Tamba clay and why it fires the way it does

If you pick up a piece from Fresh Citadel and turn it over, the foot ring will show you something the glaze covers up everywhere else: a warm, slightly speckled grey-brown clay body with small dark flecks of iron. That clay comes from the Tamba region, about 60 kilometres north of Kobe, and it has been used by potters in Hyogo Prefecture for a very long time. Understanding what it does in the kiln helps explain why the finished pieces look the way they do.

Where the clay comes from ¶

Tamba is one of Japan's six ancient kiln sites, a designation that reflects centuries of continuous ceramic production in the area. The clay there is a natural stoneware with a relatively high iron content, which gives it a warm colour when fired and a tendency to develop surface texture in reduction atmospheres. It is not a particularly plastic clay on its own, which means it can be tricky to centre on the wheel if you are used to a more forgiving body.

Sakura sources her clay from a supplier in the Tamba-Sasayama area and blends it with a small percentage of a local red clay to increase plasticity and add warmth to the fired colour. The exact ratio has been the same since 2019, arrived at after about two years of testing different combinations.

What reduction does to iron-bearing clay ¶

In a gas kiln, reduction means restricting the oxygen supply during certain phases of the firing. When iron oxide in the clay is exposed to a low-oxygen atmosphere at high temperature, it changes colour in ways that an electric kiln cannot replicate. The surface can shift from tan to grey to a deep warm brown depending on the timing and intensity of the reduction.

This is why two pieces from the same batch can look slightly different from each other. The kiln is not perfectly uniform, and the clay responds to small variations in atmosphere across the chamber. Sakura considers this a feature rather than a problem. A piece from the left side of the kiln and a piece from the right side of the same firing are related but not identical.

Practical notes for people using Tamba-blend stoneware ¶

Tamba-blend stoneware is durable. The firing temperature at Fresh Citadel is around 1,250 degrees Celsius, which produces a dense, low-absorption body. The pieces are safe for daily use, dishwasher-safe after the first month, and oven-safe up to 250 degrees Celsius.

The one thing to avoid is sudden thermal shock: do not take a cold piece directly from the refrigerator and put it in a hot oven. Let it come to room temperature first. This is true of most stoneware, not just Tamba clay, but it is worth mentioning because the dense body that makes the pieces durable also means they hold temperature changes less forgivingly than thinner porcelain.

If you want to see the clay body in person, the foot ring of any piece in the gallery is unglazed and shows the fired clay clearly. The gallery is open every Saturday from 11:00 to 17:00.