It is usually assumed that most of Bach’s instrumental music was composed before 1723, when he took up the post of cantor of St. Thomas in Leipzig, where church music became his central concern. The four surviving sonatas for flute and continuo probably belong to the years in Cöthen (1717-1723), although there is some evidence that this sonata, in E minor, was composed shortly after the move to Leipzig. These instrumental works served him well in later years when he took charge of a series of concerts given in a Leipzig coffee house, but what was played in those concerts has gone largely unrecorded, as coffee-house events inevitably would.
The Sonata is written for the side-blown traverso or “German” flute, which was rapidly supplanting the plain flauto (the recorder) with its superior tone and range. These flutes were usually made of boxwood, although ebony and ivory were sometimes used. Since there was only one key, or perhaps not even that, the player had to develop a skill in cross-fingering, derived from the fingering techniques still used in the recorder.
The four movements alternate fast and slow tempos, departing from the tonic minor key to a melodious relative major in the third. The basic dialog is between the flute and the bass in the keyboardist’s left hand, while the right hand supplies harmony and decoration as needed. In the last movement that dialog often takes the form of a canon, the one voice closely imitating the other. Bach sometimes seems to forget that the flutist has to breathe: the speedy second movement includes a passage of over 100 consecutive sixteenth-notes without a break!