Maseches Shabbos, Mishnah 1. This mishnah continues to deal with the laws of Shabbos, and the discussion now turns to cooking on Shabbos and to what one may leave in place so that it will continue cooking by itself on Shabbos. The subject of the mishnah is the law of the kirah: the kirah was an oven that served as a cooking surface, built in the shape of a rectangle, with the fire burning inside it and two openings on its upper surface upon which the pots were placed.
The question before us is this: may one place pots on top of the kirah before Shabbos so that they will remain there and cook on Shabbos, and under what circumstances? The answer depends on the type of fuel with which the kirah was heated:
"Kash u'gevava" - straw and stubble, which burn up quickly and do not leave coals behind. Therefore one may place food on the kirah and leave it there, since there is no concern that he will come to stir the coals - after all, with this kind of fuel there are generally no coals to stir.
"Gefes va'etzim" - gefes is the residue of crushed olives or crushed sesame, and it is liable to leave coals; and likewise wood, which certainly produces coals. With this kind of fuel one may not leave the pot in place before Shabbos so that it will remain there on Shabbos, unless he raked out the coals and removed them, or at the very least placed cooled ashes over the coals in order to cool them down.
Both of these methods - raking out the coals or banking them with ashes - create what we may call a 'marker': a noticeable sign indicating that the person has no interest in the coals burning hot, and it will remind him not to stir them. Doing one of these two things is the condition for leaving food on the kirah on Shabbos.
Which food are we talking about?
As the Gemara explains, this concern applies only to food that has not been fully cooked before Shabbos:
Fully cooked - there is no concern, and it is permitted.
Not fully cooked - here the laws of raking out and banking with ashes apply.
Completely raw, and not going to be ready that night - here too there is no concern, since once a person knows it will not be ready that night, there is no concern that he will come to stir the coals.
The dispute between Beis Shammai and Beis Hillel regarding what may be left in place:
Beis Shammai: one may leave only hot water in place, but not cooked food, even if he raked out the coals or placed the ashes. The reason: even when raking, it is impossible to remove all the coals, and a concern of stirring always remains.
Beis Hillel: one may leave both in place - both hot water and cooked food - provided that he did one of the two things: raked out the coals or banked them with ashes.
The law of returning a pot on Shabbos:
The question arises regarding a pot that was taken off the kirah on Shabbos when it was already fully cooked: may it be returned? Here the concern is not that he will stir the coals, but rather that it may look as though he is cooking for the first time - and even though this is not the case, that is how it appears. Therefore raking or banking is certainly required here: removing the coals or placing the ashes.
Beis Shammai: one may remove the pot - and in their view only hot water is under discussion - but one may not return it to the kirah.
Beis Hillel: one may even return it, provided that he raked out the coals or placed ashes there.
The Gemara adds further conditions for returning the pot: that he did not let go of the pot and did not set it down somewhere else. If he set it down, it looks as though he is placing it on the kirah for the first time.
It is worth noting that we have explained the mishnah according to one of the approaches found in the Gemara, and the matter depends on a dispute among the Rishonim as to how to understand this mishnah: we explained it according to the approach of the Bartenura, the Rif and the Rambam, while Rashi and Tosafos explain it differently.