Glazed Earthenware and the Law of Absorption in Chukat

However, it is necessary to say: these glazed dishes that are used with boiling liquids — do they have absorbed matter that must be purged, or not? For since they are originally vessels of earthenware, and one has coated them with glaze, does that coat of glaze upon them protect, at the time the glaze is upon them, so that the earthenware does not absorb — because glaze is a metal, and purging suffices for it; or perhaps it does not protect, and the earthenware absorbs, and there is no remedy for it? Come and hear, for they inquired of Meremar concerning leaven on Passover: these glazed vessels — do they have absorbed matter, and is it permitted to use them on Passover, or not? Those that have cracks in them, there is no question to them, for it has absorbed in the place of the cracks. The green ones there is no question to us, for they have alum. When there is a question to us, it is concerning the white or the black. He said to them: I saw concerning them that when they put hot liquids into them, it sweats from them — infer from this that they absorb. Therefore they are like vessels of earthenware. And these words are with regard to leaven on Passover, and with regard to use by worshippers of stars, whose use of them is with boiling liquid. But with regard to libation wine, whose use is with cold liquid: the green ones are forbidden, because they have alum; the black and the white, if they have cracks in them, are forbidden; if they have no cracks in them, mere rinsing suffices for them — therefore they are like vessels of metal. For we hold the law is according to Rav Zevid, for Rav Zevid said: these glazed vessels, the white and the black are permitted, the green are forbidden, and if they have cracks in them they are all forbidden. And so is the law.