Owners of a 1718 Stradivarius violin have loaned it to the concertmaster of the Detroit Symphony. But only for two-and-a-half weeks. Then it’s back to the vault in which it lives. Why? The instrument is for sale, and it’s good publicity to get it played. But the instruments are so expensive – this one valued at about $3 million – that very few musicians could ever afford to play, let alone own one.