The forth (and brief) chapter of Shabbat discusses the prohibition of hatmana – loosely translated as insulation. While the Mishnayot discussed insulating materials covered by the prohibition, timing and retrieval of the insulated objects, we shall try and probe the parameters of the prohibition.
First we must however ask why hatmana is prohibited. The Gemara (Shabbat 34b) provides two reasons for the two prohibited scenarios: any insulation during Shabbat and insulating with substances that can increase heat. In the former case, the concern is that one might heat the food thereby transgressing the prohibition of cooking. In the latter case, insulating before Shabbat using a substance that increases heat is prohibited, as there was a concern that one could cover the food in hot coals or ash and then might stoke them on Shabbat thereby violating the prohibition of kindling a flame and cooking.
The Tosfot Yom Tov notes that this appears to be a gezeira on a gezeira, which is not usually allowed. One gezeira is in case hot coals are used. The next is that if they are used, one might stoke them. He answers that the issue of a double gezeira is only when they are each made a different times. If however they are made at the same time, it is possible that the Chachamim saw the need for both understanding that the first gezeira could not stand without the second.
Let us now understand what qualifies as hatmana.1 The first point to note is that that the Rema (OC 253:1) explains that if the insulation is open at the top it is not considered hatmana. How open is a subject for further discussion.
The next qualifier is that Rabban Shimon ben Gamliel in the Gemara(Shabbat 51a) explains that it is not prohibited to wrap the food with a material that does not increase heat on Shabbat if it is has been transferred to a kli other than the one it was heated in, i.e. it is in kli sheni. In other words the prohibition of insulating on Shabbat with a material that does not increase heat only applies to a kli rishon – a utensil that was placed on the fire (see Shulchan Aruch 257:5).
Another exception found in the Shulchan Aruch (257:2) is that if one covers a pot simply to protect the food, for example its lid, then even though it assists in preserving the heat, such an action is permitted on Shabbat.
Finally, the Shulchan Aruch (257:8) also teaches that the prohibition of hatmana is only when the material is in direct contact with the pot (or food). The example the mechaber provides to illustrate a permissible scenario is if one placed a wider plate on top of a pot and then draped clothing over the pot. Since the clothing that hangs over the plate does not touch the pot, it is not considered hatmana.
An interesting case that touches on a number of these points relates to whether one can place water from an urn in a thermos flask on Shabbat. The Piskei Teshuvot (257:2) brings a number of opinions regarding this case. The first opinion is that it is permitted. Firstly, the thermos would be considered a kli sheni (the second point listed above). Secondly, citing the Chazon Ish, he explains that it is possible that a thermos is not considered hatmana at all. Hatmana is where on takes, for example, materials and wraps it around a kli that contains food. This case however is similar to pour food into a kli that already is surrounded by the insulating material.
The Piskei Teshuvot however brings other opinions that rule stringently requiring one to pour the water into an intermediate kli prior to pouring the water into the thermos. He adds that there are others that suggest that if one did not fill the thermos to the top then they can combine the leniency that the hatmana is only when the insulating material is direct contact on all sides with the food and thereby permit the use of the thermos.
As is the disclaimer with all Mishnah Yomit articles, one should not derive any practical conclusions from the content provided, but instead seek rabbinic council.
1These four qualifiers were gleaned from The 39 Melachos, Rabbi Dovid Ribiat.
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