In 1720, when Bach was 35, his employer Prince Leopold of Köthen decided to spend a couple months at the spa in Carlsbad (about 200 miles away) and took his retinue of musicians with him, including Bach. On their return Bach learned that Maria Barbara, his wife of 13 years, had inexplicably died (leaving behind their four children).
In those days if you were a musician, you either worked for royalty or you worked for the church. If you worked for royalty, your job was to provide new pleasing music for your employer — "Hey, Bach: we’re having the Grand Duchess to tea next week, let’s have a new trio sonata for the occasion." That is, a musician was a craftsman, with a status not dissimilar to the silversmith who made the lovely tea set. If you worked for the church your status was about the same.
To be sure some composers and some silversmiths were more capable than others, and were honored as such. But there was nothing like the ’hero worship’ of the "Great Artist who descends from an Olympian height to bring us Great Art" — a notion popular from the 19th century to the present day. Neither royalty nor the church were interested in the inner life of a craftsman commoner.
And so it would not have been in Bach’s world to decide "I’m going to write a piece of music about my loss" – such a piece would have been considered out of place in both court and church. (Personally, I think that "whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable" (Ph. 4:8) belongs in church, particularly when it wrestles with the big questions of Death, Love, Eternity, Suffering and God.)
At the time of his wife’s death Bach was working on a set of sonatas for solo violin. My personal feeling is that as Bach was working on them, enormous feelings of loss welled up and he set them to paper in this Chaconne ("sha-KONE"). So this set of sonatas is something like a lovely drive through the Appalachian Mountains during the spring, only to encounter Mount Kilimanjaro towering in one’s path.
Even in his day Bach was regarded as old fashioned, so upon his death his music was immediately relegated to the attic, studied only by students of composition like Mozart and Beethoven. But in the 19th century there was a revival of interest in Bach’s music, and much of it was arranged for massive forces like orchestras larger than anything Bach could ever have imagined. But bombastic arrangements of the Chaconne have never made sense to me – a piece conceived as a ’candle in the darkness’ for the vulnerable intimacy of a solo violin. So imagine my delight in recently finding an arrangement of it for the piano (since I don’t play the violin) that honored Bach’s original conception. Beginning with arranging it for the left hand alone — brilliant! And the arranger? Johannes Brahms! Brahms’ arrangement is as remarkable for what he doesn’t do as much for what he does — limiting himself to gently reworking idiomatic violin into idiomatic piano. (It’s still a difficult piece.)
Brahms said: "To me the Chaconne is one of the most beautiful, incredible compositions. On one staff, for a small instrument, the man writes a whole world of the deepest thoughts and most powerful feelings." The story goes that Brahms wrote the piece for Clara Schumann (an eminent concert pianist of her day) who particularly adored the Bach Chaconne, and was sidelined at the time with right-hand tendinitis.
A 'chaconne' is a piece built on a repeating chord progression and bass line. The Pachelbel "Canon in D" is perhaps the most famous example. The repeating chord progression/bass line over which one does variations continues to be popular, being the underlying musical form of much of Rap and Hip Hop, and arguably the Blues.
The piece is in D minor; at about the half-way mark he changes to a hymn-like section in D major (turning from sorrow at his loss to remembering their life together?), then he returns to D minor to conclude.