Taste is everything – ta’am ke’ikar. The taste of forbidden food is treated in halacha as the forbidden food itself and is equally forbidden. If the taste of forbidden food has been absorbed into a cooking vessel, such a vessel may not be used on Pesach unless it undergoes a process known as hechsher or hagalat keilim– 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. 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 keli sheni. A keli sheni can be kashered by pouring boiling water over it 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 keli sheni by means of hagalah. One may not, however, substitute libun with hagalah. Accordingly, one cannot kasher by the process of hagalah a spit or barbecue grill or anything else that absorbed the forbidden flavor through fire.
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 forbidden food was cooked. In practice, hagalah is also used for a keli aheni unless the utensil would be destroyed or damaged by the heat.
Utensils made of wood, stone, 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 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. 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 Remah, however, if it was 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 seventy-two hours provided that the water is changed every twenty-four hours.
With these principles in mind, what can we kasher in the kitchen? 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, by the dual process of immersing it in hot water that has been removed from the fire and then by the procedure of milui veirui. According to Rav Moshe, 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, dishwashers 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 twenty-four 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, 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 twenty-four 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.
Mixers used for mixing dough or other chametz can generally not be kashered because it is almost impossible to remove the chametz particles trapped in the machine. Blenders that can be dismantled should be thoroughly cleaned and their metal parts and bowls immersed in boiling water. Coffee percolators, to the extent that they can be thoroughly cleaned of all chametz particles, can be kashered by boiling water inside them. Rabbi Shimon Eider, however, writes that they should not be kashered because it is impossible to clean out all chametz particles.
According to most opinions, whisky glasses cannot be kashered because they retain the smell and flavor of whisky. The Aruch Hashulchan (Orach Chaim 451:49), however, rules that they can be kashered by milui and irui.
Because there are so many conflicting opinions in this area of halacha, in case of doubt a rabbi should be consulted.