Among the prohibitions that apply to food preparation on Shabbat is "Hatmana," which literally means, "insulating." This term refers to insulating hot food before Shabbat in order to keep it warm for the Shabbat meal. In the times of the Gemara, it was common for people to embed pots of food in coals to preserve the food’s temperature. The Sages forbade insulating food before Shabbat out of the concern that one might stir the coals in order to preserve their heat. Stirring hot coals on Shabbat constitutes a Torah violation, and the Sages enacted the prohibition of "Hatmana" as a safeguard against this violation.
This Halacha gives rise to a question concerning the common use of crockpots for Shabbat food. A crockpot consists of a pot that is placed in a metal encasing which contains a heating element and cooks the food inside the interior pot. People commonly place food in the crockpot on Friday and leave it on, so that the food cooks through the night and is ready for consumption on Shabbat morning.
At first glance, although the use of the crockpot has become widespread, it appears to transgress the prohibition of "Hatmana." After all, here, too, one encloses a pot of food in order to preserve its heat – the precise definition of "Hatmana."
This question was recently brought to Hacham Ovadia Yosef, who ruled that one may, in fact, use a crockpot for Shabbat in the manner described above. He arrived at this conclusion based on the combination of a number of factors. Firstly, two Rishonim (Medieval scholars) – Rabbi Yeshaya Ha’rishon and Rabbenu Simha Gaon – maintained that the "Hatmana" prohibition applies only to food that one intends to eat on Friday night. Since a person has his mind on this food from the time Shabbat begins, the Sages were legitimately concerned that one may stir the coals in an effort to maintain the food’s heat. However, a person does not give too much thought to the food intended for Shabbat day. Since there is plenty of time for the food to become warm, it is unlikely that one will stir the coals to keep the food hot. The aforementioned authorities therefore held that the "Hatmana" prohibition does not apply to the food intended for Shabbat day.
Secondly, the Ashkenazim follow the position of the Rama (Rabbi Moshe Isserles, Poland, 1525-1572) that "Hatmana" means a complete enclosing of the pot. According to the Rama, one may enclose a pot to retain its heat if he leaves the top of the pot exposed. This ruling would allow the use of a crockpot, since the heating element covers only the bottom and the sides of the inner pot, and not the top. (The pot cover is considered part of the pot itself, and not part of the encasing.) And although the Hazon Ish (Rabbi Avraham Yeshaya Karelitz, 1879-1954) disputed the Rama’s ruling, and applied "Hatmana" even in cases where the pot’s top remains exposed, the case of the crockpot still differs from standard "Hatmana" in that the heat source does not directly touch the pot. There is some space between the heating element and the pot containing the food, and we may thus invoke the ruling of the Ramban (Rabbi Moshe Ben Nahman, 1194-1270) and the Ritba (Rabbi Yom Tob of Seville, 1250-1330), that "Hatmana" requires direct contact between the the pot and the encasing.
Hacham Ovadia therefore employed in this context the principle of "Sefek Sefeka," or "double doubt." Since we have several factors that according to some authorities render this situation permissible, we may combine these factors to allow the use of a crockpot on Shabbat. Rabbi Yosef Shalom Elyashiv (contemporary) similarly ruled leniently after being shown a crockpot and how it operates, and this is also the ruling of Rabbi Shemuel Wosner (contemporary).
It should be noted, however, that if one uses a crockpot that has a dial to adjust the heat level, he must either remove or cover the dial before Shabbat, so that he will not mistakenly adjust the dial on Shabbat. This point was made by Rabbi Moshe Feinstein (Russia-New York, 1895-1986) as well as Hacham Ovadia Yosef.
Summary: It is permissible to place food in a crockpot before Shabbat so that it will cook in preparation for the meal on Shabbat day. One must, however, ensure to remove or cover the dial that adjusts the heat level.
