Skip to main content

Cooking With Solar Heat, Fire, and Their Derivatives: Different Types of Heat Sources Used for Cooking (Part I)

The Israel Koschitzky Virtual Beit Midrash

The Laws of Shabbat
Yeshivat Har Etzion

Shiur 12: Cooking With Solar Heat, Fire, and Their Derivatives:

Different Types of Heat Sources Used for Cooking – Part I


by HaRav Baruch Gigi

Translated by David Silverberg



            The Mishna states in Masekhet Shabbat (38b): "One may not place an egg alongside a kettle for it to cook, and one may not wrap it in garments [that had been heated]; Rabbi Yossi allows this.  And one may not bury it in [hot] sand or dust from the roads to roast it."


            The Talmud Bavli explains this Mishna as follows: The first clause – "One may not place an egg alongside a kettle" – refers to a kettle that had been heated over fire, and all opinions agree that cooking with a derivative of fire constitutes a Torah violation.  The Mishna's second clause – "one may not wrap it in garments" – deals with garments that were warmed by the sun, and this issue, of cooking with toledot ha-chama – the heat of items that were warmed by the sun – is subject to a dispute between Rabbi Yossi and the Chakhamim.  This is the sugya's conclusion on 39a:


Rav Nachman said: In the sun – all agree that [cooking] is permissible; in derivatives of fire – all agree that [cooking] is forbidden.  When do they argue?  With regard to [cooking] in derivatives of the heat of the sun: this one holds that we issue a decree regarding derivatives of the sun out of concern for [the possibility that one might cook with] derivatives of fire, and the other holds that we do not issue such a decree.


The Yerushalmi, by contrast, explains the Mishna differently, claiming that the Chakhamim and Rabbi Yossi argue with regard to cooking with the derivatives of fire, and they both agree that one may cook on Shabbat with derivatives of solar heat.  In this shiur we will assess the principles underlying the different views taken by the Bavli and Yerushalmi.


Cooking with Fire


            The primary mode of cooking proscribed by the Torah is cooking with fire.  Strictly speaking, there is no difference in this regard between an open or covered flame, so long as it has the capacity to cook.  The Tanna'im distinguished between open and covered fire only in the context of various rabbinic enactments, as we will iy"H discuss in future shiurim.


            This category of cooking with fire includes as well cooking with electric appliances with heating elements that become very hot, because fiery hot metal has the status of fire, and heating it on Shabbat constitutes a violation of the melakha of mav'ir.  This halakha emerges from the comments of the Rambam in Hilkhot Shabbat (12:1) and of the Maggid Mishneh.  Some scholars do, however, interpret the Rambam to mean that mav'ir applies only when one heats the metal for the purpose of refining it.  Rav Shlomo Zalman Auerbach zt"l addresses this issue at length in Minchat Shelomo (1:12).  He concludes that one indeed violates the Torah prohibition of mav'ir by heating metal, and one violates bishul by cooking with electric appliances that operate with metal heating elements.


            In this same responsum Rav Shlomo Zalman addresses the issue of heating water with an immersion heater:


I find it appropriate to mention that when I was once with our rabbi the Chazon Ish zt"l, I told him that if one boils water on Shabbat with an immersion heater, whose two edges are distant from one another, and the circuit is created only through the water, as a result of which the [water] becomes hot and boils, this is, in my view, only a rabbinic prohibition.  For since an electric current is not fire, it turns out that this is like cooking with a derivative of solar heat, for which one does not bring a sin-offering.  And I said that in my view, this matter should be publicized in hospitals and the like.  But he responded unequivocally that just as one who cooks with hot water, which is a "derivative" of fire, is liable to bring a sin-offering, the same applies when one cooks with an electric current; since this is generally a process that creates fire, it is considered like the "gestation" of fire.  And just as one is liable for the toleda [derivative; literally, the "offspring"], so [is one liable] for the gestation, since generally it ultimately becomes fire, even though practically it only heats water without any fire being created.  I was amazed to hear such a thing… But even today I find this very difficult, especially since the current itself merely rubs against and heats the cord that opposes it until it becomes light and burns, and it is called "fire"; but it is not possible at all to say that the current itself is fire, neither at the beginning [of the process], nor at the end.


