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.