The fourth perek of Masechet Shabbat deals with the rabbinic prohibition of insulating hot food on Shabbat (hatmana). The Rabbis prohibited hatmana because it might lead to the melachot of bishul (cooking) and maver (kindling). The Rabbis were concerned because food is insulated in order to keep it warm, and if the food was not kept warm enough, a person may reheat the food thus performing the melacha of bishul. They may also perform maver if a flame was ignited in order to reheat the food.
Not all forms of insulation are actually prohibited. In order for the insulation to be prohibited four conditions must be present (The 39 Melachot, Rabbi Dovid Ribiat, p. 627):
The food must be completely covered by the insulation.
The food must still be in its kli rishon (the vessel in which the
food was heated).3
The intent of the hatmana must be for the purposes of insulating.
The insulating material must be in direct contact with the food or
In general, if any of these conditions is absent, then hatmana is permitted.4
Hatmana can be performed before Shabbat for food that will be consumed on Shabbat. However there is a prohibition against insulating food before Shabbat with materials that add heat (such as hot coals). For example, if such a hatmana was made on Friday morning the food would need to be uncovered before the beginning of Shabbat (Mishnah Berurah (257:1)). This prohibition extends even when the source of the heat is not the insulating material itself. As long as there is an active source of heat, the insulation will be prohibited. For example, completely wrapping a hot water urn in a towel is prohibited even before Shabbat because the heating element in the urn is the active source of heat (The 39 Melachot, p631).
The Gemara (Shabbat 34b) explains that the prohibition against insulating food before Shabbat was put in place due to a concern that one may insulate food with hot ash that has live coals mixed with it. Later, when Shabbat had begun, one may be tempted to rake the coals to make the food cook faster, or in modern terms, turn up the heat. This is the same concern which underpins the prohibition of shehiya (leaving a pot on the stove on erev Shabbat – see previous article).
One may therefore think that the same exceptions that apply to shehiya would also apply to hatmana. For example, one may think that insulation is permitted if the food is fully cooked (because there is no temptation to rake the coals or turn up the heat). However the Gemara does not mention this exception in relation to hatmana. Rashi (Shabbat 34b) explains that Rabbis wanted to avoid possible confusion. Although some Rishonim hold that the prohibition of hatmana does not apply if the food is fully cooked (Artscroll Gemara Shabbat, Introduction to Chapter 4), most Rishonim hold that the prohibition does apply in such cases. The Shulchan Aruch (257:7) clearly states that hatmana is forbidden with fully cooked food. The Rama mentions the lenient view but says that it should only be followed in places where there is already a custom to be lenient. The Mishnah Berurah explains that the lenient view only applies to the case of hatmana before Shabbat.
One final note - the prohibition of hatmana is a gezeirah (a rabbinic enactment to keep people from sinning - Steinsaltz, p108). There is a general principle that the Rabbis do not enact a gezeirah for a gezeirah. However, the prohibition of hatmana appears to be exactly that. Firstly, there is gezeirah not to insulate hot food with hot ash – this is a safeguard because the hot ash may have live coals which might be raked. Secondly, there is a further gezeirah prohibiting insulating hot food with any substance – this is a safeguard against insulating with hot ash.
Rambam explains in his commentary to the Mishnah that the principle of not decreeing a gezeirah to a gezeirah only applies when attempting to issue a decree to safeguard against an existing safeguard. However, if the Rabbis realise at the time of issuing a gezeirah that it will not be a sufficient safeguard on its own, then the Rabbis can issue a second gezeirah to support the first one, provided that both are decreed at the same time. According to the Rambam this is what the Rabbis did in the case of hatmana. (Artscroll Mishnah Shabbat, p87; Rambam’s commentary to masechet Shabbat, Ch 4).
3 However if the food cooled down below the temperature of yad soledet bo (too hot to touch) then the Mishnah Berurah (257:5) suggests that hatmana would be permitted, even in the kli rishon.
4 No practical conclusions should be drawn from this summary as there are number of qualifications and conditions that apply.
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