The Prohibition of Toledot Ha-or


            It emerges from the Talmud Bavli that according to all views one may not cook on Shabbat with toledot ha-or, derivatives of fire – meaning, anything that received its heat from fire, such as blankets that were warmed through exposure to fire.  The Gemara clearly refers to a Torah prohibition, as the Mishna holds one liable for a chatat (sin-offering) for cooking an egg alongside a kettle.  According to many Rishonim, the sugya's primary chiddush is that even though this involves cooking not with fire itself, but rather with a derivative of fire, a Torah violation is nevertheless committed.  Indeed, the Rambam writes, "One who cooks with a derivative of fire is the same as one who cooks with fire itself."[1]


            The Yerushalmi (3:3), however, maintains that the status of cooking with toledot ha-or is subject to a dispute between Rabbi Yossi and the Chakhamim.  According to the Yerushalmi, Rabbi Yossi allows cooking with derivatives of fire because "ein tavshilo tavshil barur" – it does not cook completely.  The Yerushalmi gives no indication here as to whether the Chakhamim forbid cooking with toledot ha-or on the level of Torah law, or by force of rabbinic enactment.


            The Yerushalmi in Masekhet Pesachim (7:2) comments, "Rabban Gamliel does not equate derivatives of fire with fire, and the Rabbis equated derivatives of fire with fire."  The context of the Yerushalmi's discussion is the requirement that the paschal offering be roasted over fire.  The Tanna'im argue as to whether one may roast the korban pesach over a derivative of fire, or if it must be roasted only over fire itself.  This issue applies as well to the category of nega'im (skin infections) affecting areas on the body that had been "burned by fire" (mikhvat eish – Vayikra 13:24), where the question becomes whether the Torah refers also to burns caused by derivatives of fire.


            The discussion there in the Yerushalmi perhaps relates to our issue, as well, as to whether one violates bishul by cooking with derivatives of fire.  If so, then this issue is subject to a dispute among the Tanna'im.  Alternatively, however, one might differentiate between the two contexts.  Perhaps only with regard to the korban pesach, where the Torah specifically requires tzeli eish ("roasted by fire"), and nega'im, where the Torah speaks of mikhvat eish ("a burn by fire"), the Tanna'im dispute the status of fire derivatives.[2]  When it comes to the prohibition of cooking on Shabbat, however, the discussion focuses on a different question; as the Yerushalmi in our sugya indicates, the question relates to whether or not cooking with derivatives fire can be considered bishul barur – complete cooking.


            As for the final halakha, the accepted view is that cooking with derivatives of fire constitutes a Torah violation of bishul.  The Shulchan Arukh (318:3) rules, "Just as it is forbidden to cook with fire, so is it forbidden to cook with a derivative of fire, such as placing an egg alongside a pot or roasting it by breaking it on a garment that was heated by fire."  The Magen Avraham and Mishna Berura explain that the Shulchan Arukh here refers to a Torah prohibition.


The Implications of Allowing Bishul Through Solar Heat


            As mentioned earlier, both Rabbi Yossi and the Chakhamim allow cooking in the sun.  We will now turn our attention to the concepts underlying this halakha:


1) Rashi explains, "Because this is not the usual means of cooking."  The Acharonim understood Rashi as referring to the notion of ke-le-achar yad (that a melakha is not violated if it is done in an unusual manner; see Iglei Tal, and Kovetz Shiurim, Ketubot 202).  The Kovetz Shiurim explained that this principle applies to all laws in the Torah, and establishes that one violates Torah prohibitions only by performing the given act in its usual manner.  The Iglei Tal, however, raised the question of why cooking through solar heat would not be forbidden mi-de-rabbanan (by force of rabbinic enactment), given that Chazal forbade performing melakha in a manner of ke-le-achar yad.  We may suggest two answers:


            A) Chazal forbade performing a melakha with a minor deviation from normal procedure; significant deviations, however, would be permissible.  We find, for example, that the Gemara (Shabbat 141a) – as codified in the Shulchan Arukh (321:7) – allows crushing pepper in a bowl using the handle of a knife.  Halakha permits crushing pepper in this manner because it involves two deviations – using the handle of a knife, and doing this in a bowl, rather than a mortar.[3]  If one implements only one of these two deviations, the rabbinic prohibition would obtain.  In our case, then, cooking with the heat of the sun is considered a drastic deviation from the norm and is therefore permitted.


            B) The Iglei Tal distinguishes between two different types of deviation.  In some cases, one deviates only in the mode of action, and that deviation has no impact upon the final result; this type of shinui (deviation) is proscribed by Chazal.  Other deviations from standard procedure, however, cause a difference in the final product, and in these cases the act is permissible.  In the view of the Iglei Tal, such is the case regarding cooking with solar heat: a food cooked in the sun will have a different taste than the same food cooked through fire.  This approach is far from simple and requires further clarification.


