Heat Sources
Kli Rishon A kli rishon (כלי ראשון, "first vessel") is a vessel that was heated directly on a flame or other source of heat. Even when removed from the source of heat, this vessel maintains its status as a kli rishon, and possesses the capacity to enact bishul on any type of food placed within it. This capacity remains until the pot and its contents cool below the temperature of yad soledet bo (יד סולדת בו, the degree of heat "from which the hand recoils").
Rabbi Simcha Bunin Cohen, The Shabbos Kitchen Mesorah Publications, Ltd. 1991, page 17The prohibited activity of bishul is separate and distinct from that of havarah (הבערה, "kindling a fire"). Performing bishul with a pre-existing flame is forbidden on Shabbat The prohibition of bishul, however, is not limited to the use of fire as a heat source; it is forbidden to perform bishul with any source of heat,whether it be an actual flame, or an electric stove/range, a hot plate, an urn or a microwave oven. Moreover, placing food into a kli rishon may constitute bishul in certain instances. However using heat from the sun to cook is allowed on shabbat. (talmud shabbat 39a)
Read more about this topic: Sabbath Food Preparation
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