Mozart, during his first and only visit to Leipzig remarked enthusiastically after listening to J.S. Bach’s motet 'Singet dem Herrn ein neues Lied' ‘Now, there is something one can learn from’. And so he did!
After Bach’s death his music was almost forgotten. His memory was kept alive only in Leipzig and a few other little pockets of reverent admiration. In April 1782 Mozart wrote to his father: “Every Sunday at 12 I go to Baron van Swieten (President of the Court Commission Studies in Vienna) and there we play nothing but Händel and Bach. I am just building up a collection of Bach fugues – by Sebastian as well. One of the most important phases in Mozart’s creative development begins with these Sunday afternoons in van Swieten’s home. Alfred Einstein writes: “For Mozart the encounter with these compositions resulted in a revolution and a crisis in his creative activity. He was too great and fine a musician not to feel deeply and painfully the conflict produced when his habit of thinking in terms of gallant and ‘learned’ music was shaken by the encounter with a living polyphonic style and other fundamental elements of Bach’s music.’ Mozart began by copying out and rearranging some of Bach’s works, went on by composing fugues in Bach’s style, and from there gradually blended that style into his own in a more and more personal way.
For example he arranged Five Four-Part Fugues from Bach’s Well-Tempered Clavier for Two Violins, Viola and Basso. (K.405). This was followed by a period of fugue writing for various instrumental and vocal media, in Bach’s style. One of Mozart’s most successful essays in fugue texture from this time is a four-part fugue in C minor, K.426, written in December 1783 for two pianos. In June 1788 he arranged it for strings and added an introductory Adagio. This piece is the Adagio and Fugue in C minor, K 546. It was written on the same day as the famous C major piano sonata, K.545 and at the same time as the last three great symphonies, K.543, 550 and 551 (the “Jupiter”, which concludes with a great fugue.) The solemn Adagio, strongly marked by a powerful and lofty rhythm, plumbs the depths of remote keys within the short span of 52 bars and offers a gripping contrast to the resolute and energetic fugue. The fugue is based on a theme reminiscent of Bach’s Musical Offering. Mozart appears to have been haunted by the theme and returned to it in the very last months of his life for the fugal Kyrie of his Requiem (K.626).