What did a seraph touch to Isaiah's lips to take away his sin?

Answer: A glowing coal

In Isaiah's vision of the Lord on His throne, a seraph touched his mouth with a coal from the altar.

“Then one of the seraphim flew to me, and in his hand was a glowing coal that he had taken with tongs from the altar. And with it he touched my mouth and said: “Now that this has touched your lips, your iniquity is removed and your sin is atoned for.””— Isaiah 6:6-7 (BSB)