Generally speaking, it is caused by wet walnuts drying too fast. Of course, there are other factors. The repair method is to cover it with a wet towel (you can use warm) and put it in a sealed bag for about one night. The cracks in the sky are closed a little 502, just wipe off the excess, so it doesn't hinder your play. If you have knowledge about this in the Walnut Circle Forum, you can take a look. If you don’t understand, you can directly Q1208532264 Brother Niu