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Kashering Utensils – Part III


This is the third and final part in our series on kashering utensils. The first two dealt with earthenware and glass, two natural substances used for dinner and cookware. This installment will discuss modern materials in halacha: is porcelain also considered earthenware? What experiments did the Radvaz carry out to prove his halachic point of view nearly 500 years ago? Should we be concerned porcelain production has changed in the past 500 years? How can exquisite bone china be koshered? What should a Jewish hospital do if they wish to kosher their kitchen but cannot afford a new set of dishes? And newer, heat resistant glassware such as Corelle or Pyrex, are also the topic of this week’s article. Can Ashkenazim also be lenient and kosher these materials?

Horn or shell are also discussed this week. Can they be koshered if they are not heat resistant? And what about plastic?


The previous articles focused on the halacha that the Torah teaches about earthenware: it cannot be koshered because the taste it absorbs cannot be removed. Poskim in the last few hundred years have debated about porcelain and other china. While in the past, china production was a Chinese secret (hence its name), today production of porcelain takes place in many countries and is comprised of any number of various components.

While ceramics traditionally consisted of only earth and water, production of porcelain calls for adding other components to the mixture, specifically bone (hence the name bone china), which can measure up to 50% of the product. This produces tableware which is the strongest of the porcelain or china ceramics, having very high mechanical and physical strength and chip resistance, and is known for its high levels of whiteness and translucency.

The Radvaz (III, 401) was one of the first poskim to have discussed porcelain, nearly 500 years ago. In his time, porcelain production was a Chinese national secret and rumors in the west were abound with presumptions at what it contained that gave it its quality. Some thought it contained the skin of an aquatic animal which gave it its quality and prevented food absorption. The Radvaz was skeptical of this and writes that it should be seen as pottery. (He mentions that the Geonim who resided in Islamic lands had already seen and discussed this material and determined it was a form of pottery.)

The Radvaz placed a chip of porcelain in the fire and saw that the flame took shape just as it does when it comes in contact with substances that absorb flavors. In addition, he tried weighing a new porcelain vessel, then place it inside a pot of boiling water. After a half hour of boiling, he weighed it again and was not surprised to see the vessel had gained a small amount of weight. This proved that porcelain does, indeed, absorb food.

In addition, the Pri Chadash (OC 451:26) notes another issue – forgeries. Since china was a very expensive material, the market was abound with imitation china.

Most poskim refused to permit koshering porcelain and saw it as pottery. Contemporary poskim follow that route, and especially nowadays with the constant evolving materials and techniques, most porcelain is seen as regular earthenware, which is impossible to kosher.

Some contemporary poskim, however, ruled porcelain could be koshered in extenuating circumstances, when there are other reasons for leniency. A famous example is found in Igros Moshe (YD, II: 46): a non-practicing Jew who wishes to kosher his kitchen but will be discouraged if told he’d have to get rid of his expensive china. Another example can be found in Yabia Omer (I, YD chapter 6): when he served as Chief Rabbi of Cairo, Rav Ovadya once learned that the Jewish hospital had used non-kosher meat. After convincing them to use only kosher foods, he permitted them to kosher their dishes out of concern they would otherwise not kosher their kitchens at all due to the financial expense. Therefore, based on a number of reasons, he was lenient and allowed them to kosher their porcelain.

When koshering a kitchen it is important to be in touch with a competent rov to ask about koshering dishes, especially bone china or other fine porcelain which may be extremely expensive. When presenting a question it is important to mention the type of dish, how it was used, if it was ever used in a microwave, and if it was used in the past year.

Heat-Resistant Glassware

Last week we discussed glassware. While in the past glass was not heat resistant, and was likely to break if it came in contact with hot foods, nowadays, we are all familiar with heat resistant glass many of which are microwavable or oven to tableware. Are they also considered glass, or can they be koshered?

Last week we presented the different halachic opinions of the Shulchan Aruch and the Rama – while the Shulchan Aruch maintains that glass does not absorb flavor and can be used for meat and dairy interchangeably, the Rama forbids it and rules that glass cannot even be koshered as it is produced from sand and similar to earthenware. Traditionally, Sfaradim are lenient with glass (although there are some who are not) and Ashkenazim are stringent. In specific extenuating circumstances, Ashkenazim can also be lenient, as we saw in last week’s article.

