As we have learned, the objective of libun is to incinerate all taste absorbed into a utensil. To that end, the utensil must be heated to a very high temperature (more than 300ºC). There are two ways to tell that a vessel has reached such a temperature. One is that the utensil undergoing libun becomes so hot that sparks fly from it when it is brought into contact with iron. The other indication is that its outer layer peels off, or that it reddens completely.
However, some utensils are likely to be damaged by this process. It is forbidden to kosher such utensils through libun because we are concerned that, in an attempt to protect his utensil from damage, the owner will not perform libun properly. For example, it is forbidden to kosher an earthenware vessel that has absorbed non-kosher food or ĥametz, because libun is liable to crack the vessel. Nor is hagala effective on earthenware vessels because their unique composition causes them to absorb taste but not sufficiently release it. The only way to kosher an earthenware vessel is to return it to the kiln, where it is impossible to protect it from the full force of the kiln’s furnace. As a result, the vessel will either break and be lost or survive and be kosher (SA 451:1; MB ad loc. 13, 14).
Wonder Pots (an Israeli invention used for baking on stovetops) absorb taste though fire and therefore require heavy libun. However, since they are made of aluminum they cannot endure the libun process, and hence there is no way to kosher them for Pesaĥ (although if it was used to bake only simple cakes, one may be lenient and kosher it by means of hagala, as explained above in section 4).
Baking trays designed for domestic ovens become severely damaged by heavy libun. They lose their pleasant appearance and become warped to the extent that most people would no longer consider them usable. Therefore, they cannot be koshered through libun. There are, however, industrial baking trays that do not become seriously damaged by fire, and it is permissible to perform libun on such trays for Pesaĥ.
It is possible that one who knows that he will not care if his tray becomes warped and unattractive would be permitted, in time of need, to perform libun on such a tray. After all, every time libun is performed there is a risk of some damage, and the Sages only forbade libun where there is concern that the utensil will be completely ruined. When one will not be distraught if his baking tray is damaged, he may kosher it via libun. In practice, though, one should ask a competent authority what to do in this case.