Boyd’s Bible Dictionary:
(ground). Hot stones (Gen. 18:6). Pan or brazier (Jer. 36:23).
Strong’s Dictionary of Hebrew Words:
Meaning:
of uncertain derivation; a fire-pot or chafing dish
From Manners and Customs of the Bible:
Jeremiah 36:22. The king sat in the winter house in thy ninth month: and there was a fire on the hearth burning before him.
Ach, “hearth,” is a portable furnace or stove. The rooms of Oriental houses are sometimes warmed at the present day by means of such pots or furnaces. “They have the form of a large pitcher, and are placed in a cavity sunk in the middle of the apartment. When the fire has burnt down, a frame like a table is placed over the pot, and the whole is then covered with a carpet; and those who wish to warm themselves sit upon the floor and thrust their feet and legs, and even the lower part of their bodies, under the carpet” (Robinson's Gesenius).