Kannyu (Crazing)

Kannyu (貫入) is the network of fine hairline cracks that forms in a glaze, most often on pottery (toki) rather than porcelain. It happens because the glaze and the clay body beneath it shrink at slightly different rates as a piece cools after firing, and the glaze, being less flexible, cracks into a web of tiny fissures under that stress.

Kannyu is an expected part of how many pottery pieces age, not damage, and collectors often value the pattern it creates. Because the piece’s porous body sits just beneath the glaze, liquids such as tea, coffee, and soy sauce can seep into the cracks over years of use and gradually tint the glaze along the crack lines, a slow color change some tea drinkers call nanabake (“the seven transformations”). Porcelain’s dense, non-porous body does not craze this way and stays looking the same over time.