Tosfos (Chulin 108a tipas chalav) writes that if a drop of milk fell into a pot of meat, although it gives ta’am into the piece of meat that it touches, this ta’am will not transfer into other pieces of meat, even though they are touching, provided the first piece of meat is completely above the liquid. This is because a bliya cannot transfer from one chaticha to another without liquid. (ain bliyah holeches michaticha lechaticha belo rotaiv). [It should be noted that this is only true if the issur
or the chaticha are not shamein (fatty).]
The Mordechai (Chulin Perek kol basar 691) extends this ruling, and says that kol shekain a bliya that is absorbed in a kli cannot pass from one kli to another without liquid. Therefore, for example, if a dry hot pot of meat touches a dry hot pot of milk, even if they are both on the fire, the food and pots remains mutar. The same would be true if a dry hot non-kosher pot touches a dry kosher pot. Rema paskens this halacha in Yoreh Deah (Siman 92:8). Still, the implication of Rema is that this is only mutar bidieved. Pri Migadim (m.z. 97:3) explains that lichatchila we should be concerned that the pots may spill against each other.
How dry must surfaces be?
Rebbi Yechiel M’Paris (siman 78) writes that if a pot of meat and milk touch while on the fire, even when we see it is the derech for these pots to sweat through (i.e for the milk/meat to leak out), nevertheless the custom is to be lenient. This is because, that which leaks through the pot is considered kachush (lean) and will not penetrate further than a klipa. Since the wall of the adjacent pot is thicker than a klipa, the food inside remains mutar. Chavas Da’as (92:20) likewise paskens that a slight dampness between two surfaces is not enough roteiv to transfer bliyos. He brings a proof from the above Tosfos; although pieces of meat cooking in a pot will surely be damp, obviously this is not enough to allow bliyos from one chaticha to the next. Similarly, dampness between two pots will not allow bliyos to transfer. However, the exact amount of moisture that would be
permitted is not clear, and therefore if there is dampness, one should only be maikel when there are other tzirufim. [1]
Chatzuva (stovetop Grate)
Rema (O.C. 451:4) writes that one must kasher a chatzuva with libun. Mishnah Berurah (s.k. 34) explains that in truth a chatzuva should not need any kashering, since its method of use is kli el kli b’lo rotaiv, and the fire burns up spills, but as a chumra d’pischa we kasher with libun kal. Therefore, if libun kal was not done, bidieved the food would be permitted. Igeros Moshe (Y.D. I:59 & III:10) writes that regarding a non-kosher stovetop (not for Pesach), there is no need to kasher the chatzuva at all. He explains that kli el kli b’lo rotaiv is even permitted lichatchila when there is no concern that the food will actually become assur. Regarding a chatzuva, there is no concern, since even if there would be a spill; it would burn up right away. [2] However, one should double check that the grates are clean, since the above heteirm do not apply to b’en.
Glass Top Stove
Today, many people have glass top stoves. Kashering the entire surface of a glass top stove with a torch is not a feasible option, since this can crack the glass. For Pesach, the best option is to clean the glass, not use for 24 hours, turn on the burners to kasher the areas directly above the element, and then place a metal disc or aluminum foil under the pots when cooking. Although one can get by this way for 8 days, this method is not so practical for all year long. If one moves into an apartment or house that has a used glass top stove, is covering the burners required for year round use?
One can argue that a glass top should be like a chatzuva, since its method of use is kli el kli b’lo rotaiv, and similarly spills burn up quickly. However, because the surface is flat, and the pots often stick out beyond the heated circle, spills might not burn up so fast, and there can be moisture connecting the pot and the stove. As noted above from the Chavas Daas, a minimal amount of spillage would still not be a concern. Therefore, if replacing the glass is too expensive, after cleaning the stove, waiting 24 hours and burning out the elements, one can allow using a glass top stove for year-round use [3] .
However going forward, one should designate burners for milchig and fleishig, since it is difficult to keep the burners clean.
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[1] See Beis She’arim Y.D. 160 who questions how much moisture is permitted. See also Yad
Yehuda 92:end.
[2] In another teshuva (Igeros Moshe O.C. I:124), Rav Moshe implies that there would
certainly be no concern if the chatzuva was an aino ben yomo.
[3] Another tziruf is that Mechaber (O.C. 451:26) paskens (Rashba and Ran) that glass is
not boleya.