Grain can increase the heating area of iron pan. There is a trace of grain in the wok, which is caused by mildew when frying the wok. Grains can increase the heating area of the wok, store oil and reduce the oil sticking in the wok. Just like Zhangqiu wok, the inside of Zhangqiu wok is also marked with a hammer, which is also to store oil and increase the heating area, and it can not stick to the wok when cooking, so the price is relatively high.