Bowl with goldfish design in underglaze blue.

Blue underglaze

Blue and white porcelain (sometsuke) is made by painting blue underglaze decorations on white bisque-fired pottery using a cobalt-rich pigment known as zaffer (gosu). The piece is then coated with a transparent glaze and fired. This technique for making blue and white porcelain has been used in China since around the twelfth century during the Yuan Dynasty.