Some of the Basics of Kashering

Question #1: Sandwich Maker

“Can I kasher my sandwich maker for Pesach in order to toast vegetables with it?”

Question #2: Better than Boil?

“Is there a way to kasher things that is safer than placing them in boiling water in an open pot?”


Halachah assumes that when cooking food, taste residue remains in the utensil that was used. When this flavor residue, which is called ta’am, comes from something prohibited, it must be removed to allow the utensil to be used again to prepare food. When the flavor is from meat, one must extract it before using the utensil for a dairy product,* if the flavor is from chometz, the utensil must be kashered before it can be used for Pesachdik products.

Although modern appliances are not mentioned in the Torah, the basic rules for kashering all appliances lie within a careful study of the passages of the Torah, the Gemara and the early authorities on this topic. The Chumash, itself, alludes to the halachic process used to kosher a utensil when it commands, kol davar asher yavo vo’eish ta’aviru vo’eish, “Any item that entered fire, shall be passed through fire” (Bamidbar 31:23), thereby implying that kashering an appliance that became non-kosher through direct contact with a flame requires burning the appliance in a flame — no other cleaning process will sufficiently kosher this appliance.

Shabbos Hagadol

One of our responsibilities prior to Pesach is to ascertain that we know how to kasher our homes correctly. The piyutim that were traditionally added to the prayers on Shabbos Hagadol include very detailed instructions on proper kashering techniques, and we find that the baalei Tosafos discuss and correct the texts of the piyutim to accommodate the correct procedures. This week’s article will provide some introductory information to this topic, as we explore how the Gemara explains correct kashering procedures.

Let us begin by examining a passage of the Gemara that discusses kashering one’s house for Pesach. The Gemara (Pesachim 30b) quotes a beraisa (halachic source dating from the era of the Mishnah) that if beef fat was smeared onto the walls of an oven, kashering the oven to be pareve again requires firing up the oven, which means building a fire inside the oven. This heating of the oven burns out the residue of the meat fat that is absorbed into the oven walls. The Gemara then recounts that Ravina noted to Rav Ashi that the earlier amora, Rav, had declared that there is no way to kasher chometz-dik pots for Pesach-dik use. Ravina asked Rav Ashi why this was so: Why not simply fire up the pots to make them Pesachdik, just as one kashers an oven? Rav Ashi provided two answers to the question:

Metal vs. earthenware

(1) The beraisa that permits kashering an oven is referring to one made of metal, whereas Rav was discussing pots made of earthenware. Earthenware pots cannot be kashered, because once food flavor is absorbed into them, normal procedures will not physically remove the ta’am from the vessel. To quote the Gemara (Pesachim 30b, Avodah Zarah 34a), “The Torah testified that one will never be able to extract the flavor from the walls of an earthenware vessel.”

Ovens vs. pots

(2) Rav Ashi’s second answer is that an earthenware oven can be kashered by building a fire inside it, but not an earthenware pot. In those days, cooking was done by building a fire inside the oven and placing the pot inside or on top of the oven. This fire does not provide enough heat in the pot to remove the flavor (ta’am) that is absorbed inside it. Furthermore, building a fire inside the pot is also not a satisfactory method of kashering it. Chazal did not permit this method of kashering, because it may not be performed properly — the owner may be afraid that the pot might crack if it is heated long enough to kasher it (Rosh and Rabbeinu Chananel ad locum; cf. Rashi, who explains the Gemara somewhat differently.) This concern does not exist regarding an oven, presumably because this is the usual way of heating it.

Some basic rules

From this short passage of Gemara, we can derive some basic rules of kashering:

  1. When a concern exists that a particular method of kashering may break an appliance, Chazal prohibited using that method. There are many, many instances where this halachah is put into practice.

One example of this is our opening question. “Can I kasher my sandwich maker for Pesach in order to toast vegetables with it?”

Any method that might kasher the sandwich maker would very possibly ruin the machine. Therefore, it is not possible to kasher it for Pesach use.

  1. Earthenware has different properties from those of metal items, resulting in differences in halachah. Regarding metal and other types of items, there is a principle of kebol’o kach polto, that one extracts from a utensil prohibited flavor the same way the flavor was absorbed into the appliance. From the passage of Gemara quoted above, we see that there are exceptional cases when this principle does not apply. Materials such as earthenware can absorb substances that will not be removable afterwards. Rather than becoming completely extracted when one kashers them, some of the absorbed taste remains and gradually leaches out afterwards with each use, thus spreading prohibited flavor into all subsequent cooking (Tosafos, Chullin 8a s.v. Shelivna).

