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.
