All he had to do was set the door on fire, but he didn't want to trigger Potter's Trace. He disappeared into the library to look for a solution to his problem that wouldnt set off the Trace; hoping the solution would be inside one of the ancient tomes he had. Some of them were written in ancient French, he found the answer two hours later in a Renaissance volume. The author of it kept - in the middle of Dark Charms - an effective non-violent spell.