2) We find amongst the Acharonim a different explanation for why bishul does not apply to cooking with the heat of the sun, namely, that no cooking of this sort occurred in the Mishkan.  The Noda Bi-yehuda (Tanina, Y.D. 43) writes, "This is due to the fact that for Shabbat we derive [the categories of forbidden activity] from the work in the Mishkan, and we [therefore] require a derivative of fire."  Rav Moshe Feinstein (Iggerot Moshe, O.C. 3:52) similarly writes:


Rather, they mean to say that although this cannot be considered [cooking], because it is not the standard procedure to cook in the sun, for [only] if one does not have wood and the item can be cooked in the sun he will then cook it in the sun, nevertheless, since one cooks in the sun only when he has no wood, this cannot be extracted from the cooking through fire that occurred in the Mishkan, to consider it a toleda [a derivative of bishul] because it does not resemble the cooking that occurred in the Mishkan.[4]


Rav Feinstein's approach is novel in that he believes that this factor alone – that cooking by the sun did not occur in the Mishkan – is insufficient.  Had it been standard procedure to cook in the sun, one would liable for this kind of cooking on Shabbat despite the fact that cooking was not done this way in the Mishkan.  It is only because people do not customarily cook with the sun that this kind of cooking does not resemble the cooking that occurred in the Mishkan.  According to this view, if nowadays the practice has become to cook in this fashion, then this type of cooking would constitute a Torah violation despite the fact that it did not occur in the Mishkan.  If, however, even nowadays people do not cook in this manner, then the fact that this was not done in the Mishkan would render it permissible.


3) The Noda Bi-yehuda, in the responsum referred to earlier, cites an additional consideration from the one who posed to him the question: "Because it is forbidden on Shabbat to heat or add heat only with the derivatives of fire, and all these [forms of cooking] are not derivatives of fire."  Some pointed to a verse in Sefer Divrei Hayamim II (35:13), "Vayevashelu  ha-pesach ba-esh" ("They cooked the paschal offering with fire") as a possible allusion to this theory, that the term "cooking" in the Torah, unless indicated otherwise, refers only to cooking with fire or a derivative thereof.  Rav Kook zt"l elaborated upon this notion in Iggerot Ha-Re'iya (vol. 1, p. 183):


Everything in the Torah refers to the standard manner among the majority, and since most cooking is done with fire, then certainly the plain meaning of bashel in the context of Pesach means this.  The same is true regarding Shabbat and the Mishkan itself, for we follow the manner of the majority of people.[5]


This easily explains why Halakha permits cooking with solar heat.  And Chazal did not legislate against it because cooking with the sun will not be confused with cooking through fire.


4) The Meiri, in explaining the halakha permitting cooking in the sun, writes, "Because the sun does not have the status of cooking."  His intent appears to be that the sun is a natural force, and it cannot be said that the human being harnessed it for his needs and cooked with it, given that the sun works as an independent force.  If it cooks an item, we must attribute this cooking to the sun, and not to the human being, just as it is assumed that drying wet clothes in the sun entails neither bishul nor libun (laundering), because this is a natural process, whereas drying clothing through exposure to fire is forbidden.  Only cooking through fire can be attributed to the individual, because it can then be said that he has harnessed the fire for his needs, as part of man's control over natural processes.  The sun, by contrast, is not subject to human control, and its effects are those of nature, and cannot be considered the actions of a human being.


            The Or Samei'ach (Shabbat 9:2) develops a similar idea in a slightly different context, regarding the halakha permitting cooking in a keli sheni, which he perceives as the ko'ach-kocho – the result of the result – of fire:


Just as ko'ach-kocho is not considered the original entity, the same is true regarding a derivative of fire: by virtue of the distance of its relationship to the [original] fire it loses the status of fire, and its heat is a natural heat that is not caused by fire, like the hot-springs of Tiberias.


It seems to me that this is also Rashi's intent when he wrote that people do not customarily cook with the sun; he refers not to the concept of shinui, deviating from the normal manner of performing melakha, but rather to the fact that human cooking does not occur through the means of solar heat.


            With these concepts in mind, let us now discuss the status of cooking through various means that do not involve fire in the simplest sense, and determine whether or not they would violate the Torah prohibition of bishul on Shabbat:


A) Cooking with metal heating elements and immersion heaters was addressed above, in the first section.


