]. Many glazes for pottery and porcelain are said to be "high-fire glazes" that melt at over 1200 degrees [1].
3. How Glaze Melts and Cools in the Kiln to Smooth Out Bumps
The answer to smoothness lies in the glaze becoming liquid and then cooling into a glass layer. While firing, the glaze components melt into a liquid like syrup. At this time, it smooths out fine bumps and small holes on the surface, as if healing a wound. When it cools, it hardens in that shape [3]. The English Wikipedia describes this as the glaze "healing" the surface. Glaze flows more easily than the clay body. With a glaze made only of feldspar and silica, it can flow down thickly, like on a Shino tea bowl [7]. Why does the flowing liquid become a nearly horizontal surface? The sources read for this article did not explain that mechanism.
4. Why Pinholes Appear When Bubbles in the Glaze Cannot Escape
Sometimes, small holes like those made by a needle point appear on the glaze surface. These are defects called "pinholes." If bubbles inside the glaze do not escape completely while it is melting, they remain as holes after it cools and hardens. Whether they escape depends on the viscosity (how runny it is) of the glaze while it is melting [5]. The causes of bubbles vary. They can be bubbles mixed in during preparation, organic matter in the clay or glaze, or gas released from decomposing gypsum. A thick glaze can produce more gas as organic matter burns, making pinholes more likely [5].
5. Why Glaze Can Look Rough or Dull When Applied Too Thinly
According to documents from the Toki City Ceramics Testing Center, roughness or lack of gloss often happens when the glaze is applied too thinly [5]. Lack of gloss can also be caused when the grinding of glaze ingredients is insufficient. Conversely, if the ingredients are ground too finely, they begin to melt at lower temperatures, melt too much, and cause pinholes [5]. There is a "just right" range for particle fineness. If the unglazed clay body has fine bumps, areas where glaze does not cover well are created, which also makes pinholes more likely. One suggested measure is to smooth the body with fine sandpaper [5]. Note: If glaze is too thin, the surface becomes rough and lacks shine. Insufficient grinding of glaze materials also causes a lack of shine.
6. How Ingredients and Firing Cause Different Glosses and Fine Cracks
Glaze is not just simple glass. The balance of ingredients and the firing method determine whether it melts completely, leaves unmelted components or bubbles, or forms crystals. This greatly changes the color and texture [1]. Transparent glaze forms when the ingredient balance is good. If the balance is off, it becomes rough or cloudy and opaque. Increasing kaolin results in a matte (non-glossy) glaze [4]. The clay body and glaze expand and shrink differently with heat. The difference when cooling can cause fine cracks in the glaze, called crazing (fine cracks in the glaze) [5].
7. How to Look for Glaze Flaws on a Plate at Home
What you can do today: Pick one plate from home. In a bright place, shine light on it at an angle and look at the surface. You might find small holes like pinpricks (pinholes) or thin lines like cracks (crazing). Some plates have none at all. Be careful with chipped plates or those with large cracks, as they can cause injuries. Do not touch them; ask an adult for help. Official information: Prefectural or municipal ceramics testing centers in Japan sometimes publish explanations of defects and workshop notices in Japanese.