And another ANGEL, the one who has POWER over FIRE,
This may be the same angel with the golden censer (Revelation 8:3-5) and the one who "worked the miracle" in 1 Kings 18:17-41.
Physically, fire is an "oxidation" process and performs a rapid conversion of matter into other constituentants while generating heat and light. Thus the actual angel who will apparently do the conversion calls out to get on with the harvest so he can do his work (the final purging of evil within and among men).
came out from the ALTAR;
The altar in the Old Testament was where atonement was made for sins through animal sacrifices. Christ became the "final sacrifice" according to Hebrews 13:10.
Some scholars believe this is the same altar in Revelation 6:9 and Revelation 8:5. The request for vengeance has been heard.
and he called with a LOUD VOICE to him who had the SHARP SICKLE, saying,
This is the way angels talk in this chapter: verses 7, 9, and 15.
"Put in your SHARP SICKLE,
In previous verse, the angel is holding it. Again, SHARP denotes this SICKLE is "very effective and thorough". There is no chance any grapes will be left or escape being harvested. That chance to escape disappeared long ago. You have the chance TODAY to escape this harvest.
and gather the clusters from the vine of the earth, because HER GRAPES ARE RIPE."
This expression says the grapes are at their plumpest and juiciest. How deceived are these people (they took the mark of the beast, remember) to think that their life is SO RICH AND FULL? But they will be cut (Joel 3:13). Note the correlation of the "vine of the earth" to the EARTH BEAST (Revelation 13:13) complete with the implication of power over fire.