"Coal from the Altar"

For some reason, Middletown Cemetery has a preponderance of odd quotations, pithy sayings, mottos and messages on the gravestones of its occupants. This one, "The angel touched his lips with a coal from the altar", is apparently a reference to Isaiah 6:5, 6:6 and 6:7:

Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the LORD of hosts.

Then flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from off the altar.

And he laid it upon my mouth, and said, Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin purged.