Hacham David ruled that it is permitted to put a food with congealed fat back on the blech or hotplate. The congealed fat is considered a solid because that was its state when put on the blech, even though it melts after a few minutes.
Based on this reasoning, would it also be permitted to take frozen soup out of the freezer and place it on the blech to defrost and reheat? At first glance it would seem to be the same case as the congealed fat. In both instances, the food was a solid at the time of being placed on the blech, and it should, therefore, theoretically, be permitted.
However, Hacham David rules that reheating the frozen soup is forbidden. He explains the difference between the case of the congealed fat and the frozen soup using the following halachic principle: Any food that melts by itself and reverts to a liquid at room temperature is considered a liquid. Whereas, a food that only reverts to a liquid when heated is considered a solid.
According to this principle, the congealed fat is considered a solid because even if it was left on the counter at room temperature, it would remain jelly. Therefore, it is considered a dry solid and can be reheated on the blech. Frozen soup, on the other hand, would melt even at room temperature and is thus considered a liquid and may not be reheated.
1. It is permitted to reheat a food containing congealed fat on the blech.
2. It is forbidden to defrost and reheat a frozen soup on the blech.