Boyd’s Bible Dictionary:
(glow). The coal of scripture is charcoal, or embers (Prov. 26:21; John 18:18; 21:9); heated stones (1 Kings 19:6; Isa. 6:6); metaphorical (2 Sam. 22:9-13; Psa. 18:8,12-13; Rom. 12:20).
Concise Bible Dictionary:
Charcoal
Mineral coal is now known to exist in the Lebanon range, but was unknown in Biblical times. Fires were seldom needed for warmth, and were as a rule used only for the cooking of food: the fire named in John 18:18 was in the night; food was cooked by charcoal or by warming the ovens with any vegetable refuse. The coal generally referred to in the Old Testament was charcoal; but other words are used which imply the hot or glowing stones on which cakes were cooked (1 Kings 19:6; Song of Sol. 8:6; Isa. 6:6; Hab. 3:5).
Heaping coals of fire on an enemy’s head by kindness (Prov. 25:21-22; Rom. 12:20) becomes a test to him (as metal is tested by the fire), the kindness shown him will either bring about contrition and friendship, or harden him yet the more.
Strong’s Dictionary of Hebrew Words:
Meaning:
perhaps from an unused root probably meaning to be black; a coal, whether charred or live