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Parshat Tzav – Why Do We Kasher Our Dishes?

Parashat Tzav (“Command”; here, a verb) gives a detailed description of the sacrificial services. Thus, the Commandments listed in it are addressed mainly to the Kohanim.

The Parashah prescribes two important duties for them: Every morning they must remove the ashes from the Altar and keep its fire from going out. In addition to the daily sacrifices offered by the entire community, three other types of sacrifices are mentioned in this Parashah. Two of them involve only the Priests: the Kohanim who are just being inducted into the service must bring a special Minchah—a flour offering of initiation, and the Kohain Gadol, the High Priest, must bring a similar Minchah offering on his own behalf every day. The third sacrifice mentioned in the Parashah applies to all Jews: anyone who has escaped from danger must bring a thanksgiving offering—a kind of Shelamim sacrifice. The Parashah discusses the laws relating to the eating of meat from the sacrifice. These laws specify the time period during which the meat may be consumed and the place where it may be eaten. They include the prohibition of eating the blood of the animal and they specify which parts of the fat are forbidden. These last prohibitions apply not only to the one bringing the offering, but to every Jew: they are part of the laws of kashrus (the laws for preserving the sanctity of food and cooking). The Parashah ends with a description of the rituals and sacrifices with which Moshe prepared Aharon and his sons for the Temple Service.

Chapter 6 of Parashas Tzav introduces the laws concerning the vessels in which food is prepared. These laws are given in connection with cooking the meat of a Chatas sacrifice: “An earthenware vessel in which it is cooked shall be broken, but if it is cooked in a copper vessel, it shall be purged and rinsed with water” (Vayikra 6:21).

But the actions that the Torah prescribes here for cleaning vessels are clearly not related to hygiene. Why would you break a clay pot? For mere hygiene, a pot can be washed! And thus, centuries later, the Shulchan Aruch used this specific verse as a textual source for many of the laws related to kashrus—the sanctity of food, specifically, what constitutes a kosher or non-kosher pot.

In relation to the laws concerning food, we have encountered up to now only a ban on eating the non-kosher animals (Parashas Noach in Bereishis) and the bans on eating blood and specified fats discussed in Parashas Vayikra, as well as in the present Parashah. We have also seen the prohibition of mixing meat and dairy products, as in Parashas Mishpatim in the Book of Shemot. Now, before delving into a detailed discussion of kosher food, beginning in the next Parashah, Shemini, the Torah for the first time introduces the subject of kosher cookware.

You are familiar, of course, with the physical phenomenon of absorption—the penetration of particles of one substance into another. No matter how carefully we wash the dishes, some food particles and molecules penetrate the surface and can’t be washed out. If we cook kosher food in a pot in which something non-kosher was cooked previously, that pot was rendered non- kosher by the previous cooking. Particles of the prohibited food will emerge from the surface of the pot and mingle with the present contents of the pot. This causes the food now on the fire to become non-kosher. Similarly, a pot previously used for cooking a milk product is considered “dairy,” and may not (unless “kashered”) be used thenceforward for cooking meat—and vice-versa.

The above verse, via the Shulchan Aruch, teaches us how to render certain kitchenware kosher in certain culinary scenarios.

True, the verse relates to the laws of the Temple Service, not to kashrus—after all, it is obvious that only kosher animals were sacrificed and that the slaughter was carried out according to all the rules. But the matter at hand here is that meat of a sacrifice may be eaten only during a prescribed period. After that time runs out, it becomes forbidden. Accordingly, the pots into which the expired meat has been absorbed are considered desecrated and may not be used until cleaned as prescribed.

From that Temple rule, the Shulchan Aruch derives several modern-day kosher-pot rules.


Only metal and wooden utensils can be kashered.

How does the Torah indicate this? As our verse says, “An earthenware vessel… shall be broken, but… a copper vessel… shall be purged and rinsed with water.” Food particles penetrate into the walls of earthenware so deeply that it is impossible to extract them. However, when it comes nowadays not to the Temple but to the household, earthenware vessels that have been used for non-kosher foods do not have to be shattered… but they cannot be used anymore for cooking and storing food. “Copper” utensils (which includes those of any metal), on the other hand—can in fact be kashered. The Chumash does not speak of wooden dishes, but the Oral Torah later tells us how to kasher wood.

How does one kasher dishes? To begin with, you need to thoroughly clean them with detergent and water. And then, we find an interesting rule in the Talmud: Particles are expelled under the same conditions in which they are absorbed. In Hebrew, this rule is expressed as follows: k’bal’o, kach polto (“as it absorbs, so it expels”).

A cleansed metal pot in which non-kosher food was boiled in liquid can be kashered (not sooner than a day after its problematic use) by using boiling water: Simply fill with water to the top and bring to a boil; when it boils and overflows, the pot is kosher. Usually, though the water partially evaporates and therefore doesn’t spill over; to ensure spill-over, a highly-heated stone or chunk of metal is dropped into the boiling water, ensuring that the boiling water reaches—and exceeds—the lip, and thus the entire pot.

If the non-kosher food was not boiled but fried or baked (for example, on a skewer or in a frying pan), then the utensil must first be washed and cleaned, and then heated to red-hot. One may then resume using it.

To kasher smaller pots and other utensils, a large kosher pot is first filled with water and brought to a boil. Any smaller non-kosher utensils are then dropped in: pots, cutlery, and even wooden dishes are kashered this way. Then they are immediately removed and rinsed in cold water—and everything is “cured.”

Glassware is considered kosher if it was not previously used for hot food or drink. [If you can’t be sure, you may use it only in a case of extreme need—for example, if there is no way to obtain a new vessel or one that you are sure about. You can also kasher glassware as follows: Fill the vessel with water and leave standing for at least 24 hours. The water is then poured out and the vessel refilled. This procedure is repeated for a total of three times. After that, the utensil is kosher.

The preceding only summarizes the general principles of kashering. There are many details and special cases that require qualified advice, for which you need to ask a rabbi.

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