"A waterglass solution," Al answered with a satisfied smile of someone who'd just completed a very important task. "Once it set in even Septimus couldn't taste the difference. Your books should be safe unless they're periodically drenched with water before being set on fire."
He gestured toward the bookshelves. "You can test it on any of the lower-shelves books or documents or scrolls... Oh, except that one." He concentrated and attempted a wordless Accio on the contract he wrote out for Severus. As a result the sheet of paper fluttered down on the floor and then leaped toward Al's outstretched hand. "This one hasn't been fireproofed yet. It still needs signatures."