B) Microwaves

            Microwaves clearly do not involve fire, and therefore the question becomes whether the halakha permitting cooking with the sun applies to microwaves, as well.  Some of the reasons mentioned above would, indeed, apply equally to microwaves: "bishul" refers specifically to cooking through fire, and this type of cooking did not occur in the Mishkan.  (According to the Iggerot Moshe, however, even though this method of cooking was not used in the Mishkan, it would still be forbidden since it is used as a primary means of cooking.)  The other reasons, however, would not apply to this form of cooking: deviation from common practice, solar heat is a natural force that does not entail human ingenuity.  This issue is therefore at least subject to debate and uncertainty, and thus one should treat cooking in a microwave on Shabbat as potentially a Torah violation.  However, in situations where we would allow cooking on Shabbat to save a life, we might prefer using a microwave over fire, since according to some views it would not constitute a Torah prohibition.[6]


C) Cooking with chemical fire

            Certain materials can emit heat as a result of various chemical reactions, such as in the case of peat made from olive peels.  These materials are mentioned in the Mishna at the beginning of the fourth chapter of Masekhet Shabbat as items that increase the heat of foods embedded within them.  From the Rambam's remarks in his commentary to the Mishna it emerges clearly that cooking with this heat does not entail a Torah violation, since this heat does not originate from fire.  It would seem that those who explain the halakha of solar heat based on the fact that this method was not used in the Mishkan would similarly permit cooking with this chemically-generated heat.  There is some question as to what the status of this type of cooking would be according to the reasons of deviation from standard practice and the fact that solar heat is a natural force.  The straightforward implication of the Rishonim and poskim is that this does not constitute a Torah violation of bishul.[7]  In the next shiur we will discuss whether this would fall under the category of toledot ha-chama (derivatives of solar heat), cooking with which is forbidden by force of rabbinic enactment.


D) Cooking with plaster

            The Gemara (Shabbat 39a) cites a Berayta that records Rabban Shimon Ben Gamliel's comment, "One may cook on egg on a boiling hot roof, but one may not cook an egg on boiling hot plaster."  Rashi explains that hot plaster is a derivative of fire, and thus cooking with its heat constitutes bishul.  It appears that people would boil plaster in fire.  The Meiri (39a s.v. ha-sid) likewise considers plaster a derivative of fire, and claims that it is therefore subject to the Torah prohibition of bishul.  He then adds:


This "plaster" can be explained [as a reference to plaster] while it is still boiling as a result of fire. But if it cools and is then heated with the water that extinguished it, some are lenient.  But this is incorrect, because even after it has cooled it boils its location and the boiling that resulted from the fire has not left it; it rather intensifies after the moment of extinguishing.


The Meiri maintains that even when plaster cools, and water was poured on it and it regains its heat, this is not merely a chemical reaction, for the initial boiling that occurred as a result of the exposure to fire "has not left it."  (It seems that the lenient position that he cites maintains that the initial boiling is indeed negated once the plaster cools.)  In his view, there is a foundation of fire-generated heat that is strengthened by the water and the chemical process.  Since the basis of the heat is caused by fire, the plaster retains its status as toledot ha-or.


            The Shevitat Ha-Shabbat (Mevashel, 17) and Be'er Rechovot (44) cite the work Even Yekara (12) as equating plaster that was boiled in water with toledot ha-chama (derivatives of solar heat).  According to the Shevitat Ha-Shabbat, this view is identical to the lenient position cited by the Meiri.  The Shevitat Ha-Shabbat therefore wrote regarding the Meiri's comments, "I do not understand what he wrote, that they are lenient, for even if it will have a status of toledot ha-chama, it is likewise forbidden; there is no difference other than with regard to liability [meaning, whether one has violated a Torah prohibition]."  It seems to me, however, that the view expressed by the Even Yakara differs from the position cited by the Meiri.  The Even Yekara considers plaster a derivative of solar heat, cooking with which is forbidden mi-de-rabbanan, whereas the view cited by the Meiri perhaps considered plaster the same as solar heat itself, cooking with which is permissible.[8]


            As for the issue of whether plaster that was boiled in water constitutes a derivative of fire, we might draw evidence from the Rambam's commentary to the Mishna (Shabbat, beginning of chapter 4).  The Mishna introduces the prohibition of hatmana – burying a pot of food in a substance that will increase it heat, and it includes plaster in the list of substances that increase the heat of food embedded within them.  The Rambam comments: "Logic dictates that hatmana with these items is permissible, even on Shabbat, because they forbade on Shabbat adding heat or heating initially only with derivatives of fire, and these are not derivatives of fire."  The Rambam thus clearly did not consider plaster a derivative of fire, and presumably disagrees with the Meiri in this regard.[9]  He indicates that Chazal forbade using plaster only for hatmana, out of the concern that "one might say that just as increasing heat through hatmana is permissible, so is it permissible [to cook] over fire, and he will thus come to heat [food] over fire on Shabbat."  It seems that in his view, there is no prohibition against heating with these materials in a standard manner, since they are efficient in food preparation only when used for hatmana; only in cases of hatmana, then, were Chazal concerned that people would be misled.  All this requires further clarification.


