【Japanese Pottery】What is Glaze (釉薬, yuyaku)? Explanation with diagrams.

Table of Contents

• 1. What is Glaze (釉薬, yuyaku)?
• 2. Functions of Glaze
• 3. Glaze Composition and Function
• 4. The Difference Between Clay and Glaze (釉薬, yuyaku)
• 5. What Is Natural Ash Glaze (自然釉, shizen-yu)?
• 6. Types of Glazes

1. What is Glaze (釉薬, yuyaku)?

Glaze refers to the glass-like layer that forms on the surface of ceramic ware. It is also colloquially known as “uwagusuri” in Japanese, meaning a coating or surface application.

The materials used in glazes vary depending on the type, but one of the most common is ash glaze (灰釉, haiyū or kaiyū). This glaze is made by dissolving a mixture of plant ash and crushed mineral rocks such as feldspar in water. If the glaze is in liquid form, it is called a wet glaze (湿式); if in powder form, it is called a dry glaze (乾式).

The glaze is applied to the ceramic body after it has undergone bisque firing (素焼き)—a process known as glazing (施釉, seiyū). During the final firing (本焼き), the glaze melts under high heat and turns into a glossy, glassy coating on the ceramic surface.

If you compare an unglazed surface with one that is glazed, the difference in texture becomes immediately apparent. Ceramics that have been glazed are known as glazed ceramics (施釉陶器) and are now commonly seen everywhere.

2. Functions of Glaze

・Decoration – Allows expression of a wide range of colors and textures
・Durability – The glass coating increases the structural strength
・Resistance to stains – Makes the surface less absorbent to water and dirt

For example, the wide variety of tableware found in white, black, brown, and other rich colors is due to the diversity of glaze materials. Glazes not only enhance visual appeal but also serve to make the vessel more durable and resistant to contamination.

3. Glaze Composition and Function

Glazes are generally composed of three types of substances: basic (alkaline), neutral, and acidic materials.

・Basic (alkaline): These act as fluxes, helping the glaze to melt. Examples include plant ash and lime (calcium oxide).
・Neutral: These act as binders, helping the glaze adhere to the ceramic body. This role is fulfilled by alumina.
・Acidic: These are the glass-forming materials, with silica (silicon dioxide) playing the central role.

In summary, glaze is made of alkaline fluxing materials (such as ash), alumina, and silica. For those unfamiliar with alumina or silica, here is a simple breakdown of where they come from:

・Fluxing material: Lime-based compounds such as calcium oxide. These are found in all kinds of ash and in small quantities in feldspar. Ash derived from plants is called plant-based glaze (灰立て), while that made from crushed limestone is stone-based glaze (石立て).
・Alumina: A general term for aluminum oxide, mainly found in clay and feldspar.
・Silica: Also known as silicon dioxide, it is found in quartz (石英), feldspar, and ash.

To make glaze, you need a combination of ash (for melting), clay (for binding), feldspar (for melting, binding, and forming glass), and quartz (for glass).

Incidentally, the base clay used in ceramics is also typically made of clay, feldspar, and quartz, just like glaze. (Reference: The Difference Between Pottery and Porcelain – Clay Composition)

Therefore, to put it very simply: mixing clay and ash results in a basic glaze. Their functions are complementary. The chart below (omitted here) shows that feldspar, in particular, performs all three essential functions—binding, melting, and glass formation.

This diagram shows that glaze (釉薬, yuyaku) has a composition similar to that of the clay body (素地粘土, sojinen-do). This is because if their compositions are too different, the glaze and clay body cannot bond well together. This difference in composition affects whether the glaze is compatible or incompatible with a given type of clay.

What happens if we use ash, which melts into glass by itself, as a glaze on its own? It may seem like it would work well since it can form glass on its own.

However, in reality, glaze made solely from ash tends to peel off, burn, or shrink. In short, ash alone cannot function as a proper glaze.

This is because ash lacks alumina (アルミナ, arumina), the stabilizing and adhesive component found in clay and feldspar (長石, chosheki).

4. The Difference Between Clay and Glaze (釉薬, yuyaku)

The main difference between clay and glaze is whether or not they contain lime (石灰質, sekkai-shitsu). Lime plays two key roles in glaze: it helps melt the glaze and contributes to glass formation. If we remove ash (which contains lime) from glaze, what's left is basically just clay.

Although feldspar also has melting properties, lime has a much stronger melting effect.

For example, feldspar in clay helps melt and solidify the clay body. In contrast, lime melts so strongly that it causes the material to flow. Therefore, when clay is fired at high temperatures and becomes firm (melts and hardens), it’s thanks to feldspar.

If we were to replace feldspar with lime in clay, the clay would likely collapse during firing because it would melt and flow. This strong melting ability (of lime) is not suitable for clay bodies.

On the other hand, this strong melting ability is exactly what makes lime essential in glaze. Also, various coloring elements found in ash—such as copper (銅, do) or iron (鉄, tetsu)—produce different glaze colors.

5. What Is Natural Ash Glaze (自然釉, shizen-yu)?

Even without applying glaze, a glazed surface can form naturally inside the kiln when ash settles on the pottery surface. This is known as natural ash glaze (自然釉, shizen-yu).

For instance, Bizen ware (備前焼, Bizen-yaki) does not use applied glaze, yet natural ash glaze forms on the surface. A specific example is "goma" (胡麻), a natural glaze effect. Similarly, Iga ware (伊賀焼, Iga-yaki) often shows a glassy natural glaze called "bidoro" (ビードロ).

These effects occur when ash from pine firewood, for example, lands on the pottery during firing. The ash contains lime and silica (シリカ, shirika), which combine with the alumina and silica in the clay body to form a natural glaze.

This process can also be artificially replicated. By applying a glue-based adhesive to the surface and spraying ash onto it with a compressor, one can recreate the conditions needed for a natural ash glaze. After firing, a glassy glaze layer forms. This technique is known as "yobigusuri" (呼び釉), which mimics the look of natural glaze.

Another example is "tobigusuri" (飛び釉), where glaze is applied to resemble ash drips from the kiln ceiling. A well-known historical example is the Song dynasty’s flying celadon (飛青磁, tobi-seiji), where iron is applied underneath the glaze for decorative effect.

6. Types of Glazes

While ash glaze (灰釉, haiyu or kaiyu) was the main example used here, there are many types of glazes. These can be classified by material or by characteristics and type of pottery.

Material-based classifications include:
・Copper glazes (銅釉, doyu), using copper compounds
・Iron glazes (鉄釉, tetsuyu), using iron compounds
・Straw ash glazes (藁灰釉, warabaiyu), using straw ash

Classifications by characteristics or pottery type include:
・Clear glaze (透明釉, tomeiyu), which shows the base color underneath
・Shino glaze (志野釉, shinoyu), primarily made from feldspar
・Celadon glaze (青磁釉, seijiyu), made from plant ash and crushed stone materials

Glazes can also be categorized by melting temperature: ・High-fire glaze (高火度釉, kokadoyu): over 1250°C
・Mid-fire glaze (中火度釉, chukadoyu): 1100°C–1250°C
・Low-fire glaze (低火度釉, teikadoyu): under 1100°C

At first, the wide variety of glaze types may seem overwhelming. However, by observing different ceramic wares over time, you’ll naturally begin to develop a preference for certain glazes or textures. Hopefully, this overview of glaze components and structure will serve as a helpful introduction.