The Gemara in Masechet Shabbat (37) discusses the case of a coal stove upon which one had poured ashes, extinguishing the flames, but the coals somehow were reignited. Ketima – pouring ashes over coals before Shabbat to extinguish them – allows one to keep food on the coals to serve on Shabbat (Shehiya), and to return cooked food to the coals on Shabbat to be warmed up (Hazara). The Gemara cites the ruling of Rav Safra, citing Rabbi Hiya, that one may leave food on the coals and return food to the coals even if the coals became rekindled after the ashes had been poured onto the coals. Once a person demonstrated that he has no interest in the fire, we are no longer concerned that he may stoke the coals, and it is thus permissible to leave food on the coals even if the coals became reignited.
Rav Moshe Feinstein (Russia-New York, 1895-1986), in his Iggerot Moshe (Orah Haim vol. 4, 74), addresses the question of whether this Halacha would apply to a Blech, the modern-day equivalent of Ketima. Placing a Blech over a stove allows one to leave food on the Blech before Shabbat to keep it warm for the Shabbat meal, and to return food to the Blech on Shabbat. If a Blech falls off the stove on Shabbat, would it be permissible to put a pot of food directly over the fire? Seemingly, just as a Blech functions as Ketima, which is effective even after its effects end and the coals are reignited, it should be permissible to place food on the fire even after the Blech falls off the stove.
Rav Moshe, however, distinguishes between the two cases. In the case of the ashes, even after the coals are rekindled, the original ashes remain on the stove, and thus there is a remnant of one’s initial action. This is quite different from the situation of a Blech that falls off a stove, in which case nothing remains of the action of placing the Blech on the fire. Therefore, even though in the Gemara’s case one may place food on the coals even after they are rekindled, one may not place food on the fire after the Blech falls off. However, Rav Moshe writes that if the Blech did not fall off completely, and some of it remains on the stove, then one may place food over the exposed flame, since, as in the case of Ketima, some remnant of one’s original action remains.
Summary: If one had placed a Blech over the burners on the stove to allow leaving food on the stove on Shabbat, and the Blech fell off the stove, one may not then place food over the exposed flame, unless the Blech had fallen off only partially.
