Halacha divides food utensils into three categories with respect to the possibility of using them on Pesach if they had been used with Hametz. On one extreme, clear glass utensils, including drinking glasses and Pyrex glassware, may be used on Pesach despite their having been used with Hametz, and no "Koshering" procedure is required at all. (See Chazon Ovadya, page 154.) At the opposite end of the spectrum, earthenware and porcelain utensils, including chinaware and mugs, cannot be made useable for Pesach if they had been used with Hametz. One must therefore purchase new earthenware dishes and mugs for Pesach and designate them exclusively for Passover use. (See Chazon Ovadya, page 149)
In between these two extremes are metal utensils, such as pots and cutlery, which may be rendered useable for Pesach through the process of "Hag'ala," or immersion in boiling water. One places a large pot of water over the fire and brings the water to a boil until it overflows the top of the pot. Some people place a stone in the pot to ensure that the water will overflow the top. One then places the metal utensils in the boiling water, and they thereby become useable for Pesach. It is preferable to dip the utensils in a pot of cold water immediately after removing them from the boiling water.
The preferred time for performing Hag'ala is before the onset of the Hametz prohibition on Erev Pesach. If one immerses a Hametz utensil after the prohibition has taken effect, then it is possible for the Hametz particles that have been expunged from the utensil to then reenter the utensil and thus render it forbidden for use on Pesach. If, however, Hag'ala is performed before Hametz had become prohibited, then the particles expunged from the utensil cannot render the utensil forbidden once they reenter the utensil. This is due to a Halachic concept called "Notein Ta'am Bar Notein Ta'am Be'heteira," which means that a "second-degree" taste does not affect a utensil's Halachic status if that taste is currently permissible for consumption. In our case, the taste of Hametz is expunged from the utensil into the water, and then reenters the utensil; it therefore cannot affect the utensil's status, given that Hametz has yet to become forbidden.
If one did not immerse a utensil in boiling water before the time when Hametz became forbidden on Erev Pesach, he may still perform Hag'ala, provided that he remove the utensil from the boiling water immediately, so as not to allow the taste of Hametz to reenter the utensil. This is the ruling of Chacham Ovadia Yosef, in his work Chazon Ovadia (Laws of Pesach, p. 162).
If one wishes to prepare both meat and dairy utensils for Pesach use, he should immerse them separately, unless either the meat or dairy utensil had not been used within the previous twenty-four hours. If they had both been used within the previous twenty-four hours, then one must ensure to immerse them in the pot of boiling water one after the other, and not simultaneously. (ibid)
Summary: Clear glass utensils that had been used with Hametz may be used on Pesach; earthenware and porcelain utensils that had been used with Hametz may not be used on Pesach. Metal utensils that had been used with Hametz may be used on Pesach after they are immersed in a pot filled to the top with boiling water. This immersion should be done before the Hametz prohibition takes effect on Erev Pesach. Meat and dairy utensils should be immersed separately, unless either the dairy or meat utensil had not been used within the previous twenty-four hours.
