"a burning coal" (cp. Eng., "anthracite,") is used in the plural in Rom 12:20, metaphorically in a proverbial expression, "thou shalt heap coals of fire on his head" (from Pro 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.
<2,,439, anthrakia>
akin to No. 1, is "a heap of burning coals, or a charcoal fire," Joh 18:18, Joh 21:9.