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Q. I noticed in your Pesach Guide that you permit kashering countertops using irui, unlike the ruling of Mishnah Berurah 451:114 who requires an even m’lubenes. Can you please point me to the logic or source for rejecting this opinion?
A. Shulchan Aruch 451:20 says that tables should be kashered via irui kli rishon. However, Mishnah Berurah 451:114 questions this ruling, because occasionally a hot davar gush (solid food) of chametz might be placed onto the table, and we are machmir for those opinions that davar gush has the status of a kli rishon such that irui kli rishon would not be a sufficient kashering. Based on this question, Mishnah Berurah recommends that tables be kashered via irui kli rishon using an even m’lubenes to bring the level of kashering closer to that of a true kli rishon. Why then does our kashering guide says that a table can be kashered via a mere irui kli rishon and makes no note of an even m’lubenes? The answer to this question requires a deeper understanding of the halacha of “rov tashmisho”, as follows:
Shulchan Aruch 451:6 rules that – if a utensil is aino ben yomo – the method of kashering is determined by looking at the primary way the utensil is used (rov tashmisho), such that a table can be kashered via irui kli rishon, because the primary use of the table is not for a hot davar gush. Rema agrees that the letter of the law follows Shulchan Aruch’s ruling, but says that the Ashkenazic custom is to be machmir and choose a method of kashering that even suffices for the secondary uses (miut tashmisho) of the utensil. Accordingly, in the case of a table irui kli rishon is insufficient, and that is the basis for Mishnah Berurah’s point. Since it is merely a chumrah to be concerned with miut tashmisho, one is not required to follow that chumrah in cases of b’dieved (as noted in the aforementioned Mishnah Berurah and in Rema 451:6) or in cases where that will mean it is impossible to kasher the utensil (see Sha’ar HaTziun 451:51, based in essence on the ruling of Rema YD 121:5).
Accordingly, if one were able to kasher a table or counter via irui kli rishon with an even m’lubenes that would be the best way to kasher it, and in fact there are some people who do this. However, for most of the public this suggestion is impractical due to the (a) inability of many surfaces to withstand such heat and (b) the difficulty in properly using an even m’lubenes over a large surface. Therefore, we treat this situation as one where kashering based on miut tashmisho will mean that it is impossible to kasher the utensil and rely on the letter of the law that one may kasher based on rov tashmisho (i.e., irui kli rishon without an even m’lubenes).