Gosu: The Cobalt Blue Behind Japanese Sometsuke

Gosu is a blue mineral pigment based on cobalt oxide, with iron and manganese present as well. All of the indigo in sometsuke comes from gosu.

Before firing, gosu looks like nothing but blackish mud or a grey liquid. Painted onto a body that has been through bisque firing, then covered with a clear glaze and fired at around 1,300°C, it changes dramatically into clear indigo for the first time.

It was originally an expensive material imported from China. From the Edo period domestic production developed, and gosu became the basic material supporting the blue-and-white regions of Arita, Hasami and Seto.

Why the color cannot be known until it is fired

The blue of gosu appears only once the cobalt dissolves into the glassy glaze. At the stage where it is painted on the body, the cobalt is still a powdered oxide. When the glaze melts at high temperature and cobalt ions disperse through it, the way light is absorbed changes and the blue becomes visible.

So a painter draws the line without seeing the fired color. The only guide is the density of the line: laid on thickly it moves toward deep navy, thinly toward pale blue. Repeating that prediction across hundreds of pieces, the hand learns the color.

And the same gosu changes quality of blue with the kiln's temperature and air (oxidation or reduction). Reduction gives a clear blue; oxidation, a blue leaning slightly grey. The differences people describe as the blue of Arita or the blue of Tobe come largely from this difference of firing.

Types of gosu and their range of blue

The difference between the deep, settled indigo of antique sometsuke and the clear blue of modern porcelain usually comes from this difference of raw material. The more impurities, the more complex the blue; the more refined, the more vivid and uniform.

Gosu on stoneware

Gosu is not only for porcelain. As in the photograph above, a single line of gosu can also be drawn over the white glaze of a stoneware piece.

Porcelain white is uniform, so a gosu line stays sharply outlined. On stoneware clay the glaze moves more, and the line bleeds slightly into a softer expression. That the character of a line changes with the body beneath, even with the same pigment, is part of the interest in looking at gosu.

Related terms and guides

Related terms

• Sometsuke: Blue and White Porcelain, Explained
• Shita-etsuke: Underglaze Painting and Why It Never Fades
• Uwa-etsuke: How Overglaze Enamel Painting Works
• Suyaki (Bisque Firing): What Happens in the First Fire
• Celadon (Seiji): The Blue-Green Glaze of Iron and Fire
• Japanese Pottery Glossary | Terms, Techniques & Kilns

Read further

• Japanese Pottery Patterns: What the Motifs Actually Mean
• Japanese Pottery Glazes: The Main Types Explained
• Arita Ware: Japan's First Porcelain, 400 Years On
• Hasami Ware: Everyday Japanese Porcelain from Nagasaki