Meanwhile, he added a march to the 1782 Haffner Symphony, then removed it along with a minuet (now lost), drafting in flutes and clarinets. At a performance given before Emperor Joseph, Mozart kicked off with the symphony's opening movements, following that with sundry arias, concertos and fugues before finishing off with the symphony's final movement. Mozart would not understand our pious reverence which prefers to swallow symphonies whole and without applause between the movements.
The choices a conductor faces are not terribly significant with the Haffner pieces, but they render notions of authenticity provisional. For his Barbican performance with the Chamber Orchestra of Europe on Friday, Nikolaus Harnoncourt opted to include the Serenade's rarely performed march, adding to a work that is already on the long side, but he omitted the Symphony's march, preferring the later flutes- and-clarinet version. Although he has long argued that period instrument purity is not the same as authenticity, Harnoncourt retained a few old-instrument touches - sparing string vibrato and valveless trumpets - producing a satisfyingly raucous rasp.
Harnoncourt has written of Mozart's 'chiaroscuro', the expressive contrasts that shape the music. He is not afraid to emphasise those contrasts, producing a choppy Sturm und Drang turbulence reminiscent of C P E Bach. Some find it difficult to take. Not the Barbican audience, which relished the muscle Harnoncourt brings, conducting without a baton but using a hyperactive left hand to punch out the rhythm. His demeanour is that of a solicitous undertaker, albeit one whose pent-up energy is on the brink of toppling over into dance. I swear that, in the symphony's minuet, he bowed to leader Marieke Blankestijn, as if about to escort her through a dance (and in a way I suppose he was).
Players and conductor seem comfortable together. As Blankestijn played the Serenade's solo violin, Harnoncourt watched closely, content to follow her lead rather than force a tempo on her. Conversely, players are prepared to take their eyes off him for long periods, which, with such finely co-ordinated playing, bespeaks productive rehearsals.
With nine movements (including the march) spanning an hour, the Haffner Serenade can get a little wearing. Each movement seems to preface some operatic action that never comes - this is, after all, something very like background music. Harnoncourt kept the pulse racing without forcing the tempo, and Blankestijn handled the solo violin parts - written to show off Mozart's own virtuosity - with grace under extreme pressure.
The Symphony is a shorter, and greater, work. The fire and swiftness demanded by Mozart match Harnoncourt's temperament, and the orchestra's relatively small size allowed detail to emerge as if restored.
It may be impossible to make Mozart sound 'new' but Harnoncourt and the COE render him fresh and vital. In the overcrowded premier league of contemporary Mozart interpreters, this is a team challenging for the top slot.