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 O.T. 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 Solomon 8:6; Isaiah 6:6; Habakkuk 3:5 .
Heaping coals of fire on an enemy's head by kindness (Proverbs 25:21,22; Romans 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.