Question

Does earthenware absorb and not release what it absorbed? Or does it release only part of it? Because if it absorbs and does not release, then if it became non-kosher, seemingly there would be no need to kasher it even when used with heat or sharp food, or to chop on it with a knife despite the pressure of the knife. And are what people today call earthenware cups really earthenware in every respect, or are they covered with a porcelain coating, and what is the law in that case as well? When you turn these cups over, only the outer ring is earthenware, and some people sharpen knives on it.