It shouldn't have annoyed him as much as it did, but for some reason, having his expertise questioned by a teenager felt perilously close to the last straw. He put the glass down on the table, more forcefully than was necessary. "Go ahead and pour it on your arms, since you know best," he snapped. "See what happens. I'm going upstairs to get you a regenerative and some proper bandages, if that's quite alright with you." He marched the cauldron to the back of the room for cleaning, scowling. He knew little more than when he'd started - except that a Caustic Compound was, apparently, no match for the Dark Mark. He paused thoughtfully, just before setting the cauldron in the basin. But - no, that was stupid.
He looked over his shoulder as he made his way for the door. "Don't move. I'll be back in a moment."