Materials Which May and May Not Be Kashered

Introduction

As we mentioned in the previous shiur, the Torah relates to the process of kashering utensils in two places. Regarding vessels used for the preparation of the korbanchatat, the Torah (Vayikra 6:21) teaches that a copper vessel should be purged of the korban's taste, while an earthen vessel (keli cheres) must be destroyed. Similarly, regarding the vessels brought back by the soldiers after a war with Midian, the Torah (Bamidbar 31:21-23) instructs the soldiers on how to cleanse vessels made from "gold, silver, copper, tin, and lead."

The Torah only relates to the above two types of vessels: Those made from metals, which may be kashered, and those made from clay, which may not (and which therefore may not be used if they were previously used for non-kosher food). What is the halakha regarding materials not mentioned in the Torah? Are there materials that can be used without being kashered at all?

This shiur will discuss the halakhic status of different materials and whether or not they may be kashered.

Metal, Stone, and Wood

As mentioned above, the Torah explicitly says that metals (gold, silver, copper, tin, and lead) may be kashered. Although wood is not mentioned by the Torah, the Talmud states:

Rav Huna son of Rabbi Yehoshua said: The wooden ladle of a pot (etz ha-parur) must be purged with boiling water and in a keli rishon (the vessel in which the water was boiled)."  (Pesachim 30b)

The Gemara explains that wood is kashered through hagala (immersion in boiling water) in accordance with the principle ke-bol'o kakh polto – the manner in which the vessel absorbed a prohibited taste determines the manner in which it may be kashered. The Gemara seems to have assumed that it is obvious that wood may be kashered, either because the Torah did not exclude it as it excluded earthenware, or because it was simply clear to the Rabbis that wood both absorbs and expels taste.

Although the Shulchan Arukh (OC 451:8) rules accordingly that wood, like metal, may be kashered through hagala, the Acharonim note that it may sometimes be particularly difficult to kasher wood.[1]  

Regarding vessels made from stone, R. Hai Gaon (Otzar Ha-Geonim, Pesachim, pg. 28) writes that it is similar to earthenware, which the Talmud (Pesachim 30b) says absorbs but do not fully emit prohibited taste, and therefore may not be kashered. Ri Malki Tzedek disagrees, ruling that stone does not absorb and therefore does not need to be kashered.[2] Most Rishonim[3] rule that stone utensils may be kashered, similar to vessels made from metal.

The Shulchan Arukh (OC 451:1) rules that stone may be kashered, and therefore one may perform iruy (pour hot water from a keli rishon) over countertops made from stone, such as granite. We will discuss the manner in which this is done in the next shiur.

Glass

The Rishonim disagree as to whether glass may be kashered.

Avot De-Rabbi Natan (41:6) states that "glass utensils do not absorb and do not emit." Based on this passage, some Rishonim[4] maintain that there is no need for glass to be kashered. Others view glass as a form of earthenware, explaining that "it is created from sand," and therefore rule that it may not be kashered.[5] A third group of Rishonim[6] adopt a middle position: they view glass as similar to metal, and thus it may theoretically be kashered through hagala; however, they also say that glass is not kashered in practice, because we fear it may break.

R. Yosef Karo (Shulchan Arukh, OC 251:26 and YD 135:8) rules that glass may be used on Pesach without hagala; one must simply wash the vessel. This is the custom of many Sephardim.[7] The Rema, however, writes that the custom in Ashkenaz is not to kasher glass.

Why does the Rema rule this way? Some explain that the Rema, at least regarding Pesach, rules in accordance with the view that equates glass with earthenware, which cannot be kashered.[8] However, others explain that the custom not to kasher glass may simply reflect the assumption that one will fear breaking the glass and may not perform hagala properly.[9]

The Rema’s stringency led to an interesting practice among Ashkenazic Jews, referred to as miluy ve-iruy. The process itself stems from the Talmud (Avoda Zara 33a), which teaches that a barrel that stored non-kosher wine may be kashered through a procedure called miluy ve-iruy: The barrel is filled with water, sits for 24 hours, is emptied, and is then filled again and emptied every 24 hours for three days. The Rishonim debate whether miluy ve-iruy is a form of hagala, or whether the process simply dilutes and weakens the taste that was previously absorbed in the vessel (see Ran, Avoda Zara 12b). The Shulchan Arukh discusses the scope and details of this process in YD 135.

