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Earthenware pottery, china, dishes: Earthenware can only be Kashered through placing it in an oven and heating it to the point that it can be reformed. If an earthenware dish was used for only cold Chametz, although it is permitted from the letter of the law to be used even with hot foods, the custom is not place even cold Pesach foods on it during Pesach. An earthenware oven can be Kashered through Libun Gamur. Examples of earthenware vessels that cannot be Kashered: Crockpot; mugs, chinaware.
Sundried clay vessels: Vessels made of sun dried clay are Kasherable
Rock/stone: Slabs of rock and stone are Kasherable materials. [Thus, marble, such as a marble counter, is a Kasherable material. However, engineered stone is like earthenware and cannot be Kashered. Many counters today are made of engineered stone.]
Wood vessels: Wood vessels are Kasherable [so long as it does not contain uncleanable cracks and crevices]. The custom is to Lechatchilah never use any wooden vessels which were used for flour consistently, even if one cleaned it and performed Hagala. All wooden vessels may be sanded down and Kashered.
Metal vessels: Metal vessels are Kasherable. If the vessel absorbed the Chametz through cooking in water, then it suffices for it to be heated to the point that its outside reaches the point of “Yad Soledes Bo”. This can be done by either using a torch [Libun Kal] or dipping it in boiling water [Hagala]. If the vessel absorbed the Chametz directly, without any liquid, then it requires “Libun Gamur, which means that it must be heated until sparks begin to fly off from it, or until a layer of it peels off. This applies even Bedieved.
Glass: It is disputed amongst the Poskim as to whether glass vessels need to be Kashered even if used for hot Chametz foods or if it has the same status as earthenware and cannot be Kashered. Practically, the custom amongst Ashkenazi Jewry is not to Kasher for Pesach any glass vessels which have absorbed Chametz. Thus, if the glass vessel had hot Chametz poured into it, or had Chametz soak in it for 24 hours, it is not to be Kashered. This applies even if the majority usage of the vessel is for cold foods, and the vessel is not Ben Yomo. These vessels are not to be used for Pesach, and are rather to be put away with the Chametz vessels. [Sephardim, however, are lenient to allow using glass vessels even without Hagala, so long as the glass has been washed and cleaned well. It is a dispute amongst Poskim if Pyrex material glass may be Kashered.]
Glass coated vessels: If the vessel is coated with glass on its interior, in the area where the food is placed, then it may not be Kashered. If it is coated with glass only by its exterior, then if it is never commonly placed directly over a fire to cook in, such as silver vessels coated externally with glass, then it may be Kashered through Hagala. If, however, it is not uncommon to cook with it over a fire, or even to occasionally heat food in it over a fire, then it may not be Kashered.
Ceramic: Ceramic materials are a form of earthenware and cannot be Kashered.
Enamel: Enamel is a material disputed amongst the Poskim is to whether it has the status of earthenware, and hence may not be Kashered. Practically, it is customary not to Kasher Enamel coated vessels for Pesach, although many are accustomed to Kasher it regarding other Issurim during the year.
Porcelain: Porcelain materials have the same status of earthenware and cannot be Kashered.
Story Alter Rebbe and Maggid and his glass Kiddush cup:
This story was related by Chassidim in name of the Tzemach tzedek: It was customary of the students of the Great and Holy Maggid to remain by him for the Pesach Seder, and so was followed by the Alter Rebbe in one of his early years by the Maggid. The arrangement with the students was that they provide the utensils for the meal, while the Maggid provides the wine and Matzah. In his younger days, the Alter Rebbe was very poor and could not afford to even purchase a new Kiddush cup for the Seder night. He was in a dilemma; he either had to go begging for money, or be lenient to Kasher the glass cup through Miluiy Veiruiy. After a long inner debate, he concluded to Kasher the vessel and not throw himself until the public. When the night of the Seder arrived, all the students sat around the table with their cups prepared with wine to begin Kadesh. When the Maggid lifted his cup to begin the Kiddush he suddenly exclaimed “I smell Chametz.” He instructed his student, Rav Menachem Mendel of Vitebsk to search around the table to see the source of the Chametz smell. The student circled the table three times and came up empty handed. When the Maggid saw that the smell was yet to be uncovered, he exclaimed “What can I do, the Rama is standing opposite me with a very angry expression and will not allow me to make Kiddush.” When the Alter Rebbe heard this he immediately understood that he, and his Kashered glass cup, were the source of the commotion, as the Rama rules that glass cannot be Kashered. The Alter Rebbe proceeded to confess to the Maggid, explaining to him his predicament that led to the Kashering. In return, the Maggid got up and kissed the Alter Rebbe on his head, saying “You power is very great as you were able to bring the Rama to come down.” The Maggid then instructed the Alter Rebbe that he will drink the four cups of wine from his personal cup.
