Responsa על שבת 4:3

Regarding covering food up [to keep it warm on Shabbat]. Is there a hint of a transgression in our contemporary stoves, whereby we cover up the pot and sprinkle ash on top of the coals? Also regarding those who bury the pot in the ground, and bury hot coals around the pot, is all this permitted or not?¹See Tosafot on Shabbat 48a:1, which starts by questioning the practice of burying food, even when the coals are covered with ash. The Ri then gives a reason for allowing contemporary buried ovens (called אשטר"א, l'âtre or l'aistre) that have space built in between the walls and the coals. This practice and reasoning is repeated by the Rosh on Shabbat 4:2.