It is by no means certain that the Hebrews were acquainted with mineral coal, although it is found in Syria. Their common fuel was dried dung of animals and wood charcoal. Two different words are found in Hebrew to denote coal, both occurring in Pr 26:21, "As coal [Heb. peham; i.e., "black coal"] is to burning coal [Heb. gehalim]." The latter of these words is used in Job 41:21; Pr 6:28; Isa 44:19. The words "live coal" in Isa 6:6 are more correctly "glowing stone." In La 4:8 the expression "blacker than a coal" is literally rendered in the margin of the Revised Version "darker than blackness." "Coals of fire" (2Sa 22:9,13; Ps 18:8,12-13, etc.) is an expression used metaphorically for lightnings proceeding from God. A false tongue is compared to "coals of juniper" (Ps 120:4; Jas 3:6). "Heaping coals of fire on the head" symbolizes overcoming evil with good. The words of Paul (Ro 12:20) are equivalent to saying, "By charity and kindness thou shalt soften down his enmity as surely as heaping coals on the fire fuses the metal in the crucible."
pecham, "a black coal," and gachelath, "burning coals." Pr 26:21; "as coals (fuel) are to burning coals," etc.; so we speak of quarrelsome men "adding fuel to the flame." "Coals of fire" in 2Sa 22:9,13, represent the lightning of God's wrath. In Pr 25:22, "heap coals of fire upon thine enemy's head" (Ro 12:20), the meaning is, melt him into burning shame at his own unworthy hatred, and love for thee who hast overcome his evil with thy good. Either he shall be like metals melted by fire or like clay hardened by it. In Ps 120:4 "coals of juniper" rather burning brands of broom, retamim. The Arabs regard the retem (broom) the best firewood.
As their slanders burnt like coals on fire, so, by righteous retribution in kind, God will give them hot coals. Ps 140:10; 18:12-13; compare the same image of the tongue, Jas 3:6. In 2Sa 14:7 "they shall quench my coal that is left," i.e., extinguish the only surviving light of my home, my only son. In Isa 6:6 and 1Ki 19:6 the "coals" are in the Hebrew (rezeph) hot stones, on which cakes were baked and flesh cooked. In Hab 3:5 (resheph) "burning coals" poetically and figuratively express "burning diseases," as the parallel "pestilence" shows; also compare De 32:24; Ps 91:6. In La 4:8 translate as margin darker than blackness." Mineral coal protrudes through the strata to the surface of parts of Lebanon, at Cornale, eight miles from Beirut, the coal seams are three feet thick; but it seems not to have been anciently known as fuel. Charcoal is what is meant by "coal."
Mineral coal was unknown in Bible times. Wherever 'coal' (or 'coals') is mentioned, therefore, we must in the great majority of cases understand wood or charcoal. Several species of wood used for heating purposes are named in Isa 44:14-16, to which Ps 120:4 adds 'coals of broom' (Revised Version margin). In two cases, however, the 'live coal' of Isaiah's vision (Isa 6:6) and the 'coals' on which was 'a cake haken' for Elijah (1Ki 19:6), the Heb. word denotes a hot stone (so Revised Version margin
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 Joh 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. 1Ki 19:6; Cant. 8:6; Isa 6:6; Hab 3:5.
Heaping coals of fire on an enemy's head by kindness (Pr 25:21-22; Ro 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.
The first and most frequent use of the word rendered coal is a live ember, burning fuel.
coals of fire are put metaphorically for the lightnings proceeding from God.
fuel not yet lighted is clearly signified. The fuel meant in the above passage is probably charcoal, and not coal in our sense of the word.