The Mishna Berura (OC 15:156) cites the Chayei Adam, who permitted relying on miluy ve-iruy to kasher drinking glasses that may have contained a chametz beverage. The Mishna Berura adds that this method of kashering should only be used when other drinking glasses are unavailable.

Returning to the Rema, the Acharonim also debate whether the custom to refrain from kashering glass is also relevant regarding other prohibited substances, as well as basar be-chalav. Some maintain that just as the Rema is stringent for Pesach, he is stringent for all other prohibited substances, and therefore glass may never be kashered.[10] This is also the view of the Mishna Berura (451:156). Others rule that glass, like metal, may be kashered through hagala.[11] Some Acharonim, including R. Moshe Feinstein (cited by Ohalei Yeshurun, pg. 87) rule that during the year, glass does not need to be kashered at all.[12] This debate may have significant practical ramifications regarding the use of glassware in coffee shops, or in apartments rented from non-Jews.

Recent scholars have discussed whether modern forms of glass, especially those that can withstand heat, share the halakhot of glass described above. Some maintain that Pyrex and Duralex must be kashered through hagala. Others (Yechaveh Da'at 1:6) disagree and view Pyrex like any other form of glassware.

Rubber and Plastic

The Acharonim discuss whether new materials, which did not exist at the time of the Torah or during the period of Chazal, may be kashered. For example, is it possible to kasher rubber?

The Peri Megadim (Mishbetzot Zahav 451, s.v. u-kelei glalim) writes that "all materials, except earthenware, may be permitted through hagala." R. Moshe Feinstein, however, appears to disagree, and writes regarding rubber:

Regarding rubber vessels, whether hagala is effective for them: Rubber, given that it is from trees, hagala is effective. And even if there is a [halakhic] uncertainty because some chemicals are [added], it is an uncertainty concerning a Rabbinical issue, which should be ruled leniently. However, there is rubber which is not from trees, but rather from chemical compounds, which is a new substance that did not exist before the earlier Rabbis. One should not allow hagala [for utensils made from this substance]. (Iggerot Moshe OC 2:92)

R. Moshe Feinstein maintains that synthetic rubber should not be kashered. This rationale would apply to plastic and nylon as well.[13]

Although this is the common custom in American communities, the majority of halakhic authorities, including the Chelkat Yaakov (2:163), Seridei Esh (2:160), and Tzitz Eliezer (4:6), allow performing hagala to plastic, especially to hard, smooth plastic. In Israel, it is common practice to kasher plastic racks, pot handles, etc.

Stone Aggregate Countertops

As mentioned above, it is permitted to kasher stone. Therefore, countertops made from granite or other solid stones may certainly be kashered through iruy (pouring hot water over the counter), as we shall discuss in a future shiur. Recent poskim discuss whether one may kasher stone aggregates, such as "Caesarstone," made from small pieces of stone mixed and bound together with glue and other chemicals.

The Beit Yitzchak (YD 1: 146: 8) questions whether ground stone is viewed as stone, or whether it is more like dirt and therefore cannot be kashered through hagala. A number of Acharonim (Machaneh Chaim YD 2:27, Tevu’ot Shemesh 53) maintain that it is treated like solid stone. Other authorities maintain that since these materials contain a mixture of substances, they cannot be kashered.[14] Most modern kashrut agencies permit kashering these aggregate countertops.

Porcelain

As mentioned above, earthenware vessels made from clay, known as kelei cheres, cannot be kashered. The Acharonim discuss whether china and porcelain are most similar to kelei cheres, and cannot be kashered, or whether they are similar to glass, and can be kashered through hagala, or whether they do not need to be kashered at all!

