【Japanese Pottery】Nendo: The Essential Foundation of Japanese Ceramics

Table of Contents

• 1. The Origins of Clay in Ceramics
• 2. Composition of Granite
• 3.How Granite Turns into Clay
• 4. Is Clay an Unlimited Resource?

1. The Origins of Clay in Ceramics

The clay used for pottery and porcelain primarily originates from granite (花崗岩, kakōgan). However, the process of turning granite into clay takes an immense amount of time.

Granite gradually weathers and erodes over centuries, breaking down into fine particles. These tiny fragments accumulate on the earth’s surface, eventually mixing with soil, decomposed leaves, and other organic matter, slowly transforming into clay.

What Is Granite?
Granite is a type of igneous rock, formed by volcanic activity. Igneous rocks are categorized into two types:

1.Volcanic rocks – Solidified rapidly above ground due to air and water exposure.
2.Plutonic rocks (深成岩, shinseigan) – Cooled and solidified slowly underground over a long period.

Granite belongs to the second category (plutonic rocks) and is characterized by its light-colored appearance. It is commonly used in tombstones and building materials.

2. Composition of Granite

If you observe granite closely, you will notice three distinct mineral grains:

・White particles → Feldspar (長石, chōseki)
・Gray particles → Quartz (珪石, keiseki, also known as silica or 石英, seki-ei)
・Black particles → Biotite mica (黒雲母, kuro-unmo)

Over time, exposure to sunlight, rain, and wind causes these minerals to crystallize and weaken. As each component of granite separates and loses cohesion, the rock gradually crumbles into soil.
In short, weathered granite breaks down easily, and over a vast period, it slowly returns to the earth as clay.

3.How Granite Turns into Clay

Two key components of granite, feldspar and quartz, contain:
・Alumina (アルミナ, arumina) – Provides plasticity, allowing clay to be molded.
・Silica (シリカ, shirika) – A glass-forming mineral, which helps harden ceramics when fired.

These minerals mix with soil and water, gradually forming workable clay.
Once shaped, clay hardens as it dries. When fired at high temperatures, silica vitrifies (turns into glass), transforming the clay into durable ceramics.

Effects of Alumina and Silica on Clay
・High silica content → Lower plasticity, making the clay more brittle and harder to shape.
・　High alumina content → Higher heat resistance, requiring higher firing temperatures for complete vitrification.
Since the ratios of feldspar, quartz, and organic matter differ across regions, each clay deposit has unique characteristics. In essence, the balance of alumina and silica defines the "personality" of a clay body.

4. Is Clay an Unlimited Resource?

Since Japan has many active volcanoes, granite is abundant. With continuous weathering, feldspar and quartz should theoretically be plentiful. However, does this mean that clay can be found indefinitely in the same location?
Unfortunately, the answer is "No".
Simply having feldspar and quartz does not automatically create clay. Clay also requires organic matter, and its formation takes an extremely long time—ranging from tens of thousands to up to 3.5 million years.
Thus, when excavating clay in the same location, it remains a finite resource, much like other natural materials.
Once a clay mining site is depleted, it is closed. While it may be disheartening to see abandoned excavation sites, new clay is continuously forming around the world—just at an incredibly slow pace.