Why did Mozart leave the Great C Minor Mass unfinished?

The world of Mozart's Mass in C minor is full of issues, some relating to the work itself and others to the circumstances surrounding it. The main musical issue is that the work is unfinished in three quite different ways.

Some sections are complete, chorally but not instrumentally, others have survived only in an inaccurate and incomplete copy where Mozart's eight part choral writing was compressed into four or five parts because of the size of the available manuscript paper! Some sections Mozart did not set at all.

Many say ' perform the work as a noble torso, like Schubert's 'Unfinished' Symphony' . Others feel that this damages the overall proportions of the work and, what's more, the second short Hosanna makes a very poor and anti-climactic finale.

But, why did he not finish the work before the performance in Salzburg on 26 October 1783? By his standards he certainly had the time. Influenced and inspired by his recent exposure to the music of Bach and Handel, he had embarked on a vast scheme in this mass, rendering it all but useless for church purposes. Perhaps he just realised that the scheme was far too big for the classical style and maybe he was right.

For my completion, I have taken the form of the Coronation Mass K.317 as a rough template for the music of the Credo (from Et resurrexit to the end). The Et resurrexit thus becomes essentially a repeat of the music of the Credo in unum Deum, extended and developed to make a fitting climax for the Creed as a whole.

However, before the Et resurrexit comes the Crucifixus, which normally Mozart would have incorporated into the preceding Et incarnatus est. As he didn't in this case, a new movement is needed. It was not obvious what this key movement should be.

Then, I had a chance encounter in Italy with an early music group rehearsing the Masonic Funeral Music; exquisite music which is quite as weird in its own way as most of the Mass in C minor. I had found the Crucifixus, a powerful funeral march to follow the pastoral nativity scene, Et incarnatus est.

The words of the Agnus Dei come twice in the mass but, as the contexts are so very different, it is extremely important that the music should make different statements. So I have used the same music for the same words but have abridged Mozart's original Qui tollis and have totally redesigned the dynamic scheme to suit the music's new position.

If one rejects Mozart's jolly Austrian Dona nobis pacem fragment as an appropriate way to conclude such a magnificent work as this, just what does one do instead? The beautiful theme which Mozart uses only once (in the Christe eleison), taken from a vocal exercise written for his beloved Constanze, seems to be the ideal choice. More romantic than classical, perhaps, but this is true of so much of this mould-breaking work.

John Baird

John's completion of the Mozart C Minor Mass was performed by Medici Choir on Thursday 14th March 2013 At St.Martin-in-the-Fields, London.