During Pesah, we do not use the same cooking utensils or tableware that we use year-round for Hamets. It is customary, and most recommendable, to have a separate set of dishes, cookware and tableware for Pesah. If this is not possible, one can still use the year round utensils after a process known as hag'ala, a virtual sterilization that removes Hamets residues.
Before we do the hag'ala we need to make sure that we have removed all visible residues of Hamets. The hag'ala then, will expel the Hamets absorbed in the walls of the utensil, by exposing it to heat. The removal of food residues will happen upon the same conditions in which the absorption occurred in the first place (kebol'o kakh polto).
Some examples of hag'ala:
Metal: Metal tableware like forks, spoons or knives, have to be thoroughly cleaned and then immersed in a pot of boiling water. Afterwards, they need to be washed with cold water and then they can be used for Pesah. Hot water opens the pores of metal, allowing the removal of any absorbed Hamets. Cold water closes the pores back.
Glass: According to the Sephardic Minhag, glass utensils like cups or plates, used throughout the year for Hamets should be thoroughly washed and then, they can be used for Pesah without any further hag'ala. A glass utensil does not absorb anything thru its pores and therefore there is nothing to remove from its walls. This is applicable even when the glass utensil was used at high temperatures to serve hot Hamets food or even to cook Hamets food. It should be noticed that this Halakha is different for the Ashkenazi custom.
Porcelain (kele heres): Ceramic, pottery or porcelain china and tableware that we use throughout the year for Hamets should not be used for Pesah. They are an exceptional case, because Hag'ala cannot sterilize them completely. Why? Because the way these utensils absorb food in their walls and expel food from their walls is erratic. Therefore, year round porcelain or ceramic utensils should not be used for Pesah.
Cold: Since absorption takes place only at high temperatures, if a utensil was used to serve only cold food, it could be used during Pesah after thoroughly cleaning it, without any hag'ala.
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