The prohibition of Hatmana forbids "insulating" food before Shabbat in a manner that generates heat. It was customary in ancient times to keep pots warm by wrapping fabrics or garments around them. If the material does not only preserve the pot’s heat, but actually generates additional heat, then it is forbidden to insulate the pot before Shabbat in preparation for the Shabbat meal.
The Shulhan Aruch (Orah Haim 247:8) rules that even if the material used to wrap the pot does not generate heat, Hatmana will still be forbidden if the pot is placed on a fire. Since the pot is insulated and heat is being generated by the fire below, this qualifies as "Hatmana Be’dabar Ha’mosif Habal" – insulating in a manner that increases the food’s heat – and is thus forbidden.
In light of this Halacha, we might question the practice that many people have of wrapping food (such as Halla) in tin foil and placing it on the "blech" over the stove before Shabbat. Although tin foil does not add heat, this should, seemingly, be forbidden since the food is on the fire. Indeed, Rav Moshe Feinstein (1895-1986) rules in his Iggerot Moshe (4:74) that if one wraps food in tin foil for the purpose of maintaining its heat, he may not place the wrapped food on the "blech" before Shabbat. Interestingly, however, Rav Moshe’s son, Rav Reuven Feinstein, reported that his mother frequently placed kugel wrapped in tin foil on the "blech" before Shabbat. Rav Reuven explained that his father’s ruling applied only to situations where the food is wrapped for the specific intent of keeping it warm, and generally speaking, people wrap food in foil for the sake of cleanliness, either to keep the food clean or to avoid crumbs falling on the "blech." In essence, the tin foil serves as a utensil to contain the food, not as an instrument for keeping it warm. As such, placing it wrapped on the "blech" or in the oven would not constitute Hatmana, as this prohibition applies only when the intent is to preserve the food’s heat. However, it would be forbidden to wrap food if the intent is to keep it warm.
Summary: It is permissible to wrap food with tin foil and place it on the "blech" before Shabbat, as long as the food is wrapped for the purpose of cleanliness, such as to keep the food or the "blech" clean, and not for the purpose of insulating the food so it stays hot.
