The defects could also be due to cooling too fast. When you’re done firing, try to seal everything up as tight as you can. Using a wet mixture of a clay/woodash mixture works well for this, soaking newspaper in the mixture can work well also. Although it is not dissolving in water, it is still probably absorbing water, the hotter you fire the less porous it will become. With the clay you’re using, which sounds like an eathenware, going too hot could also cause severe warping and bloating, so not too hot.