The Confusion of Kashrus of Glass
Kashering glass is a most confusing issue because its status is unclear and accordingly opinions are wildly apart.
On the one hand, the Rashba (Teshuva 1:233) maintains that glass does not absorb. In fact its very transparency verifies that it does not absorb.
Accordingly, glass can be used for meat and dairy and even non-Kosher without need to Kashered. It is like using a new utensil.
At the other extreme the Mordechai asserts that glass is equivalent to earthenware, after all it is manufactured from melting sand.
The middle ground is explained by the ReAh (quoted by the RiTVa Pesachim 30b) glass is like metal, so it does absorb and in theory will emit any non-Kosher it has absorbed, provided it is properly Kashered. However, seeing that glass is fragile, we forbid Kashering because it is likely that it will not be Kashered with full intensity for fear of it breaking.
The Mechaber (OCh 451:26) rules that glass need not be Kashered, since it does not absorb. However, the Rema documents the Minhag of Ashkenaz to equate glass to earthenware. This is equally true regarding Pesach or non-Kosher. Some Acharonim [Kenesses HaGedolah brought by Pri Migadim (451:31), hold that one can be lenient to kasher other issurim, just not for Pesach. The OU does not rely on this leniency.
Still, Magen Avrohom (451:49) says that if hagalah was done, bidieved, it is acceptable.2 Although at times there is room to say “Shas hadchak k’bidieved” and permit one to do hagalah, one should only kasher glass with a psak from the office.
Common examples of glass include crystal, Pyrex, Corelle, Duralex and porcelain enamel. Porcelain enamel is made from sheets of glass which are ground, returned to the kiln and fused to other utensils.
Mechaber (451:23) writes that cheres coated with glass has the status of cheres. Although ordinary glass does not absorb, thin glass coatings that are fired onto clay3 can absorb. Therefore, according to all opinions china is cheres, even though it has a thin film of glass on its surface.
Mishna Berurah (addendum to 451:23) writes that although the Chasam Sofer was against kashering metal pots that are coated with glass with hagalah, he has heard that many Gedolim are maikel to kasher these pots when they are aino ben yomo, and are only machmir not to kasher them for Pesach. Similarly, many are maikel to kasher enamel coated ovens, such as the type that are common in homes, with libun kal. However, for Pesach one should lichatchila be machmir to do libun gamur (self clean or torching), use an oven insert, or make sure to only place covered food in the oven. In cases of difficulty, some are maikel to allow libun kal, since the Magen Avrohom said that if one kashered glass it would help bedieved.4
Glass lined reactors (glass lined steel) accordingly may be kashered with hagalah. However, Rav Schachter has said that we should also do three hagalos, to incorporate the opinion of the Ba’al Ha’itar, who holds that one can do hagalah three times to kasher aino ben yomo cheres.
Although we do not kasher glass, we allow caterers to use hotel glasses. This is because the glasses are clean and are only used with cold drink. Rama (Y.D. 121:5) writes that one is permitted to use clean non-kashered kailim derech arai (temporary fashion), such as when visiting at the house on a non-Jew. However, one may not use non-kosher glasses at one’s own home, because this is considered derech kevah (permanent type use). Caterers may not wash their own glasses in a treif hotel dishwasher, since this will make their glasses treif, and continued use is considered derech kevah.
Some pipes have a sight glass for viewing product. If hot non-kosher product flows through this pipe, can it be kashered? Rav Belsky has said that we can allow kashering when aino ben yomo, since in addition to the opinion of the Magen Avrohom that hagalah works bidieved, we can be mitztaref the opinions that are maikel by a kli shemishtamshim bo b’shefa, since a large volume of liquid passes over that small piece of pipe. Although we do not rely on kli shemishtamshim b’shefa alone, since in this case hagalah works bidieved, we can be maikel.
If glass absorbed through kavush then it may be kashered lichatchila with milui v’irui, or irui kli rishon, provided that the bliya was heter, chametz or stam yayin. Heter includes chalav stam provided that the kli being kashered is aino ben yomo. If the bliya was from other issurim, it is subject to the machlokes the Mechaber and Rama, and our minhag is not to kasher lichatchila.
Fiberglass is a material made of thin strands of glass that are pressed together to form a board. Plain fiberglass is highly porous and cannot be kashered. Even if it only absorbed heter through kavush, it may not be kashered, since it is impossible to get out the b’en. However, fiberglass mixed with resin forms a solid non-porous surface. A holding tank made of this type of fiberglass may be kashered, just like ordinary glass, provided it was kavush with wine, chametz or heter.