The Sages said that libun is the insertion of a kli into fire until “they shed their outer layer” (Avoda Zara 76a) or until the kli gives off sparks (y. Avoda Zara 5:15). Since iron generally does not peel or spark when placed in fire, it seems that the Sages’ intent was that libun should cause the food adhering to the kli to peel off from it or that the food will give off sparks.
Many maintain that libun requires heat that can incinerate and destroy any food that can adhere to the kli, until it is shed in the form of sparks – even though the fire on which ḥametz was roasted or baked never reached such an intense heat (Pri Megadim). This is a temperature or 350-400oC.
Some rule leniently, maintaining that the principle of ke-bole’o kakh polto applies to the temperature of the fire at the time of absorption. The temperature at the time of absorption is the temperature at which kashering can take place. Thus, for example, if a kli absorbed flavor at a temperature of 200oC, it can be kashered at 200oC. In their view, a baking tray that absorbed a forbidden taste in an oven can be kashered in that oven at the same temperature. In practice, halakha follows the stringent view, but in times of need, it is possible to be lenient. Therefore, if a kli is needed, or if following the stringent view might ruin the kli, one may rely on the lenient view.
There is also light libun (libun kal), which entails heating a kli by fire to the point that a piece of straw placed on the opposite side of the kli would become singed from the heat, or a fine thread stretched out over the wall of the kli would become singed and snap from the heat. Light libun is not effective where regular libun is required, but it is more effective than hagala at extracting the flavor absorbed in the kli, and it also incinerates some of the flavor absorbed in and stuck to the kli. When it is uncertain if libun is necessary, one may suffice with light libun. Light libun can be performed by putting a kli in an oven and heating it at the highest temperature for about half an hour.
At first glance, it seems that the metal of the kli itself must spark and shed a layer, but metals generally do not spark or shed layers due to fire, and according to experts, they did not do so in the past, either. Therefore, it seems that the meaning is that the foods that remained stuck to the kelim would burn, char, and shed layers or burst and release sparks. This is similar to the explanation of R. Pfeiffer (Darakh Kokhav, p. 310), and many works cite his explanation (Ma’adnei Asher, Issur Ve-hetter 159; Hora’ah Berura, YD 121:59; Divrei David, vol. 1, YD 10). Even though nowadays, when we have soap, foods do not remain stuck to kelim, the requirement of kashering remains as it was (Peninei Halakha: Kashrut 32:7-8).
As I wrote above, there is a dispute among Aḥaronim with regard to libun. Many maintain that it requires a fixed temperature that would produce sparks if there is food stuck to the kli. These include: Pri Megadim 441, Eshel Avraham 30; Responsa Maharam Schick, OḤ 213; R. Frank’s Mikra’ei Kodesh: Pesaḥ 1:80:7; Igrot Moshe, YD 1:60. Some adopt the lenient view that libun requires the same temperature at which the kli absorbed the forbidden food. These include: Responsa Arugat Ha-bosem, OḤ 119; Minḥat Yitzḥak 3:66; Tiferet Tzvi 1:30; and Nitei Gavriel (Pesaḥ 1:75, n. 3) in the name of R. Aharon Kotler and R. Y. B. of Brisk. The accepted ruling is the stringent one, as this is the view cited in contemporary works of halakha. However, since according to many the absorption of ḥametz throughout the year is considered the absorption of permissible matter (heteira bala), in which case even hagala would be effective, in times of need one may rule in accordance with the lenient view of libun, namely, that it requires the same temperature at which the kli absorbed the ḥametz. This is certainly the case more than 24 hours after the absorption, as then the flavor is spoiled, and in cases of uncertainty we follow the lenient view. In addition, since nowadays metal kelim are cleaned thoroughly with soap, there is no concern that the forbidden food will impart flavor, and the kashering requirement is based on the kli’s use with forbidden foods (Peninei Halakha: Kashrut 32:7-8). In cases of other forbidden foods, where the rationale of heteira bala (i.e., ḥametz absorbed throughout the year is considered permissible matter, so even kelim used in fire can be kashered by means of hagala) does not apply, one may only be lenient in the case of a significant loss (ibid. 33:6).
Rabbeinu Avigdor, one of the Rishonim (cited in Hagahot Maimoniyot on MT, Laws of Forbidden Foods 17:5), has an even more lenient view: Libun consists of burning a fire under the kli to the point that if one places a piece of straw on it, it will be singed. This is what the Sages meant by “until it gives off sparks.” In practice, the consensus is not to adopt his leniency, and this form of libun is called “light libun,” which is more effective than hagala. When there is uncertainty about whether a kli requires libun, many rule that it may be kashered with light libun (Terumat Ha-deshen §130; Rema 451:4). Another advantage of light libun is that it is effective for crevices that are hard to clean and that hagala does not kasher (MB 451:33). Nowadays, these kelim can be cleaned with soap, which renders everything stuck in the crevices unfit for a dog’s consumption.
The poskim further write that there is an even lighter form of libun, namely, heating the kli to the temperature of “yad soledet bo,” i.e., at which the hand reflexively recoils (Levush; Taz 451:8; SAH 10). This seems to refer to anything above 70oC. Pri Megadim (451, Eshel Avraham 30) states that this form of libun is effective for food absorbed into a kli rishon after it had been removed from the fire.