Japanese pottery often has a net of cracks that covered all the glazed surface. You can see these crack when look at Hagi, Kyo, Raku ware and many other. And you can think that this is some kind of defects, but it is not. In Japanese, those cracked patterns are called kan-nyuu (貫入) and it is actually a desirable feature of the pottery.

When clay covered with glaze is fired, it expands. The glaze melts at high temperatures and fuses with the clay. When the pottery is cooled down, it shrinks back. But the glaze shrinks more than the clay and while cooling, it covered with cracks, or kan-nyuu. Two kan-nyuu patterns can never be the same and this offers uniqueness in each piece of pottery.

Then tea seeps into these cracks, staining it every time you use the cup. The piece is changed in color with frequent use. Its character seems to grow together, develop with its user.