In the times of Chazal, cheres was made from various baked clays. In modern times common examples of cheres include earthenware and stoneware. The Gemara Pesachim (30b) tells us that a cheres utensil cannot be kashered with hagalah התורה העידה על כלי חרס שאינו יוצא מידי דופיו לעולם. Even libun gamur is not permitted in situations where there is a concern that one might not be milaben properly for fear of cracking. Therefore, one may not kasher china by putting it through a self cleaning cycle of the oven. In such cases, the only permissible kashering is to place the utensil into a potter’s kiln which gets much hotter than libun chamur. This demonstrates that one is not concerned about potential damage.
There has been much written on the subject of porcelain as to whether it is cheres. Porcelain is a material made from superheated clay (over 2,000 °F) that takes on qualities of glass, including hardness and translucence. Most importantly, it seemingly does not absorb. Rav Yaakov Emden (Shailos Yayvetz I:67) contends that porcelain should be categorized as glass. However, Rav Moshe Feinstein zt”l writes that the predominant minhag is to treat porcelain as cheres and this is the position of the OU as well.
In recent times, scientists have created new forms of materials such as alumina and zirconia. These high-tech materials have a near 100% density, are very smooth and seemingly do not absorb. Though aluminum and zirconium are metals, alumina and zirconia are non-metals. One common source for alumina is bauxite stone, from which alumina is extracted and powdered. The Yad Yehuda says that a stone which is powdered and then reconfigured by mixing with water and firing in a kiln has the status of cheres. Therefore, these new materials should also be considered cheres. However, it is the position of Rav Belsky and Rav Schachter that these materials can be kashered with hagalah. Rav Belsky explains that the Gemara (Pesachim 30b) refers to several types of cheres, and not all of their cheres exhibited the same characteristics.
ירוקא לא תיבעי לך – דודאי אסירי. כי תיבעי לך – אוכמי וחיורי מאי? והיכא דאית בהו קרטופני – לא תיבעי לך, דודאי אסירי. כי תיבעי לך – דשיעי מאי? אמר ליה: חזינא להו דמידייתי, אלמא בלעי ואסירי.
The Gemara never states as a rule that cheres must absorb. Rather, the Gemara says that we see that our cheres sweats, which is an indication that it has absorbed. The Torah testifies that once cheres absorbs, it can never be fully purged. However, Chazal leave open the possibility for the existence of a type of cheres that does not absorb. Since these new materials are less porous than porcelain and are not made from clay, they are not included in the minhag to consider porcelain as cheres. If one comes across other high tech materials, and is uncertain whether they can be kashered, the shailah should be presented to our poskim for a ruling.
One particularly sensitive piece of equipment that often contains cheres is the homogenizer. Because of their durability, piston heads are often made of cheres. If these pistons are made from alumina or zirconia, then as we just explained, they can be kashered. If they are made from materials that we consider like regular cheres, can we kasher such a homogenizer?
The Baal HaItur says that cheres can be kashered when it is aino ben yomo with three hagalos. The Rashba says that we follow this leniency of the Baal HaItur only regarding kashering after an issur dirabbanan she’ain lo ikar min Hatorah, and Shulchan Aruch (Y.D. 113:16) brings this l’halacha regarding kashering after bishul akum. There is a machlokes Achronim2 as to whether we can extend this leniency to other issurim d’rabbanon she’ain lo ikar min Hatorah such as chalav akum and gevinas akum. Although it is difficult to resolve this machlokes, Rav Schachter and Rav Belsky have agreed that a homogenizer that is aino ben yomo that was used with chalav akum or gevinas akum can be kashered with three hagalos even for use with kosher pareve. This is due to the additional consideration that there is always more than sixty times the pistons in the flow of product through the pipes. It is therefore comparable to a kli she’mishtamshim bo b’shefa.3 This combined with aino ben yomo and the lenient opinion of the Achronim is the basis for our leniency. We also allow kashering these pistons from dairy to pareve. Other issurim, and certainly issurim d’oraisah, will require replacing the piston heads.
In summary:
- We cannot kasher cheres except by returning it to a kiln.
- We consider all clay based materials such as porcelain to be types of cheres
- Alumina and zirconia materials can be kashered with hagalah
- Cheres used for bishul akum can be kashered with 3 hagalos
- Cheres pistons used for chalav or gevinas akum can be kashered with 3 hagalos when they are aino ben yomo and we can then produce kosher pareve.
- Cheres pistons used for other issurim should be replaced
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Igros Moshe (Y.D. II:46, E.H.IV:7, O.C.III,58)
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Darchei Teshuva 121:74 brings this as a machlokes the Yad Yehuda (99:28) who is machmir and the Beis Shlomo (Y.D. 186) and others who are maikel.
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Shulchan Aruch (99:7) holds that a kli she’mishtamshim bo b’shefa need not be kashered. However, Shach 122:3 and Taz Y.D. 99:15 disagree. Pri Migadim S.D. 99:23 says that one may not be lenient even when the kli is aino ben yomo. However, Kaf Hachaim and Yad Avrohom are lenient when kli is aino ben yomo. We are maikel to allow aino ben yomo, when there are other mitigating factors, such as here.