One of the most daunting preparations we make for Pesach is kashering, a process to prepare chametz utensils for Pesach use. As with all areas of halachah, those who are unsure of how to apply the rules of kashering to their situation should consult an Orthodox rabbi.
The Torah (Bamidbar 31:23) requires kashering utensils acquired from a non-Jew, as they are presumed to have been used in non-kosher cooking (and will have absorbed non-kosher flavor). Since chametz on Pesach is also forbidden, the Talmud applies the laws of kashering to chametz as well. There are four basic methods of kashering. The prescribed method depends on the utensil and how it was used.
LIBUN GAMUR (Burning)
Utensils used directly in the fire (e.g. BBQ grate), must be kashered by placing them into fire. This process has the effect of burning away any absorbed taste. To qualify as a complete libun, metal must be heated until it glows. A self-clean cycle of an oven (approx. 850° F) also qualifies as libun. There is no need to wait 24 hours before libun, though it is advised. There is no need to scrub the utensil before performing libun, since the fire will burn off residue, but some cleaning is advised.
Utensils that were used to cook non-kosher liquid can be kashered with hagalah (boiling in water). To prepare the utensil for hagalah, the utensil must be thoroughly cleaned. Only utensils that can be scrubbed clean should be kashered. Items that have narrow cracks, crevices, deep scratches or other areas that cannot be cleaned, cannot be kashered for Pesach. The following, for example, cannot be kashered for Pesach: pots with rolled lips, bottles with narrow necks, filters, colanders, knives (or other utensils) where food can get trapped between the blade and handle. After cleaning, the utensils should then be left idle for 24 hours. To kasher, every part of the utensil must make contact with boiling water. This process can be done in parts. For example, a large spoon can be immersed into a pot of boiling water for 10 seconds, turned over and then the remainder immersed. When the utensil is removed from the boiling water, it should be rinsed off in cold water. While strictly speaking these utensils may be kashered in a clean non-Pesach pot that was not used for 24 hours, the minhag, however, is to kasher the pot first, by boiling water in the pot and discarding.
IRUY KLI RISHON (Poured Boiling Water)
If the utensil only came in contact with hot liquid being poured on it (iruy), it can be kashered in the same manner. If the utensil came in contact with hot chametz solids, then one should kasher by pouring boiling water accompanied by an even melubenet, a heated stone. For example, if hot pasta fell into a sink, stones should be heated on the stove, and moved around the surface of the sink while boiling water is poured over them. In this way, the water will remain boiling on the surface of the sink. The stones may need to be reheated several times, since they cool down quickly. In all other aspects the process is identical to hagalah.
LIBUN KAL (Light Burning)
In certain cases, libun kal is sufficient. This can be accomplished by heating in an oven at 550° F for one hour. This method of kashering can be used in place of hagalah. It is also used when the need for libun is only an added stringency.
NOT EVERY MATERIAL CAN BE KASHERED.
Ceramic, such as china, and enamel coated pots cannot be kashered. It is the custom of Ashkenazim not to kasher glass as well. Some poskim do not permit kashering plastic or other synthetic materials for Pesach; however, the opinion of the OU rabbanim is that it may be kashered, if there is a need. Ask your rabbi for guidance. Composite stone (e.g. quartz counters) which is made mostly of stone, but is held together with resin, can be kashered. As a rule, materials such as metal, wood, stone, natural rubber, and fabric can be kashered.
The Kashering Checklist
SOME HELPFUL TIPS:
It is recommended that one not wait until erev yom tov to run the self-cleaning cycle to kasher an oven, as this is known to be hard on the oven and repairs may be required. Some newer self-cleaning ovens employ Aqualift technology that cleans at low heat; they should be considered like non-self-cleaning ovens (see The Modern Kitchen). Please note that kashering may discolor oven racks and stovetop burners. If racks have rubber wheels, the wheels may melt. Replacement racks for Pesach should be ordered well in advance of the holiday.