We have learned that if a kli absorbs a forbidden food by means of fire, it must be kashered by fire. This principle applies only when non-kosher food has been absorbed. For example, if one roasts non-kosher meat on a spit, the spit must be kashered through libun, because the non-kosher food was absorbed by means of fire. However, if at the time of absorption the meat was kosher, and only later became non-kosher, the kli may be kashered through hagala. Thus, if the meat of a korban was roasted on a spit, after a day passes, the meat of the korban becomes notar and may not be eaten, and the flavor absorbed by the spit is also notar. Therefore, the spit may not be used until it has been kashered. However, since the meat was permitted to eat at the moment its flavor was absorbed in the spit, it is not necessary to perform libun, and it is sufficient to perform hagala in boiling water. This is because hagala releases almost all of the flavor, and whatever is not released is considered weak flavor. When forbidden food was absorbed, even this weak flavor must be removed, but when the initial absorption was of something permitted, it is sufficient to remove the main part of the flavor by means of hagala (Avoda Zara 86a; Ramban and Ritva ad loc.).
The leading Rishonim disagree about ḥametz: Some say that the absorption of ḥametz throughout the year is considered the absorption of permissible matter (heteira bala), since ḥametz may be eaten all year round. Consequently, there is no need to kasher anything for Pesaḥ via libun, as even kelim used in fire, like roasting spits and baking pans, can be kashered by means of hagala. Others say that ḥametz is considered a forbidden food (issura bala); even though it may be eaten le-khatḥila all year round, it is always considered forbidden vis-à-vis Pesaḥ, as even something that became ḥametz before Pesaḥ is called ḥametz and is forbidden to eat on Pesaḥ. Therefore, roasting spits and baking trays must be kashered by means of libun. This is the accepted halakhic ruling (SA 551:4). However, where there are other reasons to be lenient, the lenient position is relied upon (MB 451:28).