On his desk, Rosenberg keeps folders filled with unfinished pieces and paper he’s dying to use. “Definitely the first thing I do when I'm in a European city is go to the stationery or office-supply store. But most of it is found or from things you wouldn't necessarily buy for the paper — the inside of envelopes, packaging from a Bendel’s box, a yellowed page of a book. It’s not really the kind of thing you’d buy in a store.”