The status of glass is very interesting. Glass was so rare and expensive that it is no wonder that the Talmud did not list it among the other kasherable items; metal, wood, stone, with rubber coming in later. Only the richest owned glassware. Most medieval authorities ruled that glass, being made from sand, is a form of earthenware, and extremely porous. Therefore, it is completely unkasherable. This became the ruling of most Ashkenazi rabbis to this day. RAMBAM ruled that it is similar to metal, and could be  kashered with boiling water.The situation became more complicated when, in about 900, a book of aggadah and ethics appeared, which was mistakenly attributed to a great second century Babylonian rabbi. Although it was not focused on halachah, it "threw in" that glass, being smooth (non porous) is impervious to all kinds of food, and needed no kashering. This became a popular opinion, and was accepted by the laymen in most parts of Europe. However, it was bitterly opposed by virtually all Ashkenazi rabbis. In Sepharadic lands, however, the rabbis almost always DID accept this approach. Why? It is against all major rabbinic opinions? Because we can see that glass is non porous! If wood is at least kasherable, glass HAS TO be better than that! In a conflict of sources and observable fact, Ashkenazim will usually go for earlier opinion (or custom), Sepharadim for fact. (I will once again express my gratitude to Rav Ovadia Yosef, for allowing me the privilege of become Sepharadi.) Syrian Jews, for example, will have only one set of dishes, used for meat, dairy, and Passover; with only a rinse and a little soap in between. Rabbi Dov Lior, the now retired rav of Kiryat Arba-Hevron, ruled a two years  ago Passover, that stainless steel is likewise "smooth" and non porous, and can be used for Passover with merely a cleaning. So far, few have adopted this view, as the kashering of metal is both in the Torah and the Talmud, whereas glass is not. Rabbi Lior answers this objection by pointing out that a metal so smooth and perfect, was not the norm in those days. Rabbi Abadi, of whom I spoke in a previous post, even allows glazed china to be used as glass. Others agree in theory, but are concerned that the glaze may be chipped, exposing the absorbent earthenware Sepharadic rabbis view glass variants, like Pyrex as glass. . A similar situation exists with plastic; a modern material. Whereas most rabbis ruled that plastic is no worse than wood or rubber, and is therefore kasherable, Rav Moshe Feinstein was not convinced. Therefore, most (but not all!) American rabbis, out of respect for Rav Moshe, banned kashering plastic. In Israel, this stringency is virtually unknown. In my next post, i will discuss how kashering is done, plus the alternative view of Rabbi Abadi.