The Gemara in Shabbat states that the Hachamim instituted a gezeira prohibiting leaving food cooking before Shabbat on the fire during Shabbat. The Hachamim were concerned that since people are hungry and want their food cooked faster, they are likely to unwittingly stoke the coals when they see them starting to dwindle. Stoking coals would violate the Torah prohibition of cooking or burning.
To overcome this problem, the Hachamim said that one has to create a reminder for himself before Shabbat so that he shouldn’t come to stoke to coals. One type of reminder is to perform ketima- to take ashes and to spread them over the coals, covering the open flame.
Cooking in Modern Times- Today, we generally do not cook with coals. Some Rabbis, therefore, argued that the gezeira does not apply to leaving food on our gas flames, because there are no coals to stoke. However, the Panim Meirot explained, if gas stoves would have existed in the time of Hazal, they would have included it in the gezeira. Using the knobs to turn up the gas is the equivalent of stoking the coals. Therefore, he ruled that the flame of our gas stovetops are not considered katum or garuf.
How, then, can we leave food on open stovetops going into Shabbat. Hacham Rav Ovadia a"h says: If one places a metal sheet, called a blech in Yiddish, over the stovetop, it is considered katum. Just like the spreading of ashes over the fire serve as a reminder because it is not the normal way to cook, so too, putting a blech over the fire serves as a reminder not to adjust the flame.
Hacham Rav Ovadia a"h says that using a blech makes the stovetop kosher to place on it any type of food before Shabbat. This is also the opinion of the Kaf HaChaim (commentary to the Shulhan Aruch, by Rabbi Yaakov Haim Sofer (1870-1939), Rav Moshe HaLevy and many other Hachamim.
Use of Shabbat Hotplate- Hacham Rav Ovadia a"h rules that an electric hotplate, a plata Shabbat, is even better than a blech. Since it doesn’t have buttons or knobs, there is no way that one might inadvertently adjust the heat on Shabbat. Although Hacham Ovadia Hedaya (1890-1969), one of the leading Sephardic authorities of 20th-century Israel) initially disagreed, he later retracted and said the custom is to permit the use the electric Shabbat plate (Yaskil Avdi 7).
This is also the opinion of Rav Moshe Feinstein (1895-1986, Posek in the United States) in the Igrot Moshe. He takes the argument one step further and says the original gezeira would not apply to turning the gas higher. Hazal were only concerned about stoking the existing coals, not about bringing additional wood to the fire. Increasing the gas is analogous to adding new wood and would therefore never have been prohibited. He concludes that it is clearly permitted to put a blech on top of the stovetop.
Although the Chazon Ish and the Shevet HaLEvy did not approve of it, Rav Ovadia a"h rejects their proof in his first volume of Chazon Ovadia.
Using a blech or a Shabbat hotplate is the recommended practice for the Jewish home. By doing so one avoids many complicated halachic issues as to which types of foods at which stages of cooking can or cannot be left on an open flame. This preferred way enables the Jewish woman to prepare for Shabbat without worry and concern.
Summary: It is permissible to keep food on the fire on Shabbat by using a blech or an electric Shabbat hotplate. Doing so solves many Halachic problems that would otherwise arise.
