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This week Donald Macleod explores the miraculous chamber music of Mozart's Vienna years. Today, a great act of musical generosity and a revelatory encounter with two past masters.

Mozart's attendance at the regular Sunday afternoon gatherings of the diplomat Baron Gottfried van Swieten was more than just a matter of social networking; it was here that he encountered for the first time large-scale works by Bach and Handel that had fallen into widespread neglect since the composers' deaths. The effect on Mozart's writing is palpable, but it's an influence that he absorbed fully into his own musical language - a language so distinctive that it's surprising anyone could have been taken in by his attempt to pass off his magnificent Duo in B flat for violin and viola as a work by his old Salzburg court colleague Michael Haydn - a gifted composer, but hardly in Mozart's league. Haydn had run into a spot of bother with his employer, Archbishop Colloredo, who had commissioned him to write a set of six duos; but because of illness he'd only managed to complete four. Mozart obliged by dashing off the missing pair and allowing Haydn to claim them as his own. As Donald observes, "the richness of texture and ideas that Mozart manages to conjure from just half a string quartet is truly remarkable." Difficulties of a different sort were posed by the ensemble he wrote for in his Quintet in E flat, K 452. It's written for the unusual combination of piano, oboe, clarinet, bassoon and horn, which created problems of tonal blend for which Mozart found ingenious solutions. He was clearly delighted with the result, describing it in a letter as "the best thing I have ever written in my life".

Fugue in C minor for 2 pianos, K 426
András Schiff, Peter Serkin (pianos)

Duo in B flat for violin and viola, K 424
Antje Weithaas (violin)
Tabea Zimmermann (viola)

Quintet in E flat for piano, oboe, clarinet, bassoon and horn, K 452
András Schiff (piano)
Heinz Holliger (oboe)
Elmar Schmid (clarinet)
Klaus Thunemann (bassoon)
Radovan Vlatkovic (horn).