Koshering is the process by which one makes non-kosher vessels and utensils kosher. Regarding Passover, koshering refers to the process of making chametz vessels and utensils kosher for Passover.
Year-round cooking utensils and vessels cannot be used on Passover unless they have been properly koshered, since they have absorbed chametz by contact or use. The same general procedures by which we kosher vessels that have been used for non-kosher food apply to koshering chametz vessels for use on Passover.
The following is a general description of the koshering process. The details are intricate, however, so it's best to consult with your Rabbi before you begin.
Methods of Koshering Utensils
It is important to note that there are certain kinds of utensils that cannot be koshered at all. This includes materials such as china, pottery, or enamel cookware. In addition, items that cannot be fully cleaned, such as sieves, graters, food grinders, pocketknives, etc., also cannot be koshered; neither can items that are likely to be damaged by the heat of the koshering process, such as knives with glued-on handles.
The method of koshering is based on the principle that the way the utensil absorbed the chametz is the way the utensil expels the chametz, and thus is the manner in which it is koshered. For example, a pot in which chametz (e.g., soup, pasta, etc.) had been boiled, or a spoon used to stir that pot, is koshered by immersing the pot or spoon in boiling water, thus removing the chametz with the same force by which it was absorbed.
However, if the chametz was absorbed in the utensil directly through the fire (without the medium of water) -- e.g., a baking pan or grill -- it is removed through applying direct fire to the utensil, as with a blowtorch or the like. The fire must be applied until the utensil is red hot.
To kosher a standard oven for Passover, it must not be used for 24 hours, and then it must be cleaned thoroughly and heated to the highest temperature for several hours. The racks, and the lips or grooves on which they rest, must be heated until glowing red. Some people use a special metal insert in the oven after koshering it, as an extra precaution. The method of koshering varies from oven to oven; consult a Rabbi for the particulars of your oven.
Procedure for Koshering by Immersion in Boiling Water
Utensils must be thoroughly cleaned and then not used for twenty-four hours before koshering.
One can use a chametz pot for koshering utensils, as long as it is not enamel-coated or earthenware. It must be perfectly clean, and must not have come in contact with chametz in the past 24-hour period. The pot should be filled with water, the water brought to a boil, and a hot stone or hot piece of metal dropped in, causing the water to overflow. The water should then be poured out and the pot rinsed in cold water.
The pot should be filled again and the water brought to a boil. Utensils can now be immersed in the boiling water. Items should only be put into the pot when the water is boiling, and taken out with special gloves or metal strainer when it is boiling. If the water ceases to boil, wait until it begins again. The pot should be big enough to allow all surfaces of the utensil to be touched by the water; each object should be dropped into the pot in such a way that it will not touch the sides of the pot. If there are grooves in the knives or pots, the chametz must first be burned out by heating it over a flame until the heat penetrates to the other side, and a piece of paper placed on that side will burn on contact. The utensil can then be koshered by immersion in boiling water. After immersion, it should be rinsed in cold water.
When koshering utensils in a pot, do not put in too many utensils, to ensure that there is enough room for the bubbling water to cover each utensil.
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