The prophets spoke like David described in Ps. 119 The curses would have no authority without atonement. The coals were endorsing the prophets curses by atonement.

Isa 6 At the sound of their voices the doorposts and thresholds shook and the temple was filled with smoke.
"Woe to me!" I cried. "I am ruined! For I am a man of unclean lips, and I live among a people of unclean lips, and my eyes have seen the King, the LORD Almighty."
 Then one of the seraphs flew to me with a live coal in his hand, which he had taken with tongs from the altar. With it he touched my mouth and said, "See, this has touched your lips; your guilt is taken away and your sin atoned for." Then I heard the voice of the Lord saying, "Whom shall I send? And who will go for us?"
And I said, "Here am I. Send me!"
He said, "Go and tell this people:
" 'Be ever hearing, but never understanding;
be ever seeing, but never perceiving.'
  Make the heart of this people calloused;
make their ears dull
and close their eyes. 
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts,
and turn and be healed."
Then I said, "For how long, O Lord?"
And he answered:
"Until the cities lie ruined
and without inhabitant,
until the houses are left deserted
and the fields ruined and ravaged,
until the LORD has sent everyone far away
and the land is utterly forsaken.