The reason the Rama forbade koshering glass was that in addition to the material’s similarity to pottery it was not heat resistant. Since it is not heat resistant, out of concern that it will not be koshered properly, we forbid koshering it. Heat resistant glassware can certainly withstand high temperatures. So perhaps it may be koshered nowadays?

Different Opinions

The first heat resistant glassware became available before World War II and poskim at that time discussed it. The Chavalim B’neimim (volume IV chapter 6:2) was of the opinion that the Rama’s main reason for forbidding koshering glassware was because while absorbing taste and requiring immersion in boiling water, since it was not heat resistant (at the time), immersion in boiling water was impossible out of concern they won’t be koshered properly. Therefore, he maintained that koshering heat resistant glass is possible.

Maharam Brisk (Yerushas Hapleita 20) maintains that really even Ashkenazim should have ruled according to the Shulchan Aruch that these dishes do no require koshering at all. However, since many mishaps can occur when transferring dishes from weekday to Pesach without hag’ala (especially at night before electricity was invented), Ashkenazim took upon themselves an extra stringency not to permit haga’ala for glass dishes. Therefore, he ruled that L’chatchila glass should not be koshered, especially where new dishes are readily available. Only in extenuating circumstances did he permit koshering.

The Maharam Schick (YD 141) understood that the Rama’s prohibition against koshering glass stems from seeing glass as a form of earthenware. The reason Ashkenazim are lenient in extenuating circumstances is because glass is not used for direct cooking, and therefore the flavor it absorbs is of a lower grade. If one follows this opinion, koshering heat-resistant glassware is forbidden even b’dieved because heat resistant glassware is used with kli rishon.

Ashkenazi Contemporary Poskim

Rav Elyashiv (Kovetz Teshuvos III, chapter 81:2) ruled that heat resistant glassware can be koshered in boiling water because the Chavalim Baneimim’s position is the authoritative ruling – the reason the Rama forbids koshering is because the dishes are not heat resistant. Now that they are, there is no concern that koshering won’t be done properly, and koshering is permitted. He adds that these dishes are different from the glass that the Rama forbade to kosher.

On the other hand, Rav Moshe Feinstein writes that heat-resistant glass such as Corelle, Corning Ware or Pyrex should not be koshered for Pesach use. Where there is desperate need to kosher them, or a large loss is concerned a rabbi should be consulted.

The Sridei Esh (volume I, chapter 45) writes that while they should not be koshered for Pesach, during the year they can be koshered.

Rav Wosner (Shevet Halevi, volume II, chapter 43) follows the Maharam Schick (cited above) and rules that heat resistant glass is impossible to kosher because it is used with direct heat. However, the Minchas Yitzchok (volume I, chapter 86) follows the Marharam Brisk: while the custom is not to kosher them except under extenuating circumstances, in such situations, even Pyrex oven trays or pans can be koshered with boiling water and do not require libun.

Sfaradi Contemporary Poskim

Rav Ovadya Yosef (Yabia Omer, IV OC:41) is of the opinion that heat resistant glass is no different from regular glass and Sfaradi followers can wash them well and use them on Pesach, or transfer from meat to dairy. He does however add that one who is stringent will be blessed.

Rabbi Ben Tzion Abba Shaul (Ohr Letzion III, chapter 10:12) maintains that while the basic halachah permits Sefardim to use heat resistant glass on Pesach without koshering, it is preferable to refrain from using them with kli rishon. In addition, many refrain from using chometz glass on Pesach altogether.

Rav Elyashiv (Sidur Pesach Kehilchaso, chapter 9, footnote 28) writes that since these utensils are used with boiling hot foods even Sfaradi followers should kosher them with boiling hot water.

Obviously, koshering these dishes requires the dish to be impeccably clean. Burnt in food or grease renders the dish impossible to kosher.


In the past, horn was used for spoon and pot handles, as well as baby bottles. The shape was achieved through boiling the horn and then molding it according to the desired shape. Although it is heat resistant, dipping horn in boiling water will cause it to lose its shape. How can horn be koshered?

