Free of Salzburg in 1780, Mozart established himself in Vienna as a freelance artist. He sought commissions from nobility and gave subscription concerts. He courted and married Constanze Weber, younger sister of Aloysia. Leopold harbored great resentment of Constanze and her family, fearing that after his son had broken with the archbishop he would lose what little control he had left over him. He had even withheld his blessing for the marriage until after the ceremony had taken place on 4 August 1782, knowing how hurtful that would be.
Wolfgang began working on this mass, his only one not written on commission, in late 1782. He says in a letter to his father that it represents a promise he made to Constanze to write a mass to celebrate the success of her difficult pregnancy and the birth of their first child on 17 June 1783. He never finished it, and what there is was performed only once during his lifetime, at the Benedictine abbey church of St. Peter on 26 October, at the end of the couple’s long-delayed visit to Salzburg. Nobody wrote down whether the missing parts were filled in by plainchant or by parts of other masses or how the music was received and it really doesn’t matter anymore because what there is is so good there doesn’t need to be any more. The couple left early the next morning and never returned.
Wolfgang apparently left the manuscript of “K427” behind with Leopold, who willed it to Nannerl upon his death in 1787. She in turn donated it to a monastery. Along the way, pieces of it were lost. What is left of it now resides in the German National Library. The original manuscripts of the Kyrie and Gloria have survived. So have the most crucial parts of “Credo in unum Deum” and “Et incarnatus est”, leaving modern editors room to tinker with what is missing to make them performable. There are instrumental parts in Mozart’s hand for “Sanctus”, “Hosanna”, and “Benedictus”, but no vocal parts. The vocal parts come down to us only from a copy made in about 1800 by Pater Matthäus Fischer from a set of parts that may have been Mozart’s originals. There is no indication that Mozart wrote any part of the Credo from “Crucifixus” on or anything at all for the Agnus Dei.
Since there have been so many recordings of “K427” and so many attempts to reconstruct the sketchy and missing parts, there has been no end of speculation as to what significance it had for Mozart and why he didn’t finish it during his three months in Salzburg—or thereafter.
Some pundits say the mass was intended as a peace offering from Wolfgang to Leopold. If so, it was unsuccessful, as there is no evidence that either Leopold or Nannerl received the couple with any warmth. Wolfgang was especially hurt that Leopold did not offer Constanze any of the trinkets or awards Wolfgang had received as a child. The letters between father and son after the visit take on a distinctly cooler tone than before.
We know that Mozart had become enamored of the contrapuntal music of Bach and Handel during Sunday afternoon studies in 1782 with court librarian and career diplomat Baron Gottfried van Swieten and subsequently threw a lot of what he learned into “K427”. Some say that once he felt he had mastered and updated the Baroque style, he lost interest in continuing the mass. Besides, if finished, it would have run nearly two hours and could not have been performed in any church in Vienna under the brevity reforms of Joseph II.
Bach’s “B minor mass”, which was in large part Mozart’s musical inspiration as a summing up of church style, was of similar length and also had never been performed in its entirety during his lifetime. However, although it took Bach sixteen years, he did eventually complete it.
As Mozart did not finish a number of compositions he wrote with Constanze in mind, mostly fugues and vocal exercises, some suggest that he didn’t really care about her and was still really in love with her sister Aloysia.
Some say he was just so busy with other music during the visit to Salzburg that he never got back to the mass, but the only things we know for certain that he wrote while there were two long overdue duets for violin and viola under Michael Haydn’s name and in his style, as otherwise Haydn’s alcoholism was about to get his salary withheld.
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