The Yohen Tenmoku (曜変天目) tea bowl, produced in the Chinese Southern Song dynasty (12-13th Century), is a bowl with iridescent spots scattered over its black surface. Due to its mysterious atmosphere, Yohen Tenmoku is known in the pottery world as the most extraordinary masterpiece. Just three perfectly-intact bowls remain in Japan, and all are designated as national treasures.
Tenmoku indicates the name of a glaze which develops a black colour during the firing process. This glaze, containing iron, mostly develops a glossy jet-black colour. The reason for the emergence of such an unusual colour in Yohen (曜変) Tenmoku is Yohen (窯変), a chemical change which happens in the kiln during firing*. Yet, it is still a mystery how this aesthetic feature was achieved – whether planned or coincidental. It is safe to say that such mystique has kept increasing the value of the Yohen Tenmoku tea bowls.

Thus, the only bowls which achieved a chemical change to Tenmoku glaze and its resulting spacial brilliance are given the title of “Yohen Tenmoku (曜変天目)”. However, if we look at pottery from the aspect of “Yohen Tenmoku (窯変天目)” – works that have archived a distinctive colour development through a chemical change during the firing process – there is another variety of features of pottery that we can think of.
Those bowls are often treasured in the world of tea ceremony. For example, there are several categories such as Yuki Tenmoku with small patches all over, and Nogime Tenmoku with many stripes running from the edge.
The bowls of Ginka Tenmoku by Hideki Yamashita based in Kanagawa Prefecture, on the other hand, are works not only made for tea ceremony, but also for using daily on our dining tables. They are fired in a kiln with a reduced oxygen content and a highly reducing flame with increased carbon monoxide concentration. Ginka Tenmoku is known for the oxidised, silver-like metallic resulting feature.
The chemical change of glaze is unpredictable, even to Yamashita himself, until re-opening the kiln and seeing the final result.

Even if using the same soil, forming in the same shape, applying the same glaze, every single one of the bowls will earn a different aesthetic feature. It is a mystery.
Perhaps there are people who would analyse this mystery scientifically and deliberately make a stable chemical change. But I feel that it is also important to appreciate these coincidental features and enjoy the mystery as a mystery, since pottery is not quite the same as industrial technology.
Pottery is also known as the art of soil and fire. Its charm emerges from nature beyond the control of human hands. When we enclose the sense of beauty within a strict certainty, some aspects will be lost in return.
Yohen Tenmoku in the contemporary time…. The bowls of Ginka Tenmoku are not designated as national treasures, yet, they would certainly become treasures of people who use them in their everyday life.

*It is believed that in the early 15th Century, 窯変 (Yohen) was replaced with the characters 曜変 (Yohen), with the same pronunciation as the name of Yohen Tenmoku (曜変天目). While “窯” in Chinese characters indicates “kiln”, “曜” in Chinese character indicates “star” or “shine/sparkle” (“変” in Chinese characters indicates “change”).
(Reference: Seikado Bunko Art Museum website)