When the seraph takes that coal and touches it to Isaiah’s lips, he notes “thine iniquity is taken away, and thy sin purged.” Therefore, the burning coal symbolizes Isaiah’s cleansing, making him one of the pure capable of withstanding Yahweh’s glory. As a coal from a sacrifice to Yahweh, it also associates Isaiah with the sacrifice and, hence, with the atonement for sin. Yahweh, through the sacrifice on the altar, has cleansed Isaiah of his iniquities.