Most Rishonim (Hamanhig, Hilchos Pesach chapter 30; Kol Bo chapter 48:9; Tur OC 451) maintain that koshering horn utensils is impossible for the same reason it is impossible to kosher pottery – since one will not kosher it properly out of concern for the dish. However, the Rash (quoted in the Kol Bo 48:9) maintains that one can take a different perspectve: since people are afraid the dish will become disfigured from the heat, they don’t use it with kli rishon foods, and therefore it is enough to pour boiling water over it for koshering. Another opinion is that of the Tmim Deim (35): one who doesn’t care about the shape of the dish and only wants it koshered is permitted to kosher it through immersion in boiling water.

The Shulchan Aruch rules (OC 451:7) that koshering horn is impossible. This is the final ruling of all poskim. The Pri Megadim writes (footnote 13) that even one who doesn’t care about the utensil and only wants it koshered cannot kosher it because most people do care about their dishes (why else would they kosher them?).

Post Facto Koshering

The Pri Megadim (451:13) writes that purposely koshering utensils that cannot be koshered does not help, and the dish remains useless. And if used, the food is unkosher. If it was koshered by mistake without knowing it was impossible, one can rely upon the Tmim De’im’s opinion cited above.

The Aruch Hashulchan (OC 451:20) writes that the above is merely stringencies and post hag’ala, the dish is koshered. He maintains that even purposely koshering unkosherable utensils is effective.

Other Substances

By inference we learn an important lesson – any utensil that will be ruined by boiling water cannot be koshered with it. This refers to painted utensils or other substances.

Contemporary poskim rule that when presented with a new substance not mentioned in halacha, if it cannot withstand boiling water it can be koshered only in specific situations of sha’as hadchak – extenuating circumstances, and only if used exclusively for non-kli rishon foods.


The Mishna Brura (451:55) writes that koshering is impossible even if only the handles are made of horn (or other non-kosherable materials) because halacha mandates that handles be koshered together with the vessel. While utensils that cannot tolerate boiling water may not require such koshering (according to some opinions), if only the handles are made of non-kosherable material while the rest is a pot that is used directly on the fire, since the handle absorbs taste via the rest of the pot, pouring boiling water over the handles for koshering does not suffice, and the entire pot is non-kosherable.


The Mishna Brura writes (451:51) that any dish made of shell can be koshered. Shell is unlike horn because it is hard and won’t ruin in hot water. Every type of substance requires specific halachic ruling and inferring about one substance from another is impossible.


In light of the previous ruling, contemporary poskim discuss plastic: does hag’ala kosher plastic or does it have the characteristic of earthenware, and is therefore impossible to kosher?

Rabbi Moshe Feinstein (Igros Moshe OC, II: 92) writes that new substances that were not mentioned in the poskim should not be koshered. However, rubber, made of tree sap, although it may be mixed with another substance, can be koshered, since it is a natural substance. Plastic, however, cannot. The same is true for Teflon which is a plastic coating for a metal cooking utensil, which Rav Moshe permits to kosher only after 24 hours have passed from the last use (presumably only because the body of the utensil, the metal, is kosherable).

Rav Nissim Karelitz (Chut Shani, chapter 10:10) writes that plastic cannot be koshered. Rav Elyashiv (Sidur Pesach Kehilchaso chapter 9:49) and Rav Wosner (Mibeis Halevi part III, p. 20) write that plastic should be seen like pottery. In specific cases, such as if the vessel was only used as a kli sheini, or if they are pot handles, one may be lenient. The Shvus Yitzchok adds that Rav Elyashiv was stringent when koshering plastic for Pesach or when the plastic had absorbed taste of forbidden foods, but not when koshering meat or dairy.

The Ohr Letzion (volume III, chapter 10:13), however, maintains that every substance in the world that is not earthenware can be koshered by means of immersion in boiling water. Only plastic, since it may melt in high temperatures, cannot be koshered since we are concerned koshering will not be done properly for fear of ruining the vessel.

The Tzitz Eliezer (volume IV, chapter 6) and Minchas Yitzchok (volume III, chapter 63) seem to rule more stringently with plastic and the Tzitz Eliezer writes that it clearly absorbs more taste than other substances – removal of lemon juice from a plastic cup takes much longer than from any other substance. However, in extenuating circumstances they, too, are lenient and write that only with horn there was a special gzeira, but not for any other material.



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