【Japanese Pottery】Kiln Transformations (Yōhen): Embracing Accidental Color in Ceramics

Table of Contents

• 1. About Yōhen (窯変, kiln effects)
• 2. Examples of Yōhen: Glazed Works
• 3. Yōhen Examples: Unglazed Works (無釉編)
• 4. Yōhen as Decoration

1. About Yōhen (窯変, kiln effects)

Yōhen (窯変) refers to the color changes that occur on ceramic pieces inside the kiln during firing. Because these effects are caused by the flame of the kiln, it is also sometimes called hibakari (火変わり, fire-transformation). These layered colors sometimes form patterns, adding depth and richness to the coloration of ceramics.

Interestingly, yōhen is sometimes translated as “accidental coloring” in English. When ceramic pieces are placed in the kiln and fired at high temperatures, chemical substances within the kiln interact and generate a variety of color tones on the surface of the pieces.

Even though the artist may be able to anticipate the outcome to some extent, the final coloration is not fully known until the piece is removed from the kiln.

In this sense, the word “accidental” is quite accurate. This tendency is especially strong in wood-fired kilns (makigama, 薪窯), where the flickering flames contribute to complex and varied colorations on the ceramic surfaces.

You may have come across the term yōhen in books, museums, or pottery shops. For example, a description might read: “The surface of this piece exhibits scenery formed by yōhen,” which can be interpreted as: “Color variations created by kiln effects have formed a scenic texture on the surface.”

2. Examples of Yōhen: Glazed Works

Ceramics that use glaze are referred to as shiyū (施釉, glazed) works. For example, consider Oribe ware (Oribe-yaki, 織部焼) that uses a copper glaze (dōyū, 銅釉). When the kiln atmosphere has sufficient oxygen (sanka fun'iki, 酸化雰囲気), the copper content produces a green color. Here, “atmosphere” refers to the internal condition of the kiln.

In contrast, cinnabar red glaze (shinsya, 辰砂), which also contains copper, is fired in a reduced-oxygen environment (kangen fun'iki, 還元雰囲気). In this case, the copper produces a red color.

As you can see, even glazes with similar components can yield different colors depending on whether the firing atmosphere is oxidizing or reducing. This phenomenon can also be observed in celadon (seiji, 青磁) wares.

If you search for “seiji” in images, you’ll find many pieces in blue to green hues. However, some celadon pieces appear yellow.

This yellow coloration is caused by trace amounts of iron in the glaze. When fired in an oxidizing atmosphere, the result is a yellow tone. When fired in a reducing atmosphere, the piece takes on blue to green tones.

3. Yōhen Examples: Unglazed Works (無釉編)

Yōhen (窯変) refers broadly to color changes, so it also appears in unglazed, high-fired ceramics. For example, Bizen ware (備前焼, bizenyaki), Shigaraki ware (信楽, shigarakiyaki), and Iga ware (伊賀, igayaki) are well-known for being fired with minimal or no glaze, allowing distinctive yōhen effects to emerge.

When firing in a wood-fired kiln (薪窯, makigama), the wood fuel burns down to ash. The kiln’s flame carries ash upward, depositing it on the surfaces of the pieces. At high temperatures, this ash melts into a glassy material that acts similarly to a glaze, resulting in color changes on the ceramic surface.

A classic example is goma (胡麻, sesame) in Bizen ware. Fine ash accumulates on the surface in speckled patterns, and when it melts, the piece’s brown clay body (胎土, taido) takes on spot-like patterns. Depending on the hue of these spots, terms like “yellow goma” (黄胡麻, kigoma) may be used.

In some cases, the piece can become entirely covered with ash, blocking the flame and depriving that area of oxygen. This partial reduction environment (還元雰囲気, kangen fun’iki) can cause the covered section to turn black.

In Bizen, this phenomenon is known as sangiri (桟切). Within the red-brown clay body, only the area covered by ash turns black or gray.

The word “san” (桟) generally refers to a “bridge” or “connecting piece,” as in a wharf or a connecting corridor within a multi-chamber climbing kiln (登窯, noborigama). The “san” areas are narrow spots with large amounts of flying ash and strong temperature fluctuations. Placing a piece there causes abundant ash coverage, creating a localized reduction zone—hence the term sangiri, meaning “cutting at the san.”

Thus, yōhen colors ceramics in complex ways, regardless of whether they are glazed or unglazed. Some yōhen effects directly show vibrant colors like the greens in Oribe (織部, oribeyaki) or the reds in cinnabar glaze (辰砂, shinsa). Others, like goma and sangiri, create decorative patterns from color changes.

4. Yōhen as Decoration

As noted earlier, one cannot know exactly what yōhen will look like until the piece is removed from the kiln. However, by using its underlying principles, potters can deliberately induce certain kiln effects.

Two main factors are the presence or absence of glaze and the firing atmosphere. For instance, even if the overall kiln atmosphere is oxidizing (酸化雰囲気, sanka fun’iki), one can create local reduction spots by using various techniques.

If pieces are packed tightly together, the spaces between them receive less flame contact. Alternatively, large amounts of charcoal placed around a piece can block air circulation, forming a localized reduction environment. In both scenarios, part of the surface experiences color changes.

It’s also possible to selectively apply glaze or slip (化粧土, keshō-tsuchi) to certain areas. I often discuss such possibilities with fellow potters. Imagining the “what ifs” is one way to enjoy these works from a more multifaceted perspective.

For example, consider the famous Ko-Iga (古伊賀) masterpiece Yaburebukuro (破れ袋). One entire side is covered in ash, which melts into a vivid green while unmelted ash clumps and adheres to the surface.

Is this the accidental outcome of placing the piece randomly in the kiln, or did the maker intentionally apply glaze and charcoal on that side? Of course, we may never know unless we were involved in the firing, but that space for speculation can deepen our appreciation of the piece.

Another instance is Fujiyama (不二山) by Hon’ami Kōetsu (本阿弥光悦). Some attribute the contrast of white on the upper part and black (the result of reduction/carbonization) on the lower section to pure chance. Others believe it was deliberately created through a combination of white glaze (白釉, hakuyū) and black glaze (黒釉, kokuyū), or slip.

Personally, I favor the latter theory. The interior also shows a neat division between white and black, suggesting that Kōetsu carefully planned the final design when applying his glazes. If I were to make a reproduction, I would likely apply white and black separately and then blur the boundary.

Such conjectures and informal discussions with friends are especially enjoyable. They remain speculative, but they undoubtedly enhance one’s appreciation and sense of discovery while viewing ceramics.

Notably, creating yōhen effects is not limited to wood-fired kilns. One can explore similar outcomes in electric kilns by incorporating charcoal or applying localized color glazes, thus expanding creative possibilities in contemporary practice.

In short, understanding the basics of yōhen broadens both the enjoyment of viewing ceramics and the scope of pottery-making. The personality and allure of each piece go beyond shape alone, growing richer through the dimension of yōhen.