The study of Gmara Shabbat on Daf Lamed Tet is dedicated by Benjamin Stavrash for a complete recovery for all the ill of Israel, requesting that the Almighty grant them healing of soul and body, and may their recovery be near at hand. Our analysis begins on Lamed Chet Amud Bet, starting from the Mishna at the bottom of the page, which discusses the laws of cooking on Shabbat.
The Mishna teaches that one may not place an egg next to a hot water urn for the purpose of roasting it. This refers to a case where an egg is placed against the outer wall of a copper kettle that contains hot water. Even if the kettle is no longer on the fire, it retains the status of a primary vessel. The heat from the walls of this vessel is sufficient to roast the egg to the point that it begins to roll, a physical change that indicates it has been cooked. Such an act is forbidden because it constitutes the labor of cooking on the holy day. Rashi clarifies that this roasting, even if only partial, renders one liable for the violation.
The Mishna continues with a second ruling: one may not crack an egg onto a cloth that has been heated by the sun in order to fry it. This introduces the concept of cooking with derivatives of the sun—objects that have absorbed solar heat. While everyone agrees that cooking directly in the sun is permissible, the Chachamim and Rabbi Yossi dispute the status of these derivatives. The Chachamim forbid it, while Rabbi Yossi permits it. Rabbi Yossi reasons that while cooking with fire or derivatives of fire is a Torah prohibition, the sun and its derivatives do not fall under this category. The Chachamim, however, apply a Rabbinic decree, forbidding derivatives of the sun out of a concern that people might confuse them with derivatives of fire.
A third case in the Mishna involves insulating an egg in sand or road dust to roast it. Notably, in this instance, there is no dispute; even Rabbi Yossi concurs with the Chachamim that this is forbidden. The Gemara explores why Rabbi Yossi does not maintain his permissive stance here. Rabbah suggests the prohibition is a decree to prevent one from insulating an egg in hot ashes, which would be a form of cooking with fire. Rav Yosef offers a different reason, suggesting the concern is that one might inadvertently move the earth and dig a hole, which is a separate violation. The practical difference between these two views arises in a case of loose earth that has already been dug; according to Rav Yosef, there would be no concern of digging, whereas according to Rabbah, the concern of confusing it with ashes remains.
This discussion leads to a famous incident involving the people of Tiveria. They constructed a system where a pipe of cold water ran through the hot springs of Hamei Tiveria to heat the water for use on the holy day. The Chachamim informed them that if this heating occurred on the day of rest itself, the water was considered to have been cooked on Shabbat and was therefore forbidden for both bathing and drinking. This remains true even if the process began before the day started. If the incident occurred on Yom Tov, the water would be forbidden for bathing but permitted for drinking, as the necessity of food preparation allows for cooking on a festival, but not for the purpose of full-body bathing.
The Gemara questions the halachic status of an egg that has been partially cooked, such as one roasted until it can roll. Rav Yosef rules that even this partial roasting makes one liable for a sin offering, as the egg has reached an edible state, which constitutes a Torah-level violation of cooking. Mar brei deRavina supports this by citing a rule found on Daf Kuf Mem Hey Amud Bet: anything that was fully cooked before the day of rest may be soaked in hot water, even in a primary vessel that is off the fire, because of the principle that there is no further cooking after an item has already been cooked. However, if an item was not cooked beforehand, it may only have hot water poured over it.
A specific exception is made for items that are easily cooked, such as certain types of salted fish from Sefarad. Because these items are so soft that even pouring water over them completes the cooking process, doing so is strictly forbidden. This reinforces the conclusion that any act that completes the preparation of food for edibility is considered a Torah-level violation. The Gemara notes that while direct sunlight is permitted for cooking because it cannot be confused with fire, derivatives of the sun remain a point of contention. Rav Nachman explains that the Chachamim forbid derivatives of the sun as a safeguard, while Rabbi Yossi does not feel such a decree is necessary.
Further analysis of the Mishna on Kuf Mem Vav shows that one may place food in a pit to keep it cool or place cold water in the sun to warm it. The Gemara initially wonders if this follows Rabbi Yossi, but concludes that even the Chachamim permit direct solar heating. The dispute is strictly limited to derivatives. Regarding the sand case, Rabban Shimon Ben Gamliel teaches in a Braisa that one may roast an egg on a hot roof but not in hot lime. This supports Rabbah's view; since one does not typically bury items on a roof, there is no concern of it appearing like insulation in ashes, whereas lime is used for insulation.
Rav Chisda derives a broader principle from the story of the people of Tiveria: the prohibition of insulation in a heat-adding substance applies even if the insulation was set up before the day of rest began. This is why the Chachamim forbade the water in the pipe. While the Beit Yosef and other Rishonim discuss the exact source of this law, Rav Chisda emphasizes that the consequence of such an act is that the item becomes forbidden for use. Ulla suggests that the law actually follows the people of Tiveria, implying a permissive view like Rabbi Yossi, but Rav Nachman retorts that the people of Tiveria themselves eventually conceded to the Chachamim and dismantled their system.
Finally, the Gemara clarifies the laws of bathing. While washing the entire body in hot water is generally forbidden due to a Rabbinic decree, washing only the face, hands, and feet is permitted if the water was heated before the day of rest. Rav Ika bar Chananya explains that the Mishna's prohibition regarding the Tiveria springs refers to rinsing the entire body, which is a form of showering. This follows the view of Rabbi Shimon, who permits a full-body shower in water heated before Shabbat, as opposed to Rabbi Meir, who forbids it, and Rabbi Yehudah, who distinguishes between hot and cold water. Rav Chisda notes that this dispute applies when the water is in a vessel, as people might think it was heated on the day itself; however, if the water is in the ground, some views are more lenient. Rabba bar bar Chana concludes in the name of Rabbi Yochanan that the law follows Rabbi Yehudah, meaning one may not wash the entire body in hot water, but cold water remains permissible.