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Shabbat Parashat Beshalach| 5763
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Question: After baking a challah for Shabbat, I like to wrap it in aluminum foil so it stays warm. Is that okay? Could I leave it in an oven which is turned off but still warm, where the process would be even more effective?
Answer: The problem of wrapping, which you refer to, is called hatmana (insulation). We cannot get into all the details (most of which are found in Orach Chayim 257), but we will touch on some major points, especially as they affect your cases.
The gemara (Shabbat 34a-b) forbids two basic types of hatmana. One may not wrap food on Shabbat to keep in its heat. This is a rabbinic prohibition out of fear that one who is so concerned about keeping the food hot might actually come to heat up the food before he wraps it. A second rabbinic prohibition is not to do hatmana in a situation where heat is being added to the insulated food (mosif hevel) even if this is done before Shabbat. This ruling is out of fear that the heat-adding insulation might be done with remetz, a mixture of sand and coal, in which case one might come to stoke the coals at some later point. However, in your case, you have the best of both worlds. You do the hatmana before Shabbat in a simple insulation of aluminum foil, which does not add heat. Thus, it is permitted.
Your further suggestion is much more complicated. The gemara, referring to hatmana which is mosif hevel (adds heat), discusses insulating materials which themselves add heat. Aluminum foil does not add heat itself, but it serves as a conduit for the heat of the oven. There is a machloket among the rishonim as to whether such a situation is considered mosif hevel and forbidden or not. The Shulchan Aruch (257:5) is stringent on the matter, while the Magen Avraham (257:18) and Mishna Berura (ibid.:43) report a minhag to be lenient. Logic suggests that in a case where one shuts the oven so that no new heat is added to the system, and one is showing that he is not so concerned that it stay hot over a long period of time (like inclassic mosif hevel), it might be more lenient. It would seem that if the oven will not even be yad soledet bo (approximately 113° F) when Shabbat begins, and it is continuously cooling off further, then it is permitted. However, if you would leave your oven on, even on a very low setting, then you should not wrap the challah for the purpose of insulation, as the system is designed to continuously add new heat, similar to remetz.
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