R. David ben Zimra (1479–1573), known as the Radbaz, discuss porcelain in great depth.[15] The Radbaz records that in his time, many people used porcelain for meat and cheese and chametz and Pesach, and they even used them after they were purchased directly from non-Jews. The rationale was that these dishes do not absorb at all, and therefore even hagala would be unnecessary. The Radbaz relates how he performed two scientific experiments in order to determine whether porcelain absorbs. First, he put it in the fire and waited for it to spark, which he viewed as a clear sign that, after applying heat, the walls emit "taste." Second, he weighed the porcelain, and then immersed it in hot water, in order to see whether its weight increased. He concluded that porcelain indeed absorbs, and therefore, like earthenware, it may not be kashered.

Interesting, the Kenesset Ha-Gedola (YD 121) relates that while he personally accepted the Radbaz's conclusion, he was only strict regarding chametz, and he acknowledged that the Jewish community adopted a more lenient approach.

The She'elat Ya’avetz (1:67) also discusses the status of porcelain, and compares it to glassware. He concludes that since its outer coating has the status of glass, it must be immersed in a mikveh before use (tevilat kelim) but does not need to be kashered, in accordance with the lenient opinion regarding glassware.

Although modern poskim are still divided regarding whether porcelain requires tevila, as we will discuss in a future shiur, it is customary to treat china and porcelain like earthenware and not to kasher it through hagala.[16] That said, numerous poskim leave room for leniency in cases of financial loss.[17]

In the next shiur, we will discuss the manner in which hagala and libun are performed. 

[1] The Shulchan Arukh (ibid. 3) rules that if a vessel has holes that cannot be cleaned well, hagala will not work. For that reason, the Taz (ibid. 24) cites the Bach, who rules that one should not kasher wooden vessels, as they often have holes and cracks. See also Mishna Berura ibid. 56.

[2] See Tosefta (Zevachim 10:12).

[3] For example, see Rif (Pesachim 8b) and Rambam (Hilkhot Chametz u-Matza 5:23).

[4] Rabbeinu Tam, (Tosafot, Avoda Zara 33b, s.v. kunya), Rashba (Teshuvot, 1:233).

[5] R. Yechiel of Paris, cited by the Mordechai (Pesachim 3:574); Terumat Ha-Deshen 132.

[6] See Ohr Zaru'a (v. 2, 256) and Ra'ah (cited by Ritva, Pesachim 30b, s.v. li'avid).

[7] See Yechaveh Da'at 1:12. See Ben Ish Chai, Shana Rishona, Tzav 14, who writes that Sephardim should follow the more stringent view.

[8] Bi'ur Ha-Gra (451:26), Arukh Ha-shulchan (451:50), and Mishna Berura (451:154).

[9] Yabi'a Omer OC 4:41:10 writes that the Rema's stringency is based on the assumption that glass cannot be subjected to hagala because it may break. Therefore, he writes that one may kasher Duralex and Pyrex on Pesach, even according to Ashkenazi practice, because they are stronger forms of glass. See also Tzitz Eliezer 9:26, who cites R. Tzvi Pesach Frank as ruling that Pyrex may be kashered for Pesach by doing hagala three times.

[10] Magen Avraham 451:49; Taz OC 87:2. See also Shevet Ha-Levi YD 1:43.

[11] Tzitz Eliezer (9:26), Seridei Esh (2:36), Minchat Yitzchak (1:86), Yechaveh Da'at 1:16.

[12] See also Kenesset Ha-Gedola, YD 121.

[13] R. Shimon Eider (Halachos of Pesach, pg. 138, note 10, writes that this and other materials may be kashered from other prohibited substances, aside from chametz.

[14] See Minchat Yitzchak 4:114:4 and Maharsham 1:53.

[15] Radbaz 3:401.

[16] See Mishna Berura 151:163.

[17] See Yabi'a Omer YD 1:6:22.