Often, the same kli is used at different levels of severity. How is such a kli kashered? Some are stringent, maintaining that every kli must be kashered based upon its most intense usage, so if it was once used in fire, even though it is normally used in boiling water, it must be kashered by means of libun. Likewise, a kli that was once used with boiling water but is normally used with cold water must undergo hagala, for otherwise the kli will be kashered based on majority usage, not based on most intense usage (She’iltot, Raavyah, Mordechai, Issur Ve-hetter Ha-arokh).
In contrast, many authorities are lenient and maintain that the requirement to kasher depends on how that kli is used. Therefore, if it is normally used in boiling water, but occasionally used on a flame, it is kashered by means of hagala. If it is normally used with cold water but occasionally used with boiling water, it is kashered by means of rinsing (Rabbeinu Tam, Ramban, Rashba, Ran, SA 451:6). At first glance, this is problematic: How can kashering a kli based on its majority usage eliminate the flavor it absorbed during its more intense usage? The answer is that after 24 hours, flavors absorbed into and stuck to kelim become foul, and they consequently can no longer render forbidden foods that come into contact with them (as will be explained in the next section). Therefore, the requirement of kashering is based on the kli’s majority use (Rashba, Rama Mi-Fano).
In practice, the custom le-khatḥila is to be stringent and to kasher every kli according to its most intense usage. Under extenuating circumstances, we are lenient and kasher it according to its primary use. When there is concern that kashering it based on its most intense use will ruin the kli, for example, in the case of a kli that is used in fire in a minority of its usages, and which is likely to be damaged by libun, one may kasher it le-khatḥila by means of hagala, in accordance with its primary use.
In practice, SA 451:6, 25, rules in accordance with the lenient view. According to Rema the custom is to be stringent and kasher the kelim according to their more intense usage, but we are lenient be-di’avad. According to SAH 451:27-28, such kelim should not be used on Pesaḥ. Only be-di’avad, if they were used after being kashered according to majority usage, the food is kosher. However, it seems from SHT ad loc. 144 that one may kasher a kli according to majority usage when it is difficult to kasher it according to its more intense use. Additionally, if kashering it according to its more intense use might damage the kli, according to Beit Meir ad loc. 11 (cited in SHT 51) one may kasher it according to its majority use.
Many Sephardic poskim wrote that we follow the majority usage (SA 451:6; Rama Mi-Fano; Gan Ha-melekh; Pe’ulat Tzadik; Zivḥei Tzedek, R. Yosef Messas, Yabi’a Omer 10, YD 58; Or Le-Tziyon 3:10:10-11). However, several Sephardic poskim wrote that le-khatḥila the practice is to be stringent according to the most intense usage (Knesset Ha-gedola; Shulḥan Gavo’ah; Kaf Ha-ḥayim 451:100; R. Ḥayim David Ha-Levi’s Mekor Ḥayim 186:6). SA 451:20 also states that the custom is to kasher tables by means of irui, in accordance with the more intense usage. Since we find some Sephardic poskim who ruled that one should be stringent le-khatḥila, and many Ashkenazic poskim ruled that one may be lenient when necessary, in order to minimize the differences, I wrote that we follow the stringent view le-khatḥila and that we are lenient when necessary. Therefore, forks that are sometimes used in fire should be kashered by means of hagala, since they can be damaged by libun. There is an additional reason to be lenient and require only hagala – the fork that was used in the fire is batel in the majority of forks that were not.
Several Aḥaronim wrote that we determine majority and minority usage based only on use with ḥametz that requires kashering; a kli that is primarily used for cold food that is not ḥametz but was once used for ḥametz in boiling water must undergo hagala. However, if most of the ḥametz usage was cold, as in the case of a table or countertop, even though it was occasionally used with hot ḥametz, it can be kashered with cold water, in accordance with its majority usage. So state Pri Megadim YD 91, Siftei Da’at 3; Beit Efrayim ḤM 17; Ḥazon Ish OḤ 119:15; Shevet Ha-Levi 6:116:3 (and in R. B. Frankel’s notes to §91, he is lenient and maintains that majority use is determined by all usage, including permitted use).