What is Pottery Glaze? The Ultimate Guide to Types, Ingredients, and Selection | Fukaumi Shouten

What is Pottery Glaze? The Ultimate Guide to Types, Ingredients, and Selection

Can you explain exactly what "Glaze" is made of and what role it plays?

In pottery, glaze is the most critical element that determines the "expression" of a piece.
However, the variety is vast, and the raw materials and chemical reactions are complex, making it a deep world that even professionals study for a lifetime.

In this article, Fukaumi Shouten, a pottery material manufacturer with over 100 years of history in Arita, thoroughly explains the basics of glaze, types, ingredients, and how to choose without failure, in a way that is easy to understand even for beginners.

1. What is Glaze? Meaning and 3 Roles

It is called "Yuyaku" or "Uwagusuri" in Japanese

Technically it is read as "Yuyaku," but among craftsmen, it is sometimes called "Uwagusuri" or simply "Kusuri (Medicine)" or "Yu."
In English, it is called Glaze.

Its true identity is "Glass Coating"

Simply put, glaze is a "glassy coating material applied to the surface of ceramics."
Powdered raw materials are dissolved in water, applied to the vessel, and fired at high temperatures of over 1200°C to melt into a glassy film.

3 Roles of Glaze

Why do we bother applying glaze? There are three main reasons.

1. Decoration (Beauty): Adds color and gloss, increasing artistic value.
2. Strength (Durability): Covering with a glass film makes the vessel less likely to break.
3. Waterproofing (Hygiene): Seals the pores of the clay body, preventing water leakage and stains.

2. Types of Glaze (4 Representative Types)

There are countless types of glazes, but they can be broadly classified into the following four categories.

① Transparent Glaze

As the name suggests, it is a clear, glass-like glaze.
It is used when you want to show the color of the clay underneath or the painted picture (such as Sometsuke/Blue and White). It is essential for Arita ware and porcelain.

▶ View Fukaumi Shouten's "Transparent Glazes"

② Matte Glaze

A glaze with suppressed gloss and a moist texture.
By creating fine crystals on the surface, it diffuses light, creating a modern and calm atmosphere.

▶ View Fukaumi Shouten's "Matte Glazes"

③ Colored/Milky Glaze

Glazes colored by mixing metals such as iron, copper, and cobalt.
Oribe (green), Ruri (blue), and Ame (amber/brown) are typical examples. Milky white glazes (Nyudaku) are also popular.

▶ View Fukaumi Shouten's "Milky Glazes"

④ Crystalline Glaze

A special glaze where crystals grow inside the glaze during firing, creating patterns like snowflakes or flowers.
Temperature control is difficult, but when successful, it emits a jewel-like beauty.

▶ View Fukaumi Shouten's "Crystalline Glazes"

3. What is Glaze made of? (Ingredients and Components)

Glaze is mainly made by mixing "3 elements".
It is easy to understand if you compare it to cooking.

• Feldspar / Silica (Glass base) = "Flour"
  Creates the body (skeleton) of the glaze. It is stone powder that is hard to melt on its own.
• Ash / Lime (Melting agent) = "Butter / Oil"
  Role of melting feldspar at high temperatures and turning it into glass (Flux). Wood ash etc. are used.
• Clay / Alumina (Viscosity) = "Water / Egg"
  Gives viscosity to stop the glaze from flowing down while melting during firing.

By adding "Metals (Iron, Copper, etc.)" as seasoning (Colorant) to this, glazes of various colors are created.

4. Liquid vs. Dry Powder: Which is better?

There are "Dry (Powder)" glazes that you mix with water yourself, and "Liquid" glazes that are ready to use.
Conclusion: If you don't want to fail, "Liquid" is recommended.

• Dry: Cheap, but hard to mix. Clumps tend to remain, causing pinholes.
• Liquid: Perfectly crushed and dispersed at the factory, so just shaking gives a professional finish.

5. Common Troubles and Solutions

"The color is not as per the sample." "It cracked."
In such cases, please suspect the following causes.

• Crazing (Kannyu): Cracks caused by the difference in shrinkage rate between glaze and clay.
• Pinholes: Gas holes caused by insufficient bisque firing or dirt.
• Color Difference: Differences in firing temperature or cooling time (slow cooling vs. rapid cooling).

Knowing that "color changes with firing conditions" helps you see the countermeasures.

Summary: Knowing Glaze makes Pottery Freer

Glaze is not just a material, but a crystal of chemistry and history.
If you know its properties, choose correctly, and master it, your pottery work will level up dramatically.

At Fukaumi Shouten, we deliver professional-quality glazes to everyone with knowledge and technology inherited from the Meiji era.
Please find your own "Color".