As you use your donabe over and over, it develops the nice used "wabi-sabi" character and that's something to be welcomed.The small cracks could appear on the surface (glaze) of the donabe. It's called "kannyu" in Japanese and the cracks run like veins. They are the naturally developed cracks on the glaze and won't affect the function of the donabe. They are rather the good welcome sign of authentic donabe.