"a burning coal" (cp. Eng., "anthracite,") is used in the plural in Romans 12:20 , metaphorically in a proverbial expression, "thou shalt heap coals of fire on his head" (from Proverbs 25:22 ), signifying retribution by kindness, i.e., that, by conferring a favor on your enemy, you recall the wrong he has done to you, so that he repents, with pain of heart.
akin to No. 1, is "a heap of burning coals, or a charcoal fire," John 18:18; 21:9 .