            The Shevitat Ha-Shabbat and Rav Avidan (Shabbat U-mo'ed Be-Tzahal, pp.117-119) appear to rule in accordance with the Meiri's position, which considers plaster a derivative of fire.[10]


E) Cooking with fire from a magnifying glass


            The Halakhot Ketanot (1:189) writes:


There is no fire for which one is liable other than the natural, essential fire that is attached underneath by the element of air that is ignited; thus, fire that comes into being from a stone as a result of striking, which is the absorption of the air around it – this is essential fire, and one is liable for it and all its derivatives.  But that which comes into being through glass, when a ray of the sun becomes round and is kindled – this is a derivative of the sun, as is the case regarding that which emerges from the glass lantern, such that one who cooks with it on Shabbat is exempt [from punishment].


            This contention is very difficult to understand.  The Shevitat Ha-Shabbat (ibid.) explains that once fire is kindled, there is no difference between fire that comes into being through the rubbing of stones, fire that results from striking a match, and fire that comes about from the sun's rays.  True, if one does not light a fire, but rather concentrates the sun's rays and cooks through this concentrated heat, this would, indeed, constitute cooking with the sun, as it is the sun that heats the food.  The Shevitat Ha-Shabbat deliberates as to whether this should qualify as a derivative of solar heat and thus be forbidden mi-de-rabbanan.


            The explanation of these comments might arise from the perspective discussed earlier, which claims that cooking with the sun is permitted because it constitutes a purely natural force.  In this instance, one manages to harness this natural force for his purposes, and this type of cooking should be deemed toledot ha-chama, rather than cooking directly in the sun.  In my opinion, however, we should consider this cooking with the sun itself, such that it is permissible on Shabbat.  For purposes of practical halakha, this issue requires further analysis.  The Shevitat Ha-Shabbat himself concludes, "It would seem that one may be lenient [and allow this kind of cooking] through a gentile in situations of minor illness."


F. Heat generated as a result of rubbing pieces of wood or metal against each other has the status of a derivative of solar heat.  This emerges from one of the explanations given to the Gemara's comment (39a) that one should not bury a pot of food in "the dust from the roads."  According to one view, this refers to dust that was heated as a result of the carriages traveling on the road. (Shevitat Ha-Shabbat, ibid., and Be'er Rechovot, 45)


In the next shiur, we will bs"d discuss the prohibition against cooking with derivatives of solar heat and the status of solar boilers (dudei shemesh) in this respect.




1.  In an earlier shiur about the status of cooking in a keli rishon, we noted the difficulty in explaining the view that this constitutes merely a rabbinic prohibition given the sugya's conclusion that cooking with toledot ha-or is forbidden by Torah law.  The reader is referred to our discussion there for further elaboration.

2.  See the parallel discussion in the Bavli, Masekhet Pesachim 75a, as to whether items heated by fire qualify as "fire" for purposes of roasting the korban pesach.

3.  The Beit Yosef there, citing the Shibolei Ha-leket, identifies the two deviations as deviating from a stone pestle to a wooden pestle, and then from a wooden pestle to the handle of a knife.

4.  In his view, the factor of shinui, deviating from normal practice, would not apply; since one who does not have wood does cook in the sun, we cannot consider this form of cooking a deviation from normal practice.

5.  The novelty in Rav Kook zt"l's comments is that the concept of "cooking" in the Torah is to be understood on the basis of the standard practice of most people, rather than the etymological meaning of the term.

6.  It should be emphasized that using a microwave entails additional prohibitions related to the activation of the device, even before we even come to the cooking aspect, but we will not elaborate on these issues in this context.

7.  The poskim discuss (as we will soon see) the issue of whether plaster that had been heated and then cooled is to be considered toledot ha-or; this discussion presumes that if we do not consider it toledot ha-or, then cooking with it would not entail a Torah prohibition.

8.  In the next shiur we will address the precise definition of toledot ha-chama and what this category includes.

9.  The question remains how the Rambam would understand the Berayta (39a) that forbids cooking an egg on boiling plaster.  He might explain it as referring to plaster that had been boiled by fire, or understand the prohibition differently from Rashi, who wrote that this is forbidden because plaster is a derivative of fire.

10.  They make no mention of the proofs we brought from the Rambam, but in any event one should be stringent in this regard.

This website is constantly being improved. We would appreciate hearing from you. Questions and comments on the classes are welcome, as is help in tagging, categorizing, and creating brief summaries of the classes. Thank you for being part of the Torat Har Etzion community!