Exception – kiln kashering

Although the above-quoted passage of Gemara implies that earthenware pots cannot be kashered, Tosafos notes that this rule is not absolute — there is an acceptable way to kosher them. The Gemara (Zevachim 96a) implies that all earthenware vessels, even pots, can be kashered by firing them inside a kiln used for manufacturing earthenware (Tosafos, Pesachim 30b s.v. Hatorah). The intensity of heat in a kiln, which is far greater than the temperature used when baking or cooking in an earthenware oven, will remove the non-kosher or chometz-dik absorption from the walls. Furthermore, we are not afraid that someone will not kasher the utensil adequately out of concern that it will crack, because heating in a kiln is consistent on all sides and will not cause the utensils to crack (Rosh). It is uneven heating that damages the vessel.

There is an alternative explanation for why there is no concern that the owner will not kasher his pot adequately inside the kiln for fear that it will crack. In this instance, we feel that the owner will allow the pot to remain inside long enough to kasher properly because once the owner has placed the pot inside a kiln, this demonstrates that he has no concern about the pot breaking. This halachic conclusion is followed by the Shulchan Aruch (Orach Chayim 451:1).

Purchase from gentile

We will now examine a different passage of Gemara to learn more about the rules of the kashering procedure.

The Mishnah (Avodah Zarah 75b) teaches that upon purchasing used kitchen equipment from a gentile, one kashers the equipment via one of the following procedures:

1) That which is usually used for cooking in liquid medium must be kashered in hot water, which is called hag’alah.

2) That which is used to broil or roast food directly in fire must be kashered directly in fire, called libun. As examples of the latter rule, the Mishnah chooses a barbecue spit and a grate used for roasting. Since these appliances absorbed non-kosher ta’am directly through fire, they must be kashered by burning them in fire.

Kebol’o kach polto

From this Mishnah, we learn a new rule – that there is a hierarchy in kashering. If an appliance absorbed flavor directly through fire, boiling it will not remove the residues of prohibited substance sufficiently to kasher it. This explains in more detail the rule I mentioned above, called kebol’o kach polto, which teaches that extracting food residue requires the same method that caused the absorption initially, or a method that is more intense, as I will explain shortly. Therefore, if a prohibited food was cooked in a pot, it can be kashered by hag’alah, which is a method of boiling out what was absorbed. However, if a spit or rack absorbed prohibited food directly through fire and not through a liquid medium, hag’alah will not suffice to kosher it.

Libun versus hag’alah

It is axiomatic that a stronger method of kashering will work for vessels requiring a lower level of kashering (for items other than earthenware). Thus, a metal pot used to cook non-kosher can be kashered by libun, although it is not necessary to use this method.

Iruy, miluy ve’iruy

There are other methods of kashering, such as iruy, which means pouring boiling water onto an item or surface, and miluy ve’iruy¸ which means submerging an appliance in water for three 24 hours periods. In this article, we will not discuss these methods of kashering.

How long?

At this point, we are ready to go to the next step in understanding how to kasher properly. The first question we will explore is germane to kashering directly by fire, which is called libun. The question is: How long must the spit or rack be held in a fire for it to be kashered? At what point can we assume that all the prohibited absorption will be removed?

We find two statements of the Gemara answering this question, one in the Talmud Yerushalmi and the other in the Talmud Bavli. The Talmud Yerushalmi (end of Avodah Zarah) states that one must heat it until sparks begin to shoot off. The Talmud Bavli (Avodah Zarah 76a) explains that you must keep it in the fire “until you remove the surface.” In practice, the halachah is that one needs to heat it until sparks shoot off (Shulchan Aruch, Orach Chayim 451:4).

Summing up

To sum up: From these two passages of Gemara, we have learned three basic rules of kashering:

  1. Removing the residue of a prohibited substance from an appliance requires performing on it a procedure that is similar to or stronger than what caused the absorption in the first place.
  2. When a concern exists that a particular method of kashering may break an appliance, one may not kasher it that way.
  3. One cannot kasher earthenware items through conventional household methods.


However, a different Mishnah seems to dispute one of the principles that we have just explained. The Torah teaches that there is a mitzvah to eat parts of the korbanos offered in the Beis Hamikdash, but that there is a time limit within which they may be eaten. After the korban’s time limit has passed, the leftover meat is called nosar, literally, leftover, and must be burnt. Eating it after this time violates a serious prohibition of the Torah.

What happens to the equipment used to cook the korban? The leftover flavoring remaining in the equipment becomes nosar and the equipment must be kashered. This means, essentially, that equipment used to prepare kodoshim must constantly be kashered.

How does one kasher the equipment? One would think that we would apply the same rules presented by the above-mentioned Mishnah in Avodah Zarah. However, the Mishnah states that a grill used to barbecue a korban requires only hag’alah (Zevachim 97a). This suggests that there is a one-size-fits-all approach to kashering – and that hag’alah can be used to kasher anything, even that which absorbed the food directly via fire. This approach does not fit the rule of kebol’o kach polto discussed above.

As you can imagine, we are not the first ones to raise this question. The Gemara (Avodah Zarah 76a) does, and provides several answers. The conclusion of the Gemara is that when the prohibited substance was permitted at the time of absorption, a concept that the Gemara calls heteira bala, hag’alah is sufficient to kasher it. The absorption of korban meat in equipment qualifies as heteira bala because, until the time that it becomes nosar, it is permitted to eat the meat; therefore, hag’alah suffices.

The opposite of heteira bala is issura bala, which means that the food was prohibited at the time that the absorption took place. The Mishnah in Avodah Zarah discussing used equipment purchased from a gentile is teaching the laws regarding issura bala.

Heteira bala

Why does heteira bala create a basis to be more lenient?

Some explain this phenomenon as follows: When prohibited substance is absorbed through a medium, such as by cooking in water, hag’alah, boiling out the non-kosher vessel, will remove all of the prohibited substance. However, when the substance absorbed directly by fire, boiling it will not remove all of the prohibited substance. Nevertheless, it does remove most of the substance. When the vessel initially cooked non-kosher, non-kosher food absorbed into it and must be fully removed. But when the absorbed substance was kosher at the time that it absorbed, the residue left over after the pot was boiled is not enough to be considered non-kosher.

Kashering from fleishig

The Gemara mentions the concept of heteira bala relative to the absorption of permitted kodoshim, which will later become prohibited nosar. It is obvious that if one has equipment that absorbed fleishig residues and one wants to make it pareve, this is a case of heteira bala and will require only hag’alah. Here is an actual example:

In a food service operation, some pareve baking trays had mistakenly been used to bake chicken. Assuming that the chicken was placed directly onto the trays, one might think that kashering these trays would require libun, since the absorption was direct from the meat into the tray, without any liquid medium. However, because of the principle of heteira bala, only hag’alah was required.

Is chometz considered heteira bala?

Since chometz is permitted to be eaten anytime but Pesach, it would seem that chometz should be considered heteira bala. This would mean that kashering chometz equipment for Pesach use would never require more than hag’alah. However, we find that there is a dispute among halachic authorities whether chometz is considered heteira bala or issura bala. Those who follow the stringent approach rule that at the time of its use, chometz is what was absorbed into the walls of the pot, and chometz may not be used on Pesach. The concept of heteira bala is applicable, in their opinion, to kodoshim products since, at the time that the grills were used, they were not nosar. They could not become nosar afterwards since the small remnant remaining after the hag’alah will not be considered nosar.

Whether chometz is considered heteira bala or not is very germane in practical halachic terms. If it is considered heteira bala, then hag’alah will suffice to kasher all items for Pesach, and one is never required to kosher items with libun to make them Pesachdik.

How do we rule?

Both the Shulchan Aruch and the Rema (451:4) conclude that chometz is considered issura bala. Therefore, one cannot kosher a grill used for chometz through hag’alah, but it requires libun. However, in case of major financial loss (hefsed merubeh), one may rely on the opinion that chometz is heteira bala (Mishnah Berurah 451:32, citing Elya Rabbah and Gra).

Libun kal

So far we have discussed kashering through libun, by means of a high temperature of direct fire. We have also discussed hag’alah, which is kashering through boiling in water. The rishonim discuss an in-between method of kashering, which is called libun kal, easier libun. Libun kal also uses direct heat to kasher, but it does not reach as high a temperature as does the libun we have been referring to until now, which is sometimes called libun chamur, strict libun, to avoid confusion. Libun kal is defined as heating metal hot enough that one sees that the heat has permeated through the metal fully (Mordechai, Avodah Zarah, end of 860). An alternative definition is that it is hot enough to burn straw. The poskim rule that when hag’alah would be sufficient to kasher, one may use libun kal as an alternative, but that it should not be used when there is a requirement to kasher via libun chamur (Mordechai, Avodah Zarah, end of 860).

How hot is libun kal?

At what temperature does straw burn? Based on experiments that he himself conducted, Rav Yisroel Belsky concluded that this is accomplished by a combination of temperature and time. His conclusion was that an oven heated to 550° F takes an hour to burn paper, at 450° it takes 1½ hours and at 375° it takes 2 hours. Thus, kashering with libun kal would require a longer amount of time at lower temperatures. We can thus answer another of our opening questions:

“Is there a way to kasher things that is safer than placing them in boiling water in an open pot?”

The answer is that since libun kal can be used whenever hag’alah suffices, one could kasher any items that require hag’alah by libun kal in a household oven, if one keeps the item in the oven long enough.


This article has provided a small introduction to some of the ideas of kashering, particularly to the concepts of libun and hag’alah. We have not yet dealt with several other types of kashering, including iruy, kli rishon, and miluy ve’iruy, all of which we will need to leave for a future time. We should always hope and pray that the food we prepare fulfills all the halachos that the Torah commands us.

* There is discussion among the halachic authorities whether one may kasher an appliance that is fleishig to use with dairy and vice versa. We will leave the discussion of that topic for a different time.


Many other articles germane to Pesach are available on this website. You can find them using the search words matzoh; chol hamoed; chometz; ga’al yisroel; hallel; omer.