Now that we have learned the laws of hashhaya, leaving food on a heat source before Shabbat, we will now explore the laws of hanaĥa: placing food on Shabbat somewhere that it will be warmed up. As we saw in section 3, the rule of thumb is that one may not cook on Shabbat, but one may reheat food due to the principle of ein bishul aĥar bishul. Therefore, one may remove fully cooked food from the refrigerator, such as cold chicken and kugel, and reheat them. The poskim disagree regarding reheating a liquid (as explained above in sections 5-6). However, the Sages prohibited reheating even cooked foods if one does so in a manner that resembles cooking, because he may forget that it is Shabbat and end up raising the flame and transgressing a Torah prohibition. Therefore, one may not reheat a fully cooked food over an open fire, since this is the way one normally cooks.
But one may reheat food if the method used is clearly not the normal way of cooking. Therefore, one may put a pot with cooked food on top of another pot, or on an urn that is on the fire, because this is not the normal way of cooking.
Poskim disagree, however, about placing food on a plata or a blekh.
According to many, one may not place even fully cooked food on these, because putting food on a heat source resembles cooking. However, if one places a pot or plate upside down on the plata or blekh, he may place food on them, because this is not a normal way to cook, as normally one would not place an obstruction between the heat source and the food. In practice, one may even rely upon a cover that lifts the food up only minimally (like the top from a metal coffee can) to serve as an obstruction between the plata and the food. However, aluminum foil, which does not lift the food up at all, is not acceptable. Those who are stringent maintain that one may not place even fully cooked food on an aluminum foil-covered plata or blekh.
In contrast, there are others who are lenient and maintain that since people do not normally cook upon a plata or a blekh but only over an open flame, when one places food on a blekh or plata it does not look like he is cooking, and therefore he may place cooked food directly on them on Shabbat. There are others who are lenient when it comes to a plata since it is especially designed for warming and not cooking, but are stringent regarding a blekh since it is hotter and can serve as a cooking surface.
In practice, since many poskim are stringent, it is preferable to be stringent and refrain from placing a pot with cooked food directly onto the plata or blekh. However, those who wish to be lenient may, since the law is rabbinic, and many important poskim have offered compelling reasoning in support of their lenient position. One who has a set family custom on the matter should follow his custom.
One may put fully cooked food on a heat source that is not normally used for cooking, such as a radiator, even if the radiator is very hot, since it does not resemble cooking (Igrot Moshe, OĤ 4:74, Bishul §34). Similarly, one may put fully cooked food on top of a kerosene or gas heater that is meant to heat the home as long as something separates the fire from the food and one does not warm up food or water there during the week. But if it is used during the week for heating food and water, then the laws pertaining to it are the same as those for a gas flame. Thus according to most poskim, one must put an upside-down pot on it, and only then may one reheat the food. Following the more lenient position, it is enough to put a blekh on the heater and place the food upon it.
. Included among the stringent are: R. Shlomo Zalman Auerbach (cited in SSK 1:30 and nn. 63 and 83); Or Le-Tziyon 2:17:1; Shevet Ha-Levi 1:91; and R. Meir Mazuz (cited in Menuĥat Ahava 2:10:28). In the same vein, Igrot Moshe, OĤ 1:93 is stringent about a blekh or plata that get so hot that one can cook on them; Kaf Ha-ĥayim 253:11 states similarly regarding a blekh. In their opinion, a plata is essentially a kira whose flame is covered. One may not put food directly on a plata, because even though the cover eliminates the concern that one will turn up the flame, there still remains the concern that it resembles cooking when one places things upon it. Ĥazon Ish 37:9, 11 is more stringent about leaving food on a plata or returning it to one, because, in its opinion, a plata is not considered covered since it itself is a heat source without a cover. This is also the opinion of R. Mordechai Eliyahu regarding a plata. SA 253:1 forbids leaving fully cooked food on the coals because of the prohibition of insulating; in R. Eliyahu’s opinion, when the bottom of a pot is resting on something that generates heat, this is considered insulating (hatmana) and is forbidden. In order to use a plata in a permitted fashion, one must put an additional metal piece on it that will create some space between the pot and the plata. (Or Le-Tziyon 2:17:1 rejects this reasoning, because SA was stringent only when speaking of a pot that was sinking into coals, but not regarding food that is placed on a flat and stable heat source.) R. Qafiĥ, in his commentary to MT 3:12, is stringent regarding returning pots to a plata or blekh, but not regarding leaving them on before Shabbat.Yeĥaveh Da’at 2:45, based on several Aĥaronim, takes a lenient position, as does Menuĥat Ahava 2:10:28. Tzitz Eliezer 8:26:5 inclines toward this and states that it is also the opinion of R. Frank. According to them, placing something on the plata does not resemble cooking, and the plata is comparable to a covered kira with an additional cover. Some are lenient only for a plata, where there is no concern of resembling cooking, because it is designed for reheating and not for cooking. Thus, the laws pertinent to a plata are not those of a kira but those of the sides of a bonfire, which were occasionally used to cook in the time of the Talmud (Igrot Moshe, OĤ 4:74, Bishul §35; R. Lior).
I wrote above that one who follows the stringent position may create an obstruction between the food and the plata or blekh by using an upside-down plate. This is suggested by SSK 1:44, n. 126, based on MB 253:81. (In contrast, Ĥazon Ish maintains that one may place food on the plata or blekh only if he places it on top of a pot with food in it; however, one may not put it on an empty pot that is on the plata, because that would be like stacking one blekh on another over the fire – it does not help, because it still looks like cooking.) It would seem that using the metal cover of a coffee can does not look like cooking, and moreover one may take into account the lenient position.