The raw bowls are hand-turned in porcelain, Bone China. They are glazed inside and on the edge, the grooved surface is raw / unglazed. The grooves are a long spiral that is turned from top to bottom.
The raw bowls come in different sizes, and the measurements can be seen here:
X-small: Ø = 10 cm.
Small: Ø = 15 cm.
Medium: Ø = 19 cm.
Large: Ø = 23 cm.