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Principles of Kashering

The purpose of this article is to provide a general outline of the laws of kashering and should not be used as an answer to practical questions that should be referred to your halachic authority.

Taste is everything—ta’am k’ikar. The taste of forbidden food is treated in Halacha as the forbidden food itself and is equally forbidden. If the taste of chametz has been absorbed into a cooking vessel, such a vessel may not be used on Pesach unless it undergoes a process known as hagalat kelim, popularly referred to as kashering. Kashering is the halachically prescribed way of expelling the flavor of forbidden food, such as non-kosher foods, meat and milk mixtures or chametz on Pesach from utensils and restoring them for use. The manner in which the utensil absorbs the forbidden flavor is the manner in which it must be kashered—kebolo kach polto, or in the words of the Torah, “Whatever was used over fire must be passed through fire and purged; however, that which was not used in fire need only be immersed in water.” Accordingly, utensils that have absorbed forbidden flavors while in direct contact with fire, such as spits, barbecue grills and baking tins, must be kashered by direct contact with fire until they become red hot. This process, which is the most powerful form of kashering, is known as libun.

Utensils that have absorbed forbidden flavors through water in which forbidden foods were cooked must be kashered by immersing them in a larger vessel, such as a cauldron containing boiling water. This process of kashering is known as hagalah. A vessel into which forbidden food was poured from a pot on the fire but was never directly on the flame itself, such as a cereal bowl, or a spoon used to stir the cereal in such a bowl, is known as a kli sheni. A kli sheni can be kashered by pouring boiling water over it from a vessel that was directly on the fire, the kli rishon, without actually immersing it in the boiling cauldron. This process is known as irui.

Vessels that were used for cold foods or liquids are kashered by simply cleaning them out and washing them, a process known as “shetifah.”

The kashering method can always be stepped up but can never be stepped down. Accordingly, as an alternative method to hagalah, one could kasher silverware or pots and pans by heating them with a blowtorch to a temperature hot enough to ignite straw, if it were placed on the heated surface. This process is known as libun kal. Similarly, as an alternative to irui, one could kasher a kli sheni by the process of hagalah. One may not, however, substitute libun with hagalah. Accordingly, one cannot kasher a spit or barbecue grill, or anything else that absorbs the forbidden flavor directly through fire, by the process of hagalah. Neither is it effective to substitute hagalah with irui. Therefore the process of irui cannot be used to kasher a vessel that absorbed the forbidden flavor through water, in which the forbidden food was cooked. In practice, hagalah is also used for a kli sheni, unless the utensil would be destroyed or damaged by the heat.

Utensils made of metal, stone, wood, marble, gold, silver, copper, lead, aluminum, steel, stainless steel and leather can all be kashered in the manner in which they absorbed the forbidden flavor. Earthenware, china, porcelain or enamel utensils cannot be kashered at all. Glass is controversial. According to one opinion, glass does not have to be kashered at all because its surface is so smooth that it cannot absorb any flavor. Another opinion maintains exactly the opposite. Glass cannot be kashered or used at all on Pesach because it is made of sand and is halachically considered earthenware. There is also a third opinion that maintains that glass has the halachic status of metal and can be kashered. According to this third opinion, how does one kasher glass without breaking it? If the glass never contained hot liquids, it can be kashered by simply cleaning and rinsing it, shetifah. If it was only occasionally used for hot liquids, there is a halachic debate as to whether it can be kashered. According to the Shulchan Aruch, if it was mostly used for cold liquids, it can be kashered with shetifah. According to the Rema, however, if it were occasionally used for hot liquids it must undergo hagalah and if hagalah would destroy it, it cannot be kashered at all. Accordingly, many Ashkenazi families buy new glassware for Pesach. Others have the custom to kasher glass by the process known as milui veirui. This process requires the glassware to be entirely immersed in cold water for 72 hours, provided that the water is changed every 24 hours.

Plastic and nylon materials are the subjects of halachic debate. According to Rav Moshe Feinstein, synthetic materials made of chemical mixtures, such as plastic and nylon, cannot be kashered for Pesach. According to other opinions, plastic or nylon vessels may be kashered. This can be done either by the process of hagalah, or, if this will damage the vessel, it can be done by the dual process of immersing it in a cauldron of hot water that has been removed from the fire followed by the procedure of milui veirui. According to Rav Moshe Feinstein, Pyrex, Duralex and Corningware cannot be kashered. Others permit the kashering of Pyrex and other heat-resistant glass utensils by the process of hagalah, if used for hot food, and by the process of milui veirui, if used for cold food. According to Rav Moshe Feinstein, dishwashers that are lined with plastic walls cannot be kashered. According to other opinions, a plastic-lined dishwasher can be kashered in the following way: First it must be thoroughly scrubbed. Then it should not be used for 24 hours. Then, the dishwasher should be turned on to allow boiling water to spray inside. Dishwashers lined with porcelain or enamel cannot be kashered. Gas or electric ovens can be kashered by libun, and according to some poskim, even by libun kal. According to Rav Moshe Feinstein, such ovens can be kashered by running them through the self-cleaning cycle. Microwave ovens can be kashered. They should be thoroughly cleaned and not used for 24 hours. Then a bowl of water should be placed inside. The microwave oven should then be turned on until the inside is filled with steam.


Raphael Grunfeld, a partner at the Wall Street law firm of Carter Ledyard & Milburn LLP, received smichah in Yoreh Yoreh from Mesivtha Tifereth Jerusalem of America and in Yadin Yadin from Rav Dovid Feinstein, zt’’l. This article is an extract from Raphael’s book, “Ner Eyal: A Guide to Seder Nashim, Nezikin, Kodashim, Taharot and Zerayim,” available for purchase at www.amazon.com/dp/057816731X  and “Ner Eyal: A Guide to the Laws of Shabbat and Festivals in Seder Moed” available for purchase at https://www.amazon.com/Eyal-Guide-Shabbat-Festivals-Seder/dp/0615118992. Questions for the author can be sent to [email protected].

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