With regard to a stove that was lit on Shabbat eve with straw or with rakings, scraps collected from the field, one may place a pot of cooked food atop it on Shabbat. The fire in this stove was certainly extinguished while it was still day, as both straw and rakings are materials that burn quickly. However, if the stove was lit with pomace, pulp that remains from sesame seeds, olives, and the like after the oil is squeezed from them, and if it was lit with wood, one may not place a pot atop it on Shabbat until he sweeps the coals from the stove while it is still day or until he places ashes on the coals, so that the fire will not ignite on Shabbat.
Rabba bar bar Ḥana said that Rabbi Yoḥanan said: With regard to a stove that he swept out or covered with ashes before Shabbat and subsequently reignited on Shabbat, one may leave hot water that was already completely heated and cooked food that was already completely cooked upon it, even if the coals were from the wood of a broom tree [rotem],which are very hot and long-burning.
The rotem, or broom tree, known as the desert broom, Retama raetam, is a tall bush with branches that sometimes reach the height of a tree. It grows primarily in sand and in dry riverbeds. The branches of the broom are greenish-gray, and during most of the year they do not grow leaves. The broom blooms at the end of winter with an abundance of white flowers. It was common to make coals from the roots and trunk of the broom. In the Bible (Tehillim 120:4) and in several places in the Talmud, it is emphasized that the coals made from the broom burn and retain their heat longer than other types of coals.