Question: Is it permissible to cover a pot of food on an electric hotplate with a towel on Shabbat?
Answer: Long ago, it was customary to cover a pot of food with (or immerse it in) dirt or sand in order to retain the food’s heat.
Heat Increasers vs. Heat Retainers
Some would immerse the pot of food into a substance that would add to the heat of the food, such as the sediment of olives or sesame seeds. Others would cover the pot of food with clothing which is considered a covering which merely retains the food’s heat but does not add heat. Covering it with olive or sesame sediment, however, not only retains heat but adds heat as well.
The Prohibition to Insulate Using Heat Increasers
Our Sages enacted that one may not cover a pot of food with something which adds heat even before the onset of Shabbat lest one see that the food is not hot enough and this will cause him to cover the pot with embers containing live coals which would constitute transgressing a Torah prohibition. It is therefore forbidden to insulate a pot with olive sediment, salt, and any other substance which adds heat even before Shabbat. However, insulating a pot with clothing, pillows, blankets, or anything else which does not add heat is permissible before the onset of Shabbat.
Insulation Using Heat Retainers
Nevertheless, on Shabbat itself, one may not insulate the pot even with things that do not add any heat lest one find the food to be cold and this will cause one to heat it up in a forbidden manner. It is therefore forbidden to cover a pot containing hot food on Shabbat in order to retain its heat. It is certainly forbidden to cover a pot of food on the hotplate with clothing or towels on Shabbat in order to hasten its cooking and heating time.
All of the applies on Shabbat. Before the onset of Shabbat, however, it is permissible to insulate a pot of food with a substance that merely retains heat, such as clothing. It is therefore permissible to cover a pot of food on the hotplate with a towel before the onset of Shabbat since the towel does not add any heat. In the following Halachot, we shall, G-d-willing, summarize and explain these laws further.