Even when a baking tray is coated with oil to prevent sticking, the absorption that occurs during the cooking process is considered to be by means of fire, thus requiring heavy libun to render it kosher. Only when the oil at the bottom of the vessel sizzles and bubbles is the absorption considered to be by means of a liquid.
In light of this, most Rishonim maintain that a frying pan can be koshered through hagala (Rosh, Raavya). Even if the oil is used up and the food burns, hagala is sufficient since there was oil present at the beginning of the frying process, and the taste of the food absorbed into the pan was by means of the oil, i.e., the milder form of absorption. The same applies to a pot in which a non-kosher food was cooked: the pot can be koshered by hagala even if the food dries up and burns (MB 451:63; Kaf Ha-ĥayim ad loc. 137).
However, several leading Rishonim maintain that a frying pan has the status of a baking tray and must be koshered via heavy libun, because people often fry with small amounts of oil, and the oil is often used up, causing the taste of the food to be absorbed via fire (Rashba). Although there is agreement that a pot can be koshered through hagala even if the food dries up and burns, there is reason to be more stringent about frying pans because only a small amount of oil is used from the outset, and it often gets used up.
In practice, frying pans should ideally be koshered via light libun (as will be explained in the next section), although if twenty-four hours have passed since the frying of the forbidden food, hagala is sufficient be-di’avad.
A Teflon or “non-stick” frying pan, in which food is fried without oil, cannot be koshered for Pesaĥ. In theory it is possible to kosher it via heavy libun like a baking tray, but as a practical matter this will damage the pan (as will be explained below, section 7).
A frying pan used primarily for making malawach, which is baked and heated without sizzling oil, must be koshered by heavy libun since the absorption takes place via fire. But a pan generally used for other things and infrequently used for malawach may, in extenuating circumstances, be koshered via hagala, in accordance with its primary usage (as will be explained below, section 9).
. This is according to Pri Ĥadash 451:1, quoted by SAH 451:36 as saying: “Since the dough is not sizzling with oil or fat, it means that fire alone causes the ĥametz to be absorbed.” Kaf Ha-ĥayim ad loc. 139 cites them, and this is the ruling of Yeĥaveh Da’at 1:7 (although Kaf Ha-ĥayim 451:76 states that if there was enough liquid present in the pot to wet something that touched something that was in the pot (“tofei’aĥ al menat le-hatfi’aĥ”), the pot can be koshered by hagala; this requires further investigation). Accordingly, a utensil used to cook jachnun or kugel is considered to have absorbed by means of fire, since there is no sizzling liquid (see Hagalat Kelim Le-Pesaĥ 5:23 and the supplementary material ad loc.). If a pot is generally used to cook through a liquid medium but less frequently used to make jachnun or kugel, it may be koshered by hagala in extenuating circumstances, as will be explained below in section 9.
. BHL 451:11 concludes that most poskim maintain that one may kosher a frying pan via hagala, and I have already mentioned the leading Rishonim on this topic. SA YD 121:4 rules that a frying pan generally requires libun to be koshered from forbidden foods, but for Pesaĥ hagala is sufficient. Many Aĥaronim (Gra and Shakh ad loc.) explain that regarding Pesaĥ R. Karo combines the opinion that hagala is sufficient with the view that ĥametz before Pesaĥ is considered permissible, and thus rules leniently that one may kosher a frying pan for Pesaĥ via hagala. Rema agrees in principle but states that light libun is the proper method le-khatĥila. Later Aĥaronim also disagree about this issue: Pri Ĥadash and R. Ĥayim ibn Attar require libun since ĥametz is considered a forbidden substance even before Pesaĥ, while SAH permits using hagala even le-khatĥila. See Hagalat Kelim Le-Pesaĥ 13:202.
I have written that the preferable method for koshering a frying pan is light libun, and although apparently light libun is ineffective according to those who require heavy libun, there are in fact those who maintain that light libun is as effective as heavy libun (this may be the opinion of R. Avigdor, quoted in the Hagahot Maimoniyot). Additionally, some authorities explain that the principle of ke-bole’o kakh polto also applies to the temperature of the libun, as will be explained in the next section. Consequently, light libun is sufficient, since it takes place at least at the same temperature as the initial absorption. On the other hand, it is better to use light libun since sometimes there are small grooves or cracks in the pan which are difficult to clean, in which case even those who maintain that hagala is enough to kosher a frying pan would require libun to eradicate the residue in the cracks. However, one may rely, be-di’avad, on the majority of poskim who maintain that one may kosher a frying pan via hagala, especially since once twenty-four hours have elapsed since its last use, any uncertainty pertains to a rabbinic prohibition.