Seto City in Aichi Prefecture is a pottery production center with a history of over 1,000 years.
The reason for this is that the best clay for pottery, called "Kaerume," can be extracted here.

Why is frog eye suitable for pottery?
Its main feature is its plasticity, which allows it to retain its shape like a shape-memory alloy, making it very easy to throw on a potter's wheel. It turns pure white when fired and is highly fire-resistant. If it is refined and the clay content is mixed with feldspar and silica, it can even be used to make porcelain clay.

It is truly a clay that was born for pottery. Thanks to this clay, pottery has been produced in Seto for over a thousand years. However, frogeye is a natural resource that is dug from the mountains, and as it is dug up, it disappears.

I would like more people to know about this.
We spoke to people involved in the ceramic raw materials industry and potteries who work with clay in Seto.

This time we will introduce Kasen Mine, a privately owned mine. It is a factory that elutriates the raw clay used for making Gairome (frog's eye) and turns it into clay.

The raw clay collected from the mountains cannot be used as is.
How does it become clay?
Behind the scenes at the factory where the product "clay" is made, where does the clay go ? We spoke to CEO Akihiro Kato about this.

Kasen Mine: A private mine in a residential area

Kasen Mine is located about a 15-minute walk from Owari Seto Station in a residential area.

The Kasen Mine began in the early Taisho period when potter Kato Kasen purchased a mountain to dig up soil for his pottery. At the time, the soil was of extremely high quality and was used as is without any processing.

This mine is located in the Honyama Formation, which is said to be the best quality clay layer in Seto City, as it has high plasticity that allows it to maintain its shape freely and has little impurities such as iron.

It is believed that the granite that makes up Mount Sanage in eastern Aichi Prefecture was weathered and carried by rainwater to the Seto region 6 to 10 million years ago, where it was deposited and further weathered to become the original soil for Gyome.

It was in 1956 that the company was transformed into the clay factory it is today. Until then, they had only been selling clay on a small scale, but in response to the demand for mass production after the war, they built a factory. From there, they began to manufacture clay in earnest, or more accurately, they began to work as an "elutriation" factory.

This is the product "Honzan Elutriated Frog Eyes".

"Elutriation" is the process of removing unnecessary materials such as minerals, sand, stones, and wood chips from the raw clay of "Gaerume" mined from mines, extracting only the necessary clay components, and arranging and refining the particles. Here, we manufacture this elutriated clay, or Motoyama Elutriated Gaerume Clay, as our main product.

It is used not only as a raw material for clay used to make tableware, but also in a wide range of applications including insulators, glazes, and cosmetics, and when made into a powder it is used in the electronic components industry.

From raw soil to clay

So how is this elutriated clay made?
Before touring the factory, Representative Akihiro Kato first used a small bucket to explain the process so that it would be easier to understand.

This is crushed raw clay for frog eyes. Frog eyes are a mixture of clay, kira (fine sand), and silica sand. The particles are separated in order of size, from smallest to largest, from clay to kira and then silica sand, and these particle sizes are used to separate them.

The most sought-after item at the Kasen Mine is clay. The main goal is to turn it into clay. To do this, the first step is to dissolve the raw clay and separate the contents.

Add water to the clay and swirl it around to dissolve it.

Stop turning and pour the water into another bucket, and you'll find that the largest particles of silica sand have sunk to the bottom.

If you mix the supernatant liquid further and let it sit for a while before draining the water, a fine sand called kira, which is neither clay nor silica sand, will accumulate at the bottom.

Clockwise from the top: clay, glitter, and silica sand. Of these, the one I want most is clay.

During the Showa era, Kira had no use and was thrown away, but now the high-quality parts are turned into clay to be used as a tile material. Silica sand is also shipped as a product.

The process of making clay in the factory

After getting a rough overview, we were shown around the factory.

The raw soil for frog eyes collected from the mountains is poured into the line.

The raw clay is dissolved with water and water glass to separate it into clay, glaze, and silica sand .

Once the raw soil has dissolved and turned into a liquid slurry, large pieces of wood and grass are removed using a rotary sieve.

It was once used as wood chips and fertilizer.

The bucket conveyor removes the heavy silica sand from the mud. The clay comes out through holes in the bucket conveyor, leaving the silica sand and coarse soil behind.

This is a place outside called the "Kira Groove," where the kira is allowed to settle and removed by natural sedimentation. The supernatant liquid is clay.

First the silica sand is removed, then the glitter, and what's left is the frog-eye clay.

This is the clay from the frog eyes that we finally wanted to extract. It's a pool of mud . 70 to 80% of this slurry is water.

There is too much water in this state, so we add grit to thicken the clay, making it a bit more viscous and closer to solid clay.

Excess water is removed, and finally, pressure is applied using a machine called a filter press, which squeezes it out using a filter cloth to make it solid.

Your freshly made elutriated frog eye clay is now complete.

They are sun-dried to reduce the moisture content to 7% before shipping. When freshly made, they are dark gray, but once dried, they turn pure white.

I want to convey the true value of frog eyes

From here on, I would like to share Mr. Kato's thoughts.

"Frogeye clay was very sticky and white, making it easy to make pottery cheaply. But the name of the clay is unknown. In other producing areas, the clay is given the name of the place, like Amakusa pottery stone. I think the people of Seto wanted to keep the name secret because it was so good."

In recent years, he has been trying to make more people aware of the value and preciousness of frog eyes by explaining the material in an easy-to-understand manner, and taking advantage of the fact that it is a privately owned mine, he has been giving tours of the factory together with local people.

Kira storage area. It used to be thrown away, but now it's used to make tiles.

A major motivation for wanting to convey this message was the worry that the frog-eye soil, which used to be plentiful, might run out if things continued as they were.

This sense of crisis has been discussed within the pottery industry for about 40 years. However, despite fears that pottery will disappear, it is still possible to dig it in Seto.

"The engineers at the clay manufacturer researched the necessary amount of elutriate clay and reduced the amount used. I think they used to add about 30% of the clay, but now they keep it to under 10% and reduce the amount shipped, which I think is helping to extend the life of the mine."

In addition to the depletion of resources, the number of elutriation factories is also rapidly decreasing.

"There are currently four elutriation factories in Seto, so if even one of them were to stop production, we would suddenly be in a tight spot. In the past, there were about 40 factories in the city. There were also several dozen in the neighboring Toki and Ena areas, but now the number has dwindled to just a handful."

Even if a new mine were discovered and development progressed, if the elutriation factory had gone bankrupt, the technology would be lost and elutriated clay would no longer be able to be produced.

Thoughts on frog-eye clay

The elutriated frog-eye clay produced by Kasen Mine is ordered from pottery production areas all over the country. Currently, with high-quality soil becoming scarce throughout Japan, even mixing in a small amount of frog-eye clay makes it easier to mold, so it is highly valued. It is particularly indispensable for mass-produced industrial products.

However, in the world of pottery making, the cost of the raw material clay is said to be low, and the price of frog-eye clay has remained low even as the amount harvested has decreased. Although it has been gradually increasing in recent years .

"We decided to drastically revise the prices and reduce the amount we sell. However , we received so much anger from all sides that we even considered selling the land. But I still can't let go."

He explains the reason for this:

" It's sad that frog-eye clay has a low status. Above all, if we want to preserve Seto ware, we have to develop Seto's mines. The soil is so good that anything can be done in Seto."