One may not wrap pots on the plata in towels or blankets even before Shabbat. Although the towels or blanket do not generate heat, nevertheless since the pot is on the plata and the plata generates heat, this is considered insulating with something that generates heat, which is forbidden even before Shabbat.
The prohibition of hatmana applies only when the pot is wrapped on all sides, but if it is not surrounded there is no prohibition. Therefore, one may wrap most of the pot on the plata with a towel or blanket, because as long as the pot is not entirely covered, there is no prohibition of hatmana. One may do this even on Shabbat (Rema). Similarly, one may place a pan on top of a pot and drape a towel over the pan; as long as the towel does not touch the sides of the pot, it is not considered hatmana (SA 257:8). One must take care not to place a damp towel on the pot, because drying using heat transgresses the prohibition of laundering (below 13:3).
Some prohibit using a slow cooker (a pot made of ceramic or porcelain, surrounded by a metal housing that contains a heating element (“Crock-Pot”) because of hatmana, since the pot is surrounded by the heating element. If the food inside is uncooked, they maintain that there is an additional prohibition, as this constitutes leaving a pot on an uncovered fire. In contrast, others feel there is no prohibition of hatmana in this case, since the top of the pot is visible. In order to avoid the issue of hashhaya, if the food is not fully cooked, one must cover the knobs that control the temperature. This is the halakha in practice.
. SA 253:1; 257:8 lists two stringencies pertaining to hatmana:1) If an item that does not generate additional heat, such as an item of clothing, is resting on something that generates heat, like a plata, this is considered insulating with a heat-generating item.
2) Insulating with an item that generates heat is prohibited even if the pot is only partially covered (as in the case of insulating with coals). Therefore, according to this opinion, one may not cover a pot that is on the plata with a towel even if the cover is only on top or on one side, and even if it is placed there before Shabbat, since this is considered insulating with a heat-generating item. This is also the opinion of Menuĥat Ahava 1:3:19-20.
In contrast, Yabi’a Omer 6:33 is lenient and allows one to fully wrap a pot on the plata with blankets before Shabbat. This is because according to Ramban, as long as there is space between the heat source and the pot, it is not considered insulating with a heat-generating source. In R. Ovadia’s opinion, since there is space between the plata’s heating element and the metal sheet that comes into contact with the bottom of the pot, there is no hatmana here. This is also the opinion of R. Messas (Shemesh U-magen, OĤ 3:50).
Rema has an intermediate position. If there is a heat source below the pot, even if the covers above do not generate heat, they are considered as if they do generate heat. However, if the pot is not surrounded on all sides, it is permitted; the prohibition of hatmana applies only if the pot is completely wrapped. This is the practice of Ashkenazim, as recorded in SSK 1:77. Or Le-Tziyon 2:17:10 is lenient in accordance with the Sephardic custom and Yabi’a Omer, but adds that it is preferable not to cover the pot entirely (like the Ashkenazic custom) and that when possible, it is preferable to be stringent like SA. In the main text I follow Rema’s intermediate position.
. Those who do not allow slow cookers to be used on Shabbat maintain that if most of the pot is surrounded, and certainly when all of the pot is surrounded and only the top is not, it is still considered hatmana with heat-generating material. Therefore one may not use a slow cooker on Shabbat, even if it is activated on Friday afternoon. This is the opinion of R. Shlomo Zalman Auerbach (Minĥat Shlomo 2:34:5) and Orĥot Shabbat ch. 2 n. 149, in the name of R. Yosef Shalom Elyashiv. According to them, a slow cooker could only be used if one inserts something to lift the pot and distance it from the heating element. There is also the problem of hashhaya. Since slow cookers have temperature controls, there is a concern that one might turn up the temperature. To avoid this problem, one must either cover the heating element to lessen the heat or make sure that the food is already completely cooked when Shabbat begins. Those who are permissive feel that since the pot is not entirely covered, it is not considered hatmana (based on Rema 253:1 and MB ad loc. 48). If the food is not fully cooked, one should cover the knobs (see section 17 above). This is also the opinion of R. Ovadia Yosef in Ĥazon Ovadia, Shabbat vol. 1, p. 64 and Shevet Ha-Levi 9:52. Since this is a rabbinic law, the halakha follows those who are lenient. Additionally, the lenient position is compelling. See Harĥavot.