 Admur 451:9
 Admur 451:43
 Admur 451:6-7; 461:1; 494:16
 Admur 451:25
 Admur 451:25; Piskeiy Teshuvos 451:44
 Piskeiy Teshuvos 451:44
 Engineered stone is widely used today for counter material in contrast to marble. Engineered stone is a composite material made of crushed stone bound together by an adhesive, (most commonly polymer resin, with some newer versions using cement mix). The two common stones used in producing these products are marbles and quartz. The quartz-based product is used primarily for kitchen countertops. Related materials include geopolymers and cast stone. Unlike terrazzo, the material is factory made in either blocks or slabs, cut and polished by fabricators, and assembled at the worksite.
 Piskeiy Teshuvos 451:33
 Admur 451:25
 M”B 451:56; See Admur 451:19
 Admur 451:50
Background: From the letter of the law a vessel used for only cold Chametz, such as flour, may be used even for hot Pesach food as long as it was cleaned well. Nevertheless, the custom is to Lechatchilah never use any wooden vessels which were used for flour consistently, even if one cleaned it and did to it Hagala. The reason for this is because we suspect that perhaps some Chametz has remained in the vessel. [Admur ibid]
 Admur 451:25
 Admur 451:13
Background: If the vessel absorbed the Chametz directly, without any liquid involved, such as by a baking pan, then a dispute exists as to what type of Kashering it needs. Some say that even Hagala [placing it in boiling water] suffices. Their reasoning is because Chametz before Peach is permitted, and therefore does not require such extreme heat to be Kashered. However, other Poskim rule that it requires “Libun Gamur” which means that it must be heated until sparks begin to fly off from it, or until a layer of it peels off. Practically, we rule like the latter opinion, even bedieved [such as that one already used the pot to cook in] that it needs libun gamur. [Admur ibid]
 Admur 451:73; Michaber 451:26
 Some Poskim rule that glass is a hard and smooth material which cannot absorb the taste of foods and thus does not require Hagala. [1st and Stam opinion in Admur ibid; Michaber ibid and final ruling of Beis Yosef 451:26; Ravayah 464; Rashba 1:233; Ran 2:9] Rather, one is to simply clean and wash them from any possible Chametz and they may then be used for Pesach. This applies even if they were used for cooking hot Chametz during the year or had Chametz, such as bear, stored in them for long periods of time. Other Poskim, however, rule that glass has the same status as earthenware, and can never be Kashered. Thus, if a glass vessel was used for hot Chametz, or had Chametz soak in it for over 24 hours, it may not be used during Pesach. [2nd opinion in Admur; Rama ibid; Mordechai 574; Semag L.S. 78; Iggur 737 and Hagahos Maimanis 5 in name of Yechiel Miparish; Terumos Hadeshen 1:132] Some Poskim rule that Bedieved the vessel is Kasherable through Hagalah just like metal. [Admur ibid and M”A 451:49; See P”M 451 A”A 49 that the M”A ibid holds glass is Kasherable]
Are Ashkenazim stringent regarding glass also by other Issurim? This matter is disputed in Poskim.
 Admur ibid; Rama ibid; Iggur ibid
 Bedieved: If one placed hot Pesach food in a glass Chametz vessel, then if the vessel was most commonly used for cold Chametz that did not soak in it for periods of 24 hours, while hot Chametz and soaking Chametz for 24 hours was its minority usage, the food remains Kosher [if the vessel was not Ben Yomo]. If, however, the vessel was majority used for hot Chametz foods, or for soaking cold Chametz for past 24 hours, then the Pesach food may not be eaten, if the vessel did not have Hagala or Iruiy Vemiluiy done to it. If the vessel was Kashered, then the food remains Kosher in all cases. [Admur ibid; M”A 451:49; M”B 451:155; See however Taz 451:30 who is lenient even if the vessel was not Kashered at all, as explained in P”M 451 A”A 49] If the hot Pesach food was poured from a Keli Sheiyni onto the vessel, then seemingly the food likewise remains permitted, even if the vessel was majority used for hot Chametz, and even if the vessel was not Kashered. Vetzaruch Iyun. If the vessel is not Ben Yomo at the time of use, some Poskim are lenient in a case of great loss. [M”B 451:155]
Time of need: Some Poskim rule that in a time of need, if there are no other cups or vessels available, one may be lenient to Kasher the vessel with Hagalah or Miluy Veiruiy. [Chayeh Adam; M”B 451:156 and Shaar Hatziyon 451:201]
 Beis Yosef 451:36 that so is the ruling and custom; As so rules Michaber ibid; Yechaveh Daas 1:12; However, some Sefaradim are stringent to Kasher the vessel. [Rav Poalim 3:29 that so is custom of Bagdad] Others are stringent not to use it at all as rules the Rama. [Sdei Chemed Mareches Hei 29
 See Piskeiy Teshuvos 451:54; The following Poskim say its Kasherable: Tuv Taam Vedaas 2:25; Mahram Shick Yoreh Deah 141; Yaskil Avdi 4:13; Tzitz Eliezer 8:21; 9:26; Yechaveh Daas 1:6; Yabia Omer 4:41 [Rav Ovadia holds that for Sephardim they do not need to be Kashered just like glass] ; Shearim Hametzuyanim 126:11; The following Poskim say it may not be Kashered for Pesach but may be Kashered the rest of the year: Minchas Yitzchak 1:86; Beis Avi 1:115; Yesod Yeshurun 6:170; Shraga Hameir 7:143; Shevet Halevi 2:43
 Admur 451:75
 The reason: As the custom is today to never Kasher glass through Hagala just like an earthenware vessel. [ibid]
 The reason: As although the Chametz which the glass absorbed will not come out through Hagala, and there is thus suspicion it may enter into a hot Pesach food that one places in it, nevertheless, it is allowed, as the heat of a food which is off the fire does not have ability to remove the absorbed taste of an outer coating and then bring it back into its own food. This applies even if the material of the vessel is metallic in which case we say that when part of it becomes hot it makes the entire vessel hot, as although it has the ability to heat up the entire vessel, it does not have the ability to remove the absorbed taste from the outer coating. [Admur ibid]
 The reason: As when food is heated in it over a fire, the heat of the fire has the ability to remove the Chametz taste that is still absorbed in the glass coating and never became removed through Hagala. [Admur ibid] Thus, we do not allow it to be Kashered even if one plans to never use it over the fire on Pesach, as we suspect one may come to do so. [So is implied from Admur ibid]
 Admur 451:20
 The reason: As the heat can easily ruin it and there is thus suspicion that to prevent this one will not heat the water enough for the Hagala. [Admur ibid]
 What is ceramic? A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous. Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the non crystalline glasses.
 Piskeiy Teshuvos 451:49
What is enamel? Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C (1380 and 1560 °F). The powder melts, flows, and then hardens to a smooth, durable vitreous coating on metal, and also glass or ceramics, although the use of the term “enamel” is often restricted to work on metal, which is all that this article covers; enameled glass is also called “painted”.
 Some Poskim rule it retains the status of earthenware and is not Kasherable. [Tuv Taam Vadaas Kama 183; ] Other Poskim rule it is a metallic type of coating and is hence Kasherable with Hagala. [Maharash Engel 5:96; Avnei Nezer Y.D. 2:112] Other Poskim rule it is Kasherable with Libun Kal. [Pischeiy teshuvah Y.D. 121:2 in name of Chasam Sofer Y.D. 113; Mahariy Asad 76; Imrei Yosher 2:144]
 Shevet Halevi 2:43; Cheshev Haeifod 1:117; Piskeiy Teshuvos ibid
 M”B 451:163; Piskeiy Teshuvos 451:56; See Darkei Teshuvah Y.D. 121:26; Sdei Chemed Mareches Hei 30
What is porcelain? Kaolin clay is the primary material from which porcelain is made, even though clay minerals might account for only a small proportion of the whole. The word “paste” is an old term for both the unfired and fired material. A more common terminology these days for the unfired material is “body”, for example, when buying materials a potter might order an amount of porcelain body from a vendor.
 See Piskeiy Teshuvos 451:35
What is Teflon? Polytetrafluoroethylene (PTFE) is a synthetic of that finds numerous applications. The most well known brand name of PTFE is Teflon by DuPont Co. PTFE is a solid, as it is a high- compound consisting wholly of carbon and fluorine. PTFE is neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high of fluorine. PTFE has one of the lowest coefficients of friction against any solid. PTFE is used as a coating for pans and other . It is very non-reactive, partly because of the strength and so it is often used in containers and pipe work for reactive and corrosive chemicals.
 The Poskim suggest several reasons behind why it is not Kasherable: a) As one uses it to fry things without any oil and it hence requires Libun Chamor, which cannot be done due to fear of destroying the pan. B) As Teflon is a very absorbent material and contains the same characteristics as earthenware. [See Piskeiy Teshuvos ibid footnote 187]
 See Piskeiy Teshuvos 451:53
 Some Poskim rule it has the same status as earthenware, and is thus not Kasherable. [Igros Moshe 2:92; Kinyan Torah 2:84; Lehoros Nasan 6:69] Others rule it is Kasherable. [Chelkas Yaakov 2:163; Minchas Yitzchak 3:67; Tzitz Eliezer 4:6; Chazon Ovadia; Beir Moshe 2:53; ]