One should not use vessels that had previously been used for chametz without making them kosher [=valid for Pesach use]. The way to make them kosher is in the same way that they were used for chametz. If, for example, they were used for hot liquids then the way to make them kosher is by immersing them in boiling water. The vessel should not be placed in the water unless it is boiling, but if it has ceased from actively boiling more fuel should be added to the fire in order to boil the water, and then the vessel may be placed in it. It should remain a few moments in the boiling water, enough that the water could enter into the vessel and the taste of the old food would leave (as mentioned in the Shulchan Aruch of R. Zalman of blessed memory). Still, it should stay for a short time only, and before being immersed one should scrub the pots and remove any rust. If there are any cracks from which rust cannot be removed, one needs to treat these places with libun (=extreme heat from a flame). The handles of the vessels also need to be immersed in boiling water. All this process needs to be done before the fifth hour on the eve of Pesach, in order to avoid several complications. Even if one does this before the fifth hour, one needs to be sure that the vessels have not been used in the last day in order not to get confused with the usual process of making vessels kosher. For at other times than Pesach, if one wants to make a vessel kosher it needs to have not been used for a full day. If the vessels are submerged into the water using tongs, the vessel needs to be turned [so that the water touches the place that the tongs were holding]. The lid of the pot needs to be immersed in boiling water too. In R. Haim Vital's writings, a warning is brought in the name of the Ari of blessed memory. He says that it is inadvisable to use molten lead for the libun of copper vessels without first immersing them in boiling water. For the molten lead becomes a covering over the copper and prevents the chametz from leaving the vessel if it is immersed afterwards, and the libun by molten lead isn't itself an effective way to make vessels kosher since the sages said that there need to be sparks flying, which is not the case with molten lead. It is therefore preferable to first immerse the dishes in boiling water, and then have a non-Jew plate it with molten lead. Nonetheless, one should be careful that this non-Jew does not then cool it off in water that was used for chametz vessels. Such is the advice of the Ari of blessed memory, yet here in the city of Baghdad, may God preserve it, we do not act accordingly. The prevalent custom is to first heat with molten lead and then immerse in boiling water. The rabbis do not protest, because in our city it is not the practice to use such pots for actual chametz, like a dish with flour and bread, and we only use pans for frying dough, and even in that case it is not the regular usage of the vessel. Therefore, we do not protest and require them to act according to the aforementioned practice of the Ari of blessed memory, yet one who fears heaven should certainly do so, as well as for dishes which have become unkosher through the mixtures of meat and milk or any other issue of kashrut that might require libun with molten lead - the immersion should happen first.