A "KANNYU" is a crack pattern in the glaze that occurs during cooling after firing due to the difference in shrinkage between the glaze and the base material, and is different from cracks or scratches that occur when the piece breaks.
Ordinary pottery is fired at a high temperature of 1,200 to 1,300 degrees Celsius, depending on the type of glaze, after a glaze is applied on top of the base material.
During this process, the glaze melts and forms a glass-like layer that covers the top of the pottery.
After firing, the temperature of the pottery itself decreases, and the degree of shrinkage differs between the glaze and the base of the pottery.
This is called "penetration.