The first and most frequent use of the word rendered coal is a live ember, burning fuel. (Proverbs 26:21) In (2 Samuel 22:9,13) "coals of fire" are put metaphorically for the lightnings proceeding from God. (Psalms 18:8,12,13; 140:10) In (Proverbs 26:21) 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.
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Smith, William, Dr. "Entry for 'Coal'". "Smith